MOSFET output stage [Archive]

Kevin McMurtrie[_2_]

September 18th 08, 08:00 AM

In article >,
"Trevor Wilson" > wrote:

> "RichD" > wrote in message
> ...
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
> don't.
>
> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost
> as good as BJTs.
>
> MOSFETs are very tough, have an exceptional ability to deliver high power,
> high frequency audio (and RF), but distortion is very high. They require
> lots of Global NFB in order to operate linearly.

That's nonsense. MOSFETs have a lower voltage gain, more capacitive
load, and infinite DC current gain. If you swap your bipolar
transistors for MOSFETs and crank up the bias voltage, yeah, it will run
like crap because the local feedback gain is totally wrong. It might
even incapacitate every radio within two blocks of your home then
violently explode. The circuits to drive MOSFETs and bipolars are a
little different.

> MOSFETs do not sound better than BJTs. At best, they can sound as good. All
> things being equal.

--
Google is a pro-spamming service. I will not see your reply if you use Google.

Randy Yates

September 18th 08, 10:19 AM

"Phil Allison" > writes:

> "Trevor Wilson"
> "RichD"
>>
>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>> driver?
>>
>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>> don't.
>>
>> As a device,
>
>
> ** Same old red-herring ******** from know nothing Wilson.
>
> It is totally irrelevant how DEVICES behave on their own.
>
> Only when they are properly configured and suitably biased in a
> complimentary output stage can performances be compared.
>
>
> operating at typical bias currents (say) 10-50mA, MOSFETs
>> exhibit VASTLY more THD than BJTs.
>
>
> ** UTTER ******** !!!

No it's not.

> When configured in typical amplifier topologies, THD is vanishingly small at
> all power levels and with minimal bias current.

That may be true, but only because of the negative feedback that Trevor
referred to. The basic device IS more nonlinear.

It comes down to device physics. MOSFETs, when operating in the
saturation region, have a square law relationship between Vgs and
Ids. It is only through either a) the application of negative feedback,
or b) the reduction of input level (the smaller the operating range, the
more approximating linear the Ids/Vgs relationship becomes) that the
device distortion is mitigated.
--
% Randy Yates % "Maybe one day I'll feel her cold embrace,
%% Fuquay-Varina, NC % and kiss her interface,
%%% 919-577-9882 % til then, I'll leave her alone."
%%%% > % 'Yours Truly, 2095', *Time*, ELO
http://www.digitalsignallabs.com

Jorden Verwer

September 18th 08, 10:25 AM

RichD wrote:
> Who do MOSFET sound better than bipolar, as an audio amp output
> driver?

The device properties of BJTs are superior to those of MOSFETs in all
respects, except for offset - there MOSFETs have the advantage. Whether you
will actually hear this depends on many more factors.

Randy Yates

September 18th 08, 10:35 AM

Randy Yates > writes:

> "Phil Allison" > writes:
>
>> "Trevor Wilson"
>> "RichD"
>>>
>>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>>> driver?
>>>
>>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>>> don't.
>>>
>>> As a device,
>>
>>
>> ** Same old red-herring ******** from know nothing Wilson.
>>
>> It is totally irrelevant how DEVICES behave on their own.
>>
>> Only when they are properly configured and suitably biased in a
>> complimentary output stage can performances be compared.
>>
>>
>> operating at typical bias currents (say) 10-50mA, MOSFETs
>>> exhibit VASTLY more THD than BJTs.
>>
>>
>> ** UTTER ******** !!!
>
> No it's not.
>
>> When configured in typical amplifier topologies, THD is vanishingly small at
>> all power levels and with minimal bias current.
>
> That may be true, but only because of the negative feedback that Trevor
> referred to. The basic device IS more nonlinear.
>
> It comes down to device physics. MOSFETs, when operating in the
> saturation region, have a square law relationship between Vgs and
> Ids. It is only through either a) the application of negative feedback,
> or b) the reduction of input level (the smaller the operating range, the
> more approximating linear the Ids/Vgs relationship becomes) that the
> device distortion is mitigated.

PS: See, e.g., [gray] and [razavi].

--Randy

@book{gray,
title = "Analysis and Design of Analog Integrated Circuits",
author = "Paul R. Gray and Paul J. Hurst and Stephen H. Lewis and Robert G. Meyer",
publisher = "Wiley",
edition = "fourth",
year = "2001"}
@book{razavi,
title = "Design of Analog CMOS Integrated Circuits",
author = "Behzad Razavi",
publisher = "McGraw-Hill",
year = "2001"}

--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% > % 'Waterfall', *Face The Music*, ELO
http://www.digitalsignallabs.com

Gareth Magennis

September 18th 08, 10:44 AM

"RichD" > wrote in message
...
> Who do MOSFET sound better than bipolar, as an audio amp output
> driver?
>
>
> --
> Rich

Well this is probably an urban myth/load of ********, but years ago lots of
people were talking about power Mosfets exhibiting different distortion
characteristics than BJT's in the same way thermionic valves distort with
differening harmonics and sound nicer.

Fact or crapology?

Gareth.

Eeyore

September 18th 08, 10:49 AM

Kevin McMurtrie wrote:

> In article >,
> Damon Hill > wrote:
>
> > RichD > wrote in news:b78fc9c2-fe9c-444c-8ac5-
> > :
> >
> > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > driver?
> >
> > They do? Seems like it's possible to design good amplifiers either
> > way.
> >
> > --Damon
>
> Exactly - zero difference in quality capabilities. It's usually a
> matter of impedance matching. Silicon transistors have a fixed loss
> around 0.5 volts. MOSFETs have a resistive loss inversely proportional
> to their voltage rating. That usually makes MOSFETs less expensive for
> low impedances and transistors less expensive for high impedances.

TOTAL AND COMPLETE GARBAGE.

Eeyore

September 18th 08, 10:53 AM

Trevor Wilson wrote:

> "RichD" > wrote
>
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
> don't.
>
> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost
> as good as BJTs.
>
> MOSFETs are very tough, have an exceptional ability to deliver high power,
> high frequency audio (and RF), but distortion is very high. They require
> lots of Global NFB in order to operate linearly.
>
> MOSFETs do not sound better than BJTs. At best, they can sound as good. All
> things being equal.

With the required amount of feedback (which isn't rocket science - just good
design) they have no trouble meeting VERY low THDs such as one I designed with
0.0008% THD @ 1 kHz.

They sound better because of the predominance of 2nd harmonic distortion and
almost complete absence of 3rd and ditto up the range AIUI with even and odd
harmonics.

GRaham

Trevor Wilson[_2_]

September 18th 08, 10:55 AM

"Kevin McMurtrie" > wrote in message
...
> In article >,
> "Trevor Wilson" > wrote:
>
>> "RichD" > wrote in message
>> ...
>> > Who do MOSFET sound better than bipolar, as an audio amp output
>> > driver?
>>
>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>> don't.
>>
>> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
>> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
>> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are
>> almost
>> as good as BJTs.
>>
>> MOSFETs are very tough, have an exceptional ability to deliver high
>> power,
>> high frequency audio (and RF), but distortion is very high. They require
>> lots of Global NFB in order to operate linearly.
>
> That's nonsense.

**Er, nope. Here are the specs for a modern high power BJT:

http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7890.pdf

Note the hFE vs. Ic. Particularly at elevated temps. It is almost a straight
line, from less than 10mA to several Amps. I call that spectacular
linearity.

Now, I draw your attention to a high power MOSFET:

http://www.magnatec-uk.com/pdf/magnatec/BUZ900.pdf

Note the characteristics of this device. They're pretty good, but as good as
a modern BJT.

Here is another, older, worse example:

http://www.datasheetarchive.com/pdf-datasheets/Datasheets-8/DSA-158541.html

Quite ordinary lineariy.

MOSFETs have a lower voltage gain, more capacitive
> load, and infinite DC current gain. If you swap your bipolar
> transistors for MOSFETs and crank up the bias voltage, yeah, it will run
> like crap because the local feedback gain is totally wrong.

**I never suggested anything of the sort. I mentioned ONLY the intrinsic
linearity of the devices. MOSFETs are inferior to BJTs. For now. When
installed in an appropriate topology, it is likely that there will be little
audible, nor measurable difference between a MOSFET amp and a BJT amp.
Device linearity is another story.

It might
> even incapacitate every radio within two blocks of your home then
> violently explode. The circuits to drive MOSFETs and bipolars are a
> little different.

**Again, not in dispute. The intrinsic linearity, is what I refer to.

--
Trevor Wilson
www.rageaudio.com.au

Eeyore

September 18th 08, 10:58 AM

Phil Allison wrote:

> "Trevor Wilson"
> "RichD"
> >
> >> Who do MOSFET sound better than bipolar, as an audio amp output
> >> driver?
> >
> > **Non-sequitur. If you're saying that MOSFET outputs sound better, they
> > don't.
> >
> > As a device,
>
> ** Same old red-herring ******** from know nothing Wilson.
>
> It is totally irrelevant how DEVICES behave on their own.
>
> Only when they are properly configured and suitably biased in a
> complimentary output stage can performances be compared.
>
> operating at typical bias currents (say) 10-50mA, MOSFETs
> > exhibit VASTLY more THD than BJTs.
>
> ** UTTER ******** !!!
>
> When configured in typical amplifier topologies, THD is vanishingly small at
> all power levels and with minimal bias current.
>
> This is true for LATERAL mosfets as made by Hitachi and Semelab ( ie 2SK176,
> BUZ905 etc ).
>
> Switching mosfets ( ie IRFPxxx etc) are not the same.

Yes.

Just for fun once I configured a lateral mosfet output stage with zero feedback.
Its THD at almost all power levels was ~ 1%. From that you can get a good idea
of the negative feedback required to obtain any THD you like.

Lateral mosfets, correctly biased also exhibit virtually NO crossover
distortion. This is all but impossible with bipolars due to their inherent
transfer characteristics unless operated in pure Class A.

Graham

Eeyore

September 18th 08, 11:00 AM

Kevin McMurtrie wrote:

> That's nonsense. MOSFETs have a lower voltage gain

Mosfets are used as source followers in an audio output stage you inane idiot.

Voltage gain is NOT AN ISSUE, regardless of whether or not your statement is true.

Graham

Eeyore

September 18th 08, 11:04 AM

Randy Yates wrote:

> "Phil Allison" > writes:
>
> > When configured in typical amplifier topologies, THD is vanishingly small at
> > all power levels and with minimal bias current.
>
> That may be true, but only because of the negative feedback that Trevor
> referred to. The basic device IS more nonlinear.

But when given suitable NFB, the transfer characteristic IS 'cleaner'. Very little
odd harmonic distortion (the nasty stuff) and very very low crossover distortion
(which makes a big difference).

I know, I've designed audio amps with both types. As in designed from the ground
up for commercial pro-audio sale.

Graham

Eeyore

September 18th 08, 11:05 AM

Jorden Verwer wrote:

> RichD wrote:
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> The device properties of BJTs are superior to those of MOSFETs in all
> respects,

How about SOA for one you UTTER MORON ?

Do you even know what SOA is ?

> except for offset - there MOSFETs have the advantage. Whether you
> will actually hear this depends on many more factors.

YET MORE INSANE ********

Is this all down to techs not knowing their chops ?

Eeyore

September 18th 08, 11:07 AM

Gareth Magennis wrote:

> "RichD" > wrote
>
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> Well this is probably an urban myth/load of ********, but years ago lots of
> people were talking about power Mosfets exhibiting different distortion
> characteristics than BJT's in the same way thermionic valves distort with
> differening harmonics and sound nicer.
>
> Fact or crapology?

Fact. Different transfer characteristics. Fundamental device physics.

Graham

Fred Bartoli[_2_]

September 18th 08, 11:21 AM

Eeyore a écrit :
>
> Trevor Wilson wrote:
>
>> "RichD" > wrote
>>
>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>> driver?
>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>> don't.
>>
>> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
>> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
>> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost
>> as good as BJTs.
>>
>> MOSFETs are very tough, have an exceptional ability to deliver high power,
>> high frequency audio (and RF), but distortion is very high. They require
>> lots of Global NFB in order to operate linearly.
>>
>> MOSFETs do not sound better than BJTs. At best, they can sound as good. All
>> things being equal.
>
> With the required amount of feedback (which isn't rocket science - just good
> design) they have no trouble meeting VERY low THDs such as one I designed with
> 0.0008% THD @ 1 kHz.
>
> They sound better because of the predominance of 2nd harmonic distortion and
> almost complete absence of 3rd and ditto up the range AIUI with even and odd
> harmonics.
>

Uhhh.

Please explain how you obtain even distortion products from a
symmetrical (suppose paired mosfets) push-pull.

Any symmetrical function produces odd (and no even) harmonics.

:-) ...unless you used single ended output stages.

--
Thanks,
Fred.

Phil Allison

September 18th 08, 11:26 AM

"Randy Yates"
> "Phil Allison"
>>
>>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>>> don't.
>>>
>>> As a device,
>>
>>
>> ** Same old red-herring ******** from know nothing Wilson.
>>
>> It is totally irrelevant how DEVICES behave on their own.
>>
>> Only when they are properly configured and suitably biased in a
>> complimentary output stage can performances be compared.
>>
>>
>> operating at typical bias currents (say) 10-50mA, MOSFETs
>>> exhibit VASTLY more THD than BJTs.
>>
>>
>> ** UTTER ******** !!!
>
> No it's not.

** Yes it is you pathetic, know nothing MORON,

>> When configured in typical amplifier topologies, THD is vanishingly small
>> at
>> all power levels and with minimal bias current.
>
> That may be true, but only because of the negative feedback that Trevor
> referred to.

** Bull.

> The basic device IS more nonlinear.

** Irrelevant.

Can't you read - dickhead.

..... Phil

Gareth Magennis

September 18th 08, 11:32 AM

"Eeyore" > wrote in message
...
>
>
> Gareth Magennis wrote:
>
>> "RichD" > wrote
>>
>> > Who do MOSFET sound better than bipolar, as an audio amp output
>> > driver?
>>
>> Well this is probably an urban myth/load of ********, but years ago lots
>> of
>> people were talking about power Mosfets exhibiting different distortion
>> characteristics than BJT's in the same way thermionic valves distort with
>> differening harmonics and sound nicer.
>>
>> Fact or crapology?
>
> Fact. Different transfer characteristics. Fundamental device physics.
>
> Graham
>

Well in that case the OP's question is thus answered, as far as I can tell.
It sure is noisy in here.

Gareth.

Phil Allison

September 18th 08, 11:33 AM

"Gareth Magennis"
>
>
> Well this is probably an urban myth/load of ********, but years ago lots
> of people were talking about power Mosfets exhibiting different distortion
> characteristics than BJT's in the same way thermionic valves distort with
> differening harmonics and sound nicer.
>
> Fact or crapology?

** Audiophool crapology - repeated parrot fashion, ad nauseam.

The THD residual of even the simplest topology, lateral mosfet audio
amplifiers is vanishingly small.

Sub the threshold of audibility.

..... Phil

Phil Allison

September 18th 08, 11:36 AM

"Gareth Magennis"

>
> Well in that case the OP's question is thus answered, as far as I can
> tell.

** It can never be answered.

Since it was a UTTERLY meaningless TROLL.

You pathetic fool.

..... Phil

Gareth Magennis

September 18th 08, 11:46 AM

"Phil Allison" > wrote in message
...
>
> "Gareth Magennis"
>>
>>
>> Well this is probably an urban myth/load of ********, but years ago lots
>> of people were talking about power Mosfets exhibiting different
>> distortion characteristics than BJT's in the same way thermionic valves
>> distort with differening harmonics and sound nicer.
>>
>> Fact or crapology?
>
>
> ** Audiophool crapology - repeated parrot fashion, ad nauseam.
>
> The THD residual of even the simplest topology, lateral mosfet audio
> amplifiers is vanishingly small.
>
> Sub the threshold of audibility.
>
>
>
> .... Phil
>

Not if you clip them like you might a valve amp for example. Would a Mosfet
amp clip more "nicely" than a BJT amp? I'm thinking driving bass speakers.
Is this what some of these "audiophools" or pehaps PA guys are getting at by
saying they sound better?

Gareth.

Phil Allison

September 18th 08, 11:50 AM

"Gareth Magennis"
> "Phil Allison">
>>>
>>> Well this is probably an urban myth/load of ********, but years ago lots
>>> of people were talking about power Mosfets exhibiting different
>>> distortion characteristics than BJT's in the same way thermionic valves
>>> distort with differening harmonics and sound nicer.
>>>
>>> Fact or crapology?
>>
>>
>> ** Audiophool crapology - repeated parrot fashion, ad nauseam.
>>
>> The THD residual of even the simplest topology, lateral mosfet audio
>> amplifiers is vanishingly small.
>>
>> Sub the threshold of audibility.
>>
>
> Not if you clip them like you might a valve amp for example.

** Only ****ing half-wits listen to clipped audio from a home hi-fi.

> Would a Mosfet amp clip more "nicely" than a BJT amp?

** No.

Any more ****wit MOSFET myths you want to give an airing here ?

Imbecile.

...... Phil

Gareth Magennis

September 18th 08, 11:59 AM

"Phil Allison" > wrote in message
...
>
> "Gareth Magennis"
>> "Phil Allison">
>>>>
>>>> Well this is probably an urban myth/load of ********, but years ago
>>>> lots of people were talking about power Mosfets exhibiting different
>>>> distortion characteristics than BJT's in the same way thermionic valves
>>>> distort with differening harmonics and sound nicer.
>>>>
>>>> Fact or crapology?
>>>
>>>
>>> ** Audiophool crapology - repeated parrot fashion, ad nauseam.
>>>
>>> The THD residual of even the simplest topology, lateral mosfet audio
>>> amplifiers is vanishingly small.
>>>
>>> Sub the threshold of audibility.
>>>
>>
>> Not if you clip them like you might a valve amp for example.
>
>
> ** Only ****ing half-wits listen to clipped audio from a home hi-fi.
>
>
>> Would a Mosfet amp clip more "nicely" than a BJT amp?
>
>
> ** No.
>
> Any more ****wit MOSFET myths you want to give an airing here ?
>
> Imbecile.
>
>
>
> ..... Phil
>

Er, perhaps. Mosfets don't have the same thermal characteristics as BJT's,
which are more prone to thermal runaway. So perhaps when you are runing
Mosfets hard, there is some kind of compression thing going on which sounds
nicer than a BJT amp exploding after clipping a lot.

Gareth.

Eeyore

September 18th 08, 12:11 PM

Fred Bartoli wrote:

> Eeyore a écrit :
> > Trevor Wilson wrote:
> >> "RichD" > wrote
> >>
> >>> Who do MOSFET sound better than bipolar, as an audio amp output
> >>> driver?
> >> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
> >> don't.
> >>
> >> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> >> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
> >> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost
> >> as good as BJTs.
> >>
> >> MOSFETs are very tough, have an exceptional ability to deliver high power,
> >> high frequency audio (and RF), but distortion is very high. They require
> >> lots of Global NFB in order to operate linearly.
> >>
> >> MOSFETs do not sound better than BJTs. At best, they can sound as good. All
> >> things being equal.
> >
> > With the required amount of feedback (which isn't rocket science - just good
> > design) they have no trouble meeting VERY low THDs such as one I designed with
> > 0.0008% THD @ 1 kHz.
> >
> > They sound better because of the predominance of 2nd harmonic distortion and
> > almost complete absence of 3rd and ditto up the range AIUI with even and odd
> > harmonics.
>
> Uhhh.
>
> Please explain how you obtain even distortion products from a
> symmetrical (suppose paired mosfets) push-pull.

If you're such an expert, built one and see for yourself. And use LATERAL mosfets
designed for audio.

TWIT.

Graham

Eeyore

September 18th 08, 12:18 PM

Gareth Magennis wrote:

> "Eeyore" wrote
> > Gareth Magennis wrote:
> >> "RichD" wrote
> >>
> >> > Who do MOSFET sound better than bipolar, as an audio amp output
> >> > driver?
> >>
> >> Well this is probably an urban myth/load of ********, but years ago lots
> >> of people were talking about power Mosfets exhibiting different distortion
> >> characteristics than BJT's in the same way thermionic valves distort with
> >> differening harmonics and sound nicer.
> >>
> >> Fact or crapology?
> >
> > Fact. Different transfer characteristics. Fundamental device physics.
>
> Well in that case the OP's question is thus answered, as far as I can tell.
> It sure is noisy in here.

It is indeed, with a whole bunch of nutcases spouting nonsense.

It's a fact that 'open loop' with no NFB, a mosfet output stage will indeed
distort more than a bipolar one, but the distortion characteristics are
different. Put some gain on the front and close the loop and the mosfet wins
every time (admittedly requiring more overall NFB but it can take it).

That 0.0008% THD amp I mentioned, you could not see ANY vestiges of crossover
distortion on an Audio Precision analyser output, and what you could see was
mainly 2nd harmonic.

Oh and its THD floor is 0.0007%, so doubtless the amp was rather better than the
display said.

Graham

Eeyore

September 18th 08, 12:20 PM

Gareth Magennis wrote:

> Not if you clip them like you might a valve amp for example. Would a Mosfet
> amp clip more "nicely" than a BJT amp?

Not really.

> I'm thinking driving bass speakers.
> Is this what some of these "audiophools" or pehaps PA guys are getting at by
> saying they sound better?

I know of no 'PA' amps currently using mosfets. Everyone wants cheap these days
and bipolars are cheaper.

Graham

Eeyore

September 18th 08, 12:21 PM

Gareth Magennis wrote:

> Er, perhaps. Mosfets don't have the same thermal characteristics as BJT's,
> which are more prone to thermal runaway. So perhaps when you are runing
> Mosfets hard, there is some kind of compression thing going on which sounds
> nicer than a BJT amp exploding after clipping a lot.

Competently designed BJT amps don't 'explode'. It's not difficult, but the
Chinese haven't quite mastered it yet.

Graham

Phil Allison

September 18th 08, 12:43 PM

"Gareth Magennis"
> "Phil Allison"
>>>
>>>
>>> Not if you clip them like you might a valve amp for example.
>>
>>
>> ** Only ****ing half-wits listen to clipped audio from a home hi-fi.
>>
>>
>>> Would a Mosfet amp clip more "nicely" than a BJT amp?
>>
>>
>> ** No.
>>
>> Any more ****wit MOSFET myths you want to give an airing here ?
>>
>> Imbecile.
>
>
> Er, perhaps.

** Wot a glutton for punishment we have here - folks.

> Mosfets don't have the same thermal characteristics as BJT's, which are
> more prone to thermal runaway. So perhaps when you are runing Mosfets
> hard, there is some kind of compression thing going on which sounds nicer
> than a BJT amp exploding after clipping a lot.

** Another 100% ****wit MOSFET myth.............

Go away - you IDIOT !!!

....... Phil

Gareth Magennis

September 18th 08, 12:44 PM

"Eeyore" > wrote in message
...
>
>
> Gareth Magennis wrote:
>
>> Er, perhaps. Mosfets don't have the same thermal characteristics as
>> BJT's,
>> which are more prone to thermal runaway. So perhaps when you are runing
>> Mosfets hard, there is some kind of compression thing going on which
>> sounds
>> nicer than a BJT amp exploding after clipping a lot.
>
> Competently designed BJT amps don't 'explode'. It's not difficult, but the
> Chinese haven't quite mastered it yet.
>
> Graham
>

The point is that when people prefer one amp over another it may not be easy
to tell technically what it is they prefer. If a Mosfet amp compresses the
bottom end slightly over a BJT, for example, this might in the long term be
a nicer sounding amp. Maybe absolutely nothing to do with crossover
distorion, linearity, feedback blah blah blah.

Gareth.

Gareth Magennis

September 18th 08, 12:53 PM

"Phil Allison" > wrote in message
...
>
> "Gareth Magennis"
>> "Phil Allison"
>>>>
>>>>
>>>> Not if you clip them like you might a valve amp for example.
>>>
>>>
>>> ** Only ****ing half-wits listen to clipped audio from a home hi-fi.
>>>
>>>
>>>> Would a Mosfet amp clip more "nicely" than a BJT amp?
>>>
>>>
>>> ** No.
>>>
>>> Any more ****wit MOSFET myths you want to give an airing here ?
>>>
>>> Imbecile.
>>
>>
>> Er, perhaps.
>
>
> ** Wot a glutton for punishment we have here - folks.
>
>
>> Mosfets don't have the same thermal characteristics as BJT's, which are
>> more prone to thermal runaway. So perhaps when you are runing Mosfets
>> hard, there is some kind of compression thing going on which sounds nicer
>> than a BJT amp exploding after clipping a lot.
>
>
> ** Another 100% ****wit MOSFET myth.............
>
> Go away - you IDIOT !!!
>
>
>
> ...... Phil
>

No, this is good fun, and you might eventually tell me why my supposition is
untrue.

Gareth.

Arny Krueger

September 18th 08, 01:02 PM

"RichD" > wrote in message

> Who do MOSFET sound better than bipolar, as an audio amp
> output driver?

Who do? That's voodoo!

Arny Krueger

September 18th 08, 01:04 PM

"Eeyore" > wrote in
message
> Kevin McMurtrie wrote:
>
>> In article >,
>> Damon Hill > wrote:
>>
>>> RichD > wrote in
>>> news:b78fc9c2-fe9c-444c-8ac5-
>>> :
>>>
>>>> Who do MOSFET sound better than bipolar, as an audio
>>>> amp output driver?
>>>
>>> They do? Seems like it's possible to design good
>>> amplifiers either way.
>>>
>>> --Damon
>>
>> Exactly - zero difference in quality capabilities. It's
>> usually a matter of impedance matching. Silicon
>> transistors have a fixed loss around 0.5 volts. MOSFETs
>> have a resistive loss inversely proportional to their
>> voltage rating. That usually makes MOSFETs less
>> expensive for low impedances and transistors less
>> expensive for high impedances.
>
> TOTAL AND COMPLETE GARBAGE.

Agreed. Actually, what Kevin said is the exact reverse of generally accepted
practical knowledge. Bipolar is generally more efficient when the impedances
get really low. MOSFETs were trendy for a while, but most new power amp
designs seem to have bipolar outputs.

Arny Krueger

September 18th 08, 01:05 PM

> wrote in message

> On Sep 17, 6:12 pm, RichD > wrote:
>> Who do MOSFET sound better than bipolar, as an audio amp
>> output driver?

> MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> odd harmonic distortin. on and on.

Not in any relevant way for audio power amps.

Phil Allison

September 18th 08, 01:08 PM

"Eeysore"

> I know of no 'PA' amps currently using mosfets.

** Huh ????

All those hundreds of thousands of MOSFET audio power amps made since the
mid 1980s have not justs disappeared you know - power amps made by H-H &
Harrison Electronics plus C-Audio in the UK , Perreaux and ZPE of NZ,
Australian Monitor and ARX (still in full production) and Jands here in
Aussie - plus many other less well known brands.

The majority are STILL in use, maybe looking just a bit the worse for wear.

Even in the UK - Chevin Research ( based in Yorkshire) A-series amps are
all lateral mosfet designs.

http://www.chevin-research.com/products_a_series.php

...... Phil

Phil Allison

September 18th 08, 01:11 PM

"Gareth Magennis"

** Bugger off - ******.

>> Gareth Magennis wrote:

> If a Mosfet amp compresses the bottom end slightly over a BJT,

..... blah, blah , blah .....

Phil Allison

September 18th 08, 01:13 PM

"Gareth Magennis"

> No, this is good fun, and you might eventually tell me why my supposition
> is untrue.

** Cos nothing makes it true.

YOU ****ING Z- GRADE IMBECILE !!!

..... Phil

Arny Krueger

September 18th 08, 01:14 PM

"Gareth Magennis" > wrote in
message

> Not if you clip them like you might a valve amp for
> example.

Straw man. It is entirely practical to build power amps that are never
clipped in actual use. If you don't like how your power amp clips? Get one
with enough output so that it never clips.

> Would a Mosfet amp clip more "nicely" than a
> BJT amp?

Power amps aren't just devices, they are circuits. Circuit design can
easily trump device characteristics.

In fact the sharpness of the clipping of a power amp relates to things like
how much negative feedback it has, all things considered. If you have a
circuit with lots of negative feedback, it is very likely to clip very
sharply and ideally. If you have a circuit with less negative feedback, the
clipping will be softer, but will occupy a larger proportion of the transfer
characteristic.

> I'm thinking driving bass speakers.

If they need lots of power, get a powerful power amp. Forget about what's
inside the box, worry about how the box works.

> Is this
> what some of these "audiophools" or perhaps PA guys are
> getting at by saying they sound better?

This whole "amp sounds better" stuff was fully debunked 30 years ago. Good
power amps sound the same and they sound like a piece of wire with gain.

There are tons of power amps that can't be distinguished from a piece of
wire with gain, while driving well-designed speakers.

There are quite a few amps that meet the same criteria while driving even
the weirdest speaker load.

A really good power amp will destroy a poorly-designed speaker before it
starts sounding bad, and really good power amps aren't all that unique.

Audio Expert

September 18th 08, 01:34 PM

On a sunny day (Wed, 17 Sep 2008 16:12:25 -0700 (PDT)) it happened RichD
> wrote in
>:

>Who do MOSFET sound better than bipolar, as an audio amp output
>driver?

MoSfEtS zound batter becuaze thOze work wiz EleKtRONS, LikE TubeS.
Traanzisters WOrk wiz HOLES, AND Thoze HoLES YoU wILL HeAr in Ze Muzick.

Vladimir Vassilevsky

September 18th 08, 03:06 PM

RichD wrote:

> Who do MOSFET sound better than bipolar, as an audio amp output
> driver?

In most cases, this is fallacy, and the result is just the opposite.
Reason: FETs have lower transconductance compared to BJTs. It is
impossible to build a half bridge stage with an ideal transfer curve.

However there are few special cases when a FET output stage has an
advantage:

1) With FETs, it is simpler to control bias current, because of the
negative dependency from the temperature. That simplifies the life.

2) If the global warming is not an issue, then the class A stage made of
drain follower loaded by the current source can be very linear indeed.

3) FETs are free from BJT high injection effects and the charge
accumulation in the base.

4) FETs do not require high base currents of BJTs; that simplifies the
driver stage.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Eeyore

September 18th 08, 03:36 PM

Gareth Magennis wrote:

> "Eeyore" wrote
> > Gareth Magennis wrote:
> >
> >> Er, perhaps. Mosfets don't have the same thermal characteristics as
> >> BJT's, which are more prone to thermal runaway. So perhaps when you are
> runing
> >> Mosfets hard, there is some kind of compression thing going on which
> >> sounds nicer than a BJT amp exploding after clipping a lot.
> >
> > Competently designed BJT amps don't 'explode'. It's not difficult, but the
> > Chinese haven't quite mastered it yet.
>
>
> The point is that when people prefer one amp over another it may not be easy
> to tell technically what it is they prefer.

Flatter frequency reponse perhaps ?

> If a Mosfet amp compresses the
> bottom end slightly over a BJT, for example, this might in the long term be
> a nicer sounding amp. Maybe absolutely nothing to do with crossover
> distorion, linearity, feedback blah blah blah.

How would this 'compression' occur ?

Graham

Eeyore

September 18th 08, 03:37 PM

Arny Krueger wrote:

> "RichD" > wrote
>
> > Who do MOSFET sound better than bipolar, as an audio amp
> > output driver?
>
> Who do? That's voodoo!

Only to be expected from Arny "Any amp with less than 0.1% THD at full
power sounds the same as all other amps."

Not quite as inane as some comments so far.

Graham

Eeyore

September 18th 08, 03:40 PM

Arny Krueger wrote:

> > wrote
> > RichD wrote:
> >> Who do MOSFET sound better than bipolar, as an audio amp
> >> output driver?
>
> > MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> > odd harmonic distortin. on and on.
>
> Not in any relevant way for audio power amps.

The first two are highly relevevant in ANY circuit using NFB. Basic
stability criteria.

For Christ's sake has everyone except Phil's and my brain turned to jelly
overnight ?

Graham

Eeyore

September 18th 08, 03:43 PM

Phil Allison wrote:

> "Eeysore"
>
> > I know of no 'PA' amps currently using mosfets.
>
> ** Huh ????
>
> All those hundreds of thousands of MOSFET audio power amps made since the
> mid 1980s have not justs disappeared you know - power amps made by H-H &
> Harrison Electronics plus C-Audio in the UK , Perreaux and ZPE of NZ,
> Australian Monitor and ARX (still in full production) and Jands here in
> Aussie - plus many other less well known brands.

Not to mention Studiomaster's Mosfet series of old. Initiated by none other than
myself.

> The majority are STILL in use, maybe looking just a bit the worse for wear.
>
> Even in the UK - Chevin Research ( based in Yorkshire) A-series amps are
> all lateral mosfet designs.
>
> http://www.chevin-research.com/products_a_series.php

Goodness are they still going ?

By current, I did mean in current manufacture. So there are a few. But all the
big boys are bipolar only now.

Graham

Eeyore

September 18th 08, 03:45 PM

Arny Krueger wrote:

> "Gareth Magennis" wrote
>
> > Not if you clip them like you might a valve amp for
> > example.
>
> Straw man. It is entirely practical to build power amps that are never
> clipped in actual use. If you don't like how your power amp clips? Get one
> with enough output so that it never clips.
>
> > Would a Mosfet amp clip more "nicely" than a
> > BJT amp?
>
> Power amps aren't just devices, they are circuits. Circuit design can
> easily trump device characteristics.
>
> In fact the sharpness of the clipping of a power amp relates to things like
> how much negative feedback it has, all things considered. If you have a
> circuit with lots of negative feedback, it is very likely to clip very
> sharply and ideally. If you have a circuit with less negative feedback, the
> clipping will be softer, but will occupy a larger proportion of the transfer
> characteristic.

Precisely so.

Graham

Eeyore

September 18th 08, 03:48 PM

Arny Krueger wrote:

> This whole "amp sounds better" stuff was fully debunked 30 years ago. Good
> power amps sound the same and they sound like a piece of wire with gain.
>
> There are tons of power amps that can't be distinguished from a piece of
> wire with gain, while driving well-designed speakers.
>
> There are quite a few amps that meet the same criteria while driving even
> the weirdest speaker load.
>
> A really good power amp will destroy a poorly-designed speaker before it
> starts sounding bad, and really good power amps aren't all that unique.

The speaker is a large part of it. Highly reactive ( i.e. lots of inductance or
capacitance ) speakers can prematurely trigger device protection. We discovered
that EV's SX500 was particularly bad in this respect in PA for example. Now that
is certainly audible.

Graham

Eeyore

September 18th 08, 03:49 PM

Vladimir Vassilevsky wrote:

> RichD wrote:
>
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> In most cases, this is fallacy, and the result is just the opposite.

Another one off with the faeries.

Graham

Eeyore

September 18th 08, 03:54 PM

Vladimir Vassilevsky wrote:

> 4) FETs do not require high base currents of BJTs; that simplifies the
> driver stage.

So what charges and discharges the damn gate capacitance you dozy bugger ?
The classic Class A voltage gain driver stage probably needs as much current
as if you were driving darlington output devices with a simple design..

In my ultra-low THD design I had a bipolar complementary Class A emitter
follower driving the mosfet gates ! It also removes the capacitive loading
from the voltage gain stage, increasing phase margin and loop HF response.

Graham

Don Pearce

September 18th 08, 04:02 PM

Eeyore wrote:
>
> Vladimir Vassilevsky wrote:
>
>> 4) FETs do not require high base currents of BJTs; that simplifies the
>> driver stage.
>
> So what charges and discharges the damn gate capacitance you dozy bugger ?
> The classic Class A voltage gain driver stage probably needs as much current
> as if you were driving darlington output devices with a simple design..
>
> In my ultra-low THD design I had a bipolar complementary Class A emitter
> follower driving the mosfet gates ! It also removes the capacitive loading
> from the voltage gain stage, increasing phase margin and loop HF response.
>
> Graham
>

Where did you put the dominant pole cap? - still around the voltage
amplifier I'm guessing. It is vital that the dominant pole cap is the
only one that shows up in the phase response up to unity O/L gain, so
buffering of the fet gate caps is pretty much a given.

The big problem you need to overcome with mosfets is that they have an
essentially log response, resulting in (or from, depending how you look
at it) huge - maybe 10/1 Gm changes over the operating range of
currents. This is all easily taken care of by global nfb, of course, but
it does make the sewing together of the crossover point a bit trickier
than a bipolar design. The Gms are dropping at a much sharper angle and
the doubling spike consequently more obvious.

All unimportant, of course, given enough nfb.

d

Eeyore

September 18th 08, 04:12 PM

Don Pearce wrote:

> Eeyore wrote:
> > Vladimir Vassilevsky wrote:
> >
> >> 4) FETs do not require high base currents of BJTs; that simplifies the
> >> driver stage.
> >
> > So what charges and discharges the damn gate capacitance you dozy bugger ?
> > The classic Class A voltage gain driver stage probably needs as much current
> > as if you were driving darlington output devices with a simple design..
> >
> > In my ultra-low THD design I had a bipolar complementary Class A emitter
> > follower driving the mosfet gates ! It also removes the capacitive loading
> > from the voltage gain stage, increasing phase margin and loop HF response.
>
> Where did you put the dominant pole cap? - still around the voltage
> amplifier I'm guessing. It is vital that the dominant pole cap is the
> only one that shows up in the phase response up to unity O/L gain, so
> buffering of the fet gate caps is pretty much a given.

It wasn't dominant pole compensated. But you got the location right after a
fashion.

> The big problem you need to overcome with mosfets is that they have an
> essentially log response, resulting in (or from, depending how you look
> at it) huge - maybe 10/1 Gm changes over the operating range of
> currents.

Not on laterals.

Graham

Don Pearce

September 18th 08, 04:14 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Vladimir Vassilevsky wrote:
>>>
>>>> 4) FETs do not require high base currents of BJTs; that simplifies the
>>>> driver stage.
>>> So what charges and discharges the damn gate capacitance you dozy bugger ?
>>> The classic Class A voltage gain driver stage probably needs as much current
>>> as if you were driving darlington output devices with a simple design..
>>>
>>> In my ultra-low THD design I had a bipolar complementary Class A emitter
>>> follower driving the mosfet gates ! It also removes the capacitive loading
>>> from the voltage gain stage, increasing phase margin and loop HF response.
>> Where did you put the dominant pole cap? - still around the voltage
>> amplifier I'm guessing. It is vital that the dominant pole cap is the
>> only one that shows up in the phase response up to unity O/L gain, so
>> buffering of the fet gate caps is pretty much a given.
>
> It wasn't dominant pole compensated. But you got the location right after a
> fashion.
>

How did you keep it stable?

>
>> The big problem you need to overcome with mosfets is that they have an
>> essentially log response, resulting in (or from, depending how you look
>> at it) huge - maybe 10/1 Gm changes over the operating range of
>> currents.
>
> Not on laterals.
>
> Graham
>

Don't know laterals - haven't kept up. Gimme some numbers and I'll go
have a look.

d

Eeyore

September 18th 08, 04:48 PM

Don Pearce wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >> Eeyore wrote:
> >>> Vladimir Vassilevsky wrote:
> >>>
> >>>> 4) FETs do not require high base currents of BJTs; that simplifies the
> >>>> driver stage.
> >>> So what charges and discharges the damn gate capacitance you dozy bugger ?
> >>> The classic Class A voltage gain driver stage probably needs as much current
> >>> as if you were driving darlington output devices with a simple design..
> >>>
> >>> In my ultra-low THD design I had a bipolar complementary Class A emitter
> >>> follower driving the mosfet gates ! It also removes the capacitive loading
> >>> from the voltage gain stage, increasing phase margin and loop HF response.
> >> Where did you put the dominant pole cap? - still around the voltage
> >> amplifier I'm guessing. It is vital that the dominant pole cap is the
> >> only one that shows up in the phase response up to unity O/L gain, so
> >> buffering of the fet gate caps is pretty much a given.
> >
> > It wasn't dominant pole compensated. But you got the location right after a
> > fashion.
>
> How did you keep it stable?

There's an extra pole and zero. Hence not dominant pole. Works a charm. I've used
that on every amp design since ~ 1990. Sometimes in more than one place in the loop.

> >> The big problem you need to overcome with mosfets is that they have an
> >> essentially log response, resulting in (or from, depending how you look
> >> at it) huge - maybe 10/1 Gm changes over the operating range of
> >> currents.
> >
> > Not on laterals.
>
> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
> have a look.

You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
manufactured. Equivalent types now sourced by Semelab and Exicon.

http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet

Graham

Eeyore

September 18th 08, 04:56 PM

Don Pearce wrote:

> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
> have a look.

Had better luck locating the later plastic package devices and they actually show
the relevant transfer characteristic.

http://www.datasheetcatalog.com/datasheets_pdf/2/S/K/1/2SK1058.shtml
http://www.datasheetcatalog.com/datasheets_pdf/2/S/J/1/2SJ162.shtml

Graham

Don Pearce

September 18th 08, 05:07 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Don Pearce wrote:
>>>> Eeyore wrote:
>>>>> Vladimir Vassilevsky wrote:
>>>>>
>>>>>> 4) FETs do not require high base currents of BJTs; that simplifies the
>>>>>> driver stage.
>>>>> So what charges and discharges the damn gate capacitance you dozy bugger ?
>>>>> The classic Class A voltage gain driver stage probably needs as much current
>>>>> as if you were driving darlington output devices with a simple design..
>>>>>
>>>>> In my ultra-low THD design I had a bipolar complementary Class A emitter
>>>>> follower driving the mosfet gates ! It also removes the capacitive loading
>>>>> from the voltage gain stage, increasing phase margin and loop HF response.
>>>> Where did you put the dominant pole cap? - still around the voltage
>>>> amplifier I'm guessing. It is vital that the dominant pole cap is the
>>>> only one that shows up in the phase response up to unity O/L gain, so
>>>> buffering of the fet gate caps is pretty much a given.
>>> It wasn't dominant pole compensated. But you got the location right after a
>>> fashion.
>> How did you keep it stable?
>
> There's an extra pole and zero. Hence not dominant pole. Works a charm. I've used
> that on every amp design since ~ 1990. Sometimes in more than one place in the loop.
>

Ok.

>
>
>>>> The big problem you need to overcome with mosfets is that they have an
>>>> essentially log response, resulting in (or from, depending how you look
>>>> at it) huge - maybe 10/1 Gm changes over the operating range of
>>>> currents.
>>> Not on laterals.
>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
>> have a look.
>
> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
> manufactured. Equivalent types now sourced by Semelab and Exicon.
>
> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
>
> Graham
>

Oh, I thought there was something new going on. They have exactly the Gm
characteristic I was talking about. Here's the thing. Bipolar Gm is
enormously bigger than mosfet, and you can use that in an output stage.
You do it with local feedback by emitter degeneration - a fraction of an
ohm does it. By the time you have added enough to bring the Gm down
close to that of a mosfet the bipolar is almost perfectly linear, while
the mosfet necessarily shows its square law.

And the next problem is how tidily the devices run out of Gm at low
current. The bipolar drops away gently while the mosfet simply runs
straight into the ground - there is no place you can make a clean
crossover without a lot of effort in controlling the crossover point.

d

Eeyore

September 18th 08, 05:24 PM

Don Pearce wrote:

> Eeyore wrote:
>
> >> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
> >> have a look.
> >
> > You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
> > Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
> > manufactured. Equivalent types now sourced by Semelab and Exicon.
> >
> > http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
>
>
> Oh, I thought there was something new going on. They have exactly the Gm
> characteristic I was talking about. Here's the thing. Bipolar Gm is
> enormously bigger than mosfet, and you can use that in an output stage.

Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
Exicon data sheet doesn't have the equivalent plot for some reason.

The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
crap and that's where crossover distortion comes from. You just can't get rid of it.

Graham

Eeyore

September 18th 08, 05:34 PM

Don Pearce wrote:

> Here's the thing. Bipolar Gm is
> enormously bigger than mosfet, and you can use that in an output stage.
> You do it with local feedback by emitter degeneration - a fraction of an
> ohm does it.

Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least
which would be intolerable in practical designs for obvious loss reasons.

There is a cleverer way to do it which blows away all the classic ideas of biasing bipolar
output stages but it's my secret. That basic design was good for 0.008% THD and I wasn't
even trying hard. It borrows on your idea though, just not the same way.

Graham

Don Pearce

September 18th 08, 05:39 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>
>>>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
>>>> have a look.
>>> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
>>> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
>>> manufactured. Equivalent types now sourced by Semelab and Exicon.
>>>
>>> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
>>
>> Oh, I thought there was something new going on. They have exactly the Gm
>> characteristic I was talking about. Here's the thing. Bipolar Gm is
>> enormously bigger than mosfet, and you can use that in an output stage.
>
> Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
> curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
> Exicon data sheet doesn't have the equivalent plot for some reason.
>
> The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
> crap and that's where crossover distortion comes from. You just can't get rid of it.
>
> Graham
>

Yup, looking now. The transfer characteristic is the alomst-square-law
curve I was expecting; I don't think you can get anything else from a
mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
0.6! You can cross over a bipolar output stage long before you hit that
kind of number.

d

Rich Grise

September 18th 08, 05:44 PM

On Wed, 17 Sep 2008 16:12:25 -0700, RichD wrote:

> Who do MOSFET sound better than bipolar, as an audio amp output driver?

Because you believe they will.

Cheers!
Rich

Eeyore

September 18th 08, 05:48 PM

Don Pearce wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >> Eeyore wrote:
> >>
> >>>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
> >>>> have a look.
> >>> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
> >>> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
> >>> manufactured. Equivalent types now sourced by Semelab and Exicon.
> >>>
> >>> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
> >>
> >> Oh, I thought there was something new going on. They have exactly the Gm
> >> characteristic I was talking about. Here's the thing. Bipolar Gm is
> >> enormously bigger than mosfet, and you can use that in an output stage.
> >
> > Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
> > curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
> > Exicon data sheet doesn't have the equivalent plot for some reason.
> >
> > The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
> > crap and that's where crossover distortion comes from. You just can't get rid of it.
>
> Yup, looking now. The transfer characteristic is the alomst-square-law
> curve I was expecting; I don't think you can get anything else from a
> mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
> drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
> 0.6!

It averages out including higher currents at about 1S.

> You can cross over a bipolar output stage long before you hit that
> kind of number.

And you'll still get crossover distortion.

Graham

Don Pearce

September 18th 08, 05:49 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Here's the thing. Bipolar Gm is
>> enormously bigger than mosfet, and you can use that in an output stage.
>> You do it with local feedback by emitter degeneration - a fraction of an
>> ohm does it.
>
> Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least
> which would be intolerable in practical designs for obvious loss reasons.
>

0.1 ohm is plenty in a high current stage - it makes all the difference
and brings the Gm down to a pretty flat 9ish - there isn't much to be
gained going lower. The variable resistance it is fighting is 25/(Ic *
1000). By the time you hit 200mA or so that is pretty much
inconsequential - everything above controlled by the external resistor.

> There is a cleverer way to do it which blows away all the classic ideas of biasing bipolar
> output stages but it's my secret. That basic design was good for 0.008% THD and I wasn't
> even trying hard. It borrows on your idea though, just not the same way.
>

Obviously I can't comment. You can reveal it here safely of course -
just cite the posts as proof of prior art if anyone tries to nick it.

d

Eeyore

September 18th 08, 05:49 PM

Rich Grise wrote:

> RichD wrote:
>
> > Who do MOSFET sound better than bipolar, as an audio amp output driver?
>
> Because you believe they will.

Because they CAN. By a country mile. But all people want today is cheap. And
if they want esoteric they want the high distortion of tubes instead of
ultra-low THD of well-engineered mosfet amps.

Graham

Don Pearce

September 18th 08, 05:50 PM

GregS[_3_]

September 18th 08, 05:52 PM

In article >, Eeyore > wrote:
>
>
>Rich Grise wrote:
>
>> RichD wrote:
>>
>> > Who do MOSFET sound better than bipolar, as an audio amp output driver?
>>
>> Because you believe they will.
>
>Because they CAN. By a country mile. But all people want today is cheap. And
>if they want esoteric they want the high distortion of tubes instead of
>ultra-low THD of well-engineered mosfet amps.
>
>Graham

Seems like most people like the Hafler amp of old. Is the design still around ?

greg

Jorden Verwer

September 18th 08, 05:55 PM

Eeyore wrote:
> Jorden Verwer wrote:
> > RichD wrote:
> > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > driver?
> >
> > The device properties of BJTs are superior to those of MOSFETs in all
> > respects,
>
> How about SOA for one you UTTER MORON ?
Fine, fine, but that doesn't have any direct influence on what you'll hear,
because it's a boundary condition.

> Do you even know what SOA is ?
Like I said, it's a boundary condition. It can influence the performance of
the circuit, but only indirectly, through other design decisions.

> > except for offset - there MOSFETs have the advantage. Whether you
> > will actually hear this depends on many more factors.
>
> YET MORE INSANE ********
Now you're being the moron (not that I admit to being a moron before). It
seems that either you don't know what you're talking about, or personal
attacks are a hobby of yours. Because, frankly, everything I said was
true...

Fact - MOSFETs have lower offset than BJTs.
Fact - The fidelity of the sound depends on much more than just device
properties.

Eeyore

September 18th 08, 06:03 PM

Don Pearce wrote:

> Eeyore wrote:
> >
> > Don Pearce wrote:
> >
> >> Here's the thing. Bipolar Gm is
> >> enormously bigger than mosfet, and you can use that in an output stage.
> >> You do it with local feedback by emitter degeneration - a fraction of an
> >> ohm does it.
> >
> > Actually NO. A fraction of an ohm isn't enough. You'd have to use about 1 ohm at least
> > which would be intolerable in practical designs for obvious loss reasons.
> >
>
> 0.1 ohm is plenty in a high current stage - it makes all the difference
> and brings the Gm down to a pretty flat 9ish - there isn't much to be
> gained going lower. The variable resistance it is fighting is 25/(Ic *
> 1000). By the time you hit 200mA or so that is pretty much
> inconsequential - everything above controlled by the external resistor.

Sorry 0.1 is not enough. I don't even go that low for current sharing purposes ! More like 0.15,
0.22 or even 0.33.

You're still missing the point about crossover distortion though. OK for 0.1 % THD maybe but
who's going to buy that ?

Graham

Eeyore

September 18th 08, 06:05 PM

Don Pearce wrote:

> You get crossover distortion whatever the topology.

The mosfet curves match into each other far far better, plus you're already using more feedback
too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's
output.

Graham

Eeyore

September 18th 08, 06:07 PM

GregS wrote:

> Eeyore wrote:
> >Rich Grise wrote:
> >> RichD wrote:
> >>
> >> > Who do MOSFET sound better than bipolar, as an audio amp output driver?
> >>
> >> Because you believe they will.
> >
> >Because they CAN. By a country mile. But all people want today is cheap. And
> >if they want esoteric they want the high distortion of tubes instead of
> >ultra-low THD of well-engineered mosfet amps.
>
> Seems like most people like the Hafler amp of old. Is the design still around ?

I hope not.

Some people like antiques too.

Graham

Eeyore

September 18th 08, 06:10 PM

Jorden Verwer wrote:

> Eeyore wrote:
> > Jorden Verwer wrote:
> > > RichD wrote:
> > > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > > driver?
> > >
> > > The device properties of BJTs are superior to those of MOSFETs in all
> > > respects,
> >
> > How about SOA for one you UTTER MORON ?
> Fine, fine, but that doesn't have any direct influence on what you'll hear,
> because it's a boundary condition.

It TOTALLY proves wrong your assertion "The device properties of BJTs are
superior to those of MOSFETs in all respects"

> > Do you even know what SOA is ?
> Like I said, it's a boundary condition. It can influence the performance of
> the circuit, but only indirectly, through other design decisions.
>
> > > except for offset - there MOSFETs have the advantage. Whether you
> > > will actually hear this depends on many more factors.
> >
> > YET MORE INSANE ********
> Now you're being the moron (not that I admit to being a moron before). It
> seems that either you don't know what you're talking about, or personal
> attacks are a hobby of yours. Because, frankly, everything I said was
> true...

What the **** is this 'offset' you're talking about. Do you mean biasing ?

> Fact - MOSFETs have lower offset than BJTs.
> Fact - The fidelity of the sound depends on much more than just device
> properties.

You're a COMPLETE IDIOT. You need a boundary condition up the backside. It's
clear that you know zilch about high-performance audio, whilst I've been doing
it for 37 years.

Graham

Don Pearce

September 18th 08, 06:11 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Don Pearce wrote:
>>>> Eeyore wrote:
>>>>
>>>>>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
>>>>>> have a look.
>>>>> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
>>>>> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
>>>>> manufactured. Equivalent types now sourced by Semelab and Exicon.
>>>>>
>>>>> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
>>>> Oh, I thought there was something new going on. They have exactly the Gm
>>>> characteristic I was talking about. Here's the thing. Bipolar Gm is
>>>> enormously bigger than mosfet, and you can use that in an output stage.
>>> Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
>>> curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
>>> Exicon data sheet doesn't have the equivalent plot for some reason.
>>>
>>> The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
>>> crap and that's where crossover distortion comes from. You just can't get rid of it.
>> Yup, looking now. The transfer characteristic is the alomst-square-law
>> curve I was expecting; I don't think you can get anything else from a
>> mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
>> drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
>> 0.6!
>
> It averages out including higher currents at about 1S.
>
>
>> You can cross over a bipolar output stage long before you hit that
>> kind of number.
>
> And you'll still get crossover distortion.
>
> Graham
>

Take a look at these two graphs I copied from Doug Self's power
amplifier book. They show the voltage gain of the output pair against
operating point (input volts)for a variety of bias conditions. This can
be used to select a best bias. For the bipolars at the top, the fourth
curve up is clearly the best with gain varying from 0.97 down to 0.963
across the range. This is easily tamed, and even a small error doesn't
do much damage.

Now contrast this with the fets (2SK135/2Sj50) below. First there is no
stable flat line - the gain goes on rising all the way out to the 15V
which is the maximum he measured. Secondly there is no decent bias
current that will control the crossover. I guess that again the fourth
from the bottom is as good as it gets but that gives a gain variation
from 0.83 down to 0.77, with a much sharper turnaround into the Gm
doubling region (spiky crossover products result).

http://89.174.169.10/odds/crossover.gif

That is why it is so much easier to control crossover distortion in
bipolars.

d

Don Pearce

September 18th 08, 06:12 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> You get crossover distortion whatever the topology.
>
> The mosfet curves match into each other far far better, plus you're already using more feedback
> too. That amp I designed, quite seriously had invisible crossover distortion on an AP analyser's
> output.
>
> Graham
>

I'm sure it did, but that is the amp, not the output stage. See my next
post.

d

Jorden Verwer

September 18th 08, 06:39 PM

Eeyore wrote:
> Jorden Verwer wrote:
> > Eeyore wrote:
> > > Jorden Verwer wrote:
> > > > RichD wrote:
> > > > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > > > driver?
> > > >
> > > > The device properties of BJTs are superior to those of MOSFETs in
all
> > > > respects,
> > >
> > > How about SOA for one you UTTER MORON ?
> > Fine, fine, but that doesn't have any direct influence on what you'll
hear,
> > because it's a boundary condition.
>
> It TOTALLY proves wrong your assertion "The device properties of BJTs are
> superior to those of MOSFETs in all respects"
No, it doesn't.

> > > Do you even know what SOA is ?
> > Like I said, it's a boundary condition. It can influence the performance
of
> > the circuit, but only indirectly, through other design decisions.
> >
> > > > except for offset - there MOSFETs have the advantage. Whether you
> > > > will actually hear this depends on many more factors.
> > >
> > > YET MORE INSANE ********
> > Now you're being the moron (not that I admit to being a moron before).
It
> > seems that either you don't know what you're talking about, or personal
> > attacks are a hobby of yours. Because, frankly, everything I said was
> > true...
>
> What the **** is this 'offset' you're talking about. Do you mean biasing ?
No, of course not. If I'd meant biasing I would've said biasing. BTW, I
don't see how one component's biasing can be "better" than another one's -
it's simply a design step that's necessary to make it work. I mean, nobody
would say "this amplifier's frequency compensation is so much nicer than
that one's"...

As for offset, here's one explanation (in the context of opamps):
http://en.wikipedia.org/wiki/Operational_amplifier#DC_imperfections

Note that I never claimed that this is relevant in audio applications - but
it's there.

As for your personal attacks towards me, I should mention that I'm under the
impression that your experience with electronics outside audio applications
is fairly limited, given that you've apparently never heard of the term
offset.

Eeyore

September 18th 08, 06:51 PM

Don Pearce wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >> Eeyore wrote:
> >>> Don Pearce wrote:
> >>>> Eeyore wrote:
> >>>>
> >>>>>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
> >>>>>> have a look.
> >>>>> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
> >>>>> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
> >>>>> manufactured. Equivalent types now sourced by Semelab and Exicon.
> >>>>>
> >>>>> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
> >>>> Oh, I thought there was something new going on. They have exactly the Gm
> >>>> characteristic I was talking about. Here's the thing. Bipolar Gm is
> >>>> enormously bigger than mosfet, and you can use that in an output stage.
> >>> Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
> >>> curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
> >>> Exicon data sheet doesn't have the equivalent plot for some reason.
> >>>
> >>> The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
> >>> crap and that's where crossover distortion comes from. You just can't get rid of it.
> >> Yup, looking now. The transfer characteristic is the alomst-square-law
> >> curve I was expecting; I don't think you can get anything else from a
> >> mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
> >> drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
> >> 0.6!
> >
> > It averages out including higher currents at about 1S.
> >
> >
> >> You can cross over a bipolar output stage long before you hit that
> >> kind of number.
> >
> > And you'll still get crossover distortion.
>
> Take a look at these two graphs I copied from Doug Self's power
> amplifier book. They show the voltage gain of the output pair against
> operating point (input volts)for a variety of bias conditions. This can
> be used to select a best bias. For the bipolars at the top, the fourth
> curve up is clearly the best with gain varying from 0.97 down to 0.963
> across the range. This is easily tamed, and even a small error doesn't
> do much damage.
>
> Now contrast this with the fets (2SK135/2Sj50) below. First there is no
> stable flat line - the gain goes on rising all the way out to the 15V
> which is the maximum he measured. Secondly there is no decent bias
> current that will control the crossover. I guess that again the fourth
> from the bottom is as good as it gets but that gives a gain variation
> from 0.83 down to 0.77, with a much sharper turnaround into the Gm
> doubling region (spiky crossover products result).
>
> http://89.174.169.10/odds/crossover.gif
>
> That is why it is so much easier to control crossover distortion in
> bipolars.

Nonsense. I've seen Doug Self's 'blameless amplifier' diagrams. The crossover 'pip' is clearly
visible in all of them. It's quite good but not that good. I can beat that standing on my head.

When I said the crossover of my big Mosfet amp was 'invisible' that IS what I meant. INVISIBLE
and a THD 14% above the AP analyser residual.

For some reason I can't reach your gif btw.

Graham

Eeyore

September 18th 08, 06:54 PM

Don Pearce wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >
> >> You get crossover distortion whatever the topology.
> >
> > The mosfet curves match into each other far far better, plus you're already using more > feedback
> too. That amp I designed, quite seriously had invisible crossover distortion on an AP > analyser's
> output.
>
> I'm sure it did, but that is the amp, not the output stage. See my next
> post.

What good is an amp without an output stage ? Or vice-versa. I'm not interested in how many fairies
can dance on the head of a pin. I'm intereted in real products you can build reliably in quantity
production..

Graham

Don Pearce

September 18th 08, 06:55 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Don Pearce wrote:
>>>> Eeyore wrote:
>>>>> Don Pearce wrote:
>>>>>> Eeyore wrote:
>>>>>>
>>>>>>>> Don't know laterals - haven't kept up. Gimme some numbers and I'll go
>>>>>>>> have a look.
>>>>>>> You're a bit late now they've almost come and gone. Arrived ~ 1980 courtesy of
>>>>>>> Hitachi. 2SJ56 and 2SK176 were a classic complementary pair but no longer
>>>>>>> manufactured. Equivalent types now sourced by Semelab and Exicon.
>>>>>>>
>>>>>>> http://www.profusionplc.com/pro/gex/prodGen.html?prdtyp=lateral%20mosfet
>>>>>> Oh, I thought there was something new going on. They have exactly the Gm
>>>>>> characteristic I was talking about. Here's the thing. Bipolar Gm is
>>>>>> enormously bigger than mosfet, and you can use that in an output stage.
>>>>> Oh sure. But look at the Hitachi data I posted a few mins later. Look how linear that
>>>>> curve is especially beyond the typical 100mA quiescent operating current.. Id vs Vgs The
>>>>> Exicon data sheet doesn't have the equivalent plot for some reason.
>>>>>
>>>>> The high gm of bipolars is great until you get to a few mA or tens of mA of Ic when it's
>>>>> crap and that's where crossover distortion comes from. You just can't get rid of it.
>>>> Yup, looking now. The transfer characteristic is the alomst-square-law
>>>> curve I was expecting; I don't think you can get anything else from a
>>>> mosfet. As for Gm, the 0.4V change in Vgs from -1.2 to -1.6 yields a
>>>> drain current change of 0.24A (-0.35 to -0.59A) at 75C. That is a Gm of
>>>> 0.6!
>>> It averages out including higher currents at about 1S.
>>>
>>>
>>>> You can cross over a bipolar output stage long before you hit that
>>>> kind of number.
>>> And you'll still get crossover distortion.
>> Take a look at these two graphs I copied from Doug Self's power
>> amplifier book. They show the voltage gain of the output pair against
>> operating point (input volts)for a variety of bias conditions. This can
>> be used to select a best bias. For the bipolars at the top, the fourth
>> curve up is clearly the best with gain varying from 0.97 down to 0.963
>> across the range. This is easily tamed, and even a small error doesn't
>> do much damage.
>>
>> Now contrast this with the fets (2SK135/2Sj50) below. First there is no
>> stable flat line - the gain goes on rising all the way out to the 15V
>> which is the maximum he measured. Secondly there is no decent bias
>> current that will control the crossover. I guess that again the fourth
>> from the bottom is as good as it gets but that gives a gain variation
>> from 0.83 down to 0.77, with a much sharper turnaround into the Gm
>> doubling region (spiky crossover products result).
>>
>> http://89.174.169.10/odds/crossover.gif
>>
>> That is why it is so much easier to control crossover distortion in
>> bipolars.
>
> Nonsense. I've seen Doug Self's 'blameless amplifier' diagrams. The crossover 'pip' is clearly
> visible in all of them. It's quite good but not that good. I can beat that standing on my head.
>
> When I said the crossover of my big Mosfet amp was 'invisible' that IS what I meant. INVISIBLE
> and a THD 14% above the AP analyser residual.
>
> For some reason I can't reach your gif btw.
>
> Graham
>

Sorry, wrong address

http://81.174.169.10/odds/crossover.gif

d

Don Pearce

September 18th 08, 06:57 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Eeyore wrote:
>>> Don Pearce wrote:
>>>
>>>> You get crossover distortion whatever the topology.
>>> The mosfet curves match into each other far far better, plus you're already using more > feedback
>> too. That amp I designed, quite seriously had invisible crossover distortion on an AP > analyser's
>> output.
>>
>> I'm sure it did, but that is the amp, not the output stage. See my next
>> post.
>
> What good is an amp without an output stage ? Or vice-versa. I'm not interested in how many fairies
> can dance on the head of a pin. I'm intereted in real products you can build reliably in quantity
> production..
>
> Graham
>

We're talking about the difference between fet and bipolar output
stages. It is simple to reduce distortions in amplifiers with either to
negligible proportions, which is why, for the purposes of the chat, it
is necessary to restrict the chat to output stages per se.

d

Eeyore

September 18th 08, 06:57 PM

Jorden Verwer wrote:

> Eeyore wrote:
> >
> > What the **** is this 'offset' you're talking about. Do you mean biasing ?
> No, of course not. If I'd meant biasing I would've said biasing. BTW, I
> don't see how one component's biasing can be "better" than another one's -
> it's simply a design step that's necessary to make it work. I mean, nobody
> would say "this amplifier's frequency compensation is so much nicer than
> that one's"...
>
> As for offset, here's one explanation (in the context of opamps):
> http://en.wikipedia.org/wiki/Operational_amplifier#DC_imperfections
>
> Note that I never claimed that this is relevant in audio applications - but
> it's there.

Oh for Christ's sake grow up. An amp is a closed loop (often DC) servo almost.
Any offset depends on the INPUT transistors you brainless jerk.

You have the tiniest idea what you're talking about. You must be a lecturer to
be this stupid.

Graham

Eeyore

September 18th 08, 07:00 PM

Jorden Verwer wrote:

> As for your personal attacks towards me, I should mention that I'm under the
> impression that your experience with electronics outside audio applications
> is fairly limited,

Radar, video, embedded control, computer graphics, DSP.

> given that you've apparently never heard of the term
> offset.

Offset is IRRELEVANT to output devices you complete MORON !

Do do you know what a 'closed loop' means ?

Don, can you smack this guy round the chops for me. He must be a recent product
of our educashun system.

Graham

Eeyore

September 18th 08, 07:02 PM

Don Pearce wrote:

> Sorry, wrong address
>
> http://81.174.169.10/odds/crossover.gif

Exactly. You CANNOT get rid of the crossover distortion.

Graham

Eeyore

September 18th 08, 07:05 PM

Don Pearce wrote:

> Eeyore wrote:
> > Don Pearce wrote:
> >> Eeyore wrote:
> >>> Don Pearce wrote:
> >>>
> >>>> You get crossover distortion whatever the topology.
> >>> The mosfet curves match into each other far far better, plus you're already using more > >>>
> feedback too. That amp I designed, quite seriously had invisible crossover distortion on an >>> AP
> analyser's output.
> >>
> >> I'm sure it did, but that is the amp, not the output stage. See my next
> >> post.
> >
> > What good is an amp without an output stage ? Or vice-versa. I'm not interested in how many > fairies
> can dance on the head of a pin. I'm intereted in real products you can build reliably in > quantity
> production..
>
>
> We're talking about the difference between fet and bipolar output
> stages. It is simple to reduce distortions in amplifiers with either to
> negligible proportions, which is why, for the purposes of the chat, it
> is necessary to restrict the chat to output stages per se.

I don't recall that being the OP's question.

Care to run a sim of the harmonic spectrum of a bipolar output stage vs lateral mosfet ?

Graham

Kevin Aylward[_3_]

September 18th 08, 07:15 PM

Trevor Wilson wrote:
> "RichD" > wrote in message
> ...
>> Who do MOSFET sound better than bipolar, as an audio amp output
>> driver?
>
> **Non-sequitur. If you're saying that MOSFET outputs sound better,
> they don't.
>
> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> exhibit VASTLY more THD than BJTs. Only when bias currents are
> elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics
> which are almost as good as BJTs.
>
> MOSFETs are very tough, have an exceptional ability to deliver high
> power, high frequency audio (and RF), but distortion is very high.
> They require lots of Global NFB in order to operate linearly.
>
> MOSFETs do not sound better than BJTs. At best, they can sound as
> good. All things being equal.

Ahmm.... welll....here we go...

Well, I like mosfet outputs because they are easier to design with, imo.
Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
tad harder to stabilse the feedback loop because of each stage pole.

Basically, you only need about 6 low current transistors, or so, to achieve
silly distortion and bandwidth figures, with mosfets.

As far as "sounds better", that's all moot. Any competently designed amp
should have thd, imd below audibility.

Anyone that claims that a general purpose PA amp, sounds bad or not good, if
it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
delusional. Roll on the £200 oxygen free mains cable I say....

Kevin Aylward

www.anasoft.co.uk
SuperSpice

Jorden Verwer

September 18th 08, 07:26 PM

Eeyore wrote:
> > given that you've apparently never heard of the term
> > offset.
>
> Offset is IRRELEVANT to output devices you complete MORON !
I know that, and I never claimed otherwise. Offset is a form of noise, in a
sense. And like noise, it is caused almost completely by the input
transistors. I'm well aware of all that.

> Do do you know what a 'closed loop' means ?
Yes.

Eeyore

September 18th 08, 07:32 PM

Kevin Aylward wrote:

> Trevor Wilson wrote:
> > "RichD" wrote
> >
> >> Who do MOSFET sound better than bipolar, as an audio amp output
> >> driver?
> >
> > **Non-sequitur. If you're saying that MOSFET outputs sound better,
> > they don't.
> >
> > As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> > exhibit VASTLY more THD than BJTs. Only when bias currents are
> > elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics
> > which are almost as good as BJTs.
> >
> > MOSFETs are very tough, have an exceptional ability to deliver high
> > power, high frequency audio (and RF), but distortion is very high.
> > They require lots of Global NFB in order to operate linearly.
> >
> > MOSFETs do not sound better than BJTs. At best, they can sound as
> > good. All things being equal.
>
> Ahmm.... welll....here we go...
>
> Well, I like mosfet outputs because they are easier to design with, imo.
> Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
> tad harder to stabilse the feedback loop because of each stage pole.
>
> Basically, you only need about 6 low current transistors, or so, to achieve
> silly distortion and bandwidth figures, with mosfets.
>
> As far as "sounds better", that's all moot. Any competently designed amp
> should have thd, imd below audibility.

Seems to be a few that miss that though. Esp those Chinese copies of copies.

> Anyone that claims that a general purpose PA amp, sounds bad or not good, if
> it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
> delusional. Roll on the £200 oxygen free mains cable I say....

Why stop at £200 ? Oh !
http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm

Kevin, you'd be just the guy to do a harmonic analysis of the distortion
spectrum of a properly biased bipolar vs lateral mosfet amp wouldn't you ? Any
chance ?

Graham

Eeyore

September 18th 08, 07:33 PM

Jorden Verwer wrote:

> Eeyore wrote:
> > > given that you've apparently never heard of the term
> > > offset.
> >
> > Offset is IRRELEVANT to output devices you complete MORON !
> I know that, and I never claimed otherwise.

Yes you did.

BUGGER OFF YOU IMBECILE.

Eeyore

September 18th 08, 07:34 PM

Jorden Verwer wrote:

> Offset is a form of noise, in a sense.

Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?

Graham

Jorden Verwer

September 18th 08, 07:45 PM

Eeyore wrote:
> Jorden Verwer wrote:
> > Offset is a form of noise, in a sense.
>
> Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?
Let me put it this way...

Noise-like phenomena, in increasing order of bandwith:
Offset ==> 1/f noise ==> white noise

RichD

September 18th 08, 07:54 PM

On Sep 17, Eeyore > wrote:
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> As a driver ?
>
> Now if you said as an output stage it might make sense.

There's a difference?

--
Rich

Eeyore

September 18th 08, 07:55 PM

Jorden Verwer wrote:

> Eeyore wrote:
> > Jorden Verwer wrote:
> > > Offset is a form of noise, in a sense.
> >
> > Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?
> Let me put it this way...
>
> Noise-like phenomena, in increasing order of bandwith:
> Offset ==> 1/f noise ==> white noise

Noise is AC
Offset is DC

End of story.

Eeyore

September 18th 08, 07:57 PM

RichD wrote:

> Eeyore wrote:
> > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > driver?
> >
> > As a driver ?
> >
> > Now if you said as an output stage it might make sense.
>
> There's a difference?

Damn right there is.

Graham

RichD

September 18th 08, 07:57 PM

On Sep 18, "Kevin Aylward" > wrote:
> >> Who do MOSFET sound better than bipolar, as an audio amp output
> >> driver?
>
> > **Non-sequitur. If you're saying that MOSFET outputs sound better,
> > they don't.
>
> > As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> > exhibit VASTLY more THD than BJTs. Only when bias currents are
> > elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics
> > which are almost as good as BJTs.
>
> > MOSFETs are very tough, have an exceptional ability to deliver high
> > power, high frequency audio (and RF), but distortion is very high.
> > They require lots of Global NFB in order to operate linearly.
>
> > MOSFETs do not sound better than BJTs. At best, they can sound as
> > good. All things being equal.
>
> Well, I like mosfet outputs because they are easier to design with, imo.
> Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
> tad harder to stabilse the feedback loop because of each stage pole.
>
> Basically, you only need about 6 low current transistors, or so, to achieve
> silly distortion and bandwidth figures, with mosfets.
>
> As far as "sounds better", that's all moot. Any competently designed amp
> should have thd, imd below audibility.
>
> Anyone that claims that a general purpose PA amp, sounds bad or not good, if
> it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
> delusional.

Do you have any experience designing audio amps?
Which did you use?

--
Rich

Spehro Pefhany

September 18th 08, 08:05 PM

On Thu, 18 Sep 2008 19:55:35 +0100, Eeyore
> wrote:

>
>
>Jorden Verwer wrote:
>
>> Eeyore wrote:
>> > Jorden Verwer wrote:
>> > > Offset is a form of noise, in a sense.
>> >
>> > Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?
>> Let me put it this way...
>>
>> Noise-like phenomena, in increasing order of bandwith:
>> Offset ==> 1/f noise ==> white noise
>
>Noise is AC
>Offset is DC
>
>End of story.

Everything is AC when you get right down to it.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Jorden Verwer

September 18th 08, 08:10 PM

Eeyore wrote:
> Noise is AC
> Offset is DC
>
> End of story.
Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
drift. And it does drift. Likewise, white noise also has an (infinitesimal)
component at DC. It isn't all as black and white as you claim it to be. I
realize this is a very theoretical approach, but I stand by my claim that
offset and noise are conceptually similar.

Eeyore

September 18th 08, 10:16 PM

RichD wrote:

> "Kevin Aylward" > wrote:
> > >> Who do MOSFET sound better than bipolar, as an audio amp output
> > >> driver?
> >
> > > **Non-sequitur. If you're saying that MOSFET outputs sound better,
> > > they don't.
> >
> > > As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
> > > exhibit VASTLY more THD than BJTs. Only when bias currents are
> > > elevated (around 0.5A - 1A) do MOSFETs exhibit THD characteristics
> > > which are almost as good as BJTs.
> >
> > > MOSFETs are very tough, have an exceptional ability to deliver high
> > > power, high frequency audio (and RF), but distortion is very high.
> > > They require lots of Global NFB in order to operate linearly.
> >
> > > MOSFETs do not sound better than BJTs. At best, they can sound as
> > > good. All things being equal.
> >
> > Well, I like mosfet outputs because they are easier to design with, imo.
> > Bipolars, often need an equivelent of 3 stage darlingtons. This makes it a
> > tad harder to stabilse the feedback loop because of each stage pole.
> >
> > Basically, you only need about 6 low current transistors, or so, to achieve
> > silly distortion and bandwidth figures, with mosfets.
> >
> > As far as "sounds better", that's all moot. Any competently designed amp
> > should have thd, imd below audibility.
> >
> > Anyone that claims that a general purpose PA amp, sounds bad or not good, if
> > it has thd and imd < 0.005% and slew rates of 100V/us, is pretty much
> > delusional.
>
> Do you have any experience designing audio amps?

LMAO ! Yes, Kevin had indeed had as have I. Amps that sold commercially into the
pro-audio market. I can't even estimate how many tens of thousands in my case.

Graham

Eeyore

September 18th 08, 10:17 PM

Spehro Pefhany wrote:

> Eeyore wrote:
> >Jorden Verwer wrote:
> >> Eeyore wrote:
> >> > Jorden Verwer wrote:
> >> > > Offset is a form of noise, in a sense.
> >> >
> >> > Every one a cracker. Maybe he hasn't heard of DC vs AC analysis ?
> >> Let me put it this way...
> >>
> >> Noise-like phenomena, in increasing order of bandwith:
> >> Offset ==> 1/f noise ==> white noise
> >
> >Noise is AC
> >Offset is DC
> >
> >End of story.
>
> Everything is AC when you get right down to it.

I call it moving DC. It never will stay still. ;~)

Graham

Eeyore

September 18th 08, 10:18 PM

Jorden Verwer wrote:

> Eeyore wrote:
> > Noise is AC
> > Offset is DC
> >
> > End of story.
> Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
> drift.

Drift is a different phenomenon unrelated to noise. Usually thermally caused.

Graham

Jorden Verwer

September 18th 08, 10:55 PM

Eeyore wrote:
> Jorden Verwer wrote:
> > Eeyore wrote:
> > > Noise is AC
> > > Offset is DC
> > >
> > > End of story.
> > Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
> > drift.
>
> Drift is a different phenomenon unrelated to noise. Usually thermally
caused.
I meant drift of the offset. Of course drift is different from noise.
But offset really does have a lot in common with noise, when you think about
it.

Also, noise is often thermally caused as well. ;)

Eeyore

September 18th 08, 11:27 PM

Jorden Verwer wrote:

> Eeyore wrote:
> > Jorden Verwer wrote:
> > > Eeyore wrote:
> > > > Noise is AC
> > > > Offset is DC
> > > >
> > > > End of story.
> > > Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
> > > drift.
> >
> > Drift is a different phenomenon unrelated to noise. Usually thermally
> caused.
> I meant drift of the offset. Of course drift is different from noise.
> But offset really does have a lot in common with noise, when you think about
> it.

Not at all in any remote way shape or form.

> Also, noise is often thermally caused as well. ;)

Do you think you're telling me something I don't already know. I keep
Boltzmann's Constant in my head for that pirpose.

Graham

Tony[_11_]

September 18th 08, 11:39 PM

Exactly. I believe that even the very worst designers were generally able to get a
half-decent result with MOSFETs, whereas bipolars require a little more skill. So it may
well be that MOSFETs DO sound better in car audio, consumer-grade home theatre and cheap
"pro" sound systems, where the standards of the products and the designers are low.
Tony

On Wed, 17 Sep 2008 20:52:28 -0500, Damon Hill > wrote:

>RichD > wrote in news:b78fc9c2-fe9c-444c-8ac5-
:
>
>> Who do MOSFET sound better than bipolar, as an audio amp output
>> driver?
>
>They do? Seems like it's possible to design good amplifiers either
>way.
>
>--Damon

Jorden Verwer

September 19th 08, 12:06 AM

Eeyore wrote:
> Jorden Verwer wrote:
> > Eeyore wrote:
> > > Jorden Verwer wrote:
> > > > Eeyore wrote:
> > > > > Noise is AC
> > > > > Offset is DC
> > > > >
> > > > > End of story.
> > > > Not at all. You're oversimplifying. If offset were truly DC, it
wouldn't
> > > > drift.
> > >
> > > Drift is a different phenomenon unrelated to noise. Usually thermally
> > caused.
> > I meant drift of the offset. Of course drift is different from noise.
> > But offset really does have a lot in common with noise, when you think
about
> > it.
>
> Not at all in any remote way shape or form.
Don't you at least agree there are many similarities between 1/f noise and
offset?

Phil Allison

September 19th 08, 12:24 AM

"Vladimir Vassilevsky"

>
> However there are few special cases when a FET output stage has an
> advantage:
>
> 1) With FETs, it is simpler to control bias current, because of the
> negative dependency from the temperature.

** Completely wrong.

The tempco of typical (ie switching) fets is strongly positive - the gate
threshold voltage drops by about 6mV per degree C.

Bias compensation is non trivial and crucial to get right or thermal runaway
is likely.

...... Phil

Chris Hornbeck

September 19th 08, 01:15 AM

On Thu, 18 Sep 2008 17:49:59 +0100, Eeyore
> wrote:

>> > Who do MOSFET sound better than bipolar, as an audio amp output driver?
>>
>> Because you believe they will.
>
>Because they CAN. By a country mile. But all people want today is cheap. And
>if they want esoteric they want the high distortion of tubes instead of
>ultra-low THD of well-engineered mosfet amps.

And just to complicate an already messy thread, vacuum valves
can easily have lower intrinsic distortion than any modern
device. Lots of other issues, of course, but transfer curves
with exponents below 1.05 were delivered before we were born.
Before they could even be measured, in fact.

But as you say, all people want today is cheap, and there
are cheaper ways of delivering a finished product of low
distortion than using devices of highest intrinsic linearity.

Your attempts to keep the discussion focussed on the small
end of signal level are admirable. Too many folks make too
many assumptions of monotonicity that aren't (ever!, actually)
true.

Much thanks, as always,
Chris Hornbeck

Mr.T

September 19th 08, 03:59 AM

"Arny Krueger" > wrote in message
...
>Good
> power amps sound the same and they sound like a piece of wire with gain.
>
> There are tons of power amps that can't be distinguished from a piece of
> wire with gain, while driving well-designed speakers.

Ah, but since audiophools argue endlessly about cable and wire
characteristics, you need to specify exactly what TYPE of wire with gain.
:-) :-)

MrT.

Mr.T

September 19th 08, 04:04 AM

"Fred Bartoli" <" "> wrote in message
...
> Please explain how you obtain even distortion products from a
> symmetrical (suppose paired mosfets) push-pull.
>
> Any symmetrical function produces odd (and no even) harmonics.

But isn't that part of the problem, producing Mosfets with identically
symmetrical characteristics in N-ch and P-ch ?
Of course good circuit design can compensate for many such factors, so the
final result is what is important, not some evangelical belief, IMO.

MrT.

Kevin McMurtrie[_2_]

September 19th 08, 04:06 AM

In article >,
"Arny Krueger" > wrote:

> "Eeyore" > wrote in
> message
> > Kevin McMurtrie wrote:
> >
> >> In article >,
> >> Damon Hill > wrote:
> >>
> >>> RichD > wrote in
> >>> news:b78fc9c2-fe9c-444c-8ac5-
> >>> :
> >>>
> >>>> Who do MOSFET sound better than bipolar, as an audio
> >>>> amp output driver?
> >>>
> >>> They do? Seems like it's possible to design good
> >>> amplifiers either way.
> >>>
> >>> --Damon
> >>
> >> Exactly - zero difference in quality capabilities. It's
> >> usually a matter of impedance matching. Silicon
> >> transistors have a fixed loss around 0.5 volts. MOSFETs
> >> have a resistive loss inversely proportional to their
> >> voltage rating. That usually makes MOSFETs less
> >> expensive for low impedances and transistors less
> >> expensive for high impedances.
> >
> > TOTAL AND COMPLETE GARBAGE.
>
> Agreed. Actually, what Kevin said is the exact reverse of generally accepted
> practical knowledge. Bipolar is generally more efficient when the impedances
> get really low. MOSFETs were trendy for a while, but most new power amp
> designs seem to have bipolar outputs.

Time to check the specs for this decade. Visit IRF. You can get a
surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.

Now I remember why I left this newsgroup.

--
Google is a pro-spamming service. I will not see your reply if you use Google.

Phil Allison

September 19th 08, 04:13 AM

"Mr.T"
> "Fred Bartoli"
> Eeyore
>>
>> Please explain how you obtain even distortion products from a
>> symmetrical (suppose paired mosfets) push-pull.
>>
>> Any symmetrical function produces odd (and no even) harmonics.
>
> But isn't that part of the problem, producing Mosfets with identically
> symmetrical characteristics in N-ch and P-ch ?

** For switching MOSFETs ( ie Hexfets etc ) there are no true
complimentary P and N pairs - not even close. The P ch types all have
much higher "on" resistances and lower max drain current ratings compared to
their Nch counterparts.

OTOH - lateral MOSFETs ( ie 2SK176 and 2SJ56 etc ) are near perfect
Nch and Pch compliments - varying slightly in gate capacitance.

More symmetrical in an audio output stage than nearly any BJT compliments
around.

...... Phil

Phil Allison

September 19th 08, 04:18 AM

"Kevin McMurtrie"

>
> Time to check the specs for this decade. Visit IRF. You can get a
> surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
> on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.

** So you can use it to replace the engine starter relay in a car.

Yawnnnnnnn ....

....... Phil

John Larkin

September 19th 08, 04:23 AM

On Thu, 18 Sep 2008 20:06:43 -0700, Kevin McMurtrie
> wrote:

>In article >,
> "Arny Krueger" > wrote:
>
>> "Eeyore" > wrote in
>> message
>> > Kevin McMurtrie wrote:
>> >
>> >> In article >,
>> >> Damon Hill > wrote:
>> >>
>> >>> RichD > wrote in
>> >>> news:b78fc9c2-fe9c-444c-8ac5-
>> >>> :
>> >>>
>> >>>> Who do MOSFET sound better than bipolar, as an audio
>> >>>> amp output driver?
>> >>>
>> >>> They do? Seems like it's possible to design good
>> >>> amplifiers either way.
>> >>>
>> >>> --Damon
>> >>
>> >> Exactly - zero difference in quality capabilities. It's
>> >> usually a matter of impedance matching. Silicon
>> >> transistors have a fixed loss around 0.5 volts. MOSFETs
>> >> have a resistive loss inversely proportional to their
>> >> voltage rating. That usually makes MOSFETs less
>> >> expensive for low impedances and transistors less
>> >> expensive for high impedances.
>> >
>> > TOTAL AND COMPLETE GARBAGE.
>>
>> Agreed. Actually, what Kevin said is the exact reverse of generally accepted
>> practical knowledge. Bipolar is generally more efficient when the impedances
>> get really low. MOSFETs were trendy for a while, but most new power amp
>> designs seem to have bipolar outputs.
>
>Time to check the specs for this decade. Visit IRF. You can get a
>surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
>on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
>
>Now I remember why I left this newsgroup.

IR is famous for creating incredible specs, with tiny footnotes
retracting the wilder numbers. No TO-220 is going to last long at 300
watts; not many milliseconds.

John

Chronic Philharmonic

September 19th 08, 05:03 AM

"Eeyore" > wrote in message
...
>
>
> Arny Krueger wrote:
>
>> > wrote
>> > RichD wrote:
>> >> Who do MOSFET sound better than bipolar, as an audio amp
>> >> output driver?
>>
>> > MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
>> > odd harmonic distortin. on and on.
>>
>> Not in any relevant way for audio power amps.
>
> The first two are highly relevevant in ANY circuit using NFB. Basic
> stability criteria.

Only if the bandwidth and the phase margins are small relative to the target
audio bandwidth, which is unlikely.

Eeyore

September 19th 08, 11:35 AM

Chris Hornbeck wrote:

> Eeyore wrote:
>
> >> > Who do MOSFET sound better than bipolar, as an audio amp output driver?
> >>
> >> Because you believe they will.
> >
> >Because they CAN. By a country mile. But all people want today is cheap. And
> >if they want esoteric they want the high distortion of tubes instead of
> >ultra-low THD of well-engineered mosfet amps.
>
> And just to complicate an already messy thread, vacuum valves
> can easily have lower intrinsic distortion than any modern
> device. Lots of other issues, of course, but transfer curves
> with exponents below 1.05 were delivered before we were born.
> Before they could even be measured, in fact.
>
> But as you say, all people want today is cheap, and there
> are cheaper ways of delivering a finished product of low
> distortion than using devices of highest intrinsic linearity.
>
> Your attempts to keep the discussion focussed on the small
> end of signal level are admirable. Too many folks make too
> many assumptions of monotonicity that aren't (ever!, actually)
> true.

Thanks for your comments Chris. I consider the small signal area vitally
important, since in practive most listening is done there.

Another cute little twist is that AIUI (it was reported to me by a Singapore
distributor) is that the classic QSC grounded collector circuit has a low level
montonicity problem due to the voltage gain of the out darlington output
arrangement 'jumping' as the large devices turn on (they're normally off).

Graham

Eeyore

September 19th 08, 11:38 AM

"Mr.T" wrote:

> "Fred Bartoli" <" "> wrote
>
> > Please explain how you obtain even distortion products from a
> > symmetrical (suppose paired mosfets) push-pull.
> >
> > Any symmetrical function produces odd (and no even) harmonics.
>
> But isn't that part of the problem, producing Mosfets with identically
> symmetrical characteristics in N-ch and P-ch ?
> Of course good circuit design can compensate for many such factors, so the
> final result is what is important, not some evangelical belief, IMO.

Fred is thinking of push-pull transformer coupled arrangements. He needs to
advance about 4-5 decades.

That's one reason I never call modern amps push-pull but complementary
symmetry instead.

Graham

Eeyore

September 19th 08, 11:41 AM

Kevin McMurtrie wrote:

> "Arny Krueger" wrote:
> > "Eeyore" wrote
> > > Kevin McMurtrie wrote:
> > >> Damon Hill > wrote:
> > >>> RichD > wrote in
> > >>>
> > >>>> Who do MOSFET sound better than bipolar, as an audio
> > >>>> amp output driver?
> > >>>
> > >>> They do? Seems like it's possible to design good
> > >>> amplifiers either way.
> > >>
> > >> Exactly - zero difference in quality capabilities. It's
> > >> usually a matter of impedance matching. Silicon
> > >> transistors have a fixed loss around 0.5 volts. MOSFETs
> > >> have a resistive loss inversely proportional to their
> > >> voltage rating. That usually makes MOSFETs less
> > >> expensive for low impedances and transistors less
> > >> expensive for high impedances.
> > >
> > > TOTAL AND COMPLETE GARBAGE.
> >
> > Agreed. Actually, what Kevin said is the exact reverse of generally accepted
> > practical knowledge. Bipolar is generally more efficient when the impedances
> > get really low. MOSFETs were trendy for a while, but most new power amp
> > designs seem to have bipolar outputs.
>
> Time to check the specs for this decade. Visit IRF. You can get a
> surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
> on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.

These are NOT used in audio amps.

Learn something about LATERAL mosfets that were designed for audio. I've already
given part number and links to data sheets.

Graham

Eeyore

September 19th 08, 11:43 AM

John Larkin wrote:

> IR is famous for creating incredible specs, with tiny footnotes
> retracting the wilder numbers. No TO-220 is going to last long at 300
> watts; not many milliseconds.

You simply can't get that many watts through that tiny tab of copper fast enough. I
think the highest continuous ratings are in the 50-70W area.

Graham

Eeyore

September 19th 08, 11:45 AM

Chronic Philharmonic wrote:

> "Eeyore" > wrote in message
> > Arny Krueger wrote:
> >> > wrote
> >> > RichD wrote:
> >> >> Who do MOSFET sound better than bipolar, as an audio amp
> >> >> output driver?
> >>
> >> > MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> >> > odd harmonic distortin. on and on.
> >>
> >> Not in any relevant way for audio power amps.
> >
> > The first two are highly relevevant in ANY circuit using NFB. Basic
> > stability criteria.
>
> Only if the bandwidth and the phase margins are small relative to the target
> audio bandwidth, which is unlikely.

Modern audio amplifiers are expected to have a bandwidth far greater than 20kHz.
If not, you may experience slew limiting problems and associated THD and IMD
IIRC.

Yet another one totally out of date with modern practice.

Graham

Spehro Pefhany

September 19th 08, 01:24 PM

On Thu, 18 Sep 2008 20:23:29 -0700, the renowned John Larkin
> wrote:

>On Thu, 18 Sep 2008 20:06:43 -0700, Kevin McMurtrie
> wrote:
>
>>In article >,
>> "Arny Krueger" > wrote:
>>
>>> "Eeyore" > wrote in
>>> message
>>> > Kevin McMurtrie wrote:
>>> >
>>> >> In article >,
>>> >> Damon Hill > wrote:
>>> >>
>>> >>> RichD > wrote in
>>> >>> news:b78fc9c2-fe9c-444c-8ac5-
>>> >>> :
>>> >>>
>>> >>>> Who do MOSFET sound better than bipolar, as an audio
>>> >>>> amp output driver?
>>> >>>
>>> >>> They do? Seems like it's possible to design good
>>> >>> amplifiers either way.
>>> >>>
>>> >>> --Damon
>>> >>
>>> >> Exactly - zero difference in quality capabilities. It's
>>> >> usually a matter of impedance matching. Silicon
>>> >> transistors have a fixed loss around 0.5 volts. MOSFETs
>>> >> have a resistive loss inversely proportional to their
>>> >> voltage rating. That usually makes MOSFETs less
>>> >> expensive for low impedances and transistors less
>>> >> expensive for high impedances.
>>> >
>>> > TOTAL AND COMPLETE GARBAGE.
>>>
>>> Agreed. Actually, what Kevin said is the exact reverse of generally accepted
>>> practical knowledge. Bipolar is generally more efficient when the impedances
>>> get really low. MOSFETs were trendy for a while, but most new power amp
>>> designs seem to have bipolar outputs.
>>
>>Time to check the specs for this decade. Visit IRF. You can get a
>>surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
>>on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
>>
>>Now I remember why I left this newsgroup.
>
>IR is famous for creating incredible specs, with tiny footnotes
>retracting the wilder numbers. No TO-220 is going to last long at 300
>watts; not many milliseconds.
>
>John

Anything approaching 1600A would blow the leadwires off a TO-220. The
fusing current of a long wire of that cross-section area is only in
the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Phil Allison

September 19th 08, 02:09 PM

"Spehro Pefhany"

> Anything approaching 1600A would blow the leadwires off a TO-220.

** Not if it is only for a few microseconds.

That is how the things are usually rated.

...... Phil

Arny Krueger

September 19th 08, 02:23 PM

"Eeyore" > wrote in
message

> Arny Krueger wrote:

>> This whole "amp sounds better" stuff was fully debunked
>> 30 years ago. Good power amps sound the same and they
>> sound like a piece of wire with gain.

>> There are tons of power amps that can't be distinguished
>> from a piece of wire with gain, while driving
>> well-designed speakers.

>> There are quite a few amps that meet the same criteria
>> while driving even the weirdest speaker load.

>> A really good power amp will destroy a poorly-designed
>> speaker before it starts sounding bad, and really good
>> power amps aren't all that unique.

> The speaker is a large part of it. Highly reactive ( i.e.
> lots of inductance or capacitance ) speakers can
> prematurely trigger device protection.

Agreed. Speakers that require heroic amplifiers (heroic amps definitely
exist and are easy enough to obtain) are IMO not well designed.

> We discovered that
> EV's SX500 was particularly bad in this respect in PA for
> example. Now that is certainly audible.

Agreed. However we have only part of the equation - the name of the
speaker. What was the amp?

The SX 500i EDS suggests that it might be a handful for weak-kneed amps. Its
impedance curve does go down to 4 ohms at several points below 1 KHz.

http://www.electrovoice.com/documents/ev/sx500-eds.pdf

Using pairs paralleled can be a short road to amp problems.

In contrast, the sequel ZX-5 stays above 8 ohms over the same range:

http://www.electrovoice.com/documents/ev/ZX560PI_EDS.pdf

Needless to say, I have 4 ZX-5s. ;-)

MooseFET

September 19th 08, 02:25 PM

On Sep 18, 5:05*am, "Arny Krueger" > wrote:
> > wrote in message
>
>
>
> > On Sep 17, 6:12 pm, RichD > wrote:
> >> Who do MOSFET sound better than bipolar, as an audio amp
> >> output driver?
> > MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> > odd harmonic distortin. on and on.
>
> Not in any relevant way for audio power amps.

This is not true.

The phase shift in the output section of a power amplifier is a
significant issue. If you want to have much of a gain sacrifice
factor in the audio band, the gain cross over point will be well above
the audio band.

NFB can't make a silk purse out of a sows ear. It can only lower the
distortion by about the gain sacrifice factor. This makes if better
to start with a low distortion topology.

Rumors aside MOSFETs are not really easier to operate in parallel.
You still need emitter / source resistors.

MooseFET

September 19th 08, 02:26 PM

On Sep 18, 7:40*am, Eeyore >
wrote:
> Arny Krueger wrote:
> > > wrote
> > > RichD wrote:
> > >> Who do MOSFET sound better than bipolar, as an audio amp
> > >> output driver?
>
> > > MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> > > odd harmonic distortin. on and on.
>
> > Not in any relevant way for audio power amps.
>
> The first two are highly relevevant in ANY circuit using NFB. Basic
> stability criteria.
>
> For Christ's sake has everyone except Phil's and my brain turned to jelly
> overnight ?

Checking brain ..... Mashed potatoes detected

Mine hasn't.

>
> Graham

Arny Krueger

September 19th 08, 02:30 PM

"Kevin McMurtrie" > wrote in message

> In article >,
> "Arny Krueger" > wrote:
>
>> "Eeyore" > wrote
>> in
>> message
>>> Kevin McMurtrie wrote:
>>>
>>>> In article
>>>> >, Damon
>>>> Hill > wrote:
>>>>
>>>>> RichD > wrote in
>>>>> news:b78fc9c2-fe9c-444c-8ac5-
>>>>> :
>>>>>
>>>>>> Who do MOSFET sound better than bipolar, as an audio
>>>>>> amp output driver?
>>>>>
>>>>> They do? Seems like it's possible to design good
>>>>> amplifiers either way.
>>>>>
>>>>> --Damon
>>>>
>>>> Exactly - zero difference in quality capabilities.
>>>> It's usually a matter of impedance matching. Silicon
>>>> transistors have a fixed loss around 0.5 volts.
>>>> MOSFETs have a resistive loss inversely proportional
>>>> to their voltage rating. That usually makes MOSFETs
>>>> less expensive for low impedances and transistors less
>>>> expensive for high impedances.
>>>
>>> TOTAL AND COMPLETE GARBAGE.
>>
>> Agreed. Actually, what Kevin said is the exact reverse
>> of generally accepted practical knowledge. Bipolar is
>> generally more efficient when the impedances get really
>> low. MOSFETs were trendy for a while, but most new
>> power amp designs seem to have bipolar outputs.

> Time to check the specs for this decade.

Been there done that.

> Visit IRF. You
> can get a surface mount MOSFET in package similar to
> TO-220 that has 0.0008 Ohms on resistance, 300W
> dissipation, 24V rating, and a 1600A surge rating.

The low voltage rating takes this device out of consideration for serious
audio amps for home or professional use. It may or may not be up to snuff
for automotive use.

Try coming up with a relevant example - something that can take at least 100
volts.

> Now I remember why I left this newsgroup.

I lack the time or bad judgment required to feel any remorse at all over
that. If you can't stand the heat - get out of the kitchen!

MooseFET

September 19th 08, 02:31 PM

On Sep 18, 2:25*am, "Jorden Verwer" > wrote:
> RichD wrote:
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> The device properties of BJTs are superior to those of MOSFETs in all
> respects, except for offset - there MOSFETs have the advantage. Whether you
> will actually hear this depends on many more factors.

Radiation hardness
Safe operating area
Bandwidth

Arny Krueger

September 19th 08, 02:32 PM

"Phil Allison" > wrote in message

> "Kevin McMurtrie"
>
>>
>> Time to check the specs for this decade. Visit IRF. You can get a
>> surface mount MOSFET in package similar to
>> TO-220 that has 0.0008 Ohms on resistance, 300W
>> dissipation, 24V rating, and a 1600A surge rating.
>
>
> ** So you can use it to replace the engine starter relay
> in a car.
> Yawnnnnnnn ....

Actually, it is marginal even for that. Most automotive guys like devices
that are safe at higher voltages. However, the solar panel inverter guys
might love it to death.

Phil Allison

September 19th 08, 02:34 PM

"MooseFET"

Rumors aside MOSFETs are not really easier to operate in parallel.
You still need emitter / source resistors.

** Which MOSFETS ????

Amazing how so many MOOSE like idiots have no idea there are TWO kinds
!!!!!!!!!!!!!!

The kind known as " lateral " share current just fine when in parallel in
LINEAR applications with no source ballast resistors.

Septic Tank Imbecile.

...... Phil

Arny Krueger

September 19th 08, 02:35 PM

"Eeyore" > wrote in
message
> Arny Krueger wrote:
>
>> > wrote
>>> RichD wrote:
>>>> Who do MOSFET sound better than bipolar, as an audio
>>>> amp output driver?
>>
>>> MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
>>> odd harmonic distortin. on and on.

>> Not in any relevant way for audio power amps.

> The first two are highly relevevant in ANY circuit using
> NFB. Basic stability criteria.

Yes, but we're talking about modern devices, not legacy devices. A ton of
very acceptable power amps were built with power devices with FT on the
order of a MHz. Every modern bipolar device designed for power amps that I
see has FT above 10 MHz.

I'm not saying that 100 KHz devices are optimal for audio, I'm saying that
in 2008, higher bandwidth than commonly available devices for audio power
amps is irrelevant.

MooseFET

September 19th 08, 02:38 PM

On Sep 18, 9:55*am, "Jorden Verwer" > wrote:
> Eeyore wrote:
> > Jorden Verwer wrote:
> > > RichD wrote:
> > > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > > driver?
>
> > > The device properties of BJTs are superior to those of MOSFETs in all
> > > respects,
>
> > How about SOA for one you UTTER MORON ?
>
> Fine, fine, but that doesn't have any direct influence on what you'll hear,
> because it's a boundary condition.

When I hear BLAM-FIZZZT-POP-KAPOW from an SOA problem, I gain and
understanding of how the SOA of a BJT is not better than that of a
MOSFET.

>
> > Do you even know what SOA is ?
>
> Like I said, it's a boundary condition. It can influence the performance of
> the circuit, but only indirectly, through other design decisions.

You can design around a problem with a device but that doesn't make
the device not have that problem.

>
> > > except for offset - there MOSFETs have the advantage. Whether you
> > > will actually hear this depends on many more factors.
>
> > YET MORE INSANE ********
>
> Now you're being the moron (not that I admit to being a moron before). It
> seems that either you don't know what you're talking about, or personal
> attacks are a hobby of yours. Because, frankly, everything I said was
> true...
>
> Fact - MOSFETs have lower offset than BJTs.

How are you defining offset?

> Fact - The fidelity of the sound depends on much more than just device
> properties.

Yes a Class-C audio output sounds quite bad regardless of the type of
device used. It sure gets rid of that annoying hiss.

Eeyore

September 19th 08, 02:41 PM

Spehro Pefhany wrote:

> the renowned John Larkin wrote:
> >Kevin McMurtrie wrote:
> >> "Arny Krueger" > wrote:
> >>> "Eeyore" > wrote in
> >>> > Kevin McMurtrie wrote:
> >>> >> Damon Hill > wrote:
> >>> >>> RichD > wrote in
> >>> >>>
> >>> >>>> Who do MOSFET sound better than bipolar, as an audio
> >>> >>>> amp output driver?
> >>> >>>
> >>> >>> They do? Seems like it's possible to design good
> >>> >>> amplifiers either way.
> >>> >>>
> >>> >>> --Damon
> >>> >>
> >>> >> Exactly - zero difference in quality capabilities. It's
> >>> >> usually a matter of impedance matching. Silicon
> >>> >> transistors have a fixed loss around 0.5 volts. MOSFETs
> >>> >> have a resistive loss inversely proportional to their
> >>> >> voltage rating. That usually makes MOSFETs less
> >>> >> expensive for low impedances and transistors less
> >>> >> expensive for high impedances.
> >>> >
> >>> > TOTAL AND COMPLETE GARBAGE.
> >>>
> >>> Agreed. Actually, what Kevin said is the exact reverse of generally accepted
> >>> practical knowledge. Bipolar is generally more efficient when the impedances
> >>> get really low. MOSFETs were trendy for a while, but most new power amp
> >>> designs seem to have bipolar outputs.
> >>
> >>Time to check the specs for this decade. Visit IRF. You can get a
> >>surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
> >>on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
> >>
> >>Now I remember why I left this newsgroup.
> >
> >IR is famous for creating incredible specs, with tiny footnotes
> >retracting the wilder numbers. No TO-220 is going to last long at 300
> >watts; not many milliseconds.
> >
> >John
>
> Anything approaching 1600A would blow the leadwires off a TO-220. The
> fusing current of a long wire of that cross-section area is only in
> the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.

Hence those ceramic power puks and the like.

Graham

Arny Krueger

September 19th 08, 02:42 PM

"Eeyore" > wrote in
message
> Arny Krueger wrote:
>
>> "RichD" > wrote
>>
>>> Who do MOSFET sound better than bipolar, as an audio amp
>>> output driver?
>>
>> Who do? That's voodoo!
>
> Only to be expected from Arny "Any amp with less than
> 0.1% THD at full power sounds the same as all other amps."

As usual, you're misrepresenting me.

Nonlinearity corresponding to 0.1% THD can be audible given choice of
musical program material. So, that number is way too high. Measurements
made at just one power level, like full power, are obviously bogus.

Obviously, the amp has to be able to perform within spec while driving a
real-world tough (but not badly designed) speaker load.

That all said, the world is full of good power amps. There are some bad
ones, too.

MooseFET

September 19th 08, 02:44 PM

On Sep 18, 10:39*am, "Jorden Verwer" > wrote:

[... BJTs are better ....]
> As for offset, here's one explanation (in the context of opamps):http://en.wikipedia.org/wiki/Operational_amplifier#DC_imperfections
>
> Note that I never claimed that this is relevant in audio applications - but
> it's there.

You may not have claimed it mattered but you sure implied it. That
aside since we are talking about the output section of an amplifier,
this sort of offset has no effect at all on the quality of the op-amp.

If you look for an op-amp with an extremely low offset voltage, you
will find that it uses MOSFETs to obtain that extreme low offset.

>
> As for your personal attacks towards me, I should mention that I'm under the
> impression that your experience with electronics outside audio applications
> is fairly limited, given that you've apparently never heard of the term
> offset.

Eeyore

September 19th 08, 02:47 PM

Arny Krueger wrote:

> "Eeyore" wrote
> > Arny Krueger wrote:
>
> >> This whole "amp sounds better" stuff was fully debunked
> >> 30 years ago. Good power amps sound the same and they
> >> sound like a piece of wire with gain.
>
> >> There are tons of power amps that can't be distinguished
> >> from a piece of wire with gain, while driving
> >> well-designed speakers.
>
> >> There are quite a few amps that meet the same criteria
> >> while driving even the weirdest speaker load.
>
> >> A really good power amp will destroy a poorly-designed
> >> speaker before it starts sounding bad, and really good
> >> power amps aren't all that unique.
>
> > The speaker is a large part of it. Highly reactive ( i.e.
> > lots of inductance or capacitance ) speakers can
> > prematurely trigger device protection.
>
> Agreed. Speakers that require heroic amplifiers (heroic amps definitely
> exist and are easy enough to obtain) are IMO not well designed.
>
> > We discovered that
> > EV's SX500 was particularly bad in this respect in PA for
> > example. Now that is certainly audible.
>
> Agreed. However we have only part of the equation - the name of the
> speaker. What was the amp?

With our own D Series amps. It was the only speaker we had trouble with. The VI
limiting was quite generous for a 4 ohm rated amp but not enough for an SX500. I
slackened off the VI protection and in the later AX series took a totally
different approach to protection.

> The SX 500i EDS suggests that it might be a handful for weak-kneed amps. Its
> impedance curve does go down to 4 ohms at several points below 1 KHz.

It's the phase angle that's insane more than the modulus of impedance. A D
series amp would drive 3 typical 8 ohm speakers in parallel quite happily.

> http://www.electrovoice.com/documents/ev/sx500-eds.pdf
>
> Using pairs paralleled can be a short road to amp problems.
>
> In contrast, the sequel ZX-5 stays above 8 ohms over the same range:
>
> http://www.electrovoice.com/documents/ev/ZX560PI_EDS.pdf
>
> Needless to say, I have 4 ZX-5s. ;-)

A better choice I imagine. I didn't even like the sound of the SX500s, very
harsh and boxy.

Graham

Eeyore

September 19th 08, 02:48 PM

MooseFET wrote:

> Rumors aside MOSFETs are not really easier to operate in parallel.
> You still need emitter / source resistors.

Not needed for lateral mosfets ! It's a whole different ball game.

Graham

Eeyore

September 19th 08, 02:50 PM

Arny Krueger wrote:

> "Kevin McMurtrie" > wrote in message
>
> > Visit IRF. You
> > can get a surface mount MOSFET in package similar to
> > TO-220 that has 0.0008 Ohms on resistance, 300W
> > dissipation, 24V rating, and a 1600A surge rating.
>
> The low voltage rating takes this device out of consideration for serious
> audio amps for home or professional use. It may or may not be up to snuff
> for automotive use.
>
> Try coming up with a relevant example - something that can take at least 100
> volts.

More like 230-250V !

Graham

Arny Krueger

September 19th 08, 02:55 PM

"Eeyore" > wrote in
message
> Arny Krueger wrote:
>
>> "Kevin McMurtrie" > wrote in message
>>
>>> Visit IRF. You
>>> can get a surface mount MOSFET in package similar to
>>> TO-220 that has 0.0008 Ohms on resistance, 300W
>>> dissipation, 24V rating, and a 1600A surge rating.
>>
>> The low voltage rating takes this device out of
>> consideration for serious audio amps for home or
>> professional use. It may or may not be up to snuff for
>> automotive use.
>>
>> Try coming up with a relevant example - something that
>> can take at least 100 volts.
>
> More like 230-250V !

Well, yes, now that they are so easy to obtain.

I was trying to make things easy for him.

In fact we built a lot of pretty good power amps with 60-90 volt parts, back
in the day.

Eeyore

September 19th 08, 02:57 PM

MooseFET wrote:

> "Jorden Verwer"wrote:
> > RichD wrote:
> > > Who do MOSFET sound better than bipolar, as an audio amp output
> > > driver?
> >
> > The device properties of BJTs are superior to those of MOSFETs in all
> > respects, except for offset - there MOSFETs have the advantage. Whether you
> > will actually hear this depends on many more factors.
>
> Radiation hardness
> Safe operating area
> Bandwidth

Not too worried about the first of those but gimme loads of the next two !

Hmmm - turn-off time 60ns @ 4A. That's why it pays to drive them from a
complementary emitter follower stage. Just like SMPSs.

Graham

Eeyore

September 19th 08, 02:59 PM

Phil Allison wrote:

> "MooseFET"
>
> Rumors aside MOSFETs are not really easier to operate in parallel.
> You still need emitter / source resistors.
>
> ** Which MOSFETS ????
>
> Amazing how so many MOOSE like idiots have no idea there are TWO kinds
> !!!!!!!!!!!!!!
>
> The kind known as " lateral " share current just fine when in parallel in
> LINEAR applications with no source ballast resistors.
>
> Septic Tank Imbecile.

It's quite clear that there are just TWO people in this thread who really
understand the use of lateral audio mosfets.

Graham

John Larkin

September 19th 08, 03:00 PM

On Fri, 19 Sep 2008 11:41:15 +0100, Eeyore
> wrote:

>
>
>Kevin McMurtrie wrote:
>
>> "Arny Krueger" wrote:
>> > "Eeyore" wrote
>> > > Kevin McMurtrie wrote:
>> > >> Damon Hill > wrote:
>> > >>> RichD > wrote in
>> > >>>
>> > >>>> Who do MOSFET sound better than bipolar, as an audio
>> > >>>> amp output driver?
>> > >>>
>> > >>> They do? Seems like it's possible to design good
>> > >>> amplifiers either way.
>> > >>
>> > >> Exactly - zero difference in quality capabilities. It's
>> > >> usually a matter of impedance matching. Silicon
>> > >> transistors have a fixed loss around 0.5 volts. MOSFETs
>> > >> have a resistive loss inversely proportional to their
>> > >> voltage rating. That usually makes MOSFETs less
>> > >> expensive for low impedances and transistors less
>> > >> expensive for high impedances.
>> > >
>> > > TOTAL AND COMPLETE GARBAGE.
>> >
>> > Agreed. Actually, what Kevin said is the exact reverse of generally accepted
>> > practical knowledge. Bipolar is generally more efficient when the impedances
>> > get really low. MOSFETs were trendy for a while, but most new power amp
>> > designs seem to have bipolar outputs.
>>
>> Time to check the specs for this decade. Visit IRF. You can get a
>> surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
>> on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
>
>These are NOT used in audio amps.
>
>Learn something about LATERAL mosfets that were designed for audio. I've already
>given part number and links to data sheets.
>
>Graham

That doesn't really matter. The transfer function only needs to be
continuous so that you can close a loop around it, and the fet needs
to be able to stand the peak power dissipation. That can easily be
done with vertical "switching" type fets. A modern FLOOD architecture
[1] works great with most any kind of fet. Lots of things have changed
in the last few decades.

John

[1] Of course you've never heard the term before. I just invented it.

Eeyore

September 19th 08, 03:02 PM

Arny Krueger wrote:

> "Eeyore" wrote
> > Arny Krueger wrote:
> >> > wrote
> >>> RichD wrote:
> >>>> Who do MOSFET sound better than bipolar, as an audio
> >>>> amp output driver?
> >>
> >>> MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
> >>> odd harmonic distortin. on and on.
>
> >> Not in any relevant way for audio power amps.
>
> > The first two are highly relevevant in ANY circuit using
> > NFB. Basic stability criteria.
>
> Yes, but we're talking about modern devices, not legacy devices.

It's relevant regardless ALWAYS. "Basic stability criteria" as I said above.

Those Hitachi style lateral mosfets are STILL faster than any audio specific
bipolar. Plus no SOA issues.

Show me an audio bipolar that'll switch off in 60ns.

Graham

Eeyore

September 19th 08, 03:04 PM

Arny Krueger wrote:

> I'm not saying that 100 KHz devices are optimal for audio

Even early 2N3055s were 8x better than that.

> I'm saying that in 2008, higher bandwidth than commonly available devices
> for audio power
> amps is irrelevant.

Plenty still have an fT of only 4MHz (published).

Graham

Don Pearce

September 19th 08, 03:08 PM

Eeyore wrote:
>
> Arny Krueger wrote:
>
>> I'm not saying that 100 KHz devices are optimal for audio
>
> Even early 2N3055s were 8x better than that.
>
>
>> I'm saying that in 2008, higher bandwidth than commonly available devices
>> for audio power
>> amps is irrelevant.
>
> Plenty still have an fT of only 4MHz (published).
>
> Graham
>
>

High gain bandwidth products are important for a good reason. If you can
keep device phase shifts well up in frequency, the dominant pole (I know
you didn't implement one specifically, but I'm sure it was still there)
can also move up in frequency. That means that global feedback will
carry on working instead of disappearing towards the top of the audio
band, at which point it has usually changed to local feedback in the
voltage amp, leaving output stage non-linearities uncorrected.

d

MooseFET

September 19th 08, 03:11 PM

On Sep 18, 7:06*am, Vladimir Vassilevsky >
wrote:
> RichD wrote:
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> In most cases, this is fallacy, and the result is just the opposite.
> Reason: FETs have lower transconductance compared to BJTs. It is
> impossible to build a half bridge stage with an ideal transfer curve.
>
> However there are few special cases when a FET output stage has an
> advantage:
>
> 1) With FETs, it is simpler to control bias current, because of the
> negative dependency from the temperature. That simplifies the life.

Beware: Mosfets like the STW55NM60 have a decreasing threshold
voltage for increasing temperature. This means that biasing them to a
low idle current isn't so easy.

Eeyore

September 19th 08, 03:16 PM

Arny Krueger wrote:

> "Eeyore" wrote
> > Arny Krueger wrote:
> >> "RichD" > wrote
> >>
> >>> Who do MOSFET sound better than bipolar, as an audio amp
> >>> output driver?
> >>
> >> Who do? That's voodoo!
> >
> > Only to be expected from Arny "Any amp with less than
> > 0.1% THD at full power sounds the same as all other amps."
>
> As usual, you're misrepresenting me.

It's not 'as usual' at all. I can clearly recall you making that claim.

> Nonlinearity corresponding to 0.1% THD can be audible given choice of
> musical program material.

I'm sure I've mentioned before I did a test when I thought the 0.1% spec was
pretty good against another amp with 0.01% (both bipolars). It was chalk and
cheese. The differences were NOT subtle. Nor were they 'level matching'
issues or whatever. It stood out like a sore thumb. One sounded 'gratey' the
other sounded 'clean'.

Since then a 0.01% spec is my minimum target. I like to get into the 0.00X%
area though.

> So, that number is way too high. Measurements
> made at just one power level, like full power, are obviously bogus.

I agree. As Chris Hornbeck has noted, low-level linearity is crucial.
Full-power specs are almost verging on the irrelevant but make good
advertising copy.

> Obviously, the amp has to be able to perform within spec while driving a
> real-world tough (but not badly designed) speaker load.
>
> That all said, the world is full of good power amps. There are some bad
> ones, too.

I'm not quite sure at the moment whether the overall trend is positive or
negative. It's certainly debatable.

Graham

Eeyore

September 19th 08, 03:23 PM

Arny Krueger wrote:

> "Eeyore" wrote
> > Arny Krueger wrote:
> >> "Kevin McMurtrie" > wrote in message
> >>
> >>> Visit IRF. You
> >>> can get a surface mount MOSFET in package similar to
> >>> TO-220 that has 0.0008 Ohms on resistance, 300W
> >>> dissipation, 24V rating, and a 1600A surge rating.
> >>
> >> The low voltage rating takes this device out of
> >> consideration for serious audio amps for home or
> >> professional use. It may or may not be up to snuff for
> >> automotive use.
> >>
> >> Try coming up with a relevant example - something that
> >> can take at least 100 volts.
> >
> > More like 230-250V !
>
> Well, yes, now that they are so easy to obtain.
>
> I was trying to make things easy for him.
>
> In fact we built a lot of pretty good power amps with 60-90 volt parts, back
> in the day.

That was a while back ! Even 2N3773s were good for 140V IIRC. I still have half
a tray of them somewhere from when I used to replace crap transistors with
something that would handle the job and not go phut.

Graham

Eeyore

September 19th 08, 03:28 PM

John Larkin wrote:

> Eeyore wrote:
> >Kevin McMurtrie wrote:
> >> "Arny Krueger" wrote:
> >> > "Eeyore" wrote
> >> > > Kevin McMurtrie wrote:
> >> > >> Damon Hill > wrote:
> >> > >>> RichD > wrote in
> >> > >>>
> >> > >>>> Who do MOSFET sound better than bipolar, as an audio
> >> > >>>> amp output driver?
> >> > >>>
> >> > >>> They do? Seems like it's possible to design good
> >> > >>> amplifiers either way.
> >> > >>
> >> > >> Exactly - zero difference in quality capabilities. It's
> >> > >> usually a matter of impedance matching. Silicon
> >> > >> transistors have a fixed loss around 0.5 volts. MOSFETs
> >> > >> have a resistive loss inversely proportional to their
> >> > >> voltage rating. That usually makes MOSFETs less
> >> > >> expensive for low impedances and transistors less
> >> > >> expensive for high impedances.
> >> > >
> >> > > TOTAL AND COMPLETE GARBAGE.
> >> >
> >> > Agreed. Actually, what Kevin said is the exact reverse of generally accepted
> >> > practical knowledge. Bipolar is generally more efficient when the impedances
> >> > get really low. MOSFETs were trendy for a while, but most new power amp
> >> > designs seem to have bipolar outputs.
> >>
> >> Time to check the specs for this decade. Visit IRF. You can get a
> >> surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
> >> on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
> >
> >These are NOT used in audio amps.
> >
> >Learn something about LATERAL mosfets that were designed for audio. I've already
> >given part number and links to data sheets.
>
> That doesn't really matter. The transfer function only needs to be
> continuous so that you can close a loop around it, and the fet needs
> to be able to stand the peak power dissipation. That can easily be
> done with vertical "switching" type fets. A modern FLOOD architecture
> [1] works great with most any kind of fet. Lots of things have changed
> in the last few decades.
>
> John
>
> [1] Of course you've never heard the term before. I just invented it.

Fine. Can you elaborate some more on it ? Laterals have some truly lovely features
for audio. The only downside being a slightly highish Ron. Not really a problem when
(as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They
also match beautifully with no need for source balance resistors (so some of the Ron
loss 'goes away').

Graham

Eeyore

September 19th 08, 03:36 PM

Don Pearce wrote:

> Eeyore wrote:
> > Arny Krueger wrote:
> >
> >> I'm not saying that 100 KHz devices are optimal for audio
> >
> > Even early 2N3055s were 8x better than that.
> >
> >> I'm saying that in 2008, higher bandwidth than commonly available devices
> >> for audio power
> >> amps is irrelevant.
> >
> > Plenty still have an fT of only 4MHz (published).
>
>
> High gain bandwidth products are important for a good reason. If you can
> keep device phase shifts well up in frequency, the dominant pole (I know
> you didn't implement one specifically, but I'm sure it was still there)
> can also move up in frequency.

Yes, there was ultimately one after the initial pole-zero which as you know
helps keep phase response (and therefore margin) under control.

> That means that global feedback will
> carry on working instead of disappearing towards the top of the audio
> band, at which point it has usually changed to local feedback in the
> voltage amp, leaving output stage non-linearities uncorrected.

Interesting. In the distant past, I've toyed with seperate feedback paths from
both those points.

Graham

Eeyore

September 19th 08, 03:37 PM

MooseFET wrote:

> Beware: Mosfets like the STW55NM60 have a decreasing threshold
> voltage for increasing temperature. This means that biasing them to a
> low idle current isn't so easy.

Indeed. You'll need lossy ballast resistors. Laterals are different that
way.

Graham

Fred Bartoli[_2_]

September 19th 08, 03:38 PM

Eeyore a écrit :
>
> "Mr.T" wrote:
>
>> "Fred Bartoli" <" "> wrote
>>
>>> Please explain how you obtain even distortion products from a
>>> symmetrical (suppose paired mosfets) push-pull.
>>>
>>> Any symmetrical function produces odd (and no even) harmonics.
>> But isn't that part of the problem, producing Mosfets with identically
>> symmetrical characteristics in N-ch and P-ch ?
>> Of course good circuit design can compensate for many such factors, so the
>> final result is what is important, not some evangelical belief, IMO.
>
> Fred is thinking of push-pull transformer coupled arrangements. He needs to
> advance about 4-5 decades.
>

Uhhh?

What I wrote is just basic maths.
Try a pencil and paper...

> That's one reason I never call modern amps push-pull but complementary
> symmetry instead.
>
> Graham
>

--
Thanks,
Fred.

Fred Bartoli[_2_]

September 19th 08, 03:40 PM

Eeyore a écrit :
>
> Fred Bartoli wrote:
>
>> Eeyore a écrit :
>>> Trevor Wilson wrote:
>>>> "RichD" > wrote
>>>>
>>>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>>>> driver?
>>>> **Non-sequitur. If you're saying that MOSFET outputs sound better, they
>>>> don't.
>>>>
>>>> As a device, operating at typical bias currents (say) 10-50mA, MOSFETs
>>>> exhibit VASTLY more THD than BJTs. Only when bias currents are elevated
>>>> (around 0.5A - 1A) do MOSFETs exhibit THD characteristics which are almost
>>>> as good as BJTs.
>>>>
>>>> MOSFETs are very tough, have an exceptional ability to deliver high power,
>>>> high frequency audio (and RF), but distortion is very high. They require
>>>> lots of Global NFB in order to operate linearly.
>>>>
>>>> MOSFETs do not sound better than BJTs. At best, they can sound as good. All
>>>> things being equal.
>>> With the required amount of feedback (which isn't rocket science - just good
>>> design) they have no trouble meeting VERY low THDs such as one I designed with
>>> 0.0008% THD @ 1 kHz.
>>>
>>> They sound better because of the predominance of 2nd harmonic distortion and
>>> almost complete absence of 3rd and ditto up the range AIUI with even and odd
>>> harmonics.
>> Uhhh.
>>
>> Please explain how you obtain even distortion products from a
>> symmetrical (suppose paired mosfets) push-pull.
>
> If you're such an expert, built one and see for yourself. And use LATERAL mosfets
> designed for audio.
>
> TWIT.
>

You don't need to build one. It's just basic maths that you don't seem
to grasp...

Oh, and vertical or lateral mosfets has nothing to do with that.

--
Thanks,
Fred.

Eeyore

September 19th 08, 03:42 PM

Fred Bartoli wrote:

> Eeyore a écrit :
> > "Mr.T" wrote:
> >> "Fred Bartoli" <" "> wrote
> >>
> >>> Please explain how you obtain even distortion products from a
> >>> symmetrical (suppose paired mosfets) push-pull.
> >>>
> >>> Any symmetrical function produces odd (and no even) harmonics.
> >> But isn't that part of the problem, producing Mosfets with identically
> >> symmetrical characteristics in N-ch and P-ch ?
> >> Of course good circuit design can compensate for many such factors, so the
> >> final result is what is important, not some evangelical belief, IMO.
> >
> > Fred is thinking of push-pull transformer coupled arrangements. He needs to
> > advance about 4-5 decades.
>
> Uhhh?
>
> What I wrote is just basic maths.
> Try a pencil and paper...

Try measuring one !

Graham

Kevin Aylward[_3_]

September 19th 08, 04:21 PM

it.
>>
>> Not at all in any remote way shape or form.
> Don't you at least agree there are many similarities between 1/f
> noise and offset?

Actually, I do. By and large, they amount to the same thing. Its all low
frequency variations. For example, if one designs a chopper amp to get low
offset, it also kills/corrects for 1/f noise as well. If one has 1/f
problems in an system, one immediately thinks about using a chopper..well I
do any way...

Kevin Aylward
www.anasoft.co.uk
SuperSpice

Phil Allison

September 19th 08, 04:22 PM

"MooseFET"
Vladimir Vassilevsky

>
> 1) With FETs, it is simpler to control bias current, because of the
> negative dependency from the temperature. That simplifies the life.

Beware: Mosfets like the STW55NM60 have a decreasing threshold
voltage for increasing temperature.

** Every switching MOSFET made has a negative gate threshold voltage tempco.

Equates to a VERY positive drain current tempco.

Many times more severe than a BJT.

...... Phil

Kevin Aylward[_3_]

September 19th 08, 04:23 PM

RichD wrote:
> On Sep 18, "Kevin Aylward" > wrote:
>>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>>> driver?
>>
>>> **Non-sequitur. If you're saying that MOSFET outputs sound better,
>>> they don't.
>>
>>> As a device, operating at typical bias currents (say) 10-50mA,
>>> MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents
>>> are elevated (around 0.5A - 1A) do MOSFETs exhibit THD
>>> characteristics which are almost as good as BJTs.
>>
>>> MOSFETs are very tough, have an exceptional ability to deliver high
>>> power, high frequency audio (and RF), but distortion is very high.
>>> They require lots of Global NFB in order to operate linearly.
>>
>>> MOSFETs do not sound better than BJTs. At best, they can sound as
>>> good. All things being equal.
>>
>> Well, I like mosfet outputs because they are easier to design with,
>> imo. Bipolars, often need an equivelent of 3 stage darlingtons. This
>> makes it a tad harder to stabilse the feedback loop because of each
>> stage pole.
>>
>> Basically, you only need about 6 low current transistors, or so, to
>> achieve silly distortion and bandwidth figures, with mosfets.
>>
>> As far as "sounds better", that's all moot. Any competently designed
>> amp should have thd, imd below audibility.
>>
>> Anyone that claims that a general purpose PA amp, sounds bad or not
>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
>> pretty much delusional.
>
> Do you have any experience designing audio amps?

er... yes...e.g.

http://www.studiomaster.com/1984%20-%201986.htm

"This was the amplifier pro sound companies were waiting for; many buy up to
100 units. "

> Which did you use?

Right now I use a few, one of which is a studiomaster AX2500 (750+750),
which I did not design. I also have a Behringer 1280S mixer/amp, and a
Carlsbro (600+600) PA..and a.etc...etc...

Kevin Aylward
www.anasoft.co.uk
SuperSpice

Kevin Aylward[_3_]

September 19th 08, 04:28 PM

Chronic Philharmonic wrote:
> "Eeyore" > wrote in message
> ...
>>
>>
>> Arny Krueger wrote:
>>
>>> > wrote
>>>> RichD wrote:
>>>>> Who do MOSFET sound better than bipolar, as an audio amp
>>>>> output driver?
>>>
>>>> MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
>>>> odd harmonic distortin. on and on.
>>>
>>> Not in any relevant way for audio power amps.
>>
>> The first two are highly relevevant in ANY circuit using NFB. Basic
>> stability criteria.
>
> Only if the bandwidth and the phase margins are small relative to the
> target audio bandwidth, which is unlikely.

Ahmmmm....

Lets say we are after a 50khz closed loop BW, i.e. so that there is
negligible loss at 20khz. Now suppose we are after silly distortion levels,
say , 0.005% at 20Khz. Typically this means large amounts of feedback, say a
minimum of 40db at 20khz, maybe up to 60db even. This means that we need
say, a loop unity gain bandwidth of say 5Mhz min. Now, if the Ft of a big,
high voltage high current, bipolar was say 50Mhz, which is a tad on the fast
side, its current gain would have dropped to 10, which aint so great.
Furthermore, it would be really pushing a 50Mhz ft transistor to get an
overall stable loop at 5Mhz., not forgetting that there will already be, by
design, a dominant pole rolloff, prior to the output stage.

Lets do some sums:

Cin of a bipolar ~= gm/2.pi.ft X re/RL, in emiter follower mode, i.e. Cin =
1/(2.pi.ft.RL)

For a 50Mhz bipolor this would be 800pf. at 4 ohms.

A mosfet, would be Cin ~= Cgs/(gm.RL), which at typically 1A/V and 600p,
would be Cin=125pf.

So, despite the much larger gm of a bipolar to back off its inherent large
Cbe in source follower mode, they still typically need much more high
frequency drive than mosfets. Furthermore, without additional buffering,
this larger capacitance kills the h.f gain of the class a main gain stage,
as already mentioned by Graham. Indeed, in the early 80s, such 50Mhz devices
were made from unobtainium.

There are a lot of other details, but I really don't have the time to go
into any more detailed technical design at the moment.

So... try putting full on voltage on a mosfet without a heatsink for a
while, then try that with a bipolar!.

And hopefully I haven't made any errors in my calcs;-)

Kevin Aylward
http://www.kevinaylward.co.uk/ee/index.html

Eeyore

September 19th 08, 04:30 PM

Eeyore wrote:

> It's quite clear that there are just TWO people in this thread who really
> understand the use of lateral audio mosfets.

Oops THREE. I forgot Kevin Aylward.

Graham

Kevin Aylward[_3_]

September 19th 08, 04:31 PM

Eeyore wrote:
> Kevin Aylward wrote:
>
>> Trevor Wilson wrote:
>>> "RichD" wrote
>>>
>>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>>> driver?
>>>
>>> **Non-sequitur. If you're saying that MOSFET outputs sound better,
>>> they don't.
>>>
>>> As a device, operating at typical bias currents (say) 10-50mA,
>>> MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents
>>> are elevated (around 0.5A - 1A) do MOSFETs exhibit THD
>>> characteristics which are almost as good as BJTs.
>>>
>>> MOSFETs are very tough, have an exceptional ability to deliver high
>>> power, high frequency audio (and RF), but distortion is very high.
>>> They require lots of Global NFB in order to operate linearly.
>>>
>>> MOSFETs do not sound better than BJTs. At best, they can sound as
>>> good. All things being equal.
>>
>> Ahmm.... welll....here we go...
>>
>> Well, I like mosfet outputs because they are easier to design with,
>> imo. Bipolars, often need an equivelent of 3 stage darlingtons. This
>> makes it a tad harder to stabilse the feedback loop because of each
>> stage pole.
>>
>> Basically, you only need about 6 low current transistors, or so, to
>> achieve silly distortion and bandwidth figures, with mosfets.
>>
>> As far as "sounds better", that's all moot. Any competently designed
>> amp should have thd, imd below audibility.
>
> Seems to be a few that miss that though. Esp those Chinese copies of
> copies.
>
>
>> Anyone that claims that a general purpose PA amp, sounds bad or not
>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
>> pretty much delusional. Roll on the £200 oxygen free mains cable I
>> say....
>
> Why stop at £200 ? Oh !
> http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
>
> Kevin, you'd be just the guy to do a harmonic analysis of the
> distortion spectrum of a properly biased bipolar vs lateral mosfet
> amp wouldn't you ? Any chance ?
>

It has just occurred to me, that you probably live quite close to me. I
live in Stevenage now, just a tad away from the old Luton hunting grounds. I
would say a joint visit to the pub for some Guinness might not go amiss.

Oh.. check out my current venture. Available for
bookings...www.blonddee.co.uk

Kevin Aylward
www.anasoft.co.uk
SuperSpice

John Larkin

September 19th 08, 04:35 PM

On Fri, 19 Sep 2008 15:28:33 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Eeyore wrote:
>> >Kevin McMurtrie wrote:
>> >> "Arny Krueger" wrote:
>> >> > "Eeyore" wrote
>> >> > > Kevin McMurtrie wrote:
>> >> > >> Damon Hill > wrote:
>> >> > >>> RichD > wrote in
>> >> > >>>
>> >> > >>>> Who do MOSFET sound better than bipolar, as an audio
>> >> > >>>> amp output driver?
>> >> > >>>
>> >> > >>> They do? Seems like it's possible to design good
>> >> > >>> amplifiers either way.
>> >> > >>
>> >> > >> Exactly - zero difference in quality capabilities. It's
>> >> > >> usually a matter of impedance matching. Silicon
>> >> > >> transistors have a fixed loss around 0.5 volts. MOSFETs
>> >> > >> have a resistive loss inversely proportional to their
>> >> > >> voltage rating. That usually makes MOSFETs less
>> >> > >> expensive for low impedances and transistors less
>> >> > >> expensive for high impedances.
>> >> > >
>> >> > > TOTAL AND COMPLETE GARBAGE.
>> >> >
>> >> > Agreed. Actually, what Kevin said is the exact reverse of generally accepted
>> >> > practical knowledge. Bipolar is generally more efficient when the impedances
>> >> > get really low. MOSFETs were trendy for a while, but most new power amp
>> >> > designs seem to have bipolar outputs.
>> >>
>> >> Time to check the specs for this decade. Visit IRF. You can get a
>> >> surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
>> >> on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
>> >
>> >These are NOT used in audio amps.
>> >
>> >Learn something about LATERAL mosfets that were designed for audio. I've already
>> >given part number and links to data sheets.
>>
>> That doesn't really matter. The transfer function only needs to be
>> continuous so that you can close a loop around it, and the fet needs
>> to be able to stand the peak power dissipation. That can easily be
>> done with vertical "switching" type fets. A modern FLOOD architecture
>> [1] works great with most any kind of fet. Lots of things have changed
>> in the last few decades.
>>
>> John
>>
>> [1] Of course you've never heard the term before. I just invented it.
>
>Fine. Can you elaborate some more on it ? Laterals have some truly lovely features
>for audio. The only downside being a slightly highish Ron. Not really a problem when
>(as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They
>also match beautifully with no need for source balance resistors (so some of the Ron
>loss 'goes away').
>
>Graham
>

An opamp per fet, closing a local loop, feedback from the fet source,
makes each fet look like a perfect unity-gain, fast, zero-offset
device. Opamps are cheap, but fets and heat sinks are expensive. Power
all those gate-drive opamps from a DC-DC converter floating on the
output node; DC/DC bricks are cheap nowadays, too. Do a simple output
current limit for fast overloads and back that up with a digital fet
power dissipation simulation that provides the real protections. That
will optimize the hell out of the power supply, fets, and heat sink,
giving a lot more safe power for the buck, especially in
complex-signal non-sinusoidal apps like audio and NMR gradient
drivers.

Use a bunch of smaller fets rather than a few big ones; that speeds
things up and spreads the heat out across the heat sinks better.

For the audio version, use two "hot" heat sinks, with no insulators
under the fets.

Include full BIST. It's worth it for the savings in production test
alone.

The output stage will be so quiet and linear that no overall feedback
is needed or helpful.

The audiophools will hate this.

John

Kevin Aylward[_3_]

September 19th 08, 04:42 PM

Eeyore wrote:
> Kevin Aylward wrote:

>> Anyone that claims that a general purpose PA amp, sounds bad or not
>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
>> pretty much delusional. Roll on the £200 oxygen free mains cable I
>> say....
>
> Why stop at £200 ? Oh !
> http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm

This link seems to be bad. I did find this though

http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#

All I can say, is that I am still stunned..and shocked...shocked and
stunned...all I said was that I was taller than Jesus, not that I was bigger
than Jesus...

Kevin Aylward

Eeyore

September 19th 08, 04:51 PM

Kevin Aylward wrote:

> Eeyore wrote:
> > Kevin Aylward wrote:
> >> Trevor Wilson wrote:
> >>> "RichD" wrote
> >>>
> >>>> Who do MOSFET sound better than bipolar, as an audio amp output
> >>>> driver?
> >>>
> >>> **Non-sequitur. If you're saying that MOSFET outputs sound better,
> >>> they don't.
> >>>
> >>> As a device, operating at typical bias currents (say) 10-50mA,
> >>> MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents
> >>> are elevated (around 0.5A - 1A) do MOSFETs exhibit THD
> >>> characteristics which are almost as good as BJTs.
> >>>
> >>> MOSFETs are very tough, have an exceptional ability to deliver high
> >>> power, high frequency audio (and RF), but distortion is very high.
> >>> They require lots of Global NFB in order to operate linearly.
> >>>
> >>> MOSFETs do not sound better than BJTs. At best, they can sound as
> >>> good. All things being equal.
> >>
> >> Ahmm.... welll....here we go...
> >>
> >> Well, I like mosfet outputs because they are easier to design with,
> >> imo. Bipolars, often need an equivelent of 3 stage darlingtons. This
> >> makes it a tad harder to stabilse the feedback loop because of each
> >> stage pole.
> >>
> >> Basically, you only need about 6 low current transistors, or so, to
> >> achieve silly distortion and bandwidth figures, with mosfets.
> >>
> >> As far as "sounds better", that's all moot. Any competently designed
> >> amp should have thd, imd below audibility.
> >
> > Seems to be a few that miss that though. Esp those Chinese copies of
> > copies.
> >
> >> Anyone that claims that a general purpose PA amp, sounds bad or not
> >> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
> >> pretty much delusional. Roll on the £200 oxygen free mains cable I
> >> say....
> >
> > Why stop at £200 ? Oh !
> > http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
> >
> > Kevin, you'd be just the guy to do a harmonic analysis of the
> > distortion spectrum of a properly biased bipolar vs lateral mosfet
> > amp wouldn't you ? Any chance ?
>
> It has just occurred to me, that you probably live quite close to me. I
> live in Stevenage now, just a tad away from the old Luton hunting grounds. I
> would say a joint visit to the pub for some Guinness might not go amiss.

Sounds ok to me. I'm in St Albans. Will contact you off group.

> Oh.. check out my current venture. Available for
> bookings...www.blonddee.co.uk

OK will do. I know someone who might even book you.

Graham

Eeyore

September 19th 08, 04:59 PM

Kevin Aylward wrote:

> Eeyore wrote:
> > Kevin Aylward wrote:
>
> >> Anyone that claims that a general purpose PA amp, sounds bad or not
> >> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
> >> pretty much delusional. Roll on the £200 oxygen free mains cable I
> >> say....
> >
> > Why stop at £200 ? Oh !
> > http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
>
> This link seems to be bad. I did find this though

Works here.

> http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#
>
> All I can say, is that I am still stunned..and shocked...shocked and
> stunned...all I said was that I was taller than Jesus, not that I was bigger
> than Jesus...

I'd like to start a movement to have all these liars and fraudsters shut down.

Graham

Kevin Aylward[_3_]

September 19th 08, 05:48 PM

Eeyore wrote:
> Kevin Aylward wrote:
>
>> Eeyore wrote:
>>> Kevin Aylward wrote:
>>>> Trevor Wilson wrote:
>>>>> "RichD" wrote
>>>>>
>>>>>> Who do MOSFET sound better than bipolar, as an audio amp output
>>>>>> driver?
>>>>>
>>>>> **Non-sequitur. If you're saying that MOSFET outputs sound better,
>>>>> they don't.
>>>>>
>>>>> As a device, operating at typical bias currents (say) 10-50mA,
>>>>> MOSFETs exhibit VASTLY more THD than BJTs. Only when bias currents
>>>>> are elevated (around 0.5A - 1A) do MOSFETs exhibit THD
>>>>> characteristics which are almost as good as BJTs.
>>>>>
>>>>> MOSFETs are very tough, have an exceptional ability to deliver
>>>>> high power, high frequency audio (and RF), but distortion is very
>>>>> high. They require lots of Global NFB in order to operate
>>>>> linearly.
>>>>>
>>>>> MOSFETs do not sound better than BJTs. At best, they can sound as
>>>>> good. All things being equal.
>>>>
>>>> Ahmm.... welll....here we go...
>>>>
>>>> Well, I like mosfet outputs because they are easier to design with,
>>>> imo. Bipolars, often need an equivelent of 3 stage darlingtons.
>>>> This makes it a tad harder to stabilse the feedback loop because
>>>> of each stage pole.
>>>>
>>>> Basically, you only need about 6 low current transistors, or so, to
>>>> achieve silly distortion and bandwidth figures, with mosfets.
>>>>
>>>> As far as "sounds better", that's all moot. Any competently
>>>> designed amp should have thd, imd below audibility.
>>>
>>> Seems to be a few that miss that though. Esp those Chinese copies of
>>> copies.
>>>
>>>> Anyone that claims that a general purpose PA amp, sounds bad or not
>>>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
>>>> pretty much delusional. Roll on the £200 oxygen free mains cable I
>>>> say....
>>>
>>> Why stop at £200 ? Oh !
>>> http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
>>>
>>> Kevin, you'd be just the guy to do a harmonic analysis of the
>>> distortion spectrum of a properly biased bipolar vs lateral mosfet
>>> amp wouldn't you ? Any chance ?
>>
>> It has just occurred to me, that you probably live quite close to
>> me. I live in Stevenage now, just a tad away from the old Luton
>> hunting grounds. I would say a joint visit to the pub for some
>> Guinness might not go amiss.
>
> Sounds ok to me. I'm in St Albans.

Yeah, just down the road...16.7M according to the AA. Although with a
suggested time of 20mins, obviously, the AA don't know how to drive...

>Will contact you off group.

Look foward to it. My phone # is on my websites.

>
>
>> Oh.. check out my current venture. Available for
>> bookings...www.blonddee.co.uk
>
> OK will do. I know someone who might even book you.

Oh.. nice...

Kevin Aylward

Kevin Aylward[_3_]

September 19th 08, 05:49 PM

Eeyore wrote:
> Kevin Aylward wrote:
>
>> Eeyore wrote:
>>> Kevin Aylward wrote:
>>
>>>> Anyone that claims that a general purpose PA amp, sounds bad or not
>>>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
>>>> pretty much delusional. Roll on the £200 oxygen free mains cable I
>>>> say....
>>>
>>> Why stop at £200 ? Oh !
>>> http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
>>
>> This link seems to be bad. I did find this though
>
> Works here.

worked now.

>
>
>> http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#
>>
>> All I can say, is that I am still stunned..and shocked...shocked and
>> stunned...all I said was that I was taller than Jesus, not that I
>> was bigger than Jesus...
>
> I'd like to start a movement to have all these liars and fraudsters
> shut down.
>

Indeed. If I had actually said that I was bigger than Jesus, I would have
said so...

As you may have noticed from visiting churches, the average height a while
back, was a lot smaller...

Kevin Aylward

Eeyore

September 19th 08, 05:56 PM

Kevin Aylward wrote:

> Eeyore wrote:
> > Kevin Aylward wrote:
> >> Eeyore wrote:
> >>> Kevin Aylward wrote:
> >>
> >>>> Anyone that claims that a general purpose PA amp, sounds bad or not
> >>>> good, if it has thd and imd < 0.005% and slew rates of 100V/us, is
> >>>> pretty much delusional. Roll on the £200 oxygen free mains cable I
> >>>> say....
> >>>
> >>> Why stop at £200 ? Oh !
> >>> http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm
> >>
> >> This link seems to be bad. I did find this though
> >
> > Works here.
> worked now.
>
> >> http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#
> >>
> >> All I can say, is that I am still stunned..and shocked...shocked and
> >> stunned...all I said was that I was taller than Jesus, not that I
> >> was bigger than Jesus...
> >
> > I'd like to start a movement to have all these liars and fraudsters
> > shut down.
>
> Indeed. If I had actually said that I was bigger than Jesus, I would have
> said so...
>
> As you may have noticed from visiting churches, the average height a while
> back, was a lot smaller...

By pure chance I just received some info from Russ Andrews (several booklets)
on behalf of somone who didn't want to give their identity away.

Cream of the bunch I think was a 'special' phono lead optimised for subwoofer
connections in multi-speaker systems for deeper and richer bass or whatever
nutcase story they were making up.

I really would like to see that company go down along with all the fraudulent
hi-fi rags too.

Graham

Eeyore

September 19th 08, 06:10 PM

John Larkin wrote:

> Eeyore wrote:
> >John Larkin wrote:
> >> Eeyore wrote:
> >> >
> >> >Learn something about LATERAL mosfets that were designed for audio. I've already
> >> >given part number and links to data sheets.
> >>
> >> That doesn't really matter. The transfer function only needs to be
> >> continuous so that you can close a loop around it, and the fet needs
> >> to be able to stand the peak power dissipation. That can easily be
> >> done with vertical "switching" type fets. A modern FLOOD architecture
> >> [1] works great with most any kind of fet. Lots of things have changed
> >> in the last few decades.
> >>
> >> John
> >>
> >> [1] Of course you've never heard the term before. I just invented it.
> >
> >Fine. Can you elaborate some more on it ? Laterals have some truly lovely features
> >for audio. The only downside being a slightly highish Ron. Not really a problem when
> >(as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They
> >also match beautifully with no need for source balance resistors (so some of the Ron
> >loss 'goes away').
>
>
> An opamp per fet, closing a local loop, feedback from the fet source,
> makes each fet look like a perfect unity-gain, fast, zero-offset
> device.

Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
output overshoot.

> Opamps are cheap, but fets and heat sinks are expensive. Power
> all those gate-drive opamps from a DC-DC converter floating on the
> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
> current limit for fast overloads and back that up with a digital fet
> power dissipation simulation that provides the real protections.

I've toyed in the past with doing device protection using an analogue multiplier actually
funnily enough.

> That
> will optimize the hell out of the power supply, fets, and heat sink,
> giving a lot more safe power for the buck, especially in
> complex-signal non-sinusoidal apps like audio and NMR gradient
> drivers.
>
> Use a bunch of smaller fets rather than a few big ones; that speeds
> things up and spreads the heat out across the heat sinks better.
>
> For the audio version, use two "hot" heat sinks, with no insulators
> under the fets.

Something I am very much in favour of. I like that junction to see cool aluminium as fast
as possible.

> Include full BIST. It's worth it for the savings in production test
> alone.
>
> The output stage will be so quiet and linear that no overall feedback
> is needed or helpful.
>
> The audiophools will hate this.

All the better !

Graham

Jan Panteltje

September 19th 08, 07:09 PM

On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore
> wrote in
>:

>> An opamp per fet, closing a local loop, feedback from the fet source,
>> makes each fet look like a perfect unity-gain, fast, zero-offset
>> device.
>
>Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
>output overshoot.

If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2
you will see this is exactly what is done with the lower output MOSFET, combined with opamp
makes unity gain.
The top is already a source followwr.
The over current protection same thing, opamps.
It is likely this what makes the Boucherot network not needed.

You could do the same thing with the top MOSFET in a discrete design, if you must.

>> Opamps are cheap, but fets and heat sinks are expensive. Power
>> all those gate-drive opamps from a DC-DC converter floating on the
>> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
>> current limit for fast overloads and back that up with a digital fet
>> power dissipation simulation that provides the real protections.

>I've toyed in the past with doing device protection using an analogue multiplier actually
>funnily enough.

I only see over current protection in a lot of amps, plus thermal,
the real thing was a bridge that caused foldback current limit,
couple of resistors and diodes, makes it any load proof.

Kevin Aylward[_3_]

September 19th 08, 07:13 PM

Eeyore wrote:
> John Larkin wrote:
>
>> Eeyore wrote:
>>> John Larkin wrote:
>>>> Eeyore wrote:
>>>>>
>>>>> Learn something about LATERAL mosfets that were designed for
>>>>> audio. I've already given part number and links to data sheets.
>>>>
>>>> That doesn't really matter. The transfer function only needs to be
>>>> continuous so that you can close a loop around it, and the fet
>>>> needs to be able to stand the peak power dissipation. That can
>>>> easily be done with vertical "switching" type fets. A modern FLOOD
>>>> architecture [1] works great with most any kind of fet. Lots of
>>>> things have changed in the last few decades.
>>>>
>>>> John
>>>>
>>>> [1] Of course you've never heard the term before. I just invented
>>>> it.
>>>
>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>> lovely features for audio. The only downside being a slightly
>>> highish Ron. Not really a problem when (as I have) used as many as
>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>> match beautifully with no need for source balance resistors (so
>>> some of the Ron loss 'goes away').
>>
>>
>> An opamp per fet, closing a local loop, feedback from the fet source,
>> makes each fet look like a perfect unity-gain, fast, zero-offset
>> device.
>
> Interesting idea. I'll have to chew that one over. I can see possible
> problems fron op-amp output overshoot.

I have a simple embodiment of that concept here, done a while ago, in
virtual land;-)

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

Its a push/pull gain loop around the output devices, forcing them to be
unity gain followers.

You can get lower distortion, at the expense of speed, because you have to
compensate earlier.

Common mode feedback at the second stage, allows for enormous dc/lf gain.
cascodes to allow the use of fast small transistors to do all the main work.
Emitter follower buffer to reduce the current swing in the input pair, as
per doug self. Spice says it should be in the < 0.0001% , 20Khz range,
maybe...

Kevin Aylward

Kevin Aylward[_3_]

September 19th 08, 08:39 PM

Jan Panteltje wrote:
> On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore
> > wrote in
> >:
>
>>> An opamp per fet, closing a local loop, feedback from the fet
>>> source, makes each fet look like a perfect unity-gain, fast,
>>> zero-offset device.
>>
>> Interesting idea. I'll have to chew that one over. I can see
>> possible problems fron op-amp output overshoot.
>
> If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the
> blockdiagram on page 2 you will see this is exactly what is done with
> the lower output MOSFET, combined with opamp makes unity gain.
> The top is already a source followwr.
> The over current protection same thing, opamps.
> It is likely this what makes the Boucherot network not needed.

I doubt it. I haven't seen the data sheet, but it is not usual to be able to
stabilise an amp with what I believe you are describing here.

Consider one amp feeding the other, both running open loop, with overall
feedback, to the 1st. Now consider the case where the second amp is
configured with local feedback, to make it a unity buffer, following the 1st
amp. Naively , one might argue that the 2nd stage now has a wider bandwidth,
due to feedback, such that the "new " system might be stable, i.e. one main
rolloff due to the 1st amp. However, in realty, topologically, nothing
changes. The determine the stability of the system, one needs to break the
loop at a point that breaks *all* feedback paths at once. This point will be
the point directly at the output of the 2nd amp. when this is done, it is
clear that the stabiliy is still due to the total loop gain of both amops
cascaded.

Excepting for the special cases, e.g. , where feedback is used to neutralise
r.f amps, feedback in general, cannot be used to widen bandwidth, if the
purpose of that wider bandwidth is to achieve stability, in this type of
arrangement

To wit, There is no such thing as a free lunch...

Kevin Aylward

www.superspice.co.uk

John Larkin

September 19th 08, 09:12 PM

On Fri, 19 Sep 2008 08:24:50 -0400, Spehro Pefhany
> wrote:

>On Thu, 18 Sep 2008 20:23:29 -0700, the renowned John Larkin
> wrote:
>
>>On Thu, 18 Sep 2008 20:06:43 -0700, Kevin McMurtrie
> wrote:
>>
>>>In article >,
>>> "Arny Krueger" > wrote:
>>>
>>>> "Eeyore" > wrote in
>>>> message
>>>> > Kevin McMurtrie wrote:
>>>> >
>>>> >> In article >,
>>>> >> Damon Hill > wrote:
>>>> >>
>>>> >>> RichD > wrote in
>>>> >>> news:b78fc9c2-fe9c-444c-8ac5-
>>>> >>> :
>>>> >>>
>>>> >>>> Who do MOSFET sound better than bipolar, as an audio
>>>> >>>> amp output driver?
>>>> >>>
>>>> >>> They do? Seems like it's possible to design good
>>>> >>> amplifiers either way.
>>>> >>>
>>>> >>> --Damon
>>>> >>
>>>> >> Exactly - zero difference in quality capabilities. It's
>>>> >> usually a matter of impedance matching. Silicon
>>>> >> transistors have a fixed loss around 0.5 volts. MOSFETs
>>>> >> have a resistive loss inversely proportional to their
>>>> >> voltage rating. That usually makes MOSFETs less
>>>> >> expensive for low impedances and transistors less
>>>> >> expensive for high impedances.
>>>> >
>>>> > TOTAL AND COMPLETE GARBAGE.
>>>>
>>>> Agreed. Actually, what Kevin said is the exact reverse of generally accepted
>>>> practical knowledge. Bipolar is generally more efficient when the impedances
>>>> get really low. MOSFETs were trendy for a while, but most new power amp
>>>> designs seem to have bipolar outputs.
>>>
>>>Time to check the specs for this decade. Visit IRF. You can get a
>>>surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
>>>on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
>>>
>>>Now I remember why I left this newsgroup.
>>
>>IR is famous for creating incredible specs, with tiny footnotes
>>retracting the wilder numbers. No TO-220 is going to last long at 300
>>watts; not many milliseconds.
>>
>>John
>
>Anything approaching 1600A would blow the leadwires off a TO-220. The
>fusing current of a long wire of that cross-section area is only in
>the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.
>
>
>Best regards,
>Spehro Pefhany

Their selection tables show this as a 280 amp TO-220:

https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430

but the datasheet qualifies this as merely 75:

http://www.irf.com/product-info/datasheets/data/irf2804.pdf

They do rate it for 330 watts!

John

John Larkin

September 19th 08, 09:46 PM

On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Eeyore wrote:
>> >John Larkin wrote:
>> >> Eeyore wrote:
>> >> >
>> >> >Learn something about LATERAL mosfets that were designed for audio. I've already
>> >> >given part number and links to data sheets.
>> >>
>> >> That doesn't really matter. The transfer function only needs to be
>> >> continuous so that you can close a loop around it, and the fet needs
>> >> to be able to stand the peak power dissipation. That can easily be
>> >> done with vertical "switching" type fets. A modern FLOOD architecture
>> >> [1] works great with most any kind of fet. Lots of things have changed
>> >> in the last few decades.
>> >>
>> >> John
>> >>
>> >> [1] Of course you've never heard the term before. I just invented it.
>> >
>> >Fine. Can you elaborate some more on it ? Laterals have some truly lovely features
>> >for audio. The only downside being a slightly highish Ron. Not really a problem when
>> >(as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They
>> >also match beautifully with no need for source balance resistors (so some of the Ron
>> >loss 'goes away').
>>
>>
>> An opamp per fet, closing a local loop, feedback from the fet source,
>> makes each fet look like a perfect unity-gain, fast, zero-offset
>> device.
>
>Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
>output overshoot.

It needs a little loop tweaking, roughly....

V+
|\ |
-----------| + |
| d
| out-------+------Rg--------g
| | s
+------| - | |
| |/ Cf |
| | |
+--------------------+------Rf---------+
|
|
Rs
|
|
+------------- output rail

and an opamp that can slam the gate hard enough, not a big problem
nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
can be small, and the quiescent bias voltage can be small, because the
opamp offset voltage can be tiny. The fets share the load exactly, and
the standing bias current can be designed in, exactly, with no
adjustments.

The driver stage sees only opamps, so doesn't have to work very hard.

>
>
>> Opamps are cheap, but fets and heat sinks are expensive. Power
>> all those gate-drive opamps from a DC-DC converter floating on the
>> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
>> current limit for fast overloads and back that up with a digital fet
>> power dissipation simulation that provides the real protections.
>
>I've toyed in the past with doing device protection using an analogue multiplier actually
>funnily enough.

I digitize everything - heatsink temp, supply currents, load voltage -
and run a realtime simulation of fet power dissipation and resulting
junction temperature, with shutdown at, say, 140 C.

An adaptive fan speed would be a nice touch... no fan until it's
really needed. Maybe next time.

John

Kevin Aylward[_3_]

September 19th 08, 10:13 PM

John Larkin wrote:
> On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore
> > wrote:
>
>>
>>
>> John Larkin wrote:
>>
>>> Eeyore wrote:
>>>> John Larkin wrote:
>>>>> Eeyore wrote:
>>>>>>
>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>> audio. I've already given part number and links to data sheets.
>>>>>
>>>>> That doesn't really matter. The transfer function only needs to be
>>>>> continuous so that you can close a loop around it, and the fet
>>>>> needs to be able to stand the peak power dissipation. That can
>>>>> easily be done with vertical "switching" type fets. A modern
>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>> Lots of things have changed in the last few decades.
>>>>>
>>>>> John
>>>>>
>>>>> [1] Of course you've never heard the term before. I just invented
>>>>> it.
>>>>
>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>> lovely features for audio. The only downside being a slightly
>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>> match beautifully with no need for source balance resistors (so
>>>> some of the Ron loss 'goes away').
>>>
>>>
>>> An opamp per fet, closing a local loop, feedback from the fet
>>> source, makes each fet look like a perfect unity-gain, fast,
>>> zero-offset device.
>>
>> Interesting idea. I'll have to chew that one over. I can see
>> possible problems fron op-amp output overshoot.
>
>
> It needs a little loop tweaking, roughly....
>
>
> V+
> |\ |
> -----------| + |
> | d
> | out-------+------Rg--------g
> | | s
> +------| - | |
> | |/ Cf |
> | | |
> +--------------------+------Rf---------+
> |
> |
> Rs
> |
> |
> +------------- output rail
>
>
> and an opamp that can slam the gate hard enough, not a big problem
> nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
> can be small, and the quiescent bias voltage can be small, because the
> opamp offset voltage can be tiny. The fets share the load exactly, and
> the standing bias current can be designed in, exactly, with no
> adjustments.
>
> The driver stage sees only opamps, so doesn't have to work very hard.
>

However....there are some issues with using whole op-amps, rather than
discrete transistors as I have in the noted circuit in my other post. You
may need 200V+ ratings, and a very fast one at that!!!

Kevin Aylward

www.kevinaylward.co.uk

Jan Panteltje

September 19th 08, 10:29 PM

On a sunny day (Fri, 19 Sep 2008 19:39:51 GMT) it happened "Kevin Aylward"
> wrote in
>:

>> If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the
>> blockdiagram on page 2 you will see this is exactly what is done with
>> the lower output MOSFET, combined with opamp makes unity gain.
>> The top is already a source followwr.
>> The over current protection same thing, opamps.
>> It is likely this what makes the Boucherot network not needed.
>
>I doubt it. I haven't seen the data sheet, but it is not usual to be able to
>stabilise an amp with what I believe you are describing here.

It is always a good idea to lookuop what we are talking about.

>Consider one amp feeding the other, both running open loop, with overall
>feedback, to the 1st. Now consider the case where the second amp is
>configured with local feedback, to make it a unity buffer, following the 1st
>amp. Naively , one might argue that the 2nd stage now has a wider bandwidth,
>due to feedback, such that the "new " system might be stable, i.e. one main
>rolloff due to the 1st amp. However, in realty, topologically, nothing
>changes.

Some people are really good at that stuff, sort of reasonin gI mean.

But if you look at the combination MOSFET - opamp, as John Larkin is describing
in an other post in this thread, then you can treat that as one 'perfect MOSFET'.
Of course it is not really perfect, but you can make that as stable or unstable as you want.
Then basically what you do is chaining stable blocks together.
If you then apply feedback, you have to use the phase characteristic of all those,
and, as long as you prevent too high frequencies from circulating, it should be stable,
and largely independent of the load.
I note the TDA9274 has one capacitor to roll of in the driver...
This is normal, at least in the amps I designed.

>The determine the stability of the system, one needs to break the
>loop at a point that breaks *all* feedback paths at once. This point will be
>the point directly at the output of the 2nd amp. when this is done, it is
>clear that the stabiliy is still due to the total loop gain of both amops
>cascaded.

You mean 'open loop gain?'
Yes, but he second amp would have gain 1.

>Excepting for the special cases, e.g. , where feedback is used to neutralise
>r.f amps, feedback in general, cannot be used to widen bandwidth, if the
>purpose of that wider bandwidth is to achieve stability, in this type of
>arrangement

I was not suggesting to widen bandwidth, although strong local feedback would of course
widen the bandwidth of a stage,

Honestly, I have to think about this a bit, maybe run it in spice.
Fact remains that the TDA9274 is the only amp I know that needs no Boucherot circuit :-)

>To wit, There is no such thing as a free lunch...

It seems to exist for US bankers ATM.

Don Pearce

September 19th 08, 11:03 PM

Kevin Aylward wrote:
> Eeyore wrote:
>> John Larkin wrote:
>>
>>> Eeyore wrote:
>>>> John Larkin wrote:
>>>>> Eeyore wrote:
>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>> audio. I've already given part number and links to data sheets.
>>>>> That doesn't really matter. The transfer function only needs to be
>>>>> continuous so that you can close a loop around it, and the fet
>>>>> needs to be able to stand the peak power dissipation. That can
>>>>> easily be done with vertical "switching" type fets. A modern FLOOD
>>>>> architecture [1] works great with most any kind of fet. Lots of
>>>>> things have changed in the last few decades.
>>>>>
>>>>> John
>>>>>
>>>>> [1] Of course you've never heard the term before. I just invented
>>>>> it.
>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>> lovely features for audio. The only downside being a slightly
>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>> match beautifully with no need for source balance resistors (so
>>>> some of the Ron loss 'goes away').
>>>
>>> An opamp per fet, closing a local loop, feedback from the fet source,
>>> makes each fet look like a perfect unity-gain, fast, zero-offset
>>> device.
>> Interesting idea. I'll have to chew that one over. I can see possible
>> problems fron op-amp output overshoot.
>
> I have a simple embodiment of that concept here, done a while ago, in
> virtual land;-)
>
> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>
> Its a push/pull gain loop around the output devices, forcing them to be
> unity gain followers.
>
> You can get lower distortion, at the expense of speed, because you have to
> compensate earlier.
>
> Common mode feedback at the second stage, allows for enormous dc/lf gain.
> cascodes to allow the use of fast small transistors to do all the main work.
> Emitter follower buffer to reduce the current swing in the input pair, as
> per doug self. Spice says it should be in the < 0.0001% , 20Khz range,
> maybe...
>
> Kevin Aylward
>
>

Kevin - an interesting circuit, and I appreciate what you have done with
the output stage, but I'm still wondering why you didn't include it
within the global feedback loop - that could only have made it better,
lower output impedance, more load insensitive etc etc etc.

Can you explain the thinking?

d

RichD

September 19th 08, 11:06 PM

On Sep 18, Eeyore > wrote:
> > > >Who do MOSFET sound better than bipolar, as an audio
> > > >amp output driver?
>
> > > As a driver ?
> > > Now if you said as an output stage it might make sense.
>
> > There's a difference?
>
> Damn right there is.

I'm thinking of the bits that attach to the
copper thingy which loops around the
magnets which make the air move.

-
Rich

Eeyore

September 19th 08, 11:24 PM

Jan Panteltje wrote:

> On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore
> > wrote in
> >:
>
> >> An opamp per fet, closing a local loop, feedback from the fet source,
> >> makes each fet look like a perfect unity-gain, fast, zero-offset
> >> device.
> >
> >Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
> >output overshoot.
>
> If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2
> you will see this is exactly what is done with the lower output MOSFET, combined with opamp
> makes unity gain.
> The top is already a source followwr.
> The over current protection same thing, opamps.
> It is likely this what makes the Boucherot network not needed.
>
> You could do the same thing with the top MOSFET in a discrete design, if you must.

Thanks for the tip.

> >> Opamps are cheap, but fets and heat sinks are expensive. Power
> >> all those gate-drive opamps from a DC-DC converter floating on the
> >> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
> >> current limit for fast overloads and back that up with a digital fet
> >> power dissipation simulation that provides the real protections.
>
> >I've toyed in the past with doing device protection using an analogue multiplier actually
> >funnily enough.
>
> I only see over current protection in a lot of amps, plus thermal,
> the real thing was a bridge that caused foldback current limit,
> couple of resistors and diodes, makes it any load proof.

Except you don't want an audio amp to foldback, just shut down when it sees an 'impossible'
load.

Graham

Eeyore

September 19th 08, 11:27 PM

Kevin Aylward wrote:

> Eeyore wrote:
> > John Larkin wrote:
> >> Eeyore wrote:
> >>> John Larkin wrote:
> >>>> Eeyore wrote:
> >>>>>
> >>>>> Learn something about LATERAL mosfets that were designed for
> >>>>> audio. I've already given part number and links to data sheets.
> >>>>
> >>>> That doesn't really matter. The transfer function only needs to be
> >>>> continuous so that you can close a loop around it, and the fet
> >>>> needs to be able to stand the peak power dissipation. That can
> >>>> easily be done with vertical "switching" type fets. A modern FLOOD
> >>>> architecture [1] works great with most any kind of fet. Lots of
> >>>> things have changed in the last few decades.
> >>>>
> >>>> John
> >>>>
> >>>> [1] Of course you've never heard the term before. I just invented
> >>>> it.
> >>>
> >>> Fine. Can you elaborate some more on it ? Laterals have some truly
> >>> lovely features for audio. The only downside being a slightly
> >>> highish Ron. Not really a problem when (as I have) used as many as
> >>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
> >>> match beautifully with no need for source balance resistors (so
> >>> some of the Ron loss 'goes away').
> >>
> >>
> >> An opamp per fet, closing a local loop, feedback from the fet source,
> >> makes each fet look like a perfect unity-gain, fast, zero-offset
> >> device.
> >
> > Interesting idea. I'll have to chew that one over. I can see possible
> > problems fron op-amp output overshoot.
>
> I have a simple embodiment of that concept here, done a while ago, in
> virtual land;-)
>
> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>
> Its a push/pull gain loop around the output devices, forcing them to be
> unity gain followers.
>
> You can get lower distortion, at the expense of speed, because you have to
> compensate earlier.
>
> Common mode feedback at the second stage, allows for enormous dc/lf gain.
> cascodes to allow the use of fast small transistors to do all the main work.
> Emitter follower buffer to reduce the current swing in the input pair, as
> per doug self. Spice says it should be in the < 0.0001% , 20Khz range,
> maybe...

Not exactly short of current mirrors ! ;~)

What gave you the idea ?

Graham

Eeyore

September 19th 08, 11:29 PM

John Larkin wrote:

> Spehro Pefhany wrote:
> >
> >Anything approaching 1600A would blow the leadwires off a TO-220. The
> >fusing current of a long wire of that cross-section area is only in
> >the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.
>
> Their selection tables show this as a 280 amp TO-220:
>
> https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430
>
> but the datasheet qualifies this as merely 75:
>
> http://www.irf.com/product-info/datasheets/data/irf2804.pdf
>
> They do rate it for 330 watts!

Pulsed I hope !

Graham

Eeyore

September 19th 08, 11:39 PM

John Larkin wrote:

> Eeyore wrote:
> >John Larkin wrote:
> >>
> >> An opamp per fet, closing a local loop, feedback from the fet source,
> >> makes each fet look like a perfect unity-gain, fast, zero-offset
> >> device.
> >
> >Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
> >output overshoot.
>
> It needs a little loop tweaking, roughly....
>
> V+
> |\ |
> -----------| + |
> | d
> | out-------+------Rg--------g
> | | s
> +------| - | |
> | |/ Cf |
> | | |
> +--------------------+------Rf---------+
> |
> |
> Rs
> |
> |
> +------------- output rail
>
> and an opamp that can slam the gate hard enough, not a big problem
> nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
> can be small, and the quiescent bias voltage can be small, because the
> opamp offset voltage can be tiny. The fets share the load exactly, and
> the standing bias current can be designed in, exactly, with no
> adjustments.
>
> The driver stage sees only opamps, so doesn't have to work very hard.

It's certainly interesting.

I'm wondering what the transition from one side to the other would be like i.e the crossover
point. I'm wondering if one might get a brief dead band. What would be ideal would be if the
power device never fully turned off and left say 10mA Iq.

> >> Opamps are cheap, but fets and heat sinks are expensive. Power
> >> all those gate-drive opamps from a DC-DC converter floating on the
> >> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
> >> current limit for fast overloads and back that up with a digital fet
> >> power dissipation simulation that provides the real protections.
> >
> >I've toyed in the past with doing device protection using an analogue >multiplier actually
> funnily enough.
>
> I digitize everything - heatsink temp, supply currents, load voltage -
> and run a realtime simulation of fet power dissipation and resulting
> junction temperature, with shutdown at, say, 140 C.

Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind.

> An adaptive fan speed would be a nice touch... no fan until it's
> really needed. Maybe next time.

Oh that's dead easy, transistor on the heatsink, measure Vbe for the constant blow rate plus
add a rectified sniff of the HT line ripple. The fans powers up before the heatsink gets hot.
Ordinary DC fans work fine off a variable voltage.

(c) me. LOL.

Graham

Eeyore

September 19th 08, 11:42 PM

Kevin Aylward wrote:

> John Larkin wrote:
> > Eeyore wrote:
> >> John Larkin wrote:
> >>>
> >>> An opamp per fet, closing a local loop, feedback from the fet
> >>> source, makes each fet look like a perfect unity-gain, fast,
> >>> zero-offset device.
> >>
> >> Interesting idea. I'll have to chew that one over. I can see
> >> possible problems fron op-amp output overshoot.
> >
> >
> > It needs a little loop tweaking, roughly....
> >
> >
> > V+
> > |\ |
> > -----------| + |
> > | d
> > | out-------+------Rg--------g
> > | | s
> > +------| - | |
> > | |/ Cf |
> > | | |
> > +--------------------+------Rf---------+
> > |
> > |
> > Rs
> > |
> > |
> > +------------- output rail
> >
> >
> > and an opamp that can slam the gate hard enough, not a big problem
> > nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
> > can be small, and the quiescent bias voltage can be small, because the
> > opamp offset voltage can be tiny. The fets share the load exactly, and
> > the standing bias current can be designed in, exactly, with no
> > adjustments.
> >
> > The driver stage sees only opamps, so doesn't have to work very hard.
> >
>
> However....there are some issues with using whole op-amps, rather than
> discrete transistors as I have in the noted circuit in my other post. You
> may need 200V+ ratings, and a very fast one at that!!!

If you could clamp the input, would that do it ?

Graham

Eeyore

September 19th 08, 11:48 PM

RichD wrote:

> Eeyore > wrote:
> > > > >Who do MOSFET sound better than bipolar, as an audio
> > > > >amp output driver?
> >
> > > > As a driver ?
> > > > Now if you said as an output stage it might make sense.
> >
> > > There's a difference?
> >
> > Damn right there is.
>
> I'm thinking of the bits that attach to the
> copper thingy which loops around the
> magnets which make the air move.

An unusual design but the copper thingy is very likely a heatsink, in
which case you're referring to the actual output devices. They in turn
usually have devices called 'drivers' which precede them, although it's
less necessary with mosfets, only for ultimate performance..

Graham

John Larkin

September 19th 08, 11:56 PM

On Fri, 19 Sep 2008 23:29:40 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Spehro Pefhany wrote:
>> >
>> >Anything approaching 1600A would blow the leadwires off a TO-220. The
>> >fusing current of a long wire of that cross-section area is only in
>> >the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.
>>
>> Their selection tables show this as a 280 amp TO-220:
>>
>> https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430
>>
>> but the datasheet qualifies this as merely 75:
>>
>> http://www.irf.com/product-info/datasheets/data/irf2804.pdf
>>
>> They do rate it for 330 watts!
>
>Pulsed I hope !
>
>Graham

Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW.

John

Eeyore

September 20th 08, 12:07 AM

John Larkin wrote:

> Eeyore wrote:
> >John Larkin wrote:
> >> Spehro Pefhany wrote:
> >> >
> >> >Anything approaching 1600A would blow the leadwires off a TO-220. The
> >> >fusing current of a long wire of that cross-section area is only in
> >> >the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.
> >>
> >> Their selection tables show this as a 280 amp TO-220:
> >> https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430
> >>
> >> but the datasheet qualifies this as merely 75:
> >>
> >> http://www.irf.com/product-info/datasheets/data/irf2804.pdf
> >>
> >> They do rate it for 330 watts!
> >
> >Pulsed I hope !
>
> Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW.

You just reminded me. There was one bipolar design I developed of a range of powers which was used
extensively across a range of products including rack mount amplifiers and powered mixers. We must
have built tens of thousands of the amp modules.

Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a
strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it
and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them.

Graham

John Larkin

September 20th 08, 12:11 AM

On Fri, 19 Sep 2008 21:13:15 GMT, "Kevin Aylward"
> wrote:

>John Larkin wrote:
>> On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore
>> > wrote:
>>
>>>
>>>
>>> John Larkin wrote:
>>>
>>>> Eeyore wrote:
>>>>> John Larkin wrote:
>>>>>> Eeyore wrote:
>>>>>>>
>>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>>> audio. I've already given part number and links to data sheets.
>>>>>>
>>>>>> That doesn't really matter. The transfer function only needs to be
>>>>>> continuous so that you can close a loop around it, and the fet
>>>>>> needs to be able to stand the peak power dissipation. That can
>>>>>> easily be done with vertical "switching" type fets. A modern
>>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>>> Lots of things have changed in the last few decades.
>>>>>>
>>>>>> John
>>>>>>
>>>>>> [1] Of course you've never heard the term before. I just invented
>>>>>> it.
>>>>>
>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>>> lovely features for audio. The only downside being a slightly
>>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>>> match beautifully with no need for source balance resistors (so
>>>>> some of the Ron loss 'goes away').
>>>>
>>>>
>>>> An opamp per fet, closing a local loop, feedback from the fet
>>>> source, makes each fet look like a perfect unity-gain, fast,
>>>> zero-offset device.
>>>
>>> Interesting idea. I'll have to chew that one over. I can see
>>> possible problems fron op-amp output overshoot.
>>
>>
>> It needs a little loop tweaking, roughly....
>>
>>
>> V+
>> |\ |
>> -----------| + |
>> | d
>> | out-------+------Rg--------g
>> | | s
>> +------| - | |
>> | |/ Cf |
>> | | |
>> +--------------------+------Rf---------+
>> |
>> |
>> Rs
>> |
>> |
>> +------------- output rail
>>
>>
>> and an opamp that can slam the gate hard enough, not a big problem
>> nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
>> can be small, and the quiescent bias voltage can be small, because the
>> opamp offset voltage can be tiny. The fets share the load exactly, and
>> the standing bias current can be designed in, exactly, with no
>> adjustments.
>>
>> The driver stage sees only opamps, so doesn't have to work very hard.
>>
>
>
>However....there are some issues with using whole op-amps, rather than
>discrete transistors as I have in the noted circuit in my other post. You
>may need 200V+ ratings, and a very fast one at that!!!
>
>

The opamp power supply can be a cheap isolated dc/dc converter, +-12
volts maybe, floating on the output rail, so the opamps never see a
lot of swing. Of course, a real circuit needs some protections for
overload and startup conditions, but that's just a few diodes. The
opamp inputs need never go more than a few tenths of a volt above or
below the output rail.

The floating opamp supply allows one to truly saturate the fets and
swing the output all the way to both supply rails. (I assume some
number of N-channel and P-channel opamp-composite fets in a real amp.)
That pays for a dinky DC-DC sip thingie all by itself.

John

John Larkin

September 20th 08, 12:26 AM

On Fri, 19 Sep 2008 23:39:10 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Eeyore wrote:
>> >John Larkin wrote:
>> >>
>> >> An opamp per fet, closing a local loop, feedback from the fet source,
>> >> makes each fet look like a perfect unity-gain, fast, zero-offset
>> >> device.
>> >
>> >Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
>> >output overshoot.
>>
>> It needs a little loop tweaking, roughly....
>>
>> V+
>> |\ |
>> -----------| + |
>> | d
>> | out-------+------Rg--------g
>> | | s
>> +------| - | |
>> | |/ Cf |
>> | | |
>> +--------------------+------Rf---------+
>> |
>> |
>> Rs
>> |
>> |
>> +------------- output rail
>>
>> and an opamp that can slam the gate hard enough, not a big problem
>> nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
>> can be small, and the quiescent bias voltage can be small, because the
>> opamp offset voltage can be tiny. The fets share the load exactly, and
>> the standing bias current can be designed in, exactly, with no
>> adjustments.
>>
>> The driver stage sees only opamps, so doesn't have to work very hard.
>
>It's certainly interesting.
>
>I'm wondering what the transition from one side to the other would be like i.e the crossover
>point. I'm wondering if one might get a brief dead band. What would be ideal would be if the
>power device never fully turned off and left say 10mA Iq.

Assuming some number of composite N-channel and P-channel fet
thingies, as a complementary follower, some bias voltage has to be
applied between the pseudo-gates (N-bank and P-bank opamp inputs) to
set the idle current. When the mess starts to drive a load in one
direction, some means ought to make sure the other bank doesn't go
off, but maintains its bias current. That takes a little more "analog
logic", still floating with all the rest of the stuff. That's not a
big deal.

>
>
>> >> Opamps are cheap, but fets and heat sinks are expensive. Power
>> >> all those gate-drive opamps from a DC-DC converter floating on the
>> >> output node; DC/DC bricks are cheap nowadays, too. Do a simple output
>> >> current limit for fast overloads and back that up with a digital fet
>> >> power dissipation simulation that provides the real protections.
>> >
>> >I've toyed in the past with doing device protection using an analogue >multiplier actually
>> funnily enough.
>>
>> I digitize everything - heatsink temp, supply currents, load voltage -
>> and run a realtime simulation of fet power dissipation and resulting
>> junction temperature, with shutdown at, say, 140 C.
>
>Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind.

You can do that, simulate the Tj's, once for each bank...

Tnfet = Theatsink + K * (lowpass filter of) { (V+ - Vout) * Iout }

Tpfet = Theatsink + K * (lowpass filter of) { (V- - Vout) * Iout }

(except you have to get all the signs right)

where K relates to Theta-junction-heatsink and the lowpass filter
simulates the thermal mass of the silicon.

My NMR amps needed a uP and ADC and display anyhow, so the fancy
protections were pretty much just more code. The code runs at a few
KHz. A real PITA, but free in production.

My big amps display everything... temperatures, power supply voltages,
TRMS load current, output power, error messages, tons of stuff.

John

John Larkin

September 20th 08, 12:39 AM

On Sat, 20 Sep 2008 00:07:58 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Eeyore wrote:
>> >John Larkin wrote:
>> >> Spehro Pefhany wrote:
>> >> >
>> >> >Anything approaching 1600A would blow the leadwires off a TO-220. The
>> >> >fusing current of a long wire of that cross-section area is only in
>> >> >the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter.
>> >>
>> >> Their selection tables show this as a 280 amp TO-220:
>> >> https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430
>> >>
>> >> but the datasheet qualifies this as merely 75:
>> >>
>> >> http://www.irf.com/product-info/datasheets/data/irf2804.pdf
>> >>
>> >> They do rate it for 330 watts!
>> >
>> >Pulsed I hope !
>>
>> Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW.
>
>You just reminded me. There was one bipolar design I developed of a range of powers which was used
>extensively across a range of products including rack mount amplifiers and powered mixers. We must
>have built tens of thousands of the amp modules.
>
>Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a
>strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it
>and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them.
>
>Graham

We tested a bunch of TO-247 power mosfets to destruction, in various
sadistic ways. One was pure temperature. Vgs-th drops as temp goes up.
The fets seemed to turn on hard, with 0 gate voltage, at 300C, but
recovered when cooled. After 330C, they died, on hard, and didn't
recover.

It's hard to buy TO-3 fets any more.

John

Eeyore

September 20th 08, 12:41 AM

John Larkin wrote:

> "Kevin Aylward" wrote:
> >
> >However....there are some issues with using whole op-amps, rather than
> >discrete transistors as I have in the noted circuit in my other post. You
> >may need 200V+ ratings, and a very fast one at that!!!
>
> The opamp power supply can be a cheap isolated dc/dc converter, +-12
> volts maybe, floating on the output rail, so the opamps never see a
> lot of swing. Of course, a real circuit needs some protections for
> overload and startup conditions, but that's just a few diodes. The
> opamp inputs need never go more than a few tenths of a volt above or
> below the output rail.
>
> The floating opamp supply allows one to truly saturate the fets and
> swing the output all the way to both supply rails. (I assume some
> number of N-channel and P-channel opamp-composite fets in a real amp.)
> That pays for a dinky DC-DC sip thingie all by itself.

Yes you need some high side drive for that. I've used that in both my mosfet
and bipolar designs (to oversome multiple Vbe's (some of the drivers ran off
of the high side rail) AND a Baker clamp to stop the last voltage gain stage
transistor saturating).

You probably do only need input protection. Might need to be clever about
start up / turn-off conditions. Maybe a gate control ? I used that one on my
amps too. 100% effective against power-out damage even if the previous signal
chain generates a 'thump'.

Graham

Eeyore

September 20th 08, 12:50 AM

John Larkin wrote:

> Eeyore wrote:
> >
> >I'm wondering what the transition from one side to the other would be >like i.e the crossover
> point. I'm wondering if one might get a brief >dead band. What would be ideal would be if the
> power device never fully >turned off and left say 10mA Iq.
>
> Assuming some number of composite N-channel and P-channel fet
> thingies, as a complementary follower, some bias voltage has to be
> applied between the pseudo-gates (N-bank and P-bank opamp inputs) to
> set the idle current. When the mess starts to drive a load in one
> direction, some means ought to make sure the other bank doesn't go
> off, but maintains its bias current. That takes a little more "analog
> logic", still floating with all the rest of the stuff. That's not a
> big deal.

I wish it were that simple. It's defeated me for years. Maybe a new set of eyes can discover the
trick ?

> >> >> Opamps are cheap, but fets and heat sinks are expensive. Power
> >> >> all those gate-drive opamps from a DC-DC converter floating on the
> >> >> output node; DC/DC bricks are cheap nowadays, too. Do a simple >> >> output current limit
> for fast overloads and back that up with a >> >> digital fet power dissipation simulation that
> provides the real >> >> protections.
> >> >
> >> >I've toyed in the past with doing device protection using an >> >analogue >multiplier
> actually funnily enough.
> >>
> >> I digitize everything - heatsink temp, supply currents, load voltage -
> >> and run a realtime simulation of fet power dissipation and resulting
> >> junction temperature, with shutdown at, say, 140 C.
> >
> >Nice if you have the budget. I found a cheap NJR / JRC multiplier I had >in mind.
>
> You can do that, simulate the Tj's, once for each bank...
>
> Tnfet = Theatsink + K * (lowpass filter of) { (V+ - Vout) * Iout }
>
> Tpfet = Theatsink + K * (lowpass filter of) { (V- - Vout) * Iout }
>
> (except you have to get all the signs right)
>
> where K relates to Theta-junction-heatsink and the lowpass filter
> simulates the thermal mass of the silicon.

Absolutely. You can get all the thermal time constants in there and so on.

> My NMR amps needed a uP and ADC and display anyhow, so the fancy
> protections were pretty much just more code. The code runs at a few
> KHz. A real PITA, but free in production.
>
> My big amps display everything... temperatures, power supply voltages,
> TRMS load current, output power, error messages, tons of stuff.

Nice. The pro-audio market wouldn't buy it though. Price (and weight) is currently everything,
although there are a very few high end niche products with ethernet (or proprietary) control and
monitoring around.

Graham

Eeyore

September 20th 08, 01:27 AM

John Larkin wrote:

> Eeyore wrote:
> >
> >You just reminded me. There was one bipolar design I developed of a range of powers which >was used
> extensively across a range of products including rack mount amplifiers and powered >mixers. We
> musthave built tens of thousands of the amp modules.
> >
> >Late in the proving process I ran one up with no fan. The heatsink reached 150C and there >was a
> strong smell of hot aluminium plus creaking noises from thermal expansion before I took >pity on it
> and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat >them.
>
> We tested a bunch of TO-247 power mosfets to destruction, in various
> sadistic ways. One was pure temperature. Vgs-th drops as temp goes up.
> The fets seemed to turn on hard, with 0 gate voltage, at 300C, but
> recovered when cooled. After 330C, they died, on hard, and didn't
> recover.

Were you estimating Tj ?

> It's hard to buy TO-3 fets any more.

More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then down with
too. Need I mention the advantages of that ?

Graham

John Larkin

September 20th 08, 01:37 AM

On Sat, 20 Sep 2008 01:27:33 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> Eeyore wrote:
>> >
>> >You just reminded me. There was one bipolar design I developed of a range of powers which >was used
>> extensively across a range of products including rack mount amplifiers and powered >mixers. We
>> musthave built tens of thousands of the amp modules.
>> >
>> >Late in the proving process I ran one up with no fan. The heatsink reached 150C and there >was a
>> strong smell of hot aluminium plus creaking noises from thermal expansion before I took >pity on it
>> and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat >them.
>>
>> We tested a bunch of TO-247 power mosfets to destruction, in various
>> sadistic ways. One was pure temperature. Vgs-th drops as temp goes up.
>> The fets seemed to turn on hard, with 0 gate voltage, at 300C, but
>> recovered when cooled. After 330C, they died, on hard, and didn't
>> recover.
>
>Were you estimating Tj ?

No, we were heating the fets externally.

>
>
>> It's hard to buy TO-3 fets any more.
>
>More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then down with
>too. Need I mention the advantages of that ?

But you can put a lot more silicon into a TO-247, especially the
version without the mounting hole.

John

Eeyore

September 20th 08, 01:49 AM

John Larkin wrote:

> Eeyore wrote:
> >John Larkin wrote:
> >
> >> Eeyore wrote:
> >> >
> >> >You just reminded me. There was one bipolar design I developed of a range of powers >> >which was
> used extensively across a range of products including rack mount amplifiers and >> >powered mixers. We
> musthave built tens of thousands of the amp modules.
> >> >
> >> >Late in the proving process I ran one up with no fan. The heatsink reached 150C and there >> >was a
> strong smell of hot aluminium plus creaking noises from thermal expansion before I >> >took pity on it

And my colleagues who were looking slightly anxious.

> >> >and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat >> >them.
> >>
> >> We tested a bunch of TO-247 power mosfets to destruction, in various
> >> sadistic ways. One was pure temperature. Vgs-th drops as temp goes up.
> >> The fets seemed to turn on hard, with 0 gate voltage, at 300C, but
> >> recovered when cooled. After 330C, they died, on hard, and didn't
> >> recover.
> >
> >Were you estimating Tj ?
>
> No, we were heating the fets externally.

Sorry, I didn't read it properly.

How about by self-heating ?

> >> It's hard to buy TO-3 fets any more.
> >
> >More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then >down with
> too. Need I mention the advantages of that ?
>
> But you can put a lot more silicon into a TO-247, especially the
> version without the mounting hole.

Well, at least clips or mounting bars won't bend the tab !

Semelab/Magnatec make lateral fets with 2 matched dies in TO-3. Beat that ! 250W true continuous Pd.
http://www.magnatec-uk.co.uk/latmos.shtml

And quads too by the look of it !
http://www.magnatec-uk.co.uk/mosdata.shtml

BUZ901X4S 200V 32A 500W SOT227
BUZ906X4S -200V -32A 500W SOT227

Graham

MooseFET

September 20th 08, 02:34 AM

On Sep 19, 7:37*am, Eeyore >
wrote:
> MooseFET wrote:
> > Beware: *Mosfets like the STW55NM60 have a decreasing threshold
> > voltage for increasing temperature. *This means that biasing them to a
> > low idle current isn't so easy.
>
> Indeed. You'll need lossy ballast resistors. Laterals are different that
> way.

Yes, but once the STW55NM60 or equiv. is biased up, it makes a nice
200V at about 100KHz. Its not exactly audio but is sure isn't really
RF either.

>
> Graham

Michael A. Terrell

September 20th 08, 04:01 AM

MooseFET wrote:
>
> On Sep 19, 7:37 am, Eeyore >
> wrote:
> > MooseFET wrote:
> > > Beware: Mosfets like the STW55NM60 have a decreasing threshold
> > > voltage for increasing temperature. This means that biasing them to a
> > > low idle current isn't so easy.
> >
> > Indeed. You'll need lossy ballast resistors. Laterals are different that
> > way.
>
> Yes, but once the STW55NM60 or equiv. is biased up, it makes a nice
> 200V at about 100KHz. Its not exactly audio but is sure isn't really
> RF either.

Tell that to WWVB, who transmits at 60 kHz.

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Kevin Aylward[_3_]

September 20th 08, 10:11 AM

Don Pearce wrote:
> Kevin Aylward wrote:
>> Eeyore wrote:
>>> John Larkin wrote:
>>>
>>>> Eeyore wrote:
>>>>> John Larkin wrote:
>>>>>> Eeyore wrote:
>>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>>> audio. I've already given part number and links to data sheets.
>>>>>> That doesn't really matter. The transfer function only needs to
>>>>>> be continuous so that you can close a loop around it, and the fet
>>>>>> needs to be able to stand the peak power dissipation. That can
>>>>>> easily be done with vertical "switching" type fets. A modern
>>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>>> Lots of things have changed in the last few decades.
>>>>>>
>>>>>> John
>>>>>>
>>>>>> [1] Of course you've never heard the term before. I just invented
>>>>>> it.
>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>>> lovely features for audio. The only downside being a slightly
>>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>>> match beautifully with no need for source balance resistors (so
>>>>> some of the Ron loss 'goes away').
>>>>
>>>> An opamp per fet, closing a local loop, feedback from the fet
>>>> source, makes each fet look like a perfect unity-gain, fast,
>>>> zero-offset device.
>>> Interesting idea. I'll have to chew that one over. I can see
>>> possible problems fron op-amp output overshoot.
>>
>> I have a simple embodiment of that concept here, done a while ago, in
>> virtual land;-)
>>
>> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>>
>> Its a push/pull gain loop around the output devices, forcing them to
>> be unity gain followers.
>>
>> You can get lower distortion, at the expense of speed, because you
>> have to compensate earlier.
>>
>> Common mode feedback at the second stage, allows for enormous dc/lf
>> gain. cascodes to allow the use of fast small transistors to do all
>> the main work. Emitter follower buffer to reduce the current swing
>> in the input pair, as per doug self. Spice says it should be in the
>> < 0.0001% , 20Khz range, maybe...
>>
>> Kevin Aylward
>>
>>
>
> Kevin - an interesting circuit, and I appreciate what you have done
> with the output stage, but I'm still wondering why you didn't include
> it within the global feedback loop - that could only have made it
> better, lower output impedance, more load insensitive etc etc etc.

It is.!!!

I think the schematic is not as clear as it should be.. I have a zero volt
source near the output devices in the feedback circuit to calculate LG. The
overall loop feedback passes through this source!!!

Regards

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

Kevin Aylward[_3_]

September 20th 08, 10:11 AM

Jan Panteltje wrote:
> On a sunny day (Fri, 19 Sep 2008 19:39:51 GMT) it happened "Kevin
> Aylward" > wrote in
> >:
>
>>> If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the
>>> blockdiagram on page 2 you will see this is exactly what is done
>>> with the lower output MOSFET, combined with opamp makes unity gain.
>>> The top is already a source followwr.
>>> The over current protection same thing, opamps.
>>> It is likely this what makes the Boucherot network not needed.
>>
>> I doubt it. I haven't seen the data sheet, but it is not usual to be
>> able to stabilise an amp with what I believe you are describing here.
>
> It is always a good idea to lookuop what we are talking about.

Indeed.

>
>
>> Consider one amp feeding the other, both running open loop, with
>> overall feedback, to the 1st. Now consider the case where the second
>> amp is configured with local feedback, to make it a unity buffer,
>> following the 1st amp. Naively , one might argue that the 2nd stage
>> now has a wider bandwidth, due to feedback, such that the "new "
>> system might be stable, i.e. one main rolloff due to the 1st amp.
>> However, in realty, topologically, nothing changes.
>
> Some people are really good at that stuff, sort of reasonin gI mean.
>
> But if you look at the combination MOSFET - opamp, as John Larkin is
> describing in an other post in this thread, then you can treat that
> as one 'perfect MOSFET'. Of course it is not really perfect, but you
> can make that as stable or unstable as you want. Then basically what
> you do is chaining stable blocks together. If you then apply
> feedback, you have to use the phase characteristic of all those, and,
> as long as you prevent too high frequencies from circulating, it
> should be stable, and largely independent of the load.

If we consider calculating the actual LG=B(s).A(s) of the whole system, then
it is identical whther or not there is local feedback around the 2nd stage
or not.

Yes, you can analyse as you describe here, but the result must be identical,
as breaking all loops (if it can actually be done that is). So, it can't be
because there is a local loop that there is no zobel network.

>I note the
> TDA9274 has one capacitor to roll of in the driver... This is normal,
> at least in the amps I designed.
>
>> The determine the stability of the system, one needs to break the
>> loop at a point that breaks *all* feedback paths at once. This point
>> will be the point directly at the output of the 2nd amp. when this
>> is done, it is clear that the stabiliy is still due to the total
>> loop gain of both amops cascaded.
>
> You mean 'open loop gain?'

No. I mean the loop gain. The loop gain is the open loop gain X the beta
factor (e.g. resistive divider). It is the gain around the loop that
matters.

> Yes, but he second amp would have gain 1.

But thats my point. Its irrelevent that the closed lop gain of the second
stage is unity when the loop is closed. When you do the stability analyisis
you need to break all the loops. As I said, if you break the loop directly
at the output of the second amp, which breaks both feedback loops at once,
it is obvious that there overal loop gain is not effected by having the 2nd
stage. It is the same loop gain

>
>
>> Excepting for the special cases, e.g. , where feedback is used to
>> neutralise r.f amps, feedback in general, cannot be used to widen
>> bandwidth, if the purpose of that wider bandwidth is to achieve
>> stability, in this type of arrangement
>
> I was not suggesting to widen bandwidth, although strong local
> feedback would of course widen the bandwidth of a stage,

But not the BW of the final, overall amp, so the BW of the internal stages
is irrelevant if it is the result of feedback.

What the local loop can buy you is reduced distortion at lower frequencies.

I ran these two circuits up quite a while ago. One has the UGB at the
output, one doesn't.

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp1.jpg
http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

The UGB version had to be compensated earlier in frequency, i.e the non UGB
version was significantly faster. Unfortunately, I cant remember much of the
data, and I seem to have lost the SS files;-)

I think one had 0.0001% at 20khz.

>
> Honestly, I have to think about this a bit, maybe run it in spice.
> Fact remains that the TDA9274 is the only amp I know that needs no
> Boucherot circuit :-)

I would wager it's "non-optimum" designed. You give up a bit if the output
load is not defined, usually. Without the zobel, the load reflected to the
gain stages is all over the place. My guess is that they had a design goal
of minimising the number of external components, which is standard practice
in designing ics, but consequently, gave up some potential performance
improvement.

>
>
>> To wit, There is no such thing as a free lunch...
>
> It seems to exist for US bankers ATM.

Kevin Aylward
www.kevinaylward.co.uk
www.blonddee.co.uk

Kevin Aylward[_3_]

September 20th 08, 10:18 AM

Eeyore wrote:
> Kevin Aylward wrote:
>
>> Eeyore wrote:
>>> John Larkin wrote:
>>>> Eeyore wrote:
>>>>> John Larkin wrote:
>>>>>> Eeyore wrote:
>>>>>>>
>>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>>> audio. I've already given part number and links to data sheets.
>>>>>>
>>>>>> That doesn't really matter. The transfer function only needs to
>>>>>> be continuous so that you can close a loop around it, and the fet
>>>>>> needs to be able to stand the peak power dissipation. That can
>>>>>> easily be done with vertical "switching" type fets. A modern
>>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>>> Lots of things have changed in the last few decades.
>>>>>>
>>>>>> John
>>>>>>
>>>>>> [1] Of course you've never heard the term before. I just invented
>>>>>> it.
>>>>>
>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>>> lovely features for audio. The only downside being a slightly
>>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>>> match beautifully with no need for source balance resistors (so
>>>>> some of the Ron loss 'goes away').
>>>>
>>>>
>>>> An opamp per fet, closing a local loop, feedback from the fet
>>>> source, makes each fet look like a perfect unity-gain, fast,
>>>> zero-offset device.
>>>
>>> Interesting idea. I'll have to chew that one over. I can see
>>> possible problems fron op-amp output overshoot.
>>
>> I have a simple embodiment of that concept here, done a while ago, in
>> virtual land;-)
>>
>> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>>
>> Its a push/pull gain loop around the output devices, forcing them to
>> be unity gain followers.
>>
>> You can get lower distortion, at the expense of speed, because you
>> have to compensate earlier.
>>
>> Common mode feedback at the second stage, allows for enormous dc/lf
>> gain. cascodes to allow the use of fast small transistors to do all
>> the main work. Emitter follower buffer to reduce the current swing
>> in the input pair, as per doug self. Spice says it should be in the
>> < 0.0001% , 20Khz range, maybe...
>
> Not exactly short of current mirrors ! ;~)
>
> What gave you the idea ?
>
> Graham

The loop around the output came about from studying a patent that could not
work as claimed, around 1983. This patent claimed distortion reduction by
eliminating miller effect. It had a standard class A gain stage, but used a
cascode. However, the cascode transistor base was connected to the output in
a local feedback loop. This was the claim for distortion reduction. However,
to1st order, the voltage on the base of a cascode transistor, don't effect
anything. The input is current feed, irespective of the base potential,
therfore the could not be any signal feedback. This led me to consider a way
of doing it properly. I also noted that as the base was connected to the
output, the emmiter of the cascode, and the collecter of the driving
transister. hence the collecter still swings the full output, hence, millor
effect is still there. So, that patent was complete nonsense, and my final
circuit was completly different, but it followed from the thought flow of
that patent.

The key point in this approach was to get a push pull drive to both
outputs.,i.e. to avoid low turn off resisters, which kills gain and drive.

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

Don Pearce

September 20th 08, 10:22 AM

Kevin Aylward wrote:
> Don Pearce wrote:
>> Kevin Aylward wrote:
>>> Eeyore wrote:
>>>> John Larkin wrote:
>>>>
>>>>> Eeyore wrote:
>>>>>> John Larkin wrote:
>>>>>>> Eeyore wrote:
>>>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>>>> audio. I've already given part number and links to data sheets.
>>>>>>> That doesn't really matter. The transfer function only needs to
>>>>>>> be continuous so that you can close a loop around it, and the fet
>>>>>>> needs to be able to stand the peak power dissipation. That can
>>>>>>> easily be done with vertical "switching" type fets. A modern
>>>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>>>> Lots of things have changed in the last few decades.
>>>>>>>
>>>>>>> John
>>>>>>>
>>>>>>> [1] Of course you've never heard the term before. I just invented
>>>>>>> it.
>>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>>>> lovely features for audio. The only downside being a slightly
>>>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>>>> match beautifully with no need for source balance resistors (so
>>>>>> some of the Ron loss 'goes away').
>>>>> An opamp per fet, closing a local loop, feedback from the fet
>>>>> source, makes each fet look like a perfect unity-gain, fast,
>>>>> zero-offset device.
>>>> Interesting idea. I'll have to chew that one over. I can see
>>>> possible problems fron op-amp output overshoot.
>>> I have a simple embodiment of that concept here, done a while ago, in
>>> virtual land;-)
>>>
>>> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>>>
>>> Its a push/pull gain loop around the output devices, forcing them to
>>> be unity gain followers.
>>>
>>> You can get lower distortion, at the expense of speed, because you
>>> have to compensate earlier.
>>>
>>> Common mode feedback at the second stage, allows for enormous dc/lf
>>> gain. cascodes to allow the use of fast small transistors to do all
>>> the main work. Emitter follower buffer to reduce the current swing
>>> in the input pair, as per doug self. Spice says it should be in the
>>> < 0.0001% , 20Khz range, maybe...
>>>
>>> Kevin Aylward
>>>
>>>
>> Kevin - an interesting circuit, and I appreciate what you have done
>> with the output stage, but I'm still wondering why you didn't include
>> it within the global feedback loop - that could only have made it
>> better, lower output impedance, more load insensitive etc etc etc.
>
> It is.!!!
>
> I think the schematic is not as clear as it should be.. I have a zero volt
> source near the output devices in the feedback circuit to calculate LG. The
> overall loop feedback passes through this source!!!
>
> Regards
>
> Kevin Aylward
> www.blonddee.co.uk
> www.kevinaylward.co.uk
>
>

Ah, is that what it was? I thought you were putting actual voltage
sources in there that would be replaced by some small circuit in an
actual design. I hope you are connecting the sensing point of the
feedback resistor to the final summed speaker connection, not some
random point in amongst the bunch of fets (teaching granny to suck
eggs?) ;-)

Actually the "right" place to connect the feedback sensing resistor is
right out at the speaker itself, via a third sensing wire.

d

Eeyore

September 20th 08, 11:40 AM

Kevin Aylward wrote:

> Jan Panteltje wrote:
> >
> > Honestly, I have to think about this a bit, maybe run it in spice.
> > Fact remains that the TDA9274 is the only amp I know that needs no
> > Boucherot circuit :-)
>
> I would wager it's "non-optimum" designed. You give up a bit if the output
> load is not defined, usually. Without the zobel, the load reflected to the
> gain stages is all over the place. My guess is that they had a design goal
> of minimising the number of external components, which is standard practice
> in designing ics, but consequently, gave up some potential performance
> improvement.

Absolutely. I always use an isolating inductor twixt amp and load and the series
RC to GND to define an accurate HF load.

Never heard it called a "Boucherot circuit" though. Zobel network is the popular
one here, although technically isn't that across the load itself ?

There's a lot to be said for impedance compensation of speaker drivers too.

Graham

Eeyore

September 20th 08, 11:48 AM

Don Pearce wrote:

> Actually the "right" place to connect the feedback sensing resistor is
> right out at the speaker itself, via a third sensing wire.

You don't realise just how true that is. I've tuned pcb layouts for THD just by
moving that node. PCB layout guys look perplexed but thankfully usually do it. In
fact there's loads of layout tricks the PCB guys are hopeless at, loops in
particular.

In my 1200B design, all the power fets were mounted on a separate pcb to which
power, drive and feedback wires were attached, manually soldered. If you didn't
get the feedback wire bang in the centre up went the 2nd harmonic THD.

Graham

Eeyore

September 20th 08, 11:58 AM

Don Pearce wrote:

> Kevin Aylward wrote:
> > Don Pearce wrote:
> >> Kevin Aylward wrote:
> >>> Eeyore wrote:
> >>>> John Larkin wrote:
> >>>>> Eeyore wrote:
> >>>>>> John Larkin wrote:
> >>>>>>> Eeyore wrote:
> >>>>>>>> Learn something about LATERAL mosfets that were designed for
> >>>>>>>> audio. I've already given part number and links to data sheets.
> >>>>>>> That doesn't really matter. The transfer function only needs to
> >>>>>>> be continuous so that you can close a loop around it, and the fet
> >>>>>>> needs to be able to stand the peak power dissipation. That can
> >>>>>>> easily be done with vertical "switching" type fets. A modern
> >>>>>>> FLOOD architecture [1] works great with most any kind of fet.
> >>>>>>> Lots of things have changed in the last few decades.
> >>>>>>>
> >>>>>>> John
> >>>>>>>
> >>>>>>> [1] Of course you've never heard the term before. I just invented
> >>>>>>> it.
> >>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
> >>>>>> lovely features for audio. The only downside being a slightly
> >>>>>> highish Ron. Not really a problem when (as I have) used as many as
> >>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
> >>>>>> match beautifully with no need for source balance resistors (so
> >>>>>> some of the Ron loss 'goes away').
> >>>>> An opamp per fet, closing a local loop, feedback from the fet
> >>>>> source, makes each fet look like a perfect unity-gain, fast,
> >>>>> zero-offset device.
> >>>> Interesting idea. I'll have to chew that one over. I can see
> >>>> possible problems fron op-amp output overshoot.
> >>> I have a simple embodiment of that concept here, done a while ago, in
> >>> virtual land;-)
> >>>
> >>> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
> >>>
> >>> Its a push/pull gain loop around the output devices, forcing them to
> >>> be unity gain followers.
> >>>
> >>> You can get lower distortion, at the expense of speed, because you
> >>> have to compensate earlier.
> >>>
> >>> Common mode feedback at the second stage, allows for enormous dc/lf
> >>> gain. cascodes to allow the use of fast small transistors to do all
> >>> the main work. Emitter follower buffer to reduce the current swing
> >>> in the input pair, as per doug self. Spice says it should be in the
> >>> < 0.0001% , 20Khz range, maybe...
> >>>
> >>> Kevin Aylward
> >>>
> >>>
> >> Kevin - an interesting circuit, and I appreciate what you have done
> >> with the output stage, but I'm still wondering why you didn't include
> >> it within the global feedback loop - that could only have made it
> >> better, lower output impedance, more load insensitive etc etc etc.
> >
> > It is.!!!
> >
> > I think the schematic is not as clear as it should be.. I have a zero volt
> > source near the output devices in the feedback circuit to calculate LG. The
> > overall loop feedback passes through this source!!!
>
> Ah, is that what it was? I thought you were putting actual voltage
> sources in there that would be replaced by some small circuit in an
> actual design. I hope you are connecting the sensing point of the
> feedback resistor to the final summed speaker connection, not some
> random point in amongst the bunch of fets (teaching granny to suck
> eggs?) ;-)
>
> Actually the "right" place to connect the feedback sensing resistor is
> right out at the speaker itself, via a third sensing wire.

Better still, have differential sensing, to compensate for the volt drop in the
groundy side wire too !

Graham

Jan Panteltje

September 20th 08, 12:44 PM

On a sunny day (Sat, 20 Sep 2008 09:11:49 GMT) it happened "Kevin Aylward"
> wrote in
>:

>> I was not suggesting to widen bandwidth, although strong local
>> feedback would of course widen the bandwidth of a stage,
>
>But not the BW of the final, overall amp, so the BW of the internal stages
>is irrelevant if it is the result of feedback.
>
>What the local loop can buy you is reduced distortion at lower frequencies.
>
>I ran these two circuits up quite a while ago. One has the UGB at the
>output, one doesn't.
>
>http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp1.jpg

Now that gets complicated, Q4 and the 2 diodes in the emitter...

>http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>
>The UGB version had to be compensated earlier in frequency, i.e the non UGB
>version was significantly faster. Unfortunately, I cant remember much of the
>data, and I seem to have lost the SS files;-)
>
>I think one had 0.0001% at 20khz.

Yes, very good. Why use dotted paper ;-)

Jan Panteltje

September 20th 08, 12:45 PM

On a sunny day (Sat, 20 Sep 2008 10:22:09 +0100) it happened Don Pearce
> wrote in
>:

>Actually the "right" place to connect the feedback sensing resistor is
>right out at the speaker itself, via a third sensing wire.

Ha, why did I never think of that...
This will eliminate my massive gold feed rods to the woofer.

You will need 2 sensing wires, and a diff amp.

Jan Panteltje

September 20th 08, 12:45 PM

On a sunny day (Fri, 19 Sep 2008 23:24:02 +0100) it happened Eeyore
> wrote in
>:

>> I only see over current protection in a lot of amps, plus thermal,
>> the real thing was a bridge that caused fold back current limit,
>> couple of resistors and diodes, makes it any load proof.
>
>Except you don't want an audio amp to foldback, just shut down when it sees an 'impossible'
>load.
>
>Graham

That depends, if you just current limit, and have thermal protection, then
that takes time, perhaps enough time to melt some silicon.

If an amp is designed for 4 Ohm minimum load, and somebody connects 2 speakers
in parallel (for example), then current limit will step in.
The voltage drop will be small over the 2 Ohm (or lower, in case of a near short),
so say the transistors will have near full voltage at max current.
It is clear that when current limiting, then you can lower the reference for the current limit
circuit for low output voltages.
This reduces dissipation.

In fact (but this may hurt audio-people's egos perhaps), an audio amp
is nothing but 2 symmetrical regulated power supplies ;-)
In such a power supply one often also uses fold back for protection against
bad loads, example 7805 regulator...

Kevin Aylward[_3_]

September 20th 08, 01:03 PM

Eeyore wrote:
> Don Pearce wrote:
>
>> Actually the "right" place to connect the feedback sensing resistor
>> is right out at the speaker itself, via a third sensing wire.
>
> You don't realise just how true that is. I've tuned pcb layouts for
> THD just by moving that node. PCB layout guys look perplexed but
> thankfully usually do it. In fact there's loads of layout tricks the
> PCB guys are hopeless at, loops in particular.

Yes indeed. This is exactly what happened on the 1st PCB prototype of the
MOSFET 1000. Ian did the layout. When it came back, I was only getting 0.02%
thd. Now..this is *exactly* how it happened. I looked at the routing,
immediately noticed that the feedback point was picked up along a high
current trace, took a piece of wire and jumpered across the offending trace,
and thd dropped down to 0.002%. Like, simple putting a wire in || with
another one (that had a tee in it) worked magic!

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk

Eeyore

September 20th 08, 01:04 PM

Jan Panteltje wrote:

> On a sunny day it happened Don Pearce wrote
>
> >Actually the "right" place to connect the feedback sensing resistor is
> >right out at the speaker itself, via a third sensing wire.
>
> Ha, why did I never think of that...
> This will eliminate my massive gold feed rods to the woofer.
>
> You will need 2 sensing wires, and a diff amp.

LOL ! See one of my other posts.

Graham

Don Pearce

September 20th 08, 02:28 PM

Eeyore wrote:
>
> Don Pearce wrote:
>
>> Kevin Aylward wrote:
>>> Don Pearce wrote:
>>>> Kevin Aylward wrote:
>>>>> Eeyore wrote:
>>>>>> John Larkin wrote:
>>>>>>> Eeyore wrote:
>>>>>>>> John Larkin wrote:
>>>>>>>>> Eeyore wrote:
>>>>>>>>>> Learn something about LATERAL mosfets that were designed for
>>>>>>>>>> audio. I've already given part number and links to data sheets.
>>>>>>>>> That doesn't really matter. The transfer function only needs to
>>>>>>>>> be continuous so that you can close a loop around it, and the fet
>>>>>>>>> needs to be able to stand the peak power dissipation. That can
>>>>>>>>> easily be done with vertical "switching" type fets. A modern
>>>>>>>>> FLOOD architecture [1] works great with most any kind of fet.
>>>>>>>>> Lots of things have changed in the last few decades.
>>>>>>>>>
>>>>>>>>> John
>>>>>>>>>
>>>>>>>>> [1] Of course you've never heard the term before. I just invented
>>>>>>>>> it.
>>>>>>>> Fine. Can you elaborate some more on it ? Laterals have some truly
>>>>>>>> lovely features for audio. The only downside being a slightly
>>>>>>>> highish Ron. Not really a problem when (as I have) used as many as
>>>>>>>> 6 in parallel (12 mosfets per channel / 24 per amp). They also
>>>>>>>> match beautifully with no need for source balance resistors (so
>>>>>>>> some of the Ron loss 'goes away').
>>>>>>> An opamp per fet, closing a local loop, feedback from the fet
>>>>>>> source, makes each fet look like a perfect unity-gain, fast,
>>>>>>> zero-offset device.
>>>>>> Interesting idea. I'll have to chew that one over. I can see
>>>>>> possible problems fron op-amp output overshoot.
>>>>> I have a simple embodiment of that concept here, done a while ago, in
>>>>> virtual land;-)
>>>>>
>>>>> http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg
>>>>>
>>>>> Its a push/pull gain loop around the output devices, forcing them to
>>>>> be unity gain followers.
>>>>>
>>>>> You can get lower distortion, at the expense of speed, because you
>>>>> have to compensate earlier.
>>>>>
>>>>> Common mode feedback at the second stage, allows for enormous dc/lf
>>>>> gain. cascodes to allow the use of fast small transistors to do all
>>>>> the main work. Emitter follower buffer to reduce the current swing
>>>>> in the input pair, as per doug self. Spice says it should be in the
>>>>> < 0.0001% , 20Khz range, maybe...
>>>>>
>>>>> Kevin Aylward
>>>>>
>>>>>
>>>> Kevin - an interesting circuit, and I appreciate what you have done
>>>> with the output stage, but I'm still wondering why you didn't include
>>>> it within the global feedback loop - that could only have made it
>>>> better, lower output impedance, more load insensitive etc etc etc.
>>> It is.!!!
>>>
>>> I think the schematic is not as clear as it should be.. I have a zero volt
>>> source near the output devices in the feedback circuit to calculate LG. The
>>> overall loop feedback passes through this source!!!
>> Ah, is that what it was? I thought you were putting actual voltage
>> sources in there that would be replaced by some small circuit in an
>> actual design. I hope you are connecting the sensing point of the
>> feedback resistor to the final summed speaker connection, not some
>> random point in amongst the bunch of fets (teaching granny to suck
>> eggs?) ;-)
>>
>> Actually the "right" place to connect the feedback sensing resistor is
>> right out at the speaker itself, via a third sensing wire.
>
> Better still, have differential sensing, to compensate for the volt drop in the
> groundy side wire too !
>
> Graham
>

I wrote an article for one of the Hi Fi mags a few years ago with
exactly that idea - suggested easy ways to do it too. Only problem is
when you accidentally connect the sense wires in wrong polarity - smoke
ensues.

d

MooseFET

September 20th 08, 03:28 PM

On Sep 19, 8:01*pm, "Michael A. Terrell" >
wrote:
> MooseFET wrote:
>
> > On Sep 19, 7:37 am, Eeyore >
> > wrote:
> > > MooseFET wrote:
> > > > Beware: *Mosfets like the STW55NM60 have a decreasing threshold
> > > > voltage for increasing temperature. *This means that biasing them to a
> > > > low idle current isn't so easy.
>
> > > Indeed. You'll need lossy ballast resistors. Laterals are different that
> > > way.
>
> > Yes, but once the STW55NM60 or equiv. is biased up, it makes a nice
> > 200V at about 100KHz. Its not exactly audio but is sure isn't really
> > RF either.
>
> * *Tell that to WWVB, who transmits at 60 kHz.

Good point. I wasn't radiating, at least not on purpose.

A lot of echo sounders work up at 500KHz so 100KHz wasn't really too
high to be called sound too.

isw

September 21st 08, 05:02 AM

In article >,
Jan Panteltje > wrote:

> On a sunny day (Sat, 20 Sep 2008 10:22:09 +0100) it happened Don Pearce
> > wrote in
> >:
>
> >Actually the "right" place to connect the feedback sensing resistor is
> >right out at the speaker itself, via a third sensing wire.
>
> Ha, why did I never think of that...
> This will eliminate my massive gold feed rods to the woofer.
>
> You will need 2 sensing wires, and a diff amp.

If you really think that's important (and I don't), why not just put the
amplifiers near the speakers? That way, there won't be any nasty
stability problems to deal with.

Isaac

Eeyore

September 21st 08, 04:36 PM

isw wrote:

> Jan Panteltje > wrote:
> > it happened Don Pearce wrote
> >
> > >Actually the "right" place to connect the feedback sensing resistor is
> > >right out at the speaker itself, via a third sensing wire.
> >
> > Ha, why did I never think of that...
> > This will eliminate my massive gold feed rods to the woofer.
> >
> > You will need 2 sensing wires, and a diff amp.
>
> If you really think that's important (and I don't), why not just put the
> amplifiers near the speakers? That way, there won't be any nasty
> stability problems to deal with.

They're called active speakers and are widely used by many professionals.

Graham

Kevin Aylward[_3_]

September 21st 08, 05:20 PM

Eeyore wrote:
> isw wrote:
>
>> Jan Panteltje > wrote:
>>> it happened Don Pearce wrote
>>>
>>>> Actually the "right" place to connect the feedback sensing
>>>> resistor is right out at the speaker itself, via a third sensing
>>>> wire.
>>>
>>> Ha, why did I never think of that...
>>> This will eliminate my massive gold feed rods to the woofer.
>>>
>>> You will need 2 sensing wires, and a diff amp.
>>
>> If you really think that's important (and I don't), why not just put
>> the amplifiers near the speakers? That way, there won't be any nasty
>> stability problems to deal with.
>
> They're called active speakers and are widely used by many
> professionals.

Indeed.

I don't actually like powered speakers though. They need *two* leads. Just
more hassle in setting up for the gig.

Kevin Aylward
www.blonddee.co.uk
www.anasoft.co.uk

Don Pearce

September 21st 08, 05:26 PM

Kevin Aylward wrote:
> Eeyore wrote:
>> isw wrote:
>>
>>> Jan Panteltje > wrote:
>>>> it happened Don Pearce wrote
>>>>
>>>>> Actually the "right" place to connect the feedback sensing
>>>>> resistor is right out at the speaker itself, via a third sensing
>>>>> wire.
>>>> Ha, why did I never think of that...
>>>> This will eliminate my massive gold feed rods to the woofer.
>>>>
>>>> You will need 2 sensing wires, and a diff amp.
>>> If you really think that's important (and I don't), why not just put
>>> the amplifiers near the speakers? That way, there won't be any nasty
>>> stability problems to deal with.
>> They're called active speakers and are widely used by many
>> professionals.
>
> Indeed.
>
> I don't actually like powered speakers though. They need *two* leads. Just
> more hassle in setting up for the gig.
>

Wouldn't want them on a gig - very good for studio monitoring though.

d

Eeyore

September 21st 08, 05:39 PM

Kevin Aylward wrote:

> Eeyore wrote:
> > isw wrote:
> >> Jan Panteltje > wrote:
> >>> it happened Don Pearce wrote
> >>>
> >>>> Actually the "right" place to connect the feedback sensing
> >>>> resistor is right out at the speaker itself, via a third sensing
> >>>> wire.
> >>>
> >>> Ha, why did I never think of that...
> >>> This will eliminate my massive gold feed rods to the woofer.
> >>>
> >>> You will need 2 sensing wires, and a diff amp.
> >>
> >> If you really think that's important (and I don't), why not just put
> >> the amplifiers near the speakers? That way, there won't be any nasty
> >> stability problems to deal with.
> >
> > They're called active speakers and are widely used by many
> > professionals.
>
> Indeed.
>
> I don't actually like powered speakers though. They need *two* leads. Just
> more hassle in setting up for the gig.

I know exactly what you mean. Far too much hassle and just more leads getting
in the way for live.

When The Horn had an upgrade last year, all the PA amps, crossover (controller
now actually) went into a back room in a dedicated amp rack with ventilation
and speaker lines (in 4mm2) were run in trunking to the appropriate points.
The monitors sit on a raised surface at the front of the stage and the runs to
them terminate in connectors mounted in purpose made pressed metal panels
mounted in the performer's side of the upright. This means you need only 1 or
2 m leads to wire them up. It's lovely.

Graham

MooseFET

September 21st 08, 05:53 PM

On Sep 21, 12:02 pm, isw > wrote:
> In article >,
> Jan Panteltje > wrote:
>
> > On a sunny day (Sat, 20 Sep 2008 10:22:09 +0100) it happened Don Pearce
> > > wrote in
> > >:
>
> > >Actually the "right" place to connect the feedback sensing resistor is
> > >right out at the speaker itself, via a third sensing wire.
>
> > Ha, why did I never think of that...
> > This will eliminate my massive gold feed rods to the woofer.
>
> > You will need 2 sensing wires, and a diff amp.
>
> If you really think that's important (and I don't), why not just put the
> amplifiers near the speakers? That way, there won't be any nasty
> stability problems to deal with.

You can do about as well if you characterize what a linear system does
and then correct for it. If a system is nonlinear it is harder to do.

If you want to go all the way to having feedback from the speaker, it
seems to me that moving the feedback detection as close to the sound
output as practical is the way to go. Sensing actual cone movement is
better than sensing the driven voice coil's voltage. Some years back I
saw a really nice design for a subwoofer that did this. Here is
basically what the design did:

There was no voice coil at all. The cone was moved by a fast servo
motor with a metal belt drive that drove a rod on the cone center..
This allowed motions of several inches but didn't have a very flat
frequency response.

A position sensor on the rod on the cone center and a pressure
transducer where combined to form the feedback signal. The position
sensor couldn't know about the cone flex and the pressure sensor
couldn't know the very low frequency components. The combined signal
covered the entire range of interest.

There was a really massive servo amplifier driving the motor giving
the system the ability to work up to several Hz.

It seems to me that this sort of thing could be done today and make it
up into the 10s of Hz.

Vladimir Vassilevsky

September 21st 08, 06:19 PM

MooseFET wrote:

> If you want to go all the way to having feedback from the speaker, it
> seems to me that moving the feedback detection as close to the sound
> output as practical is the way to go. Sensing actual cone movement is
> better than sensing the driven voice coil's voltage.

The idea of the feedback directly from the cone is no new. The main
problem is economical: it is easier to make a reasonable conventional
speaker rather then trying to make a good speaker from the bad one by
the means of feedback and feedforward compensation.

> Some years back I
> saw a really nice design for a subwoofer that did this. Here is
> basically what the design did:
>
> There was no voice coil at all. The cone was moved by a fast servo
> motor with a metal belt drive that drove a rod on the cone center..
> This allowed motions of several inches but didn't have a very flat
> frequency response.
>
> A position sensor on the rod on the cone center and a pressure
> transducer where combined to form the feedback signal. The position
> sensor couldn't know about the cone flex and the pressure sensor
> couldn't know the very low frequency components. The combined signal
> covered the entire range of interest.
>
> There was a really massive servo amplifier driving the motor giving
> the system the ability to work up to several Hz.
>
> It seems to me that this sort of thing could be done today and make it
> up into the 10s of Hz.

In geophysics, they use the powerful hydraulic vibrators which operate
at the frequencies up to 100Hz. It is interesting that the flat response
and the linearity are important for geophysical application; so some
sort of compensation is applied. This technology can be used for the
woofers if we are after the power levels of 10kW or higher.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Eeyore

September 21st 08, 06:43 PM

Don Pearce wrote:

> Kevin Aylward wrote:
> > Eeyore wrote:
> >> isw wrote:
> >>> Jan Panteltje > wrote:
> >>>> it happened Don Pearce wrote
> >>>>
> >>>>> Actually the "right" place to connect the feedback sensing
> >>>>> resistor is right out at the speaker itself, via a third sensing
> >>>>> wire.
> >>>> Ha, why did I never think of that...
> >>>> This will eliminate my massive gold feed rods to the woofer.
> >>>>
> >>>> You will need 2 sensing wires, and a diff amp.
> >>> If you really think that's important (and I don't), why not just put
> >>> the amplifiers near the speakers? That way, there won't be any nasty
> >>> stability problems to deal with.
> >> They're called active speakers and are widely used by many
> >> professionals.
> >
> > Indeed.
> >
> > I don't actually like powered speakers though. They need *two* leads. Just
> > more hassle in setting up for the gig.
>
> Wouldn't want them on a gig - very good for studio monitoring though.

Exactly. KRK have a very good name in that respect at the moment and I think
they do actives.

Graham

Eeyore

September 21st 08, 06:46 PM

MooseFET wrote:

> isw > wrote:
> > Jan Panteltje > wrote:
> > > it happened Don Pearce wrote
> >
> > > >Actually the "right" place to connect the feedback sensing resistor is
> > > >right out at the speaker itself, via a third sensing wire.
> >
> > > Ha, why did I never think of that...
> > > This will eliminate my massive gold feed rods to the woofer.
> >
> > > You will need 2 sensing wires, and a diff amp.
> >
> > If you really think that's important (and I don't), why not just put the
> > amplifiers near the speakers? That way, there won't be any nasty
> > stability problems to deal with.
>
> You can do about as well if you characterize what a linear system does
> and then correct for it. If a system is nonlinear it is harder to do.
>
> If you want to go all the way to having feedback from the speaker, it
> seems to me that moving the feedback detection as close to the sound
> output as practical is the way to go. Sensing actual cone movement is
> better than sensing the driven voice coil's voltage.

Philips did that decades ago with a piezo transducer IIRC. No idea why it
didn't take off other than Philips aren't exactly reknowned for hi-fi.

Graham

Eeyore

September 21st 08, 06:48 PM

Vladimir Vassilevsky wrote:

> MooseFET wrote:
>
> > If you want to go all the way to having feedback from the speaker, it
> > seems to me that moving the feedback detection as close to the sound
> > output as practical is the way to go. Sensing actual cone movement is
> > better than sensing the driven voice coil's voltage.
>
> The idea of the feedback directly from the cone is no new. The main
> problem is economical: it is easier to make a reasonable conventional
> speaker rather then trying to make a good speaker from the bad one by
> the means of feedback and feedforward compensation.

The other problem is which part of the speaker cone do you sense ? They
'bend' and resonate etc etc in use.

Graham

Jan Panteltje

September 21st 08, 07:00 PM

On a sunny day (Sun, 21 Sep 2008 18:43:24 +0100) it happened Eeyore
> wrote in
>:

>> > I don't actually like powered speakers though. They need *two* leads. Just
>> > more hassle in setting up for the gig.
>>
>> Wouldn't want them on a gig - very good for studio monitoring though.
>
>Exactly. KRK have a very good name in that respect at the moment and I think
>they do actives.
>
>Graham

I think it should be possible [I could] design powered speakers with a WiFi interface.
Each speaker would have its own IP address, or perhaps its own port on one IP,
and from the [new] mixer only digital Ethernet to a wireless access point.
No bandwidth problem I think.
56 Mbits / second, should be enough for a few channels.
You will have power cables to the speaker, but not a lot of audio wiring.

mmm maybe do the mikes too ;-)

Vladimir Vassilevsky

September 21st 08, 07:41 PM

Jan Panteltje wrote:

> I think it should be possible [I could] design powered speakers with a WiFi interface.

How would you synchronize the different channels?

> Each speaker would have its own IP address, or perhaps its own port on one IP,
> and from the [new] mixer only digital Ethernet to a wireless access point.
> No bandwidth problem I think.
> 56 Mbits / second, should be enough for a few channels.

The real 802.11G throughput is 2.8MB/s at the best. An uncompressed
audio channel takes roughly 100KB/s.

> You will have power cables to the speaker, but not a lot of audio wiring.
> mmm maybe do the mikes too ;-)

The big problem with WiFi for audio is the synchronization between the
different WiFi units while maintaining the reasonable delay. This is
hard (if possible at all) to attain with the WiFi equipment.

AFAIK the solutions for audio via Ethernet (CobraNet and such) used the
special protocol stacks and were not fully compatible with the standard
networking stuff. In the general, Ethernet is not good as the network
for the multimedia; it was not designed for that purpose.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Jan Panteltje

September 21st 08, 07:50 PM

On a sunny day (Sun, 21 Sep 2008 13:41:43 -0500) it happened Vladimir
Vassilevsky > wrote in
>:

>
>
>Jan Panteltje wrote:
>
>> I think it should be possible [I could] design powered speakers with a WiFi interface.
>
>How would you synchronize the different channels?

Yes, good point, timestamp would be one way, but that does not solve the delay.
the delay would be fatal in a live application.

>> Each speaker would have its own IP address, or perhaps its own port on one IP,
>> and from the [new] mixer only digital Ethernet to a wireless access point.
>> No bandwidth problem I think.
>> 56 Mbits / second, should be enough for a few channels.
>
>The real 802.11G throughput is 2.8MB/s at the best. An uncompressed
>audio channel takes roughly 100KB/s.

There is no reason not to use a compressed format, mp2 or AC3 would be cool.

>> You will have power cables to the speaker, but not a lot of audio wiring.
>> mmm maybe do the mikes too ;-)
>
>The big problem with WiFi for audio is the synchronization between the
>different WiFi units while maintaining the reasonable delay. This is
>hard (if possible at all) to attain with the WiFi equipment.

See above.

>AFAIK the solutions for audio via Ethernet (CobraNet and such) used the
>special protocol stacks and were not fully compatible with the standard
>networking stuff. In the general, Ethernet is not good as the network
>for the multimedia; it was not designed for that purpose.

OK, it looks like we have to scrap this idea, in its current form, but
the basic principle could work.

>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com

Good observation!

krw[_3_]

September 21st 08, 07:53 PM

In article >,
says...
>
>
> Jan Panteltje wrote:
>
> > I think it should be possible [I could] design powered speakers with a WiFi interface.
>
> How would you synchronize the different channels?

Don't. One WiFi channel.

> > Each speaker would have its own IP address, or perhaps its own port on one IP,
> > and from the [new] mixer only digital Ethernet to a wireless access point.
> > No bandwidth problem I think.
> > 56 Mbits / second, should be enough for a few channels.
>
> The real 802.11G throughput is 2.8MB/s at the best. An uncompressed
> audio channel takes roughly 100KB/s.

You're going to have to justify that number. 100MB/s? I can see
20KB, *maybe*. Five channels at perhaps three times this...

> > You will have power cables to the speaker, but not a lot of audio wiring.
> > mmm maybe do the mikes too ;-)
>
> The big problem with WiFi for audio is the synchronization between the
> different WiFi units while maintaining the reasonable delay. This is
> hard (if possible at all) to attain with the WiFi equipment.

Why is that so hard? All of the recievers get the same stream.
Each takes what it needs and throws the rest in the bit bucket.

> AFAIK the solutions for audio via Ethernet (CobraNet and such) used the
> special protocol stacks and were not fully compatible with the standard
> networking stuff. In the general, Ethernet is not good as the network
> for the multimedia; it was not designed for that purpose.

It may not be good, but easily could be good enough.

--
Keith

Eeyore

September 21st 08, 08:18 PM

Vladimir Vassilevsky wrote:

> Jan Panteltje wrote:
>
> > I think it should be possible [I could] design powered speakers with a WiFi interface.
>
> How would you synchronize the different channels?
>
> > Each speaker would have its own IP address, or perhaps its own port on one IP,
> > and from the [new] mixer only digital Ethernet to a wireless access point.
> > No bandwidth problem I think.
> > 56 Mbits / second, should be enough for a few channels.
>
> The real 802.11G throughput is 2.8MB/s at the best. An uncompressed
> audio channel takes roughly 100KB/s.
>
> > You will have power cables to the speaker, but not a lot of audio wiring.
> > mmm maybe do the mikes too ;-)
>
> The big problem with WiFi for audio is the synchronization between the
> different WiFi units while maintaining the reasonable delay. This is
> hard (if possible at all) to attain with the WiFi equipment.

i.e. latency. All the specialist solutions to this issue have addressed this problem
specifically.

> AFAIK the solutions for audio via Ethernet (CobraNet and such) used the
> special protocol stacks and were not fully compatible with the standard
> networking stuff. In the general, Ethernet is not good as the network
> for the multimedia; it was not designed for that purpose.

Cobranet causes trouble does it ? I know they have some kind of 'sync' signal.

Graham

Eeyore

September 21st 08, 08:18 PM

Jan Panteltje wrote:

> There is no reason not to use a compressed format, mp2 or AC3 would be cool.

You vile vermin !

Graham

Eeyore

September 21st 08, 08:23 PM

krw wrote:

> You're going to have to justify that number. 100MB/s?

He said kBps.

> I can see 20KB, *maybe*.

44.1k samples / sec x 2 bytes each = 88.2kBps. Allow overhead for collisions etc. Actually if
you transmit the full AES/EBU SPDIF data it's THREE bytes.

100kBps sounds about right for one mono channel. Or 150 kBps with flags etc.

Graham

Jan Panteltje

September 21st 08, 08:24 PM

On a sunny day (Sun, 21 Sep 2008 20:18:50 +0100) it happened Eeyore
> wrote in
>:

>
>
>Jan Panteltje wrote:
>
>> There is no reason not to use a compressed format, mp2 or AC3 would be cool.
>
>You vile vermin !
>
>Graham

mm actually, I had the intention to use separate amps in the speakers
for woofer, midrange, and tweeter, and do the filtering _in the mixer_,
so then you could compensate per speaker so to speak :-)
Anyways, for people with supernatural hearing, like some animals, that
would perhaps not give full satisfaction.
Most people are satisfied with AC3 though.

Jorden Verwer

September 21st 08, 08:38 PM

Kevin Aylward wrote:
> > Don't you at least agree there are many similarities between 1/f
> > noise and offset?
>
>
> Actually, I do. By and large, they amount to the same thing. Its all low
> frequency variations. For example, if one designs a chopper amp to get low
> offset, it also kills/corrects for 1/f noise as well. If one has 1/f
> problems in an system, one immediately thinks about using a chopper..well
I
> do any way...
Yes, that's exactly what I meant. :)

Jorden Verwer

September 21st 08, 08:40 PM

MooseFET wrote:
> If you look for an op-amp with an extremely low offset voltage, you
> will find that it uses MOSFETs to obtain that extreme low offset.
Quite right. In hindsight, I could've been more clear about this not being
relevant here, I just didn't want any "You're wrong because MOSFETs have
lower offset"-type responses, so that's why I mentioned it...

Jorden Verwer

September 21st 08, 08:48 PM

MooseFET wrote:
> Radiation hardness
Boundary condition.

> Safe operating area
Boundary condition.

> Bandwidth
AFAIK, BJTs still hold the world record for cutoff frequency.

John Larkin

September 21st 08, 08:57 PM

On Thu, 18 Sep 2008 09:25:39 -0000, "Jorden Verwer"
> wrote:

>RichD wrote:
>> Who do MOSFET sound better than bipolar, as an audio amp output
>> driver?
>
>The device properties of BJTs are superior to those of MOSFETs in all
>respects, except for offset - there MOSFETs have the advantage. Whether you
>will actually hear this depends on many more factors.
>

Offset? Two BJTs of the same part number will have delta-Vbe well
below a tenth of a volt. A similar pair of mosfets will have Vgs-th
and transfer curves that can differ by a volt or more. And the mosfet
thresholds will change with time a lot more then the bjt's.

Mosfets have zero storage/desaturation time, far better SOAR specs,
and are easier to drive.

John

Jorden Verwer

September 21st 08, 09:24 PM

Jan Panteltje wrote:
> I think it should be possible [I could] design powered speakers with a
WiFi
> interface.
Yuck, wireless...

Don Pearce

September 21st 08, 09:25 PM

Jorden Verwer wrote:
> Jan Panteltje wrote:
>> I think it should be possible [I could] design powered speakers with a
> WiFi
>> interface.
> Yuck, wireless...
>
>

Around the house these days Homeplug is far better than WiFi.

d

Jan Panteltje

September 21st 08, 09:41 PM

On a sunny day (Sun, 21 Sep 2008 20:24:20 -0000) it happened "Jorden Verwer"
> wrote in
t>:

>Jan Panteltje wrote:
>> I think it should be possible [I could] design powered speakers with a
>WiFi
>> interface.
>Yuck, wireless...

You mean you do not watch TV and listen to radio?
And have no cellphone too?

Jorden Verwer

September 21st 08, 09:44 PM

Jan Panteltje write:
> You mean you do not watch TV and listen to radio?
> And have no cellphone too?
I mean I try to avoid wireless whenever possible. So when I'm at home, I
watch cable TV and listen cable radio and use my landline phone...

Jan Panteltje

September 21st 08, 09:52 PM

On a sunny day (Sun, 21 Sep 2008 21:25:33 +0100) it happened Don Pearce
> wrote in
>:

>Jorden Verwer wrote:
>> Jan Panteltje wrote:
>>> I think it should be possible [I could] design powered speakers with a
>> WiFi
>>> interface.
>> Yuck, wireless...
>>
>>
>
>Around the house these days Homeplug is far better than WiFi.
>
>d

I have looked a bit at homeplug, because the new TVs seem to support it,
but really, I am a step ahead, I use the laptop as portable TV monitor to
watch satellite TV, and any file on the server I like.
Using WiFi ! so wireless.
This is an old blog entry, have added much cool stuff since:
http://panteltje.com/panteltje/blog/index.html
Playing nice slide shows now too.
http://panteltje.com/panteltje/jpg_to_mjpegtools_yuv/

It is in this that it shows, and is proof of,
that WiFi is not in any way a real bandwidth problem
This is all Linux soft of course, so highly efficient.

The worlds is just spitting out one marketing term after the other, now
it is 'home plug'.
Really, as far as network is concerned, ssh, netcat, scp, transport stream,
etc... rules it all.

They (Philips, others), want to sell you an 'internet radio', a 'digital picture frame',
a this, a that, while any laptop running Linux with a 5 line script does it all.
With more possibilities and more flexible, and future proof, and wireless too.
Often for less money.

Jan Panteltje

September 21st 08, 09:58 PM

On a sunny day (Sun, 21 Sep 2008 20:44:55 -0000) it happened "Jorden Verwer"
> wrote in
t>:

>Jan Panteltje write:
>> You mean you do not watch TV and listen to radio?
>> And have no cellphone too?
>I mean I try to avoid wireless whenever possible. So when I'm at home, I
>watch cable TV and listen cable radio and use my landline phone...

OK, whatever you like best.
See also my other posting.

Chronic Philharmonic

September 21st 08, 10:17 PM

"RichD" > wrote in message
...
> Who do MOSFET sound better than bipolar, as an audio amp output
> driver?

A lot of bits have been spilled on this thread so far, but I think it is the
same question as "Why do tubes sound better than transistors?" or "How many
angels can dance on the head of a pin?"

First of all, it implies a truth that may not objectively be the case. But
tubes, BJTs and MOSFETs all have very different characteristics and
engineering trade-offs. I think it has been reasonably well established that
it is possible to make excellent sounding amplifiers with any of those
technologies.

In the end, I think it boils down to the experience and skill of the
engineer in working with a given technology, and how that impacts the
economics of producing the end product using said technology.

For example, I think we no longer use vacuum tubes for mainstream audio
power amplifiers because they:

1. Require an additional power supply and heat to operate.
2. Are bulky compared to their solid-state alternatives.
3. Are not available in complimentary pairs (e.g., P-channel/N-channel)
4. Have output impedance that usually requires an output transformer.

None of these are insurmountable obstacles, but they do increase the size,
cost, power consumption and heat load for a given output power.

Similar, but less obvious considerations affect the choice of BJTs and
MOSFETs. They have different characteristics, and the skilled engineer will
exploit them for optimal effect -- sonic quality, economy, reliability and
so on. The marketing department will define the requirements for the product
based on the target market, and the engineer will attempt to design a
product that fits within those constraints. In all that I have read here, I
have not seen anything that would consistently make me select MOSFETs over
BJTs. But I might have a preference based on a *given set* of product
requirements.

Vladimir Vassilevsky

September 21st 08, 10:46 PM

Jan Panteltje wrote:
> On a sunny day (Sun, 21 Sep 2008 13:41:43 -0500) it happened Vladimir
> Vassilevsky > wrote in
> >:
>
>
>>
>>Jan Panteltje wrote:
>>
>>
>>>I think it should be possible [I could] design powered speakers with a WiFi interface.
>>
>>How would you synchronize the different channels?
>
> Yes, good point, timestamp would be one way, but that does not solve the delay.
> the delay would be fatal in a live application.

Here is the idea: using the power frequency as the common timing
reference. In the local WiFi network, the ping time would be at the
order of 1ms, so all channels could be PLLed to the same half period of
the AC power without an ambiguity. With the sufficient amount of
buffering, that should allow streaming multiple synchronized channels.
Sooo simple... I bet somebody already got a patent on that.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Jan Panteltje

September 21st 08, 10:59 PM

On a sunny day (Sun, 21 Sep 2008 16:46:27 -0500) it happened Vladimir
Vassilevsky > wrote in
>:

>
>
>Jan Panteltje wrote:
>> On a sunny day (Sun, 21 Sep 2008 13:41:43 -0500) it happened Vladimir
>> Vassilevsky > wrote in
>> >:
>>
>>
>>>
>>>Jan Panteltje wrote:
>>>
>>>
>>>>I think it should be possible [I could] design powered speakers with a WiFi interface.
>>>
>>>How would you synchronize the different channels?
>>
>> Yes, good point, timestamp would be one way, but that does not solve the delay.
>> the delay would be fatal in a live application.
>
>Here is the idea: using the power frequency as the common timing
>reference. In the local WiFi network, the ping time would be at the
>order of 1ms, so all channels could be PLLed to the same half period of
>the AC power without an ambiguity. With the sufficient amount of
>buffering, that should allow streaming multiple synchronized channels.
>Sooo simple... I bet somebody already got a patent on that.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com

Clever!
How about this: we give each speaker a GPS.
It will also send back its position, and the 'mixer' will
then calculate the optimum sound pattern for 5.1.
GPS also has a very precise clock.

Eeyore

September 21st 08, 11:03 PM

Jan Panteltje wrote:

> Most people are satisfied with AC3 though.

Dumbing down wins again.

Graham

Eeyore

September 21st 08, 11:04 PM

Jorden Verwer wrote:

> Kevin Aylward wrote:
> > > Don't you at least agree there are many similarities between 1/f
> > > noise and offset?
> >
> > Actually, I do. By and large, they amount to the same thing. Its all low
> > frequency variations. For example, if one designs a chopper amp to get low
> > offset, it also kills/corrects for 1/f noise as well. If one has 1/f
> > problems in an system, one immediately thinks about using a chopper..well
> > I do any way...
> Yes, that's exactly what I meant. :)

And a chopper amp has exactly WHAT to do with ultra high quality audio ?

Graham

Eeyore

September 21st 08, 11:05 PM

Jorden Verwer wrote:

> MooseFET wrote:
> > Radiation hardness
> Boundary condition.
>
> > Safe operating area
> Boundary condition.
>
> > Bandwidth
> AFAIK, BJTs still hold the world record for cutoff frequency.

In devices suitable for audio amps ?

You are totally incapable of staying on-topic.

Graham

Eeyore

September 21st 08, 11:09 PM

John Larkin wrote:

> "Jorden Verwer" wrote:
> >RichD wrote:
> >> Who do MOSFET sound better than bipolar, as an audio amp output
> >> driver?
> >
> >The device properties of BJTs are superior to those of MOSFETs in all
> >respects, except for offset - there MOSFETs have the advantage. Whether you
> >will actually hear this depends on many more factors.
>
> Offset? Two BJTs of the same part number will have delta-Vbe well
> below a tenth of a volt.

IME typically within a 10mV range. That saved a trimpot and the line time to
adjust it on more than one occasion.

> A similar pair of mosfets will have Vgs-th
> and transfer curves that can differ by a volt or more.

For vertical types, yes. Not for laterals.

> And the mosfet thresholds will change with time a lot more then the bjt's.

No comment. Never known it be a problem with laterals.

> Mosfets have zero storage/desaturation time,

Well, 60ns on the data sheet was good enough for me.

> far better SOAR specs, and are easier to drive.

True.

Graham

Eeyore

September 21st 08, 11:12 PM

Chronic Philharmonic wrote:

> "RichD" wrote
>
> > Who do MOSFET sound better than bipolar, as an audio amp output
> > driver?
>
> A lot of bits have been spilled on this thread so far, but I think it is the
> same question as "Why do tubes sound better than transistors?" or "How many
> angels can dance on the head of a pin?"
>
> First of all, it implies a truth that may not objectively be the case. But
> tubes, BJTs and MOSFETs all have very different characteristics and
> engineering trade-offs.

Actually, certain mosfets are not at all unlike triodes.

More to the point is whether you are anti-science and despise negative feedback.
And / or LIKE your music deliberately distorted as some seem to do.

Graham

Vladimir Vassilevsky

September 21st 08, 11:18 PM

Jan Panteltje wrote:

> On a sunny day (Sun, 21 Sep 2008 16:46:27 -0500) it happened Vladimir
> Vassilevsky > wrote in
>>Jan Panteltje wrote:
>>>On a sunny day (Sun, 21 Sep 2008 13:41:43 -0500) it happened Vladimir
>>>Vassilevsky > wrote in
>>>>Jan Panteltje wrote:

>>>>>I think it should be possible [I could] design powered speakers with a WiFi interface.

>>>>How would you synchronize the different channels?

>>>Yes, good point, timestamp would be one way, but that does not solve the delay.
>>>the delay would be fatal in a live application.

>>Here is the idea: using the power frequency as the common timing
>>reference. In the local WiFi network, the ping time would be at the
>>order of 1ms, so all channels could be PLLed to the same half period of
>>the AC power without an ambiguity. With the sufficient amount of
>>buffering, that should allow streaming multiple synchronized channels.
>>Sooo simple... I bet somebody already got a patent on that.

> How about this: we give each speaker a GPS.

Yes, something like GPS but it should be 10 times more precise and be
able to operate indoors. Local positioning system is another perpetual
problem; it would be a good thing to have regardless.

> It will also send back its position, and the 'mixer' will
> then calculate the optimum sound pattern for 5.1.
> GPS also has a very precise clock.

Lots of things can be done; however the consumer audio is very sensitive
to the cost. Many suggestions in this thread, although technically
standing, are totally missing this point.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Jorden Verwer

September 22nd 08, 12:13 AM

Eeyore wrote:
> And a chopper amp has exactly WHAT to do with ultra high quality audio ?
Well, it proves my point.

John Larkin

September 22nd 08, 12:20 AM

On Sun, 21 Sep 2008 23:09:11 +0100, Eeyore
> wrote:

>
>
>John Larkin wrote:
>
>> "Jorden Verwer" wrote:
>> >RichD wrote:
>> >> Who do MOSFET sound better than bipolar, as an audio amp output
>> >> driver?
>> >
>> >The device properties of BJTs are superior to those of MOSFETs in all
>> >respects, except for offset - there MOSFETs have the advantage. Whether you
>> >will actually hear this depends on many more factors.
>>
>> Offset? Two BJTs of the same part number will have delta-Vbe well
>> below a tenth of a volt.
>
>IME typically within a 10mV range. That saved a trimpot and the line time to
>adjust it on more than one occasion.
>
>
>> A similar pair of mosfets will have Vgs-th
>> and transfer curves that can differ by a volt or more.
>
>For vertical types, yes. Not for laterals.

Why? The physics is the same... oxide thickness, trapped charge, all
the things that modulate gate threshold.

The datasheets I've seen show a volt or so spread of Vgs-th spec,
plenty enough to defeat simple biasing/balancing schemes for
paralleled devices.

John

John Larkin

September 22nd 08, 12:21 AM

On Sun, 21 Sep 2008 19:48:17 -0000, "Jorden Verwer"
> wrote:

>MooseFET wrote:
>> Radiation hardness
>Boundary condition.
>
>> Safe operating area
>Boundary condition.
>
>> Bandwidth
>AFAIK, BJTs still hold the world record for cutoff frequency.
>

No. Google "widest bandwidth transistor" or some such.

John

Jamie[_2_]

September 22nd 08, 12:29 AM

Jorden Verwer wrote:

> Eeyore wrote:
>
>>And a chopper amp has exactly WHAT to do with ultra high quality audio ?
>
> Well, it proves my point.
>
>
LOL!
good one.

http://webpages.charter.net/jamie_5"

Phil Allison

September 22nd 08, 12:42 AM

"Chronic Philharmonic"

> Similar, but less obvious considerations affect the choice of BJTs and
> MOSFETs.

** Which mosfets ?

You must not confuse switching types with laterals.

> They have different characteristics,

** Which mosfets ?

You must not confuse switching types with laterals.

> and the skilled engineer will exploit them for optimal effect -- sonic
> quality, economy, reliability and so on.

** Which mosfets ?

You must not confuse switching types with laterals.

> The marketing department will define the requirements for the product
> based on the target market, and the engineer will attempt to design a
> product that fits within those constraints.

** It don't have to work that idiotic way at all.

> In all that I have read here, I have not seen anything that would
> consistently make me select MOSFETs over BJTs.

** Which mosfets ?

You must not confuse switching types with laterals.

> But I might have a preference based on a *given set* of product
> requirements.

** Pigs might fly too.

Wot an posturing old wind bag you are.

...... Phil

Eeyore

September 22nd 08, 01:03 AM

John Larkin wrote:

> Eeyore wrote:
> >John Larkin wrote:
> >> "Jorden Verwer" wrote:
> >> >RichD wrote:
> >> >> Who do MOSFET sound better than bipolar, as an audio amp output
> >> >> driver?
> >> >
> >> >The device properties of BJTs are superior to those of MOSFETs in all
> >> >respects, except for offset - there MOSFETs have the advantage. Whether you
> >> >will actually hear this depends on many more factors.
> >>
> >> Offset? Two BJTs of the same part number will have delta-Vbe well
> >> below a tenth of a volt.
> >
> >IME typically within a 10mV range. That saved a trimpot and the line time to
> >adjust it on more than one occasion.
> >
> >
> >> A similar pair of mosfets will have Vgs-th
> >> and transfer curves that can differ by a volt or more.
> >
> >For vertical types, yes. Not for laterals.
>
> Why? The physics is the same... oxide thickness, trapped charge, all
> the things that modulate gate threshold.

I'm telling you that's how laterals are in practice.

Better processing perhaps ?

> The datasheets I've seen show a volt or so spread of Vgs-th spec,
> plenty enough to defeat simple biasing/balancing schemes for
> paralleled devices.

Laterals work beautifully like that.

Graham

Chronic Philharmonic

September 22nd 08, 01:41 AM

"Eeyore" > wrote in message
...
>
>
> Chronic Philharmonic wrote:
>
>> "RichD" wrote
>>
>> > Who do MOSFET sound better than bipolar, as an audio amp output
>> > driver?
>>
>> A lot of bits have been spilled on this thread so far, but I think it is
>> the
>> same question as "Why do tubes sound better than transistors?" or "How
>> many
>> angels can dance on the head of a pin?"
>>
>> First of all, it implies a truth that may not objectively be the case.
>> But
>> tubes, BJTs and MOSFETs all have very different characteristics and
>> engineering trade-offs.
>
> Actually, certain mosfets are not at all unlike triodes.

I almost said that, but decided to leave it because many other physical
and/or electrical parameters are distinctly different, and I figured I'd get
flamed for that.

> More to the point is whether you are anti-science and despise negative
> feedback.
> And / or LIKE your music deliberately distorted as some seem to do.

I think negative feedback is an extremely powerful tool in electronic,
economic, social and natural systems. But people don't seem to understand
it, so they don't trust it.