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Phil Allison Phil Allison is offline
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"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






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"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.


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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.


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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
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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



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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


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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


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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

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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

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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




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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

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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

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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
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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

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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


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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/...teral%20mosfet

Graham

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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/data.../2SK1058.shtml
http://www.datasheetcatalog.com/data...1/2SJ162.shtml

Graham



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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/...teral%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
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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/...teral%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

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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



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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/...teral%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
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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


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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/...teral%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

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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
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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



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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/...teral%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


You get crossover distortion whatever the topology.

d
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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
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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.


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Default MOSFET output stage



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

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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



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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

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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


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Default MOSFET output stage

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/...teral%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
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Default MOSFET output stage

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
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Jorden Verwer Jorden Verwer is offline
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Posts: 29
Default MOSFET output stage

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/Operati..._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.




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Default MOSFET output stage



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/...teral%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

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Default MOSFET output stage



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

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Don Pearce Don Pearce is offline
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Posts: 2,726
Default MOSFET output stage

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/...teral%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
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Default MOSFET output stage

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
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Default MOSFET output stage



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/Operati..._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

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