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  #11  
Old September 12th 19, 11:18 AM posted to rec.audio.high-end
~misfit~[_3_]
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Posts: 81
Default Introducing a New Horse to the Stable

On 12/09/2019 2:45 AM, Peter Wieck wrote:
> OK... getting down to basics:
>
> Electrolytic capacitors are essentially chemical engines. The materials developed over the last 20 years have greatly improved along with longevity and reliability, but they remain chemical engines. A single very large capacitor will, therefore, necessarily be slower than multiple smaller capacitors in parallel, all other things being equal. The limiting factor being real-estate in most cases.
>
> Generally, I try to run multiple caps in parallel where real-estate permits, with a small-value, high-voltage film cap across each as a snubber. This is a preference, not a requirement.
>
> For something as brute-force as a power-supply for audio purposes, the difference(s) will be manifest only at or near clipping, or when the amps are fed signal with extreme Peak-to-Average content. A cap bank will be able to deliver a *marginally* faster transient than a single very large cap. NOTE: If you are going to have the capacity (pun intended) to overdrive your output devices for these transients, you might need to install some sort of speaker protection. Solid-state devices often do not clip nicely.
>
> Peter Wieck
> Melrose Park, PA
>


Thank you Peter.

I already have speaker protection ready to install. I bought two of these
<https://www.aliexpress.com/item/1350315871.html> Probably not the best option but I don't know how
to make such things myself.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

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  #12  
Old September 12th 19, 11:18 AM posted to rec.audio.high-end
Trevor Wilson[_3_]
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Posts: 116
Default Introducing a New Horse to the Stable

On 12/09/2019 12:17 am, ~misfit~ wrote:
> On 10/09/2019 11:54 PM, Peter Wieck wrote:
>> OK, OK, I will bite! Minor rant to follow:
>>
>> Tube vs. Solid State on reliability:
>>
>> There are not so very many 60-year old components in operation these
>> days unmodified since-new. My oldest tube item turned 100 this year
>> and likely works better than when it was new based on a better
>> understanding of antenna systems, optimum tube voltages and so forth.
>> Other than moving parts (CD player), the newest component in my office
>> system was made in 1963. The system runs 9 hours per day, 5 days per
>> week. Oh, and the tubes are original as well.
>>
>> On the other hand, and given my hobby, I see a large number of SS
>> components that have blown transistors, exploded capacitors and much
>> worse, irrespective of age and source. The well made, well designed
>> stuff is serviceable, distinguishing it from the rest of the garbage
>> out there.
>>
>> I would make a fairly apt comparison: A tube amplifier is much like a
>> mid-last-century Mercedes or VW - few things were self-adjusting, and
>> they required regular and attentive care-and-feeding. With such, they
>> were good for several hundred thousand miles of reliable service. A
>> contemporary Ford, Cadillac, Plymouth would be considered remarkable
>> were it to survive 100,000 miles without heroic measures. Might run
>> very nicely when running, but that would be your basic solid-state
>> device in comparison.
>>
>> Put simply, they are different beasts designed with different things
>> in mind, but for the same basic purpose. That one is or is not
>> "BETTER" than the other is not relevant to the purpose in either case.
>>
>> Now, when I here things like "Zero global NFB" and "Critically matched
>> components", I can smell the snake-oil from a great distance, even the
>> 10,000 miles from here to Australia. I am sure that process also
>> contains descriptives of "interconnects" rolled on the thighs of
>> virgins on Walpurgis Night...
>>
>> Note that even "critically matched" solid-state components drift after
>> a very short period of time in-service. All of them, such that that
>> "less than 1%" is meaningful for perhaps 12 hours or so.
>>
>> Being as this is a hobby for me, I get to try things that are
>> otherwise unproductive, unprofitable or impractical. Such as
>> shotgunning a device with single-value capacitors and then comparing
>> it to the same device with carefully screened and matched caps. Or
>> matching driver and output transistors and comparing to a similar
>> device with disparate values. Guys and gals - you would be seriously
>> shocked to discover how little difference some things make that the
>> ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no
>> difference at all.

>
> Thanks for your input Peter. If I may ask, do you have an opinion on
> 'storage capacitors' on an amplifier power supply? What in your opinion
> is 'better', a single (or few) very large caps or multiple smaller caps
> to the same / similar capacitance?
>
> I have a long term project building my own amp based on PCBs taken from
> 100w MOSFET (two pairs of J50 / K135 devices per amp) PA amps made by a
> New Zealand company in the 1980s. (Craft, Gary Morrison's company before
> he went on to become head designer at Plinius until 2005 when he left to
> set up Pure Audio). I got my hands on a rack of four of these mono amps
> and preliminary testing using a clean source and good speakers suggest
> they will make a great stereo amp.
>
> I need to put together a power supply to feed two of these and have some
> new 10,000uF caps but was wondering if multiple smaller caps would be
> better. (In the PA amps they only had 2,200uF but obviously weren't
> called on to reproduce much bass.)
>
> As it is I'll be using fly leads from the rectifier PCB to the caps,
> then to the amps and I'm building my own case. I was thinking of maybe
> using my 10,000uF caps as well as maybe some smaller ones, perhaps 1,000
> in a bank, the best of both worlds. (There are also 100uF electros
> across the rails on the amp PCBs that I'll be replacing.) That said I
> could also just go to multiple
>
> Cheers,


**Those old MOSFETs were pretty ordinary devices (not very linear).
Evidenced by the fact that Plinius amps have always used BJTs. As Peter
has stated, multiple small value caps will usually provide a superior,
higher speed power supply. However, I would posit that those old MOSFETs
are so horrible (modern MOSFETs are far superior), that it may not be
worth the effort. Craft amps used huge amounts of global NFB, required
due to very low bias currents and the necessity to reduce the huge
levels of distortion caused by the 'knee' at low currents (A Class A, or
high bias MOSFET amp would have been much better). Anyway, the huge
levels of global NFB means that PSRR (Power Supply Rejection Ratio) will
be quite high, thus the influence of power supply changes will be
relatively small.

One more thing: Decent amounts of capacitance placed close to the output
devices is far more influential than caps placed some distance away. In
fact, long(ish) cables AFTER the main filter caps can be a serious
limiting factor on the effectiveness of a power supply in a Class A/B
amplifier. This is because the inductance of the wires can be a factor.



--
Trevor Wilson
www.rageaudio.com.au

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  #13  
Old September 12th 19, 02:32 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Posts: 104
Default Introducing a New Horse to the Stable

https://www.eeweb.com/tools/parallel-wire-inductance

This website will allow you to calculate inductance by giving it the gauge, type and nature of the wire you are using. You will find, pretty quickly, that the distances involved in the typical component of say 40 cm (16") square are such that the actual inductance realized will be infinitesimal in "real life".

Again, getting to practical matters: there are common-sense applications and techniques for wiring electronics, much dependent on the nature and use intended. Part of my hobby is the restoration of vintage Zenith TransOceanic tube radios - and wire location/component location can be critical for high-band Short Wave sensitivity. It is common sense to shield power-supplies in pre-amplifiers, especially those that contain phono or NAB pre-amp sections. And, wire-dressing is always good practice. But worrying about straight-wire inductance at audio frequencies is much akin to worrying about skin-effect...

Now, Trevor clearly has a 'thing' about negative feedback, which is entirely his choice, and doubtless for sufficient and good reasons. But, again, in the real world, negative feedback, done properly, has many more advantages than disadvantages. Done badly - Ouch! Keep in mind that in its most practical application, it dates back to 1927, and was patented by Bell Labs in 1937. So, it is a pretty well established technique, such that any thoughtful designer not totally strangled by the bean-counters will get it right very nearly all of the time.

https://en.wikipedia.org/wiki/Negati...is_of_feedback

Go down to the "distortion" section. As brief as it is, it conveys some very good information.

In point-of-fact, part of the TIP-Mod for your 120 involved increasing capacitance within the feedback loop to reduce bass roll-off.

Peter Wieck
Melrose Park, PA
  #14  
Old September 12th 19, 09:28 PM posted to rec.audio.high-end
Trevor Wilson[_3_]
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Posts: 116
Default Introducing a New Horse to the Stable

On 12/09/2019 11:32 pm, Peter Wieck wrote:
> https://www.eeweb.com/tools/parallel-wire-inductance
>
> This website will allow you to calculate inductance by giving it the gauge, type and nature of the wire you are using. You will find, pretty quickly, that the distances involved in the typical component of say 40 cm (16") square are such that the actual inductance realized will be infinitesimal in "real life".


**Some of the products I work on are significantly larger than 40cm. One
(badly designed) amplifier, from a well-known 'high end' manufacturer,
used power supply wires which were approximately 60cm long! Whilst it
made servicing easier (allowing the output device heat sinks to be
removed from the amplifier and still be fully operational, the
additional inductance damaged the ability of the output stage to deliver
fast transients. A bunch of capacitance close to the output devices made
things much better.

>
> Again, getting to practical matters: there are common-sense applications and techniques for wiring electronics, much dependent on the nature and use intended. Part of my hobby is the restoration of vintage Zenith TransOceanic tube radios - and wire location/component location can be critical for high-band Short Wave sensitivity. It is common sense to shield power-supplies in pre-amplifiers, especially those that contain phono or NAB pre-amp sections. And, wire-dressing is always good practice. But worrying about straight-wire inductance at audio frequencies is much akin to worrying about skin-effect...'


**No. Skin effect is only a worry at RF and for power line companies,
where VERY long cables can lead to significant losses (hence the rising
popularity of DC transmission systems).


>
> Now, Trevor clearly has a 'thing' about negative feedback, which is entirely his choice, and doubtless for sufficient and good reasons.


**Let me be very clear about several things:

* NFB is fine. In fact, NO audio amplifier can work without it.
* GLOBAL NFB is also fine. When properly applied.
* I have a personal preference for the amplifiers I use, which employ
lots of local NFB and no global NFB. Others may have a different opinion.
* As part of my education into the world of zero global NFB amplifiers,
I subjected myself to a couple of single (unfortunately) blind tests,
between two, otherwise identical, amplifiers. One employed zero GNFB and
one employed a modest amount of GNFB. I preferred the zero GNFB one.
Since that time, I subjected several (10) of my clients to the same test
(DBT). The zero GNFB models was preferred every time. Except one.
* Once mo I would posit that part of the reason why some listeners
prefer valve amplifiers, is due to the fact that global NFB levels are
very low, or non-existent.


But, again, in the real world, negative feedback, done properly, has
many more advantages than disadvantages.

**Again: No issue with NFB. In fact, no issue with GNFB, when done well.


Done badly - Ouch! Keep in mind that in its most practical
application, it dates back to 1927, and was patented by Bell Labs in
1937. So, it is a pretty well established technique, such that any
thoughtful designer not totally strangled by the bean-counters will get
it right very nearly all of the time.

**Sure. I learned about GNFB back when I was a teenager, having just
built my second amplifier. A mighty 10 Watt/ch, push pull amp using 6V6
output valves. It had no GNFB. It also had a gain control before the
phase splitter. After reading an old article in a local electronics
magazine about NFB, I decided to try it. With the in-loop gain control,
I found I could vary the amount of NFB right up to the point of
oscillation. Backed off a fraction, I found that GNFB improved the sound
quality significantly.

>
> https://en.wikipedia.org/wiki/Negati...is_of_feedback
>
> Go down to the "distortion" section. As brief as it is, it conveys some very good information.


**Indeed. All good and well, but that does prompt the question as to why
your preference is for an old valve amp, which employs far less GNFB
than a typical SS amp?

>
> In point-of-fact, part of the TIP-Mod for your 120 involved increasing capacitance within the feedback loop to reduce bass roll-off.
>


**Huh?


--
Trevor Wilson
www.rageaudio.com.au

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  #15  
Old September 13th 19, 11:02 AM posted to rec.audio.high-end
~misfit~[_3_]
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Posts: 81
Default Introducing a New Horse to the Stable

On 12/09/2019 10:18 PM, Trevor Wilson wrote:
> On 12/09/2019 12:17 am, ~misfit~ wrote:
>> On 10/09/2019 11:54 PM, Peter Wieck wrote:
>>> OK, OK, I will bite! Minor rant to follow:
>>>
>>> Tube vs. Solid State on reliability:
>>>
>>> There are not so very many 60-year old components in operation these days unmodified since-new.
>>> My oldest tube item turned 100 this year and likely works better than when it was new based on a
>>> better understanding of antenna systems, optimum tube voltages and so forth. Other than moving
>>> parts (CD player), the newest component in my office system was made in 1963. The system runs 9
>>> hours per day, 5 days per week. Oh, and the tubes are original as well.
>>>
>>> On the other hand, and given my hobby, I see a large number of SS components that have blown
>>> transistors, exploded capacitors and much worse, irrespective of age and source. The well made,
>>> well designed stuff is serviceable, distinguishing it from the rest of the garbage out there.
>>>
>>> I would make a fairly apt comparison: A tube amplifier is much like a mid-last-century Mercedes
>>> or VW - few things were self-adjusting, and they required regular and attentive
>>> care-and-feeding. With such, they were good for several hundred thousand miles of reliable
>>> service. A contemporary Ford, Cadillac, Plymouth would be considered remarkable were it to
>>> survive 100,000 miles without heroic measures. Might run very nicely when running, but that
>>> would be your basic solid-state device in comparison.
>>>
>>> Put simply, they are different beasts designed with different things in mind, but for the same
>>> basic purpose. That one is or is not "BETTER" than the other is not relevant to the purpose in
>>> either case.
>>>
>>> Now, when I here things like "Zero global NFB" and "Critically matched components", I can smell
>>> the snake-oil from a great distance, even the 10,000 miles from here to Australia. I am sure
>>> that process also contains descriptives of "interconnects" rolled on the thighs of virgins on
>>> Walpurgis Night...
>>>
>>> Note that even "critically matched" solid-state components drift after a very short period of
>>> time in-service. All of them, such that that "less than 1%" is meaningful for perhaps 12 hours
>>> or so.
>>>
>>> Being as this is a hobby for me, I get to try things that are otherwise unproductive,
>>> unprofitable or impractical. Such as shotgunning a device with single-value capacitors and then
>>> comparing it to the same device with carefully screened and matched caps. Or matching driver and
>>> output transistors and comparing to a similar device with disparate values. Guys and gals - you
>>> would be seriously shocked to discover how little difference some things make that the
>>> ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no difference at all.

>>
>> Thanks for your input Peter. If I may ask, do you have an opinion on 'storage capacitors' on an
>> amplifier power supply? What in your opinion is 'better', a single (or few) very large caps or
>> multiple smaller caps to the same / similar capacitance?
>>
>> I have a long term project building my own amp based on PCBs taken from 100w MOSFET (two pairs of
>> J50 / K135 devices per amp) PA amps made by a New Zealand company in the 1980s. (Craft, Gary
>> Morrison's company before he went on to become head designer at Plinius until 2005 when he left
>> to set up Pure Audio). I got my hands on a rack of four of these mono amps and preliminary
>> testing using a clean source and good speakers suggest they will make a great stereo amp.
>>
>> I need to put together a power supply to feed two of these and have some new 10,000uF caps but
>> was wondering if multiple smaller caps would be better. (In the PA amps they only had 2,200uF but
>> obviously weren't called on to reproduce much bass.)
>>
>> As it is I'll be using fly leads from the rectifier PCB to the caps, then to the amps and I'm
>> building my own case. I was thinking of maybe using my 10,000uF caps as well as maybe some
>> smaller ones, perhaps 1,000 in a bank, the best of both worlds. (There are also 100uF electros
>> across the rails on the amp PCBs that I'll be replacing.) That said I could also just go to multiple
>>
>> Cheers,

>
> **Those old MOSFETs were pretty ordinary devices (not very linear). Evidenced by the fact that
> Plinius amps have always used BJTs. As Peter has stated, multiple small value caps will usually
> provide a superior, higher speed power supply. However, I would posit that those old MOSFETs are so
> horrible (modern MOSFETs are far superior), that it may not be worth the effort.


I hooked a pair of them up to a preamp while still using their original power supplies and was very
pleased with the sound so decided to go ahead with the build.

> Craft amps used
> huge amounts of global NFB, required due to very low bias currents and the necessity to reduce the
> huge levels of distortion caused by the 'knee' at low currents (A Class A, or high bias MOSFET amp
> would have been much better). Anyway, the huge levels of global NFB means that PSRR (Power Supply
> Rejection Ratio) will be quite high, thus the influence of power supply changes will be relatively
> small.


Unfortunately I don't own a 'scope so am unable to check a lot of stuff. When I listened to them
with the original power supplies (designed for PA use) they sounded sweet and clean at low and
moderate volume levels but seemed to run out of power at higher volumes, especially when there was
a lot of bass.

> One more thing: Decent amounts of capacitance placed close to the output devices is far more
> influential than caps placed some distance away. In fact, long(ish) cables AFTER the main filter
> caps can be a serious limiting factor on the effectiveness of a power supply in a Class A/B
> amplifier. This is because the inductance of the wires can be a factor.


Thanks. The fly-leads will only be 6" tops and I'll be using at least 1.5 square mm multistrand
copper conductors. If space allows I'll put a ~1,000uF cap right at the amplifier PCB as well (or
as large as I can get away with). I may end up building a wooden case as I don't have a suitable
metal one and wood's something I have experience and the tools for.

I still haven't finalised my design yet. I might end up feeding them a few more volts than they
were getting from their original power supplies (my only suitable toroidial transformer is 10v AC
higher than original) so may parallel up a third pair of output devices onto the heatsinks using
one of the other amps as a donor. I haven't decided yet, as I said it's a long-term project and I'm
learning as I go.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.
  #16  
Old September 14th 19, 02:58 PM posted to rec.audio.high-end
Trevor Wilson[_3_]
external usenet poster
 
Posts: 116
Default Introducing a New Horse to the Stable

On 13/09/2019 8:02 pm, ~misfit~ wrote:
> On 12/09/2019 10:18 PM, Trevor Wilson wrote:
>> On 12/09/2019 12:17 am, ~misfit~ wrote:
>>> On 10/09/2019 11:54 PM, Peter Wieck wrote:
>>>> OK, OK, I will bite! Minor rant to follow:
>>>>
>>>> Tube vs. Solid State on reliability:
>>>>
>>>> There are not so very many 60-year old components in operation these
>>>> days unmodified since-new. My oldest tube item turned 100 this year
>>>> and likely works better than when it was new based on a better
>>>> understanding of antenna systems, optimum tube voltages and so
>>>> forth. Other than moving parts (CD player), the newest component in
>>>> my office system was made in 1963. The system runs 9 hours per day,
>>>> 5 days per week. Oh, and the tubes are original as well.
>>>>
>>>> On the other hand, and given my hobby, I see a large number of SS
>>>> components that have blown transistors, exploded capacitors and much
>>>> worse, irrespective of age and source. The well made, well designed
>>>> stuff is serviceable, distinguishing it from the rest of the garbage
>>>> out there.
>>>>
>>>> I would make a fairly apt comparison: A tube amplifier is much like
>>>> a mid-last-century Mercedes or VW - few things were self-adjusting,
>>>> and they required regular and attentive care-and-feeding. With such,
>>>> they were good for several hundred thousand miles of reliable
>>>> service. A contemporary Ford, Cadillac, Plymouth would be considered
>>>> remarkable were it to survive 100,000 miles without heroic measures.
>>>> Might run very nicely when running, but that would be your basic
>>>> solid-state device in comparison.
>>>>
>>>> Put simply, they are different beasts designed with different things
>>>> in mind, but for the same basic purpose. That one is or is not
>>>> "BETTER" than the other is not relevant to the purpose in either case.
>>>>
>>>> Now, when I here things like "Zero global NFB" and "Critically
>>>> matched components", I can smell the snake-oil from a great
>>>> distance, even the 10,000 miles from here to Australia. I am sure
>>>> that process also contains descriptives of "interconnects" rolled on
>>>> the thighs of virgins on Walpurgis Night...
>>>>
>>>> Note that even "critically matched" solid-state components drift
>>>> after a very short period of time in-service. All of them, such that
>>>> that "less than 1%" is meaningful for perhaps 12 hours or so.
>>>>
>>>> Being as this is a hobby for me, I get to try things that are
>>>> otherwise unproductive, unprofitable or impractical. Such as
>>>> shotgunning a device with single-value capacitors and then comparing
>>>> it to the same device with carefully screened and matched caps. Or
>>>> matching driver and output transistors and comparing to a similar
>>>> device with disparate values. Guys and gals - you would be seriously
>>>> shocked to discover how little difference some things make that the
>>>> ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no
>>>> difference at all.
>>>
>>> Thanks for your input Peter. If I may ask, do you have an opinion on
>>> 'storage capacitors' on an amplifier power supply? What in your
>>> opinion is 'better', a single (or few) very large caps or multiple
>>> smaller caps to the same / similar capacitance?
>>>
>>> I have a long term project building my own amp based on PCBs taken
>>> from 100w MOSFET (two pairs of J50 / K135 devices per amp) PA amps
>>> made by a New Zealand company in the 1980s. (Craft, Gary Morrison's
>>> company before he went on to become head designer at Plinius until
>>> 2005 when he left to set up Pure Audio). I got my hands on a rack of
>>> four of these mono amps and preliminary testing using a clean source
>>> and good speakers suggest they will make a great stereo amp.
>>>
>>> I need to put together a power supply to feed two of these and have
>>> some new 10,000uF caps but was wondering if multiple smaller caps
>>> would be better. (In the PA amps they only had 2,200uF but obviously
>>> weren't called on to reproduce much bass.)
>>>
>>> As it is I'll be using fly leads from the rectifier PCB to the caps,
>>> then to the amps and I'm building my own case. I was thinking of
>>> maybe using my 10,000uF caps as well as maybe some smaller ones,
>>> perhaps 1,000 in a bank, the best of both worlds. (There are also
>>> 100uF electros across the rails on the amp PCBs that I'll be
>>> replacing.) That said I could also just go to multiple
>>>
>>> Cheers,

>>
>> **Those old MOSFETs were pretty ordinary devices (not very linear).
>> Evidenced by the fact that Plinius amps have always used BJTs. As
>> Peter has stated, multiple small value caps will usually provide a
>> superior, higher speed power supply. However, I would posit that those
>> old MOSFETs are so horrible (modern MOSFETs are far superior), that it
>> may not be worth the effort.

>
> I hooked a pair of them up to a preamp while still using their original
> power supplies and was very pleased with the sound so decided to go
> ahead with the build.


**I haven't listened to Craft (hi fi) amps in many years. What I heard
back then was pleasing. Very wide bandwidth (ca. 1MHz), as I recall.

>
>> Craft amps used huge amounts of global NFB, required due to very low
>> bias currents and the necessity to reduce the huge levels of
>> distortion caused by the 'knee' at low currents (A Class A, or high
>> bias MOSFET amp would have been much better). Anyway, the huge levels
>> of global NFB means that PSRR (Power Supply Rejection Ratio) will be
>> quite high, thus the influence of power supply changes will be
>> relatively small.

>
> Unfortunately I don't own a 'scope so am unable to check a lot of stuff.
> When I listened to them with the original power supplies (designed for
> PA use) they sounded sweet and clean at low and moderate volume levels
> but seemed to run out of power at higher volumes, especially when there
> was a lot of bass.


**That could be due to a number of factors. Including:

* Insufficient Voltage output.
* Insufficient current output.
* Insufficient power supply.
* An unreasonable speaker impedance.

Don't forget: Those meaty looking 2SJ50/2SK135 output devices are only
rated for a meagre 7 Amps each and 100 Watts PDiss. By comparison, a
typical output BJT of the same time period was rated at a far more
respectable 20 Amps and 200 Watts PDiss (MJ15003/MJ15004). Present
production variants are rated at 25 Amps and 250 Watts.

So, a little Ohm's Law should tell you if you are demanding more current
than the output devices are capable of delivering. 14 Amps is, by high
end audio standards, a relatively modest current ability for a (say) 100
Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
benign, you should be OK. Fortunately, it is real hard to damage
MOSFETs, by 'asking' them to deliver more current than they are rated for.


>
>> One more thing: Decent amounts of capacitance placed close to the
>> output devices is far more influential than caps placed some distance
>> away. In fact, long(ish) cables AFTER the main filter caps can be a
>> serious limiting factor on the effectiveness of a power supply in a
>> Class A/B amplifier. This is because the inductance of the wires can
>> be a factor.

>
> Thanks. The fly-leads will only be 6" tops and I'll be using at least
> 1.5 square mm multistrand copper conductors. If space allows I'll put a
> ~1,000uF cap right at the amplifier PCB as well (or as large as I can
> get away with). I may end up building a wooden case as I don't have a
> suitable metal one and wood's something I have experience and the tools
> for.


**Wiring sounds good. And yeah, caps placed close to output devices is a
very good thing. A wooden case, not so much. Wood is an excellent
thermal insulator, which means heat may not escape too easily.

>
> I still haven't finalised my design yet. I might end up feeding them a
> few more volts than they were getting from their original power supplies
> (my only suitable toroidial transformer is 10v AC higher than original)
> so may parallel up a third pair of output devices onto the heatsinks
> using one of the other amps as a donor. I haven't decided yet, as I said
> it's a long-term project and I'm learning as I go.


**Well, the MOSFETs are rated for a decent 160 Volts, so a few more rail
Volts should be OK. And yes, more output devices won't hurt (refer to
Ohm's Law as before). Pay attention to the drive capabilities of the
preceding stages though.


--
Trevor Wilson
www.rageaudio.com.au

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  #17  
Old September 15th 19, 10:49 PM posted to rec.audio.high-end
[email protected]
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Posts: 332
Default Introducing a New Horse to the Stable

On Saturday, September 14, 2019 at 9:58:44 AM UTC-4, Trevor Wilson wrote:
> So, a little Ohm's Law should tell you if you are demanding more current
> than the output devices are capable of delivering. 14 Amps is, by high
> end audio standards, a relatively modest current ability for a (say) 100
> Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
> benign, you should be OK.


Hmm, that's not what a little Ohm's law tells me.

100 Watts into 8 Ohms is a tad over 3.5 amps. Let's say it's a VERY
robust 100 watt amplifier, delivering 200 Watts into 4 Ohms requires
about 7 amps, and, let's pretend it has essentially ZERO output
impedance and an effectively limitless power supply, you're not reaching
14 amps until you're driving 400 watts into 2 ohms.

So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
presents a broadband 2 ohm impedance or, conversely, what kind of
musical content would generate that kind of power requirement over
the pretty narrow band of frequencies where a loudspeaker has the
kind of pathological impedance curve that would dip to as low as
2 ohms.

(Yes, there exist SOME rare examples of loudspeakers with
2 ohm impedance, but such are confined to a VERY narrow
band of frequencies)

Okay, let's pretend we have real examples of the above. Let's assume
such a speaker has a moderately low efficiency, say the equivalent
of, oh, 86 dB SPL/1W/1M. We're blowing in 400 watts that means the speaker
is putting out 112 dB 1 meter way, a stereo pair, assuming the two
channels are uncorrelated, that's 115 dB. Really? This is a serious
requirement?

But wait, you explicitly stated:

"Provided the driver impedance is relatively benign"

and you specified 8 Ohms. So let's assume it's a nominal 8 ohm
impedance 3-way speaker using at least a 2nd-order crossover
network. The impedance will be around 6.5 ohms below system
cutoff (DC resistance of woofer voice coil), will rise to
perhaps 30 Ohms at and around system cutoff, then drop down
to perhaps 15% above the DC resistance above there, start
rising again until the woofer-midrange crossover starts working,
maybe betting to 10-12 ohms, then dip to perhaps 60% of the rated
impedance, so about 4.8 ohms, rise again to about 12 ohms or so
at the mid-tweeter crossover point, drop down to about 10-15% above
the tweeter DC resistance (which, for the purpose of argument, we'll
take to be a nominal 4 Ohm tweeter, so about 4.5 Ohms, after which
it starts rising again.

So, minimum impedance of about 4.8 ohms will occur over perhaps
a 2-octave band around 1 kHz, then about 4.5 ohms around 5 kHz.

Let's take your 14 amps, produce a musical signal where ALL the energy
is concentrated from about 500-2000 Hz and from about 4000-8000 Hz
ALONE, and see what 14 Amps does.

Well, since

P = I^R

And we'll assume the impedance at these points is largely resistive,
which is is, then:

P = 14^2 * 4.5

882 watts. And to do that, the amplifier must be capable of outputting

E = I R

E = 14 * 4.5

63 volts RMS.

Really?

Oh, wait! Everyone knows that under transient conditions, the loudspeaker
impedance can actually go well below the lowest impedance of the
speaker for brief moments due to back EMF, Otala said so.

Oh, wait! Everyone who knows that is wrong and has yet to advance any
confirmed data sowing this to be the case and, by the way, Otala DID
NOT say so: he basically said that the peak current requirement under
actual transient conditions is exactly what is expected from the actual
measure steady state impedance, and the only thing he really said
that's even remotely like this is that the peak current requirements
are greater than predicted by the "nominal" impedance of the loudspeaker.

Give me a shovel, Mr. Ohm wants to go back to sleep.
  #18  
Old September 16th 19, 02:43 AM posted to rec.audio.high-end
~misfit~[_3_]
external usenet poster
 
Posts: 81
Default Introducing a New Horse to the Stable

On 15/09/2019 1:58 AM, Trevor Wilson wrote:
> On 13/09/2019 8:02 pm, ~misfit~ wrote:
>> On 12/09/2019 10:18 PM, Trevor Wilson wrote:
>>> On 12/09/2019 12:17 am, ~misfit~ wrote:
>>>> On 10/09/2019 11:54 PM, Peter Wieck wrote:
>>>>> OK, OK, I will bite! Minor rant to follow:
>>>>>
>>>>> Tube vs. Solid State on reliability:
>>>>>
>>>>> There are not so very many 60-year old components in operation these days unmodified
>>>>> since-new. My oldest tube item turned 100 this year and likely works better than when it was
>>>>> new based on a better understanding of antenna systems, optimum tube voltages and so forth.
>>>>> Other than moving parts (CD player), the newest component in my office system was made in
>>>>> 1963. The system runs 9 hours per day, 5 days per week. Oh, and the tubes are original as well.
>>>>>
>>>>> On the other hand, and given my hobby, I see a large number of SS components that have blown
>>>>> transistors, exploded capacitors and much worse, irrespective of age and source. The well
>>>>> made, well designed stuff is serviceable, distinguishing it from the rest of the garbage out
>>>>> there.
>>>>>
>>>>> I would make a fairly apt comparison: A tube amplifier is much like a mid-last-century
>>>>> Mercedes or VW - few things were self-adjusting, and they required regular and attentive
>>>>> care-and-feeding. With such, they were good for several hundred thousand miles of reliable
>>>>> service. A contemporary Ford, Cadillac, Plymouth would be considered remarkable were it to
>>>>> survive 100,000 miles without heroic measures. Might run very nicely when running, but that
>>>>> would be your basic solid-state device in comparison.
>>>>>
>>>>> Put simply, they are different beasts designed with different things in mind, but for the same
>>>>> basic purpose. That one is or is not "BETTER" than the other is not relevant to the purpose in
>>>>> either case.
>>>>>
>>>>> Now, when I here things like "Zero global NFB" and "Critically matched components", I can
>>>>> smell the snake-oil from a great distance, even the 10,000 miles from here to Australia. I am
>>>>> sure that process also contains descriptives of "interconnects" rolled on the thighs of
>>>>> virgins on Walpurgis Night...
>>>>>
>>>>> Note that even "critically matched" solid-state components drift after a very short period of
>>>>> time in-service. All of them, such that that "less than 1%" is meaningful for perhaps 12 hours
>>>>> or so.
>>>>>
>>>>> Being as this is a hobby for me, I get to try things that are otherwise unproductive,
>>>>> unprofitable or impractical. Such as shotgunning a device with single-value capacitors and
>>>>> then comparing it to the same device with carefully screened and matched caps. Or matching
>>>>> driver and output transistors and comparing to a similar device with disparate values. Guys
>>>>> and gals - you would be seriously shocked to discover how little difference some things make
>>>>> that the ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no difference at all.
>>>>
>>>> Thanks for your input Peter. If I may ask, do you have an opinion on 'storage capacitors' on an
>>>> amplifier power supply? What in your opinion is 'better', a single (or few) very large caps or
>>>> multiple smaller caps to the same / similar capacitance?
>>>>
>>>> I have a long term project building my own amp based on PCBs taken from 100w MOSFET (two pairs
>>>> of J50 / K135 devices per amp) PA amps made by a New Zealand company in the 1980s. (Craft, Gary
>>>> Morrison's company before he went on to become head designer at Plinius until 2005 when he left
>>>> to set up Pure Audio). I got my hands on a rack of four of these mono amps and preliminary
>>>> testing using a clean source and good speakers suggest they will make a great stereo amp.
>>>>
>>>> I need to put together a power supply to feed two of these and have some new 10,000uF caps but
>>>> was wondering if multiple smaller caps would be better. (In the PA amps they only had 2,200uF
>>>> but obviously weren't called on to reproduce much bass.)
>>>>
>>>> As it is I'll be using fly leads from the rectifier PCB to the caps, then to the amps and I'm
>>>> building my own case. I was thinking of maybe using my 10,000uF caps as well as maybe some
>>>> smaller ones, perhaps 1,000 in a bank, the best of both worlds. (There are also 100uF electros
>>>> across the rails on the amp PCBs that I'll be replacing.) That said I could also just go to
>>>> multiple
>>>>
>>>> Cheers,
>>>
>>> **Those old MOSFETs were pretty ordinary devices (not very linear). Evidenced by the fact that
>>> Plinius amps have always used BJTs. As Peter has stated, multiple small value caps will usually
>>> provide a superior, higher speed power supply. However, I would posit that those old MOSFETs are
>>> so horrible (modern MOSFETs are far superior), that it may not be worth the effort.

>>
>> I hooked a pair of them up to a preamp while still using their original power supplies and was
>> very pleased with the sound so decided to go ahead with the build.

>
> **I haven't listened to Craft (hi fi) amps in many years. What I heard back then was pleasing. Very
> wide bandwidth (ca. 1MHz), as I recall.
>
>>
>>> Craft amps used huge amounts of global NFB, required due to very low bias currents and the
>>> necessity to reduce the huge levels of distortion caused by the 'knee' at low currents (A Class
>>> A, or high bias MOSFET amp would have been much better). Anyway, the huge levels of global NFB
>>> means that PSRR (Power Supply Rejection Ratio) will be quite high, thus the influence of power
>>> supply changes will be relatively small.

>>
>> Unfortunately I don't own a 'scope so am unable to check a lot of stuff. When I listened to them
>> with the original power supplies (designed for PA use) they sounded sweet and clean at low and
>> moderate volume levels but seemed to run out of power at higher volumes, especially when there
>> was a lot of bass.

>
> **That could be due to a number of factors. Including:
>
> * Insufficient Voltage output.
> * Insufficient current output.
> * Insufficient power supply.
> * An unreasonable speaker impedance.
>
> Don't forget: Those meaty looking 2SJ50/2SK135 output devices are only rated for a meagre 7 Amps
> each and 100 Watts PDiss. By comparison, a typical output BJT of the same time period was rated at
> a far more respectable 20 Amps and 200 Watts PDiss (MJ15003/MJ15004). Present production variants
> are rated at 25 Amps and 250 Watts.


So three pairs per side should be fine for a reasonably powerful amp? I've studied the PCB and the
output devices are paralleled (along with a resistor for each) so it wouldn't be hard to add a
third device to each (on very short flyleads - or even daughterboards - mounted to the same heatsink).

The speakers I'm intending to use with this are Sony SS-K90EDs.
Like these:
<https://www.stereo.net.au/forums/topic/260972-fs-sony-ss-k90ed-speakers-rare/>

> So, a little Ohm's Law should tell you if you are demanding more current than the output devices
> are capable of delivering. 14 Amps is, by high end audio standards, a relatively modest current
> ability for a (say) 100 Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
> benign, you should be OK. Fortunately, it is real hard to damage MOSFETs, by 'asking' them to
> deliver more current than they are rated for.


That's one of the things I like about MOSFETs.

>>> One more thing: Decent amounts of capacitance placed close to the output devices is far more
>>> influential than caps placed some distance away. In fact, long(ish) cables AFTER the main filter
>>> caps can be a serious limiting factor on the effectiveness of a power supply in a Class A/B
>>> amplifier. This is because the inductance of the wires can be a factor.

>>
>> Thanks. The fly-leads will only be 6" tops and I'll be using at least 1.5 square mm multistrand
>> copper conductors. If space allows I'll put a ~1,000uF cap right at the amplifier PCB as well (or
>> as large as I can get away with). I may end up building a wooden case as I don't have a suitable
>> metal one and wood's something I have experience and the tools for.

>
> **Wiring sounds good. And yeah, caps placed close to output devices is a very good thing. A wooden
> case, not so much. Wood is an excellent thermal insulator, which means heat may not escape too easily.


I have a couple of big heatsinks for the amplifier modules that will sit either side of the case,
fins outwards in free air. They'll easily handle the power dissipation being 4x bigger than the
'sinks used on the PA amp. Also I'll ventilate the top and bottom of the 'box' (if I end up going
with wood).

>> I still haven't finalised my design yet. I might end up feeding them a few more volts than they
>> were getting from their original power supplies (my only suitable toroidial transformer is 10v AC
>> higher than original) so may parallel up a third pair of output devices onto the heatsinks using
>> one of the other amps as a donor. I haven't decided yet, as I said it's a long-term project and
>> I'm learning as I go.

>
> **Well, the MOSFETs are rated for a decent 160 Volts, so a few more rail Volts should be OK. And
> yes, more output devices won't hurt (refer to Ohm's Law as before). Pay attention to the drive
> capabilities of the preceding stages though.


Thanks for this Trevor, I have saved it for future reference. My 300 VA toroid that I'm thinking of
using with this outputs 50v AC so +/- 70v DC when rectified. The original PA transformers were 40v AC.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.
  #19  
Old September 16th 19, 10:52 AM posted to rec.audio.high-end
Trevor Wilson[_3_]
external usenet poster
 
Posts: 116
Default Introducing a New Horse to the Stable

On 16/09/2019 7:49 am, wrote:
> On Saturday, September 14, 2019 at 9:58:44 AM UTC-4, Trevor Wilson wrote:
>> So, a little Ohm's Law should tell you if you are demanding more current
>> than the output devices are capable of delivering. 14 Amps is, by high
>> end audio standards, a relatively modest current ability for a (say) 100
>> Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
>> benign, you should be OK.

>
> Hmm, that's not what a little Ohm's law tells me.
>
> 100 Watts into 8 Ohms is a tad over 3.5 amps. Let's say it's a VERY
> robust 100 watt amplifier, delivering 200 Watts into 4 Ohms requires
> about 7 amps, and, let's pretend it has essentially ZERO output
> impedance and an effectively limitless power supply, you're not reaching
> 14 amps until you're driving 400 watts into 2 ohms.


**Well, no. The RMS current is certainly 3.5 Amps, but output devices
only 'care' about PEAK currents. The peak current is, of course, 3.5 X
1.414 ~ 5 Amps.

With a 4 Ohm load, the peak current required is 10 Amps. For 2 Ohms, it
is 20 Amps.

Assuming a 100 Watt amp. For a (say) 200 Watt amp, those peak current
figures become 7 Amps, 14 Amps and 28 Amps respectively. WAY past the
ability of two pairs of old Hitachi MOSFETs to deal with.

>
> So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
> presents a broadband 2 ohm impedance or, conversely, what kind of
> musical content would generate that kind of power requirement over
> the pretty narrow band of frequencies where a loudspeaker has the
> kind of pathological impedance curve that would dip to as low as
> 2 ohms.


**I have a few here that are tougher than that. Some of the Peerless
XXLS drivers dip to the low 2 Ohm region. Most ESLs fall lower than that
at HF.

>
> (Yes, there exist SOME rare examples of loudspeakers with
> 2 ohm impedance, but such are confined to a VERY narrow
> band of frequencies)


**If those frequencies happen to be in an area where the amplifier is
required to deliver a lot of power, then it matters a lot. In the bass,
for instance.

>
> Okay, let's pretend we have real examples of the above. Let's assume
> such a speaker has a moderately low efficiency, say the equivalent
> of, oh, 86 dB SPL/1W/1M. We're blowing in 400 watts that means the speaker
> is putting out 112 dB 1 meter way, a stereo pair, assuming the two
> channels are uncorrelated, that's 115 dB. Really? This is a serious
> requirement?
>
> But wait, you explicitly stated:
>
> "Provided the driver impedance is relatively benign"
>
> and you specified 8 Ohms. So let's assume it's a nominal 8 ohm
> impedance 3-way speaker using at least a 2nd-order crossover
> network. The impedance will be around 6.5 ohms below system
> cutoff (DC resistance of woofer voice coil), will rise to
> perhaps 30 Ohms at and around system cutoff, then drop down
> to perhaps 15% above the DC resistance above there, start
> rising again until the woofer-midrange crossover starts working,
> maybe betting to 10-12 ohms, then dip to perhaps 60% of the rated
> impedance, so about 4.8 ohms, rise again to about 12 ohms or so
> at the mid-tweeter crossover point, drop down to about 10-15% above
> the tweeter DC resistance (which, for the purpose of argument, we'll
> take to be a nominal 4 Ohm tweeter, so about 4.5 Ohms, after which
> it starts rising again.
>
> So, minimum impedance of about 4.8 ohms will occur over perhaps
> a 2-octave band around 1 kHz, then about 4.5 ohms around 5 kHz.
>
> Let's take your 14 amps, produce a musical signal where ALL the energy
> is concentrated from about 500-2000 Hz and from about 4000-8000 Hz
> ALONE, and see what 14 Amps does.
>
> Well, since
>
> P = I^R
>
> And we'll assume the impedance at these points is largely resistive,
> which is is, then:
>
> P = 14^2 * 4.5
>
> 882 watts. And to do that, the amplifier must be capable of outputting
>
> E = I R
>
> E = 14 * 4.5
>
> 63 volts RMS.
>
> Really?
>
> Oh, wait! Everyone knows that under transient conditions, the loudspeaker
> impedance can actually go well below the lowest impedance of the
> speaker for brief moments due to back EMF, Otala said so.


**I pay no attention to such things. I just look at the actual
impedance/phase angle curves. Like the ones at the end of this post.

>
> Oh, wait! Everyone who knows that is wrong and has yet to advance any
> confirmed data sowing this to be the case and, by the way, Otala DID
> NOT say so: he basically said that the peak current requirement under
> actual transient conditions is exactly what is expected from the actual
> measure steady state impedance, and the only thing he really said
> that's even remotely like this is that the peak current requirements
> are greater than predicted by the "nominal" impedance of the loudspeaker.
>
> Give me a shovel, Mr. Ohm wants to go back to sleep.
>


**Here are some real-world speaker impedance plots. All have parts of
their curve below 4 Ohms. Most of them are speakers I have personal
experience with. I owned a pair of Westlakes for awhile and I have a
pair of Martin Logan Quest Z as a workshop system. Both are tough loads
for any amplifier.

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

https://www.stereophile.com/content/...r-measurements

And, of course, the pathological loads I measured some years ago:

http://www.rageaudio.com.au/index.php?p=1_12


--
Trevor Wilson
www.rageaudio.com.au

---
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  #20  
Old September 16th 19, 10:53 AM posted to rec.audio.high-end
Trevor Wilson[_3_]
external usenet poster
 
Posts: 116
Default Introducing a New Horse to the Stable

On 16/09/2019 11:43 am, ~misfit~ wrote:
> On 15/09/2019 1:58 AM, Trevor Wilson wrote:
>> On 13/09/2019 8:02 pm, ~misfit~ wrote:
>>> On 12/09/2019 10:18 PM, Trevor Wilson wrote:
>>>> On 12/09/2019 12:17 am, ~misfit~ wrote:
>>>>> On 10/09/2019 11:54 PM, Peter Wieck wrote:
>>>>>> OK, OK, I will bite! Minor rant to follow:
>>>>>>
>>>>>> Tube vs. Solid State on reliability:
>>>>>>
>>>>>> There are not so very many 60-year old components in operation
>>>>>> these days unmodified since-new. My oldest tube item turned 100
>>>>>> this year and likely works better than when it was new based on a
>>>>>> better understanding of antenna systems, optimum tube voltages and
>>>>>> so forth. Other than moving parts (CD player), the newest
>>>>>> component in my office system was made in 1963. The system runs 9
>>>>>> hours per day, 5 days per week. Oh, and the tubes are original as
>>>>>> well.
>>>>>>
>>>>>> On the other hand, and given my hobby, I see a large number of SS
>>>>>> components that have blown transistors, exploded capacitors and
>>>>>> much worse, irrespective of age and source. The well made, well
>>>>>> designed stuff is serviceable, distinguishing it from the rest of
>>>>>> the garbage out there.
>>>>>>
>>>>>> I would make a fairly apt comparison: A tube amplifier is much
>>>>>> like a mid-last-century Mercedes or VW - few things were
>>>>>> self-adjusting, and they required regular and attentive
>>>>>> care-and-feeding. With such, they were good for several hundred
>>>>>> thousand miles of reliable service. A contemporary Ford, Cadillac,
>>>>>> Plymouth would be considered remarkable were it to survive 100,000
>>>>>> miles without heroic measures. Might run very nicely when running,
>>>>>> but that would be your basic solid-state device in comparison.
>>>>>>
>>>>>> Put simply, they are different beasts designed with different
>>>>>> things in mind, but for the same basic purpose. That one is or is
>>>>>> not "BETTER" than the other is not relevant to the purpose in
>>>>>> either case.
>>>>>>
>>>>>> Now, when I here things like "Zero global NFB" and "Critically
>>>>>> matched components", I can smell the snake-oil from a great
>>>>>> distance, even the 10,000 miles from here to Australia. I am sure
>>>>>> that process also contains descriptives of "interconnects" rolled
>>>>>> on the thighs of virgins on Walpurgis Night...
>>>>>>
>>>>>> Note that even "critically matched" solid-state components drift
>>>>>> after a very short period of time in-service. All of them, such
>>>>>> that that "less than 1%" is meaningful for perhaps 12 hours or so.
>>>>>>
>>>>>> Being as this is a hobby for me, I get to try things that are
>>>>>> otherwise unproductive, unprofitable or impractical. Such as
>>>>>> shotgunning a device with single-value capacitors and then
>>>>>> comparing it to the same device with carefully screened and
>>>>>> matched caps. Or matching driver and output transistors and
>>>>>> comparing to a similar device with disparate values. Guys and gals
>>>>>> - you would be seriously shocked to discover how little difference
>>>>>> some things make that the ALL-SEEING, ALL-KNOWING gurus will tell
>>>>>> you are critical. Often no difference at all.
>>>>>
>>>>> Thanks for your input Peter. If I may ask, do you have an opinion
>>>>> on 'storage capacitors' on an amplifier power supply? What in your
>>>>> opinion is 'better', a single (or few) very large caps or multiple
>>>>> smaller caps to the same / similar capacitance?
>>>>>
>>>>> I have a long term project building my own amp based on PCBs taken
>>>>> from 100w MOSFET (two pairs of J50 / K135 devices per amp) PA amps
>>>>> made by a New Zealand company in the 1980s. (Craft, Gary Morrison's
>>>>> company before he went on to become head designer at Plinius until
>>>>> 2005 when he left to set up Pure Audio). I got my hands on a rack
>>>>> of four of these mono amps and preliminary testing using a clean
>>>>> source and good speakers suggest they will make a great stereo amp.
>>>>>
>>>>> I need to put together a power supply to feed two of these and have
>>>>> some new 10,000uF caps but was wondering if multiple smaller caps
>>>>> would be better. (In the PA amps they only had 2,200uF but
>>>>> obviously weren't called on to reproduce much bass.)
>>>>>
>>>>> As it is I'll be using fly leads from the rectifier PCB to the
>>>>> caps, then to the amps and I'm building my own case. I was thinking
>>>>> of maybe using my 10,000uF caps as well as maybe some smaller ones,
>>>>> perhaps 1,000 in a bank, the best of both worlds. (There are also
>>>>> 100uF electros across the rails on the amp PCBs that I'll be
>>>>> replacing.) That said I could also just go to multiple
>>>>>
>>>>> Cheers,
>>>>
>>>> **Those old MOSFETs were pretty ordinary devices (not very linear).
>>>> Evidenced by the fact that Plinius amps have always used BJTs. As
>>>> Peter has stated, multiple small value caps will usually provide a
>>>> superior, higher speed power supply. However, I would posit that
>>>> those old MOSFETs are so horrible (modern MOSFETs are far superior),
>>>> that it may not be worth the effort.
>>>
>>> I hooked a pair of them up to a preamp while still using their
>>> original power supplies and was very pleased with the sound so
>>> decided to go ahead with the build.

>>
>> **I haven't listened to Craft (hi fi) amps in many years. What I heard
>> back then was pleasing. Very wide bandwidth (ca. 1MHz), as I recall.
>>
>>>
>>>> Craft amps used huge amounts of global NFB, required due to very low
>>>> bias currents and the necessity to reduce the huge levels of
>>>> distortion caused by the 'knee' at low currents (A Class A, or high
>>>> bias MOSFET amp would have been much better). Anyway, the huge
>>>> levels of global NFB means that PSRR (Power Supply Rejection Ratio)
>>>> will be quite high, thus the influence of power supply changes will
>>>> be relatively small.
>>>
>>> Unfortunately I don't own a 'scope so am unable to check a lot of
>>> stuff. When I listened to them with the original power supplies
>>> (designed for PA use) they sounded sweet and clean at low and
>>> moderate volume levels but seemed to run out of power at higher
>>> volumes, especially when there was a lot of bass.

>>
>> **That could be due to a number of factors. Including:
>>
>> * Insufficient Voltage output.
>> * Insufficient current output.
>> * Insufficient power supply.
>> * An unreasonable speaker impedance.
>>
>> Don't forget: Those meaty looking 2SJ50/2SK135 output devices are only
>> rated for a meagre 7 Amps each and 100 Watts PDiss. By comparison, a
>> typical output BJT of the same time period was rated at a far more
>> respectable 20 Amps and 200 Watts PDiss (MJ15003/MJ15004). Present
>> production variants are rated at 25 Amps and 250 Watts.

>
> So three pairs per side should be fine for a reasonably powerful amp?


**Again: It depends on the maximum Voltage output. 3 pairs allows for a
peak current ability of 21 Amps.

> I've studied the PCB and the output devices are paralleled (along with a
> resistor for each) so it wouldn't be hard to add a third device to each
> (on very short flyleads - or even daughterboards - mounted to the same
> heatsink).


**Sure. However, make certain the drive circuitry can cope.

>
> The speakers I'm intending to use with this are Sony SS-K90EDs.
> Like these:
> <https://www.stereo.net.au/forums/topic/260972-fs-sony-ss-k90ed-speakers-rare/>


**OK.

>
>
>> So, a little Ohm's Law should tell you if you are demanding more
>> current than the output devices are capable of delivering. 14 Amps is,
>> by high end audio standards, a relatively modest current ability for a
>> (say) 100 Watt @ 8 Ohms amplifier. Provided the driver impedance is
>> relatively benign, you should be OK. Fortunately, it is real hard to
>> damage MOSFETs, by 'asking' them to deliver more current than they are
>> rated for.

>
> That's one of the things I like about MOSFETs.


**Well, a properly designed BJT amp should demonstrate the same
robustness and reliability.

>
>>>> One more thing: Decent amounts of capacitance placed close to the
>>>> output devices is far more influential than caps placed some
>>>> distance away. In fact, long(ish) cables AFTER the main filter caps
>>>> can be a serious limiting factor on the effectiveness of a power
>>>> supply in a Class A/B amplifier. This is because the inductance of
>>>> the wires can be a factor.
>>>
>>> Thanks. The fly-leads will only be 6" tops and I'll be using at least
>>> 1.5 square mm multistrand copper conductors. If space allows I'll put
>>> a ~1,000uF cap right at the amplifier PCB as well (or as large as I
>>> can get away with). I may end up building a wooden case as I don't
>>> have a suitable metal one and wood's something I have experience and
>>> the tools for.

>>
>> **Wiring sounds good. And yeah, caps placed close to output devices is
>> a very good thing. A wooden case, not so much. Wood is an excellent
>> thermal insulator, which means heat may not escape too easily.

>
> I have a couple of big heatsinks for the amplifier modules that will sit
> either side of the case, fins outwards in free air. They'll easily
> handle the power dissipation being 4x bigger than the 'sinks used on the
> PA amp. Also I'll ventilate the top and bottom of the 'box' (if I end up
> going with wood).


**OK.

>
>>> I still haven't finalised my design yet. I might end up feeding them
>>> a few more volts than they were getting from their original power
>>> supplies (my only suitable toroidial transformer is 10v AC higher
>>> than original) so may parallel up a third pair of output devices onto
>>> the heatsinks using one of the other amps as a donor. I haven't
>>> decided yet, as I said it's a long-term project and I'm learning as I
>>> go.

>>
>> **Well, the MOSFETs are rated for a decent 160 Volts, so a few more
>> rail Volts should be OK. And yes, more output devices won't hurt
>> (refer to Ohm's Law as before). Pay attention to the drive
>> capabilities of the preceding stages though.

>
> Thanks for this Trevor, I have saved it for future reference. My 300 VA
> toroid that I'm thinking of using with this outputs 50v AC so +/- 70v DC
> when rectified. The original PA transformers were 40v AC.


**+/- 70VDC suggests a maximum power output of around 250 Watts @ 8
Ohms. If you plan on attempting to obtain that much power
(continuously), then you will need two of those toroids.


--
Trevor Wilson
www.rageaudio.com.au

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