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-   -   Bob Cordell Interview: Power Supplies (http://www.diyaudio.com/forums/solid-state/90728-bob-cordell-interview-power-supplies.html)

Bob Cordell 18th November 2006 10:20 PM

Power supply quality
 
Here's a new topic to consider: power supply quality.

We constantly hear about people installing Black Gate capacitors, or Shottky diodes, or better bypasses in power supplies and getting startling sonic improvements. Often, we hear about better bass. I believe in putting good power supplies in power amplifiers, but often I've wondered what the mechanism is for improvement in sonics.

Sure, the power supply sags during a bass transient. Sure, we want to keep EMI off the supply lines. But if the rails don't sag to the point where the output stage doesn't clip or run out of headroom, why should there be a difference? After all, source-follower and emitter follower output stages have quite good power supply rejection. If we can take the amp up to nearly full power into a 4-ohm load without seeing any 120 Hz artifacts on the output, why should there be a sonic effect?

But there is a lot of anecdotal evidence that there is. Is this just a mystery of life in audio, or is there a mechanism to explain this?

Maybe the quality of the supply and its variations and stuff on it matter to the earlier stages in the amplifier, like the input stage and the VAS. But these stages require much less current, and it should be much easier to filter them or even regulate them. So if we just supply the earlier stages with exceptionally clean and stable rails, will we no longer see a big benefit from improving the main rails beyond stock? Maybe otherwise-very-high-quality amplifiers just don't pay enough attention to the rails for the input and driver stages?

I don't know. What do you guys think?

Bob

AJT 18th November 2006 11:34 PM

Hi Bob,

thanks for your previous responses to my questions, you sure nailed them...now i am glad you opended up the topic about power supplies, i have had this suspicion that commercial amps power supplies are undesized. the manufacturer hoped that the end users will not play it at full volume all of the time.

just like car makers with their cars that are capable of running at 160km/hours, many of them hope the owners run them at speed limits of say 60 to 80 km.hr...

so for starters, let us say i have this power amp rated for 100watts per channel both channels running, what size power transformer would you specify?

AKSA 19th November 2006 12:06 AM

Bob,

I've found there are gains to be had with superior power supply caps. I use the Nichicon Gold Tune, which are made to order. Nice caps with excellent bass.

However, topology must play a part. These caps have two purposes; one to smooth the power pulses coming off the rectifier (and these are pretty savage), and the other to accommodate speaker earth return current. If both these currents coexist in the same cap, then intermodulation results, so I like to use two caps per rail, separate them with a small resistor (typically 0R22, or even a tiny inductor around half a millihenry) so that the two currents are separated.

Cheers,

Hugh

lumanauw 19th November 2006 01:46 AM

Hi, Bob,

Good topic :D In AudioExpress magazine, once there is an article by Jean Hiraga "The many faces of distortion" that is translated by our Janneman.
In that article, Jean Hiraga "listens" to power supply, to see the effect on audio reproduction.

I found 1 interesting "anomaly". While some designers are putting bulky cap (10's or 100's of thousands of uF), there is some designer that think that put minimal cap in power supply bank gives the best sound. Japanese Gainclone and Venture tube power amp likes to put small (only 1000uf or below) for supply bank. I heard that Venture desinger opinioned that big capacitance in supply rail gives "unclear/muddy" sound.

john curl 19th November 2006 06:17 AM

I agree, Hugh.

gerhard 19th November 2006 09:57 AM

Quote:

Originally posted by lumanauw
In that article, Jean Hiraga "listens" to power supply, to see the effect on audio reproduction.

I found 1 interesting "anomaly". While some designers are putting bulky cap (10's or 100's of thousands of uF), there is some designer that think that put minimal cap in power supply bank gives the best sound. Japanese Gainclone and Venture tube power amp likes to put small (only 1000uf or below) for supply bank. I heard that Venture desinger opinioned that big capacitance in supply rail gives "unclear/muddy" sound.

Isn't Hiraga the guy who brought us cable burn-in?

Just providing DC with no hum and a Zsource aproaching 0 is one of the easier tasks when designing an amp.
If one cannot do that, one is not ready to design the rest.
If a little bit of noise/hum is left over, the PSRR of the amp should be able to take care of it.
OTOH, some amps don't feature much rejection, usually because their makers don't understand grounding and/or feedback.

For a typical tube amp with Vplate of several hundred Volts, 1mF should be enough. Stored energy goes up with the square of the voltage.

48Lab with their LossCard, well, that's just a tightfisted copy of a low cost TV audio amp test circuit.
The only optimization is maximizing of the price-detriment product.
Yes, it features a tight feedback loop. Only 9 millimeters. Cargo Cult.

Disclaimer: My opinion only. Some names have been changed to protect the innocent (me :-) ).

Gerhard

Tube_Dude 19th November 2006 10:53 AM

Quote:

Originally posted by gerhard


For a typical tube amp with Vplate of several hundred Volts, 1mF should be enough. Stored energy goes up with the square of the voltage.


Hi

If that is obviously true from a energy point of vue , we must not forget thet the rail capacitor is the path that close the signal to ground.

So this parasitic series impedance must be the minimum that can be practically implemented...

Ideally it must be 0 Ohms at all signal frequencies...;)

MikeB 19th November 2006 11:01 AM

Re: Power supply quality
 
Quote:

Originally posted by Bob Cordell
Here's a new topic to consider: power supply quality.

...

Maybe the quality of the supply and its variations and stuff on it matter to the earlier stages in the amplifier, like the input stage and the VAS. But these stages require much less current, and it should be much easier to filter them or even regulate them. So if we just supply the earlier stages with exceptionally clean and stable rails, will we no longer see a big benefit from improving the main rails beyond stock? Maybe otherwise-very-high-quality amplifiers just don't pay enough attention to the rails for the input and driver stages?

I don't know. What do you guys think?

Bob


Bob, i think that this issue is one of the most underestimated ones in a lot of amps.
Even the outputstage (EF/SF) is quite sensible to variations from the supply lines because of the highish outputcapacitance (C <-> B) of power devices. Especially for higher freqs psrr degrades quickly. Because of this, EMI and other RF stuff from the powersupply easily find its way to the output / into the feedbackloop. With more outputdevices this effect is even more increased.

For the earlier stages you can use some simple RC-filtering in the supplies, filtering out most of the garbage. I always found instant increase in sonic qualities by applying these filters to the early stages.

In ClassAB/B amps, the supply rails are severely wasted with harmonics from the halfwave currents flowing into the outputdevices. Especially when reproducing treble content, the harmonics of these halfwaves become very high freq and difficult to filter because of "bad" ESR from the elyt caps and inductance of traces and wiring.

An easy way reducing this problem seems to be placing a maybe 1000F between the collectors of the upper and lower outputdevices with short traces. (smoothing the halfwave currents to mostly 2nd harmonic content) Still, as the fundamental of these half wave currents is double the frequency of the signal reproduced, it might be harmful letting these find back into the feedbackloop. This can also find its way back through inproper grounding.

Paying enough attention to these problems seems to be one of the keys to a good sounding power amp.

A difficult question: How much psrr is enough ? -90db ?

Mike

lumanauw 19th November 2006 01:45 PM

Hi, Gerhard,

Quote:

Just providing DC with no hum and a Zsource aproaching 0 is one of the easier tasks when designing an amp.
If one cannot do that, one is not ready to design the rest.
How to do that (0 Zsource)? Using overspec power transformer + 10's or 100's uf cap bank + many parrarel rectifier diodes?

Quote:

48Lab with their LossCard
:D

Gerhard, sometimes ago I tried an experiment. Comparing GainClone, with ordinary PS and with car batteries. Gainclone+car batteries have better audible sonics.
(But you won't notice this if you don't make instant A/B switch comparison)

What do you think causes it?

Bob Cordell 19th November 2006 06:05 PM

Quote:

Originally posted by AKSA
Bob,

I've found there are gains to be had with superior power supply caps. I use the Nichicon Gold Tune, which are made to order. Nice caps with excellent bass.

However, topology must play a part. These caps have two purposes; one to smooth the power pulses coming off the rectifier (and these are pretty savage), and the other to accommodate speaker earth return current. If both these currents coexist in the same cap, then intermodulation results, so I like to use two caps per rail, separate them with a small resistor (typically 0R22, or even a tiny inductor around half a millihenry) so that the two currents are separated.

Cheers,

Hugh


Thanks, Hugh

I agree completely. I also subscribe to the idea of inserting a little bit of impedance between two rail capacitors, as you mentioned. Much more bang for the filtering buck. Sometimes spending a lot of extra money on heroically low ESR is not the best approach.

Take a guy who has decided to provide 20,000 uF per rail. He can do it one of three ways, at fairly similar cost. He can put in one big 20,000 uF cap; or he can parallel a pair of smaller 10,000 uF caps; or, he can put in two 10,000 uF caps in parallel, but separated by a 0.22 ohm resistor. If you do the ESR math, the last approach is about 10 times better at filtering the nasty stuff, although total energy storage and p-p 120 Hz ripple are similar in all three cases. Of course, it is even better if the second cap is paralleled by some good-sized film caps, and maybe located very close to the output transistors, but this will increase cost a bit.

I also believe it is very important to be intentional about where the big currents flow, as they are nasty.

But I still suspect that a lot of designers overlook the bang for the buck available in making the lower-current supplies to the input and VAS stages more pristine.

If someone does an upgrade and doubles the size of the main caps, and hears better bass under normal listening conditions (i.e., not coming close to clipping), I'm guessing the effect he is hearing is better operation of the input and VAS stages because the supply they receive is moving around at low and subsonic frequencies only half as much, suggesting that they could have done just as well, if not better, by paying attention to the filtering (or regulation) of those lower-current sugnals.

Of course, the ordinary upgrader often doesn't have the option or desire to do much more than add/change existing capacitance. If that was an expensive amplifier, I would think that one should question why the designer didn't make a better power supply filtering arrangement in the first place.

Anyway, I am certainly an advocate of good power supplies, but I'm always curious about what works best, or is most efficient at making an improvement for a given $ spent.

Bob


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