Bob Cordell Interview: Power Supplies - Page 2 - diyAudio
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Old 19th November 2006, 06:43 PM   #11
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Quote:
Originally posted by Tony
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?

Your points are right on the money. Unfortunately, the answer to how many VA a power transformer should have for a given size power amplifier is not an easy one to answer, at least for me.

I could just say the bigger the better, but that would be a cop out. We all live in a world where the $ are constrained, even the Halcros and the Boulders. Do you spend the next $ on a bigger filter capacitor, or more output transistors, or a bigger heat sink, or a thicker, more polished front panel? This is what's called engineering (or a witche's brew between that and marketing).

Also, VA doesn't always mean the same thing from transformer maker to transformer maker or type to type. The biggest thing we think of is power supply stiffness, but we also must consider transformer heating, EMI radiation, and physical noise. Even stiffer is better is not always the choice, since this affects the difference between continuous power and peak power, i.e., dynamic headroom.

My criteria tend to be as follows. First, if you run the amplifier at full power into 4 ohms, both channels, for an extended period, the transformer temperature shall not exceed 50C. Secondly, if the amplifier delivers 100 watts continuous at clipping into 8 ohms both channels driven, it should deliver at least 160 watts per channel into four ohms (a perfect supply, assuming no current limiting in the output stage, would give you 200 wpc into four ohms). Some would call these criteria overly conservative, others would call them just the opposite. Relating advertized VA to achieving these criteria is difficult, at least for me. Recognize that with the usual capacitor input filter, the peak currents are extremely high, so the transformer's ability to source these very high, brief peaks is what tends to affect stiffness and regulation the most.

Hope this helps. There may be others here that can provide more concrete rules of thumb.

Bob
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Old 19th November 2006, 07:25 PM   #12
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Quote:
Originally posted by Bob Cordell
Also, VA doesn't always mean the same thing from transformer maker to transformer maker or type to type.
In Europe it's usually 105 deg C core temp at 40 or 70 deg ambient temp.
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Old 19th November 2006, 08:45 PM   #13
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Originally posted by peranders

In Europe it's usually 105 deg C core temp at 40 or 70 deg ambient temp.

Thanks for this information, P-A.

I'm certainly not a transformer expert, but I would assume that these ratings are arrived at by loading the transformer with a resistive load, as opposed to a rectifier feeding a capacitor input filter. Am I right? If this is the case, the VA rating may not be a very good indicator of regulation and stiffness in a typical amplifier power supply application.

Another thing. If the VA rating is primarily an indicator of maximum core temperature, then different transformers of the same VA rating could have vastly different useable d.c. rectified current capabilities and regulation in the real world. For example, copper is now very expensive. A transformer manufacturer could probably arrive at the same thermal VA rating by using less copper and more iron, and maybe arranging that iron to dissipate heat better. The resulting transformer would have significantly more winding resistance and would not have as good regulation, even though it had the same VA.

So I guess what I'm saying is that VA doesn't do a very good job of telling the whole story.

Bob
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Old 19th November 2006, 09:13 PM   #14
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Yes, the load is a resistance so if you have smoothing caps the VA rating will be halfed and normal sized (for audio use) somewhere in between 1.0 to 0.5.
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Old 19th November 2006, 09:46 PM   #15
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Default Re: Re: Power supply quality

Quote:
Originally posted by MikeB



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 1000µF 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
I agree with everything you have said, Mike. Those Class-AB half-wave output transistor currents are especially toublesome in regard to their radiated magnetic field. Closing those current paths right close to the output stage, with good ESR out to high frequencies, and also with attention to any needed damping to prevent resonances (snubber networks) is very important. Of course, this is a much bigger deal than just sticking in a couple of expensive Black Gate capacitors in the main power supply. I guess the point is that good engineering more often overcomes blind brute force.

I have also used the X capacitor, most recently in my Super Gain Clone amplifier. Good supply treatment is especially important in IC power amplifiers because in those we don't usually have the option of putting extra filtering in for the input and VAS stages.

Bob
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Old 19th November 2006, 09:47 PM   #16
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35 degrees Celsium overheating, or 10 percent voltage drop, for power transformers. I don't know about American or Chinese rulrs, they may be different.
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Old 19th November 2006, 10:01 PM   #17
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Quote:
Originally posted by janneman




Note to moderators: Maybe the power supply discussion as initiated by Bob should be split off in a separate thread?

Jan Didden

Jan,

I don't think the power supply discussion is worthy of splitting off. I think its useful to be able to move around within a larger topic as long as it doesn't get out of control. For example, I don't see this thread as an error-correction thread. Had that been the case, the recent useful discussions of SOA of output transistors would have been seen as inappropriate.

Bob
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Old 19th November 2006, 10:23 PM   #18
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Quote:
Originally posted by lumanauw
Hi, Gerhard,



How to do that (0 Zsource)? Using overspec power transformer + 10's or 100's uf cap bank + many parrarel rectifier diodes?



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?

I'm always intrigued when I see high-end designs run from batteries (not just a Gain Clone). Does this mean that it is not possible or economical for us to make a power supply as good as a battery? In fairness, to do as well as a battery, one might have to house the power supply in a separate box to completely avoid the effects of interference emissions from the PS.

As I mentioned earlier, I do think Gain Clones need a good, quiet, well-filtered power supply for them to yield their best performance, since the input and VAS circuits must live off of the same rails as the output transistors.

Bob
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Old 20th November 2006, 07:16 AM   #19
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Quote:
Originally posted by Bob Cordell



Jan,

I don't think the power supply discussion is worthy of splitting off. I think its useful to be able to move around within a larger topic as long as it doesn't get out of control. For example, I don't see this thread as an error-correction thread. Had that been the case, the recent useful discussions of SOA of output transistors would have been seen as inappropriate.

Bob
Hi Bob,

Indeed, this thread has developed as one of the most interesting in a long time. Still, the usual practise of a relatively bounded subject per thread is good to keep the contributions focussed. People who want to continue the Hawksford discussion but not participate in the power supply discussion now have to wade through many posts to keep track of their interest. I think splitting/regrouping would make for more focussed discussions.
And yes, the thread could/should have been split earlier to have a dedicated thread on MOSFET vs BJT.

My 2 (euro) cents worth.

Jan Didden
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Old 20th November 2006, 09:35 AM   #20
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I agree Jan.
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