Power Supply Resevoir Size

[danielwritesbac]

Originally Posted by abraxalito

There's no reason I know of to take out the diode on the positive rails - no chance of blowing it up there I'm working on a TDA7293 design at the moment and I'm going to run the positive driver pin (pin7) on a boosted, linear regulated supply. I suspect (though have no evidence) that positive PSRR is going to be better than negative so I'm borrowing a leaf from Klaus (KSTR)'s book here in referencing signals to the -ve rail.

Well, for using the diode to make the amp dull-proof even if the power supply tank is huge or the umbilical cable zero length, we can fake it and install an optional open sound generator, also fake of course.

So, it may be interesting to adopt bus rail, yet also do the polar opposite of the usual hard sound consequences. I didn't hit the middle ground yet, but this may be more fun.

In the photo, it needs RF filters, and it doesn't hurt or help if you add 2200uF either before or after the diodes. Take out or change the 0.5r if you didn't want miniscule reverb or try a 1R for a difference. It is scalable, like 0.7v for standard diode, 0.5v for fast, or 0.3v for schottky, and the resistor can be any value and 1/4 watt is okay since that much voltage drop is a light load. The main point is that the umbilical can be zero length and the power supply reservoir huge, without risk of a dulling.

[gootee]

Originally Posted by gootee (Tom: )
So the best way to think of it might be "resonant dips" and "impedance peaks". But even Henry W. Ott alternates between calling the peaks "impedance spikes", "resonant peaks", "resonant spikes", and "anti-resonance spikes".

(Frank: )At least I'm in good company ... . That said, using "impedance peaks" may help reduce confusion out there, so I shall aim to consistently use that term forthwith. But, the effect is still due to a resonance behaviour, between the L and C characteristics of separate parts, in the "anti-resonance spikes". If there were no L in the picture then these spikes would not occur, no matter what C values were in parallel; resonance is both a low and a high impedance behaviour

(Tom: )
And where decoupling needs to cover a wider range of frequencies, sometimes people attempt to choose multiple values of parallel caps so that their resonances are spread out over the needed frequency range. Problems are usually part of the result, because the multiple capacitors interact to cause high-impedance peaks in between the resonant dips, where high impedances are not wanted.

The resonance dips are usually "a good thing", in a decoupling or bypassing context. They "swallow" problem frequencies, optimally, if located there. They produce current transients optimally upon demand at frequencies where they are needed, if located there.

The impedance peaks (the "anti-resonance" points produced by interactions among multiple C values in LCL networks) are usually "a bad thing". If they are, say, 25 dB-high peaks, then noise and whatever else is around will be 25 dB higher in level, at those frequencies. And if current is needed at those frequencies, the response is very poor. And if an amplifier's high-frequency power rail feedback has content at an impedance-peak frequency, the amplifier would be likely to oscillate at that frequency.

There is usually not much of a problem if the paralleled capacitor values are within a 2-to-1 ratio, because then the spikes usually fall within the dips, greatly reducing their magnitudes. The severe problems occur mainly when the capacitor values differ by an order of magnitude or more, such as when paralleling small-value film caps and large-value electrolytics.

This is where I part ways with you, Tom, sorry. You'll have to show me with LTspice, say, where paralleling an electro and a film gets you into trouble. Note the thread, Mundorf M-Cap Supreme as bypass cap? where I talk at length about this stuff ...

Frank

Frank,

I haven't read any of that thread, yet. But I am sure that there are cases where anything could work well. And with a network analyzer, one could see exactly what one was doing and get away with even more, if necessary.

I am not an expert at any of this stuff, but, unless I misunderstood what he wrote, everything I wrote in the third quoted section above was paraphrased straight out of the latest edition of Henry W. Ott's book, "Electromagnetic Compatibility Engineering", sections 11.3 and 11.4 . He even goes as far as advocating only paralleling multiple capacitors of the same value.

Regards,

Tom

Edit: I have to leave you some "wiggle room" on the above: I just re-realized that sections 11.3. and 11.4 of Ott's book are mainly about decoupling for digital systems, and the capacitors being discussed were all small low-loss types, as far as I can see. And he does mention that the size, shape, and position of the anti-resonance peaks will depend on the ESRs of the caps (among other things).

[gootee]

Originally Posted by fas42

OK, round 2, ...

Electro's have low Q because of their very nature, their construction. That is, if you look at a curve of impedance with frequency for such it will alway look like the bottom of a bathtub, which means low Q: high ESR for the value of C and L of the unit. And that relatively high ESR will always damp any resonance with respect to any adjacent capacitor, film or electro.

But 2 high Q cap's next to each other, film say, is asking for trouble, if you get the values wrong, because the ESR of either is insufficient to damp the resonance between them.

Yes, the bypassing needs to be where the transient current is required, within millimetres of the part, for the real benefit to be felt.

The confusion is in part where you think the power supply smoothing caps should be: in my world they should also be within a tiny distance of where the current actually does its work, not on the other side of the amp, because it's convenient to have them there, or it looks nice ...

Frank

Well I have to totally agree with you on where the PSU caps OUGHT to be placed. I guess I was speaking to the "usual" placement and power distribution habits of almost everyone else.

And you could almost convince me, with your ESR argument. And I would have no problem believing that it would be the case, at least some of the time. But don't really-big electrolytics usually have tiny ESRs?

[danielwritesbac]

Russian Roulette for the deaf? Paralleling random electrolytic caps with arbitrary 100nF polypro. Same as paralleling any woofer with whatever tweeter. There's going to be a crazy peak! Treble realism will be unoptimized. Needed amplifier heatsink may be more expensive/larger and amplifier power output potential or durability may be compromised.

Same person who does careless bypassing at the power circuit will probably spend days finding the perfect bypass cap to use with input and/or NFB caps. Well, the power caps need that much attention too.

Originally Posted by gootee

But don't really-big electrolytics usually have tiny ESRs?

And higher inductance. So, the HF inefficiency happens differently for each different construction of cap.

Nichicon makes those tiny value electrolytic that can be used for bypassing larger electrolytics with much less chance of abrupt peaks. So, if one missed the value a bit, the consequences are minimal.

[fas42]

Originally Posted by gootee

Edit: I have to leave you some "wiggle room" on the above: I just re-realized that sections 11.3. and 11.4 of Ott's book are mainly about decoupling for digital systems, and the capacitors being discussed were all small low-loss types, as far as I can see. And he does mention that the size, shape, and position of the anti-resonance peaks will depend on the ESRs of the caps (among other things).

Well, I guess I'm a lucky fellow, then ...

I only have access to the previous edition, and there it's clear he's talking about high Q caps, digital is a whole different ball game compared to analogue niceties ...

Frank

[gootee]

Originally Posted by danielwritesbac

Russian Roulette for the deaf? Paralleling random electrolytic caps with arbitrary 100nF polypro. Same as paralleling any woofer with whatever tweeter. There's going to be a crazy peak! Treble realism will be unoptimized. Needed amplifier heatsink may be more expensive/larger and amplifier power output potential or durability may be compromised.

Same person who does careless bypassing at the power circuit will probably spend days finding the perfect bypass cap to use with input and/or NFB caps. Well, the power caps need that much attention too.


And higher inductance. So, the HF inefficiency happens differently for each different construction of cap.

Nichicon makes those tiny value electrolytic that can be used for bypassing larger electrolytics with much less chance of abrupt peaks. So, if one missed the value a bit, the consequences are minimal.

Interesting. Do you have Ott's book? He says, "Some people consider this approach to be the equivalent of playing "Russian Roulette" ", in section 11.4.4, "Multiple Capacitors of Many Different Values", because, as he says, "One must hope that none of the clock harmonics fall on or near any of the resonant spikes.".

[gootee]

Originally Posted by fas42

Well, I guess I'm a lucky fellow, then ...

I only have access to the previous edition, and there it's clear he's talking about high Q caps, digital is a whole different ball game compared to analogue niceties ...

Frank

Dang! I thought I had you. But I'm glad that I realized my error before the editing time-limit passed.

Actually, there's a lot in that digital decoupling section that can also be applied at audio frequencies, or adapted for them.

I'm off to bed.

[abraxalito]

Originally Posted by abraxalito

Incidentally I recently discovered that 50V 1uF X7R caps when biassed near to their maximum voltage lose more in ESR than they do in capacitance. When biassed up the capacitance is about half the nominal value but the ESR goes down by a factor of 4. The 1206 1uF shows a highly impressive 3mohm ESR on Kemet's capacitor simulation software. I plan to try a few in parallel on my next chipamp build.

After a bit more research, I have found that 4.7uF X5R is probably the sweet spot. If you buy 10k from Mouser they're $0.043 each which is more microfarads per dollar than the 1uFs. ESR isn't quite as low but its probably plenty low enough at 4mohm or so

UMK316AB7475KL-T Taiyo Yuden | Mouser

[fas42]

Originally Posted by gootee

Well I have to totally agree with you on where the PSU caps OUGHT to be placed. I guess I was speaking to the "usual" placement and power distribution habits of almost everyone else.

And you could almost convince me, with your ESR argument. And I would have no problem believing that it would be the case, at least some of the time. But don't really-big electrolytics usually have tiny ESRs?

Yes, the ESR will be low, but it is still swamped by the size of the capacitance, as you say, it has to be very skillfully engineered to get something like decent ESRs. It's all about the shape of that V impedance curve with frequency: if it looks like a dumpy U then there are no resonance issues; if it looks like the end of a needle at the bottom then it will "spike" you, .

Very simple for you to try this at home, with LTspice: drive a typical electro with a film cap in parallel, with the right parasitic values for both, and check the impedance with frequency. I spent weeks playing with values of real caps, years ago, to see how it all behaved in combinations ...

Frank

[danielwritesbac]

Originally Posted by gootee

Interesting. Do you have Ott's book? He says, "Some people consider this approach to be the equivalent of playing "Russian Roulette" ", in section 11.4.4, "Multiple Capacitors of Many Different Values", because, as he says, "One must hope that none of the clock harmonics fall on or near any of the resonant spikes.".

Ah, no. I don't have the book. I was playing with my NFB cap. It is the same value as my amp decoupling caps. What's good for the gander was good for the goose. That's by design since it is a beginner friendly project (including me). The prospect is giving equal care to the power circuit. The pedantic HF power filtering assists realistic digital replay by not further convoluting the harmonics. The result is very cool running and effective.

Perhaps one does not get stunning clarity and epic soundstage without coming to similar/same conclusions as Ott does in his book?
Monkeys with typewriters eventually get Shakespeare . . . Cowboys with NFB caps eventually get Ott.
Wow! Really similar!!
Kind of related to Edison's thousand ways not to make a light bulb.