Bob Cordell Interview: Power Supplies

[gootee]

Originally Posted by AndrewT

Yes, but I stopped following about 2months ago, because I am spending too long on this Forum and somethings have to give.

BTW,
that is a brilliant summary.
It will please Gootee as well. Have you reads his threads?

Haha! Yes!! It does!! Thank you!! I just now saw this, but hadn't been following this thread. What a pleasant surprise!

I wish that tankcircuitnoise (and everyone else) would also wander over to here:

paralleling film caps with electrolytic caps

I haven't had time to get back to it, yet, but I'm still excited about paralleling caps without any conductors in common, so that the total ESL of the paralleled cap/conductor combinations might be reduced in the same (algebraic) way that the total resistance of parallel resistances gets reduced, which I have read (in Ott's latest EMC book) can't properly occur if there is any MUTUAL inductance; ergo my belief that paralleling the conductors as well as the caps might provide a possibly-significant benefit.

I started thinking about that for cases where multiple parallel decoupling caps might need to be used across one set of pins (when planes are NOT being used, as is the case for a lot of DIY PCBs) in order to lower both the ESL and the ESR so that the (local) "power supply" impedance seen by the power pins of an amp chip, for example, could be made low-enough, to a high-enough frequency, to provide for good-enough transient response [ f = 1 / ( (π) (risetime) ), I think ].

THEN I realized that MAYBE it would also be a good idea to use multiple parallel copies of the power and ground rail traces, ALL the way from the rectifiers to the load's power pins! i.e. EACH smoothing cap (or each subset of smoothing caps) would have its own rails, all the way from the rectifier bridge (or maybe after the first big cap) to the load. And actually, for low inductance connections, in general, couldn't we just use multiple parallel traces, or, in the case of a board interconnect, a ribbon cable with multiple conducors for each rail and ground?

Anyway, I guess those things should be discussed in the thread at the link I gave, rather than here, unless they're applicable here too of course.

Cheers,

Tom

[magnoman]

Originally Posted by Speedskater

Before making slits or slots in a ground plane, I suggest reading some of Henry W. Ott's works:

I agree,
forcing currents to go via a higher impendance path can create more problems.

Thanks
-Antonio

[tankcircuitnoise]

Or read this appNote from AnalogDevices. The idea of currents taking ALL possible paths at DC, yet becoming more concentrated under the outgoing wire as frequency
increased, was a real eye-opener for me. And while reading a Schaum's Outline book
on E&M, to be told coax-cables began to be effective at about 50KHz: surprising.
Apparently near 50KHz, the inductance in paths far away from the shield becomes
enough to "encourage" return-currents to "choose" the coax-shield. Its all part of
minimizing the stored energy, I think.

Anyway, after reading this ADI Brokaw AppNote, I began to understand planes,
and even to use slits as further "encouraging" of return currents. Just do not route
ANY Any any fast signals over slits.


http://www.analog.com/static/importe...0828AN_345.pdf

[maxlorenz]

--- reduce edge-rates (use that 0.22 ohm and 100milliFarad, to keep
fast rectifier surges in the first bank of capacitor, away from low-level circuits)

Hi tankcircuitnoise,

I've used -ECdesigns'- stepped rectifers concept in both low and high power application with success. It can use 3 or more stepped diodes. I attach one of his diagrams:

https://picasaweb.google.com/lh/phot...eat=directlink

Of course parts and values can be changed to your specific needs...
What do you think?
I found it brilliant.

Cheers,
M.

[alayn91]

Hello Gootee,

Relationship is:
Bw * Tr = 0.35

Regards.
Alain.

[gootee]

Originally Posted by alayn91

Hello Gootee,

Relationship is:
Bw * Tr = 0.35

Regards.
Alain.

Interesting. The form that I gave is equivalent to

Bw * Tr = 0.318 (i.e. = 1 / Pi )

I wonder what causes the difference.

The equation that I gave came from Henry W. Ott's famous book, "Electromagnetic Compatibility Engineering". I didn't see the derivation and am too old to remember how to re-derive it.

[alayn91]

Hello,

Here is the explanation:
- Rise time - Wikipedia, the free encyclopedia

Regards.
Alain.

[Emerson Prado]

I'll take a ride in this long thread - a really nice one - to add my 2 cent:

Originally Posted by Bob Cordell

We constantly hear about people installing Black Gate capacitors, or Shottky diodes, or better bypasses in power supplies and getting startling sonic improvements...
...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?

If it's audible, it should be measureable.

It seems obvious that anything here should be related to the changes in the output devices voltages (Vce, Vds or Vpk) due to the finite supply admittance - specially regarding the tracks and wire inductances. Transients could have even more impact. Sure, that includes the current return path and eventual decouplers too.
These voltage changes would impact the output via the finite impedance of the output devices (early effect, capacitances and the like).
Some of this effect is cancelled by negative feedback, but this is also finite. And there's also the possibility of any of these impedances / admittances to be non-linear, adding insult to injury.

I was just thinking about a way to measure this. Sure, distortion, spectrum analyzers and stuff. But also there should be a way to provoke the effect.
Maybe a good way to test this would be adding series resistors and/or inductors to the PS rails - between reservoir capacitors and rail decouplers and/or between decouplers and output devices - and watching the effect with a test signal. The test signal could be a sinusoidal, where the distortion effects can be more easily measured, or a damped sinusoidal, to get the effect of a transient.
But it would tell a lot to watch the rail voltage right before the output device (collector, drain or plate). I'd bet some inductance here would cause a short and deep drop. If driven far enough, this drop would disturb the output device quite a bit. I wouldn't be surprised if some conditions actually saturated the output device, even when the output is far away from the rails.

Sadly, I don't have the resources yet to do this test. But, just in case someone luckier has good stuff and time in his/her hands, and wants to figure this out, here goes my suggestion.

Best regards all,
Emerson

[tiefbassuebertr]

Originally Posted by Emerson Prado

I'll take a ride in this long thread - a really nice one - to add my 2 cent:



If it's audible, it should be measureable.

It seems obvious that anything here should be related to the changes in the output devices voltages (Vce, Vds or Vpk) due to the finite supply admittance - specially regarding the tracks and wire inductances. Transients could have even more impact. Sure, that includes the current return path and eventual decouplers too.
These voltage changes would impact the output via the finite impedance of the output devices (early effect, capacitances and the like).
Some of this effect is cancelled by negative feedback, but this is also finite. And there's also the possibility of any of these impedances / admittances to be non-linear, adding insult to injury.

I was just thinking about a way to measure this. Sure, distortion, spectrum analyzers and stuff. But also there should be a way to provoke the effect.
Maybe a good way to test this would be adding series resistors and/or inductors to the PS rails - between reservoir capacitors and rail decouplers and/or between decouplers and output devices - and watching the effect with a test signal. The test signal could be a sinusoidal, where the distortion effects can be more easily measured, or a damped sinusoidal, to get the effect of a transient.
But it would tell a lot to watch the rail voltage right before the output device (collector, drain or plate). I'd bet some inductance here would cause a short and deep drop. If driven far enough, this drop would disturb the output device quite a bit. I wouldn't be surprised if some conditions actually saturated the output device, even when the output is far away from the rails.

Sadly, I don't have the resources yet to do this test. But, just in case someone luckier has good stuff and time in his/her hands, and wants to figure this out, here goes my suggestion.

Best regards all,
Emerson

The main issue for me in this case is the fact, that I don't find model circuits/equivalent circuits (respectively P-Spice models) for the main transformers, electrolytic caps and the mains like this
Transformer - Wikipedia, the free encyclopedia
and this:
http://www.scholar.de/studenten/lern...chaltungen.pdf

If I would have this for each transformer and other power supply parts of an certainly amp model so as the individual curvature include clipping/distortion effects of the individual mains socket in the wall, it would be an easy task to intestigate the different effects and reasons for the sonic differences while listening tests by a p-spice simulation.

[Emerson Prado]

Originally Posted by tiefbassuebertr

If I would have this (model) for each transformer and other power supply parts of an certainly amp model so as the individual curvature include clipping/distortion effects of the individual mains socket in the wall, it would be an easy task to intestigate the different effects and reasons for the sonic differences while listening tests by a p-spice simulation.

I agree, but I don't think we need to go that far to find something.
What I believe that should be found is the relationship between rails sagging and rising distortion. Surely, the transformer, the mains, etc., all affect sagging in a "long" fashion (>~10ms), by their ability to recharge the reservoir caps. But, in the end, it's just sagging - if the rails drop 1V, the output devices will have 1V less, no matter which part of the supply chain lost this V.
So my point of view is that, if some transformer or capacitor is said to be better, my guess is that it reduces sagging, so output devices suffer less.
Or, from another angle: whatever good the cap/transformer/rectifier/mains combination is, we can make it 1 ohm worse, or 1mH worse, or both, and already have a clue of the mechanism Bob is searching. If you're simulating, you can also make it 1 ohm or 1mH better...
With the complete models, then we can tell how much.
Best regards,
Emerson