Power Supply Resevoir Size

[twest820]

The LT Spice model's considerably clearer than the other documentation, as it were; I'll take a closer look when I get some time. Thanks!

[gootee]

The spreadsheet is, of course, fully documenting all of the math, if you look at the cell formulas and the VBA code.

In LT-Spice, I made the transformer model MUCH more useful by making it a component, with a symbol, and by making its parameters settable from the main circuit's schematic, where it was used.

To do that, I put something like

Vsb={Vrms_setting} Ssb={VA_setting} fb={mains_freq_set} Vpb={mains_rms_set}

in the PARAMS line in the box that pops up when I right-click on the symbol I created for the transformer.

Then I can set the params (that appear in the { }s, above) in the main circuit's ".param" spice directives, and can use them in sweeps!

It's been QUITE useful!

And if you have a variac, it's quick and easy to measure what's needed in order to make the model for a different transformer.

You can download a .asc file for a slightly-older version of the transformer model, from somewhere in this thread. And there should also be a zip file here that even includes the symbol for the transformer, etc.

[ginetto61]

Originally Posted by Mooly

Bigger isn't always best.
As you say, amplifier "tuning" or the sonic characteristics of an amp can be altered by its PSU design.
Again this is where objectivism and subjectivism might not agree.
A) The amp is better technically with the low ripple massive PSU.
B) The amp sounds better with a smaller "faster" PSU.
So which do you build ? A or B
Some good general useful info here... but it doesn't tell you how godd an amp will sound with a given PSU.
The Signal Transfer Company: Power Output

B) The amp sounds better with a smaller "faster" PSU

the consensus is general on this statement ?
What is the problem with too much capacitance in the PS ?
Actually the best amps around have huge capacitance
Some manufacturers even offer a capacitance upgrade for their amps
Thanks for any explanation.
Kind regards,
gino

[gootee]

Maybe there could be a problem if the transformer was too small. It needs to be able to recharge all of the caps and maybe also supply part of the music signal, during the short times when the rectifiers turn on. I can't think of any reason that more capacitance would be bad, especially if it had low-inductance connections. It should give a lower impedance, which should make its response faster, not slower. The signal is current, not voltage. The larger RC time constant slows the voltage droop but that's a good thing.

[twest820]

The larger the reservoir capacitance the greater the cost and complexity, both in and of itself and in the addition of components such as soft start circuits. Using large cap banks as a workaround for poor PSRR by minimizing ripple in a linear supply also reduces the diode conduction angle in the bridge, which tends to degrade both reliability and power factor. For DIY these aren't often significant concerns, but it is a bit silly to stack up 100,000uF per rail when you can get the same job done with a 50 cent regulator and 10,000uF per rail.

From a sound quality perspective, the narrower the conduction angle the broader the bandwidth of the ripple. So, in addition to there being less ripple, throwing cap at the problem results in spreading the charging energy over a larger number of mains harmonics, reducing their audibility. In an amp with insufficient PSRR to be transparent to the supplies (which would be, um, a lot of them---typical designs are in the 70dB range whereas 100+dB tends to be desirable) this means changing caps makes audible changes in the amplifer's tonal qualities. To answer Gino's question, some people like more supply dirt in their music, others like less.

[Harleyjon]

Use a ripple filter instead. Less costly and a lot cheaper!

[fas42]

Originally Posted by ginetto61

B) The amp sounds better with a smaller "faster" PSU

the consensus is general on this statement ?
What is the problem with too much capacitance in the PS ?
Actually the best amps around have huge capacitance

Like most things in audio there is not just "an answer". A playback system has to be engineered to work properly, throughout its entirety; and "easy fix" suggestions should be treated very carefully. The suggestion will have parts of the truth, but is not the truth in of itself.

If one thinks of creating a system like designing a bridge then one has a much better chance of getting it to work right. Every component is linked to everything else in some fashion, and just using some "fashionable" technique, or "impressive" idea that someone reckons is the go, is not going to cut it.

Which is a long-winded way of saying that a supply should be both low ripple and "fast". To do the two at the same time, correctly, is just an engineering issue ...

Frank

[abraxalito]

Originally Posted by twest820

For DIY these aren't often significant concerns, but it is a bit silly to stack up 100,000uF per rail when you can get the same job done with a 50 cent regulator and 10,000uF per rail.

+1 for having the regulator. I experimented with raw capacitance and reached well over 100,000uF in paralleled 2,200uFs. The LF noise on the supply was lower with a regulator and 'only' 11,000uF but still adding the caps improved the noise above around 1kHz. This could be because I have a low value inductor after the reg (an LM350T) as when I first experimented without an inductor on the reg, the sound was distinctly flakey. I have yet to determine the optimum value of the inductor (its currently about 300nH, air cored).

To return to gino's question, I suspect the problems arising from higher cap values only apply to caps fed from mains freq rectifiers - the charging pulses become shorter and richer in harmonics and are more likely to couple into other parts of the circuit. At these higher freqs, the impedance of the cap becomes ESR limited. The solution to that is paralleling many smaller ones and adding some series L along the way. I rather suspect then that there's no upper limit on improvement by adding capacitance, just increasingly smaller returns for the outlay.

[twest820]

Originally Posted by abraxalito

I rather suspect then that there's no upper limit on improvement by adding capacitance, just increasingly smaller returns for the outlay.

That's what tomchr and I found when measuring the load regulation of LM78xx and LM79xx parts, though with LDOs it depends on the loop configuration.

I think you probably meant to type ESL at high frequencies rather than ESR, or does high mean kHz in this content? Not sure why one would want to add ESL; most regulators land in the uH range on their own.

[abraxalito]

'High' meant 'above 1kHz' in this context - I agree at even higher freqs - say above audio then ESL becomes dominant. I added the explicit inductor because I suspected (though didn't measure) that the regulator had stability issues without. I have rather a lot of very low ESR ceramics on my supply too... Yes the rising output impedance of the reg looks like an inductor - back of the envelope of the order a few hundred nH.

<edit> Just worth mentioning - some incredibly large (hundreds to thousands of Farads) caps are available nowadays. They're not useful because their ESR is too high. They're very limited voltage (2.5V typically) so you have to put quite a few in series by which time the ESR kills them as audio band decouplers.