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RE: My Take - with a Step Load

Posted by Mike C (A) on January 23, 2017 at 16:13:24

In Reply to: Can someone please do a check run on psud2 for me? posted by vinnie2 on January 3, 2017 at 15:37:58:

My personal view here, which has given me good results in several units.
I was pointed in this direction by Jeff and it's based on John Swenson's work. I suggest you look up his posts about PSUD.
Note: I have not gone as far as John and Jeff in terms of LSES but I may try that sometime. What I have used is their methods of step load analysis, and for me it really works well.

The method is to edit the current load and specify a step load, with the step about 10% or so more. I make it come in after 15.1 seconds. Then observe the change of output voltage after the step (specify 'after a reporting delay' of 15 seconds).
If there is oscillation, it's a *poor* PSU which is likely to sound harsh.
If the drop is steady without oscillation but slow, the sound may be somewhat slow and verging towards boring, probably with poor leading edges.

The ideal (I am told, and believe) is a fast clean drop without oscillation, with a very slight overshoot. Ideally it stops dropping after about 0.06 seconds.

This cannot be achieved with large capacitors, so to get reasonably low ripple requires a 2 stage PSU.
My best SET amp uses a cLCLC supply tuned to give this response and it sounds *very* good, with fast clean leading edges and rhythm and dynamics enough to satisfy a diehard Naim fan friend.

Now to your PSU.
It is accepted practice to increase the C and/or L values as you go along a multi stage PSU. Your use of 10H and 5H in that order contravenes this and is most likely the reason for the oscillation on startup.

I modelled it with a step load 160mA to 180mA after 15.1 sec and this clearly shows 5 oscillations after the step.

Swapping the chokes over (5H then 10H) gives an improvement, 3 oscillations, but taking 0.3 seconds to get close to final value.

Changing C2 to 30uF ... now it gets interesting!
Smooth drop, no oscillation, no overshoot, getting there in just under 0.2 seconds; not too bad a PSU. About 19mV of ripple (analyse for say 900mS after delay of 14 seconds to get the ripple).

If you can tolerate more ripple, you can go further.
Change C3 to 50uF and now there is slight overshoot, no oscillation, and a response of 0.1 seconds; reasonable.

Change C3 to 40uF and it's ugly! Definite oscillation. Now you have to play with various values to try to get what you want.

With just a bit of playing I found good results with:
4uF; 5H 55 ohm; 20uF; 10H 65 ohm; 40uF
Smooth drop, no oscillation, just detectable overshoot, and response
under 0.1 sec (looks like about 0.75 sec). A very good PSU IMHO though I expect a few comments on this ...

I suggest you try these models and see if you can do better; also check the ripple and see if it's low enough for your needs.

I hope this helps; I repeat, it has really worked for me.

Regards
Mike
(in the freezing UK)

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