Re: Help with power supply noise., Please.

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Think I'd be schrieking with fear if my own supplies ran that
clean :).
It's capacitor ripple breaking through. Seems a very non linear
control loop from sampling the output to taking corrective action.
From pic', control action seems slightly lopsided. Maybe assymetrical
lags. I'd be inclined to lose D2 and drop R8 R17 to maybe 1/10 their
present values and then add some local linearising with a (say) 220ohm
in Q4's emitter.
Puzzled by U3, it seem way overkill.

It is. It started out as a difference amp using an OPA2277, but not having read the data *** for the AD7706
thoroughly enough I didn't realize that I needed to run it in unbuffered mode in order to read accurately down
close to ground. Before I figured that out I had changed to the INA101 and am just too lazy to change it back.
It does result in very accurate output readings, so I'll probably leave it in place.

I thought D2 might be a good idea to make sure Q4 was never reversed biased from the output of U1A, but I'll
remove it and make the other mods you mentioned and see how it does. Thanks for the suggestions. Not being an
engineer tho, I'm not sure I understand all that you said, but I'll work on it.

I thought the noise was pretty high. I see the specs for commercial supplies and a lot of those claim 0.5mv
or less noise and ripple on the output.

I sure wish I knew why the OPA2277 doesn't work for U1, it has much better drift specs than the amps I have
that do work properly.

Thanks again.

Mike


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