aluminium cap after regulator


2018-02-09 1:42 am
hello. this is regarding power for op amps.

I thought a regulator would usually have an aluminium electrolytic cap before and after the regulator. Usually a larger cap before and a smaller one after.

Would it be beneficial to add two electrolytics after this regulator below and maybe bypass the electrolytic caps with wima 0.1uf caps? The board is out and so not a problem to modify.

The circuit is to power op amps in an Eq.

Thank you for any info.

Not sure how big C17-24 are, but this doesn't look terribly ideal to me as-is. This page illustrates what can happen.

With a 500 mA fuse in the secondary, I'd suggest some modern low-noise 78M18/79M18 (or at least lower-noise 78xx/79xx if you have trouble finding those), and anywhere from 10 µF to 47 µF (@35-63 V) of regular good-quality electrolytic output cap. Do consult the datasheets as suggested.

You probably don't need to go overboard for an old EQ, they tend to be relatively noisy either way for various reasons.
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Why the urge to sprinkle bypass caps around? In most cases they do little harm but they rarely do any good. The regulator provides a low impedance output; adding a cap merely makes this worse in a certain frequency range. Is that what you want?

Decoupling caps take over at higher frequencies where the regulator's loop loses authority. Too much can destabilize the loop, so read the datasheet for any limitations for a particular regulator.

Decoupling caps also need to be close to the load, which is why they are sprinkled around - particularly faster opamps need individual ceramic decoupling, as does every digital logic chip. At higher frequencies power rails are dominated by stray inductance, and distribted decoupling enables them to remain low impedance at many places.
By "sprinkle" I did not mean 'put them in particular places where they will do some good' but 'put them in random places or everywhere where there is already another cap'. This is what that circuit appears to do - four in parallel after a regulator, when at most you need one at the regulator and one at the circuit which uses the power from the regulator.


2018-02-09 1:42 am
Thank you very much for your replies.

I shall look more into the modern regulators, that sounds interesting.

"Not sure how big C17-24 are"

I am not sure either. They look very small, blue in colour and are about 1.5 cm away from the opamps on this particular board (the board that deals with unbalancing from input). I can't see a number on them unfortunately. but these small caps are for the op-amps.NE5532.

The reason I asked is usually when I build a circuit with a regulator I put electrolytic caps after the regulator.

I shall refer to the regulator pdf from now on.

Thanks again

all the best