Ripple & hum on Audio amp power supply

[harry]

My Dell 5.1 channel audio amp has developed a loud hum in the surround sound speakers.
The power supply looks like a classic transformer with four diode bridge connected to an electrolytic capacitor and IC voltage regulator.

The power supply filter cap terminals show 300mv of ripple at 21 volts DC.
Is this too much? 

The output stages are fed directly by the output of this capacitor.

Some input stages are fed via a 12 volt output IC regulator that removes most of the ripple on those stages.

There is lots of hum on the speaker outputs, and it's waveform looks like the power supply ripple on the capacitor.   

I removed and tested the power supply capacitor.  It is "KSC" brand, labelled 6800 uf / 50 volt.
On a Der EE LCR meter, it measured 6200 uf, with a dissipation factor of .112 at 100hz.
ESR showed "0.0" which I interpret as less then 150 milliohm. 


A 4000 uf cap in connected in parallel  reduced the ripple voltage from about 300mv to 200mv, with some reduction in speaker hum.


Is 300 mv ripple too much on the PS?
The filter cap isn't great,  but not bad enuf to account for the hum on the four surround speakers.
Also, I do not hear a hum on the subwoofer, which is powered by the same amplifier.

Unfortunately, I do not have a schematic, so anything I do has to be extra simple...

Where would you look to find the source of the hum?

Thanks,
Harry

[harry]

ADDED INFO:

The frequency of the ripple is about 120 hz.
Because 120Hz is 2x the freq coming out of the transformer,  I infer that all four bridge diodes are ok.  I have also individually resistance checked the bridge diodes in-circuit and they seem ok.

[bdunham7]

It's probably not too much ripple, the output stages should reject PS ripple.  However, there may be a malfunction that causes it to not do that properly.  The fact that an additional capacitor caused a reduction in the hum is interesting, although I'm not suggesting that a larger capacitor would be the fix.

What is the amplitude of the hum output with the inputs disconnected?  Have you verified that the 12V regulated power supply has no ripple?  If so, how low was it?  Can you post a good photo or photos of the board so we can make some educated guesses as to how the circuit is set up?