How do I eliminate 120 cycle hum/noise from my solid-state amplifier?

[lundee]

I am experiencing what I think is 120 cycle hum through my Peavey Backstage 30 solid-state amp. The reason that I suspect 120 rather than 60 cycle hum is that the unwanted noise sounds more like a 'buzz' than a low frequency hum, and the amplitude of the buzzing sound is directly proportional to the positioning of either the two gain controls. At high volume, the noise is simply intolerable! Only the treble pot effects the tone of this noise and there is no 'scratching', 'pops' or 'crackling' sound from any of the pots. Does anyone actually know exactly how to eliminate this particular annoying hum that is afflicting my amplifier? Thanks...

[Enzo]

You need to determine where it is coming from.

Start by checking the power supplies for ripple on them.

If the controls turn the hum down, then the source of the hum is before the control.

Don't overlook poor ground contact on the inoput jacks. Loosen the jacks, move the star washer and retighten.

[lundee]

More information, please...

Enzo, can you explain: What do you mean by power supplies? What is ripple? How may I check for ripple on the power supplies?

[Don Moose]

The Power Supply converts the AC wall voltage to the various DC voltages the amplifier requires. These DC voltages are supposed to be pure DC - they never are, there is always a little AC left riding on top of the DC - that's ripple.

If the ripple is too big - inadequately filtered - the amplifier will amplify that noise.

If the amp controls affect the hum, then it's probably not PS ripple, more likely a loose jack or bad ground in the preamp.

Hope this helps!

[Enzo]

Hi, welcome to the forum.

Um... if you have to ask those questions, then I am not sure you can handle this repair yourself. I don't mean that unkindly. Everything electrical has to have a power source, whether a battery or a plug into a wall outlet. Your amp has positive and negative DC voltages inside it runs on. The 120 volts of AC from your wall gets converted into lower voltage DC in circuits called power supplies. The larger filter caps - cylindrical things - are part of that.

Your amp - without me looking it up - runs on positive and negative 25 volt DC power sources, power supplies. Might be 24, might be 30, whatever. That is for the power amp part. The preamp part also runs on positive and negative DC voltages, but more like 15v. In any case, these DC voltages must be smooth and steady. If it is not smooth, we say it has ripple. Like the surface of a still pond is like a mirror, very smooth. If a breeze blows across it, it ripples and is not smooth.

So to do this repair, we need you to understand what a power supply is, where it is in the circuitry inside the amp, and how to measure its voltages with a volt meter. We need you to understand the difference between AC and DC. You will need to measure the amount of ripple, which means the amount of AC included in the DC.

[lundee]

The ripple effect

Thank you, Don and Enzo. Yes, this would be embarrassing had I claimed to be an electrician! Certainly, I am no technician but if the source of the problem was something simple like a loose jack or ground wire I could certainly correct that myself, and this is what I was attempting to assess by putting my question out there. All soldered joints and jacks appear to be solid, so I'll conceed and have the amp serviced, as you suggest, by someone like yourselves who knows something about electronics. The ripple is on me. Cheers!