Amplifier hum

[splaz]

Hey. I have an amplifier that has a hum problem.

With the amp on whenever the speakers are connected it pops and they emit a low frequency hum, regardless of what input or volume setting. You can't hear anything else.

I've heard the power supply can often be the cause for this, either filter caps or rectifier. The main filter caps are Nippon Chemicon but are probably getting on, the amp I'd guess would be late 70s early 80s maybe.

Otherwise grounding ?

Can anyone shed any light, like what to check for ?

I've got a beginner level electronics knowledge I guess, poking around with a DMM and soldering in new parts is no problem I can read basic schematics, I just have no idea how to diagnose problems...

Should I find some batteries for my ESR meter which I've never used ?

[kc8adu]

Re: Amplifier hum

check for dc on the output.
sounds like shorted output transistors.
it may cook the speaker if you leave it on very long.

[splaz]

Re: Amplifier hum

check for dc on the output.
sounds like shorted output transistors.
it may cook the speaker if you leave it on very long.

Yep that looks like the case, 12v DC on the speaker posts.

Good thing I tested it with cheap speakers.

It's an integrated amp, there are no discrete output transistors, I guess the IC is at fault ?

[Krankshaft]

Re: Amplifier hum

Yes replace the amplifier output IC only AC should be coming out of the speaker posts.

DC is a killer also check the output coupling capacitors as well since their job is to make sure that no DC (when the amp IC is functioning properly) gets out of the speaker posts.

Better amps have a protection relay so if the protection circuit senses DC coming out of the amps outputs it activates the relay protecting the speakers.

[splaz]

Re: Amplifier hum

Thanks very much, just need to wait on parts from overseas now.