120 Hz hum on a solid state amp is usually...

[CA_Dan]

What are the top culprets assuming 60Hz hum and power supply caps are ruled out? I have an amp that definetly has 120Hz hum that doesn't change with volume or tone adjustments. Zener regulating diode, rectifier, other?

[Amp Kat]

Output transistors.

[Jazz P Bass]

Yeah, dc volts on the output.
ie: blown output stage.

[Enzo]

More info please. If there is a LOUD hum, then we could have DC on the output or a bad filter cap.

If you have a functioning amp with some annoying level of hum, that is different.

You don't mention what the amp is, so we can;t look up the schematic or draw from experience. Is it a commercial amp or a home build? Has it ALWAYS been hummy, or has that been a recent change?

SS amps will have typically four power supplies, +/- power amp and +/-15 or maybe 12 for the preamps. SOme older ones and for that matter some recent ones may use transistors instead of op amps in the preamp. Those may have a single supply in the preamp, but then again maybe three or four.

The supplies must be clean. A little ripple on the power amp will tend to cancel out, but the preamps want to be very clean.

But there is more to it than just filter caps going soft. Each pulse of rectified current from the power transformer, every 120th of a second, a little spike of current happens to recharge those filters. That current flows throughu the amp0 grounds among other places, and if that ground return path shares conductor with ground returns from signal stage, then that ripple current hum will be impressed upon the signal.

[Jazz P Bass]

Well said, Enzo.
I just finished up a Hartke HA3500.
Hummed when you advanced the master volume.
Turns out the -15 volt regulator was going bad.
It had a steadily increasing Vac on the output pin as the unit warmed up.

[CA_Dan]

Thanks for the replys everyone. This is on an AMPEG B4R bass amp. Customer complained of loud buzz when powered up. I replaced the filter caps and rectifier - no change. Now when I power it up the lights dim until the caps charge and it flips the amp's circuit breaker after about 5 seconds. Even without speakers hooked up I can hear a 120 Hz buzz coming from the amp internals that wasn't present before.

I've attached a schematic.

[pdf64]

Is there any Vdc at the speaker output (before the protection relays, output_A and output_B, p5 of schematic)?
If so there are likely to be blown transistors. Pete.

[R.G.]

I replaced the filter caps and rectifier - no change. Now when I power it up the lights dim until the caps charge and it flips the amp's circuit breaker after about 5 seconds. Even without speakers hooked up I can hear a 120 Hz buzz coming from the amp internals that wasn't present before.

If the rectifiers and caps are good, and it still pops breakers after a short delay in the face of a loud buzz, then it's most likely that something is just using too much power. Jazzp and pdf are on that track. Anything that uses power from the main power supply can do it. Quick question - do the output transistors/heat sink get hot?

Some possibilities are
- power transistor(s) going bad (look for DC on the output)
- power regulator(s)/circuits going bad
- wiring/soldering short causing a "soft" short
- bad/incorrect biasing setup on the output devices

It's quick and simple to test the biasing issue. Short between the collectors Q106/Q107 for output A and Q206/Q207 for output B. This prevents any biasing of the output stages, which will then have BAD crossover distortion but will not be misbiased. If this cures the overcurrent/breaker popping, delve into the biasing setup around Q106/107 and/or Q206/207. If not, the problem is elsewhere.

If all else fails start temporarily disconnecting both the + and - wires from the power supply to e.g. output power amp A, then output power amp B, then the low voltage regulators, etc., noting when the breaker quits blowing.

A light bulb limiter would be REALLY good here, as it would force the breaker to quit blowing and let you measure voltages to see what's pulling the power supply down.

[CA_Dan]

OK - Will try these ideas today. Thanks...