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Output hum

ppontiac

ppontiac

Active Member
Banned
Question: Where might my audible hum originate?
Hum with speaker A or B is selected. The hum goes away a little as the amp warms up, within a minute. Amp was silent up to six months ago. It's a Pyramid PA-310. Looks just like a Realistic PA amp at 100 watts a channel. I've checked grounds, flipped the plug-ins, cleaned the RCA jacks & then replaced them. I suspect it's internal. My other amp does not hum when I swap them around. The amp plays just fine, just hums when no source is playing. I bought the amp used for $60 a year agoand was wondering if it would be expensive to repair.
 
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Don't know. Once the music is on I can't hear it. I plugged in the amp in another room with a speaker and the hum is still there. My concern is how long will amp go until it has to be repaired, or will it damage a speaker? I have a pair of Realistic Minimus 7 running from this amp.
 
Hum stays the same. Like engine idle noise; not heard once you give it gas. Don't hear it when playing a subwoofer on speaker B selector. Minimus-7 on speaker A with an inline bass blocker.
 
Unplug all music sources and everything else connected to the amp, so there is only the amp connected to the speakers.

Now do you hear a hum, and if so does it change with volume?
 
Then it's certain that the hum is originating from the amp, we had to rule out other sources such as ground loops, interconnects, etc.

While t's tempting to automatically call out "power supply filter caps" as the source of the hum (and yes I am indeed a strong proponent of recapping old vintage amps) hum can be caused by other problems. I've had cracked/poor solder joints cause hum; same with poor connections internally. We must rule these issues out before automatically deciding to recap a fairly modern amp that hums.

Does the hum come and go? Try the old "whack test"--take the palm of your hand and whack the amp HARD on the sides, top, all around, see if the hum changes. A change in hum indicates a physical connection problem, such as the cracked solder joint or poor connection.

Then take the cover off the amp, plug in the speakers but nothing else, and carefully power up the amp. Then take something non-conductive...like a wooden chopstick... and gently poke/prod various components to see if the hum is affected. Believe it or not, this is a legitimate test method, and it's called...drum roll...the chopstick test. Careful not to electrocute yourself (and I'm not being flippant). Only use the chopstick to touch components. Pushing on a component slightly will stress the solder joint a little so a poor solder joint may then be discovered, the hum may increase or even decrease if the connection is forced to be better. Include wiring conenctions as part of the test; also look for wires that may have become disconnected.
 
No change in hum. poked around with chopsticks. Took a flashlight & magnifying glass and inspected the PCB. Just hum. It did not do this 6 months ago. The amp sounds good other than the hum at idle. Just my luck.
 
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