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Amplifier Hum - Only Amp Connected

I have read many threads on here about various amplifier hum.

I recently bought a used tube amplifier.  When I connected the amp and turned it on, I immediately heard a hum from the mid-range driver of both speakers. The hum is audible even if the pre-amplifier is turned off/muted. In fact, I unplugged all interconnects from the amplifier and still heard a hum when the amp was on. I then unplugged the the interconnects and unplugged everything else in the room from the outlets (turntable, pre-amplifier, TV, and cable box), so that the only thing in the room connected to the wall was the amp. I still hear the hum from the speakers when it is only wall>power cord>amp>speaker cable>speaker.

This has to mean that it is a problem internal to the amp, correct?
jdpawnbroker

Showing 4 responses by atmasphere

atmasphere's avatar
atmasphere
12,117 posts
 
If there are separate bias controls for each power tube then this explanation makes plenty of sense. If the currents are not balanced, the sawtooth waveform in the power supply can be heard in the speaker. When properly balanced (both bias controls set correctly) then the sawtooth (buzz) waveform is cancelled.
atmasphere's avatar
atmasphere
12,117 posts
 
This sounds to me like a power supply problem such as a failing filter capacitor in the power supply. Such a part can indeed cause more hum in one channel than the other but the problem would be in both. Sometimes you might be able to get it to stop humming for a little bit by simply turning the amp off and on again, but only sometimes. My recommendation is to seek service on the amplifier.
atmasphere's avatar
atmasphere
12,117 posts
 
I will try the cheater plug this evening. Does it make sense that a ground loop would be worse in one channel?
That hasn't been my experience but that does not mean it couldn't happen.
atmasphere's avatar
atmasphere
12,117 posts
 
It is normal for an amplifier to pick up a little bit of buzz with no input or with the preamp turned off. Does it have the buzz with the preamp turned on?

If no you're off the hook. If yes, this type of buzz can be caused by a ground loop. For testing purposes only, place a ground cheater adapter on the power cord of the amp. You can get these at almost any hardware store. Run the system with the preamp ON. Did that fix it? If yes, you have a ground loop and some exploration will be needed to sort out a solution. If no, other exploration will be needed.

Try these and report back- we'll see what the next step (if any) is needed.
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