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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
gdnrbob
4,417 posts
 
I am going through almost the same thing right now.
One question:
Are the speaker cables anywhere near the power cords?
If so, try moving them as far away as you can.
B
jdpawnbroker OP
10 posts
 
Is there any mechanical low buzz/hum emanating from the amp itself, or is it quiet and hum coming purely from the speakers?

Hum is coming only from the speakers and only from the mids as far as I can tell.

What do you recommend for a DC offset issue?

Are the speaker cables anywhere near the power cords?
If so, try moving them as far away as you can.

I don't have a ton of room, since everything is fairly tight. I will try to move things to a different room at least to experiment.
gdnrbob
4,417 posts
 
DC offset is a b***ch.
As I said, I am dealing with a similar issue.
First, I would get an electrician to go over the circuit. If possible make a clean run to your system. Replace outlets with hospital grade outlets-cheap upgrade.
And, at the very least, make sure the wires are properly installed. Sometimes the ground and neutral get mixed up, then all hell breaks loose. 

Bob
jea48
4,313 posts
 
Does the hum sound like this?  
120Hz hum.   

https://www.youtube.com/watch?v=pC4Uzt0qm2E  

If yes it could be failing electrolytic caps in the power supply. 

How old is the amp?

Manufacturer and model?

delkal
110 posts
 
DC offset should not sound like a hum. You need AC to get sound from speakers. In extreme cases DC offset might push the cone in or out some but the cones will stop there and you won’t hear anything.
On the plus side DC offset is easy to troubleshoot.   Put any multimeter set on DC across the speaker terminals.   If you see more than 100-200 mV you may have a problem (it should be zero).
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