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Amp transformer buzz/hum from DC on AC line?

I have two tube monoblocks with EI type transformers.
One amp hums much more than the other. Not through speakers. It gets somewhat louder and much warmer than its counterpart over time(3-4 hours).
They are on different circuits, I have tried swapping circuits. I have isolated it by turning off every breaker except the one to the amp, so it's not from somewhere else in the house. I have swapped all the output tubes from one to the other.
I have measured .46 volts DC on the line to both amps. Could the one be more sensitive to DC than the other?
Is this enough DC to cause a problem? I can get a PS audio Humbuster on free trial to make sure, but it sounds too good to be true. Any thoughts?
mrmatt
ghs
25 posts
 
I agree with Mlsstl, the only thing I might add is that the transformer is coupled to the chassis with some type of rubber grommet in most case's. You might try loosening the transformer and put a rubber wedge between the base of the transformer and the chassis.One of my mono amps has the same problem, but I can't hear it from where I seat.
jea48
4,473 posts
 
I have measured .46 volts DC on the line to both amps.
How did you go about measuring the DC voltage on the AC line.

DC voltage on the line can cause transformer mechanical noise.

http://sound.westhost.com/xfmr.htm#7-noise

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

http://www.altavistaaudio.com/hum.html#trafo
plato
1,543 posts
 
Are you sure your amps have the 60HZ transformers used in the US as opposed to the 50Hz transformers? Sometimes if a transformer is designed to see 50Hz putting it on a 60Hz line can cause hum.
gbart
290 posts
 
I second what Jea48 said. I have deliberately induced hum in transformers with DC as an experiment and solved a number of DC problems with simple DC blocker.
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