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Power trans coupling with speaker, causing hum
Power trans coupling with speaker, causing hum
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Old 15th January 2007, 01:59 AM   #1
slowpogo is offline slowpogo  United States
diyAudio Member
Join Date: Jan 2007
Default Power trans coupling with speaker, causing hum

I have a Fender Pro Jr tube amp that I've ugpraded/modified extensively. Hiss is way reduced, and there's no hum or buzz in the signal unless it's turned up all the way.

However, it seems the power transformer is coupling directly with the speaker, which is only about 6 inches away. In the 5-10 seconds before the tubes warm up and hiss fades in, I hear a quiet hum. Actually, it's there even if I unplug the speaker from the chassis.

I already relocated the output transformer and put a shield between it and the PT (small angled piece of sheet metal, thanks to Bill Machrone for that one), and that seems to have eliminated almost all output trans. related hum.

But how to shield the speaker from the power transformer? I'm sure the stock Pro Jr PT is just cheap, and getting a good replacement with a built-in shield would solve a lot of it. But is there any simpler solution, short of trying to construct some kind of massive homemade shield?

The hum is quiet and it's not a huge deal, but having come this far, I'd like to get rid of that last bit if at all possible...
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