Bass preamp + Edcor balanced out = HUM! - diyAudio
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Old 21st May 2011, 06:40 AM   #1
diyAudio Member
 
Join Date: Aug 2005
Default Bass preamp + Edcor balanced out = HUM!

Made a simple bass preamp, 12ax7 stage --> 12ax7 stage --> TMB stack --> 12ax7 cathode follower --> out. I included an Edcor XSM 15k:600 for a balanced output. The output of the cathode follower goes to a 1/4" unbalanced output jack and then to the Edcor primary. DC heaters.


I fired it up and it worked great but hummed like a son of a gun. Listening to it through a PA or a bass amp power amp input, and I get hum just by turning on the power and leaving it in standby mode. This is with the preamp unbalanced output connected to the power amp or PA, not only with the balanced output.

I disconnected the PT (also edcor 200V 40mA, 8V 1.5A) secondaries and still get hum. I unbolted the line out transformer so i could move it around. I got some of the noise out by moving it further from the PT and rotating it a little while watching the scope. It's pretty good I guess, but it's still there and I want to know if there's anyway to kill it. If I disconnect the line out transformer primary from the unbalanced output jack, then the hum disappears. So it is definitely induced hum getting into the line out transformer.

I've googled around and found that in the hifi and studio gear circles, these transformers have mixed reputations since they are steel core and unshielded and therefore susceptible to induced hum. I don't want to splash for a mu-metal cage, what else should i try?

Thanks for any ideas,
MIke
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