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I have an odd problem with microphony...

Occasionally my system makes a hum that sounds almost like a musical note. Two days ago it started up and, as usual, when I switched the source switch it stopped but would start up again if I tapped the preamp or the phono pre. It seems to react more strongly to the pre than the phono but I will also hear a similar sound if I tap the TT. The most odd aspect is that yesterday I played the same record and there was no sign of this hum. I have a Lamm LP2 Deluxe and a Lamm LL2 Deluxe, both tube amps, and I recently added Herbie's Tube Dampers to try to cure the problem. Any ideas out there?

128x128fosolitude
czarivey
5,946 posts
 

Go over bad tubes and replace those.

That's pretty much an idea

fosolitude's avatar
fosolitude OP
72 posts
 

I should have mentioned that I've already replaced tubes with no effect

carlsbad2's avatar
carlsbad2
1,602 posts
 

What happens if you tap the tubes?  that is how you check for microphonics.   With nothing playing, turn on the amp and let it warm up.  tap lightly on each tube with your fingernail.  If sound comes out of the speaker, that tube has microphonics.   Often tubes with microphonics are seen to be a non-problem and are resold.  I don't tolerate microphonics because the problem you are vulerable to is as you describe.  

If, OTOH, tapping on the tubes does not cause noise to come out of the speakers, then your problem isn't microphonics in your tubes.  In that case I'd say you have a loosely connected ground loop and tapping makes the connection come and go.

Jerry

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