Bad Grounding,or Cheap Pot?? - diyAudio
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Old 21st February 2008, 10:15 PM   #1
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Default Bad Grounding,or Cheap Pot??

I have built a tube preamp,and I have an issue with low level hum,but this only occurs with the gain set half way,by the way I'm using a cheap Alpha Pot,once I turn the gain all the way up the hum completely goes away and it's quiet,and I'm using star grounding!!!! any suggestions?
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Old 22nd February 2008, 03:03 AM   #2
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Hey, whaddya know, no psychics here today. Why don't you humour us and give us clues like your schematic and layout info?
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Old 22nd February 2008, 04:28 PM   #3
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Default Re: Bad Grounding,or Cheap Pot??

Quote:
Originally posted by denny9167
I have built a tube preamp,and I have an issue with low level hum,but this only occurs with the gain set half way,by the way I'm using a cheap Alpha Pot,once I turn the gain all the way up the hum completely goes away and it's quiet,and I'm using star grounding!!!! any suggestions?

Have you grounded the pot housing (body) ?

Maybe the pot can have to high a value...
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Old 23rd February 2008, 06:26 PM   #4
6f6 is offline 6f6  United States
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Denny,
My guess is that you are using the stereo pot that Radio Shack sells. I guess this because I had a similar problem with equipment that I constructed using it. I remedied the problem by connecting a small (10pf or 20pf) capacitor from the feedback tap to ground. You could also connect the same small cap in paralell with the "top" and the "bottom" of the pot.
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Old 23rd February 2008, 09:58 PM   #5
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FWIW, I had this on my aikido pre until I sorted out the grounding (which I had done incorrectly). I had this effect on both an alps pot and then a stepped attenuator.

Fran
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Old 27th February 2008, 12:34 AM   #6
6f6 is offline 6f6  United States
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Did you get it fixed?
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