Bad Grounding,or Cheap Pot??

denny9167 · 21st February 2008, 10:15 PM

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?

leadbelly · 22nd February 2008, 03:03 AM

Hey, whaddya know, no psychics here today. Why don't you humour us and give us clues like your schematic and layout info?

Tube_Dude · 22nd February 2008, 04:28 PM

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...

6f6 · 23rd February 2008, 06:26 PM

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.

woodturner-fran · 23rd February 2008, 09:58 PM

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

6f6 · 27th February 2008, 12:34 AM

Did you get it fixed?

21st February 2008, 10:15 PM	#1
denny9167 diyAudio Member Join Date: Nov 2007	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?

22nd February 2008, 03:03 AM	#2
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Hey, whaddya know, no psychics here today. Why don't you humour us and give us clues like your schematic and layout info? __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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23rd February 2008, 06:26 PM	#4
6f6 diyAudio Member Join Date: Oct 2006	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.

23rd February 2008, 09:58 PM	#5
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	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

27th February 2008, 12:34 AM	#6
6f6 diyAudio Member Join Date: Oct 2006	Did you get it fixed?

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