scratchy pot

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iC
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scratchy pot

Post by iC » Wed May 20, 2009 6:10 pm

on John Hardy M1.
for some reason i feel i shouldn't use a dab of de-oxit... am i mistaken?
thanks, kevin
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alf
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Post by alf » Thu May 21, 2009 5:51 am

Why not? Is there something weird about the pot that makes it different from others?

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iC
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Post by iC » Thu May 21, 2009 7:10 am

mmm, no, not that i know of... Just thought i read somewhere... whats odd is it seems to be scratchy ONLY when the singer is actually singing and i try to adjust the pot. (that seems weird too though, since as long as the unit is on and a mic is hooked up to it, its "passing audio".)
"There is nothing in a caterpillar that tells you it's going to be a butterfly."
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alf
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Post by alf » Fri May 22, 2009 6:29 am

So if there is no input to the unit and the vol. is all the way up, the pot can't be heard to be scratchy?

Hmm... May not be a dirty pot then. If it happens only when the audio signal is present there might be something else going on. There might be a leaky filter cap putting dc across the pot(not good).

nclayton
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Post by nclayton » Fri May 22, 2009 7:20 am

It sounds to me like for some reason the audio opamp wants to run with some DC offset (which makes the pot scratch) and that for some reason the servo's only able to compensate it out when there's no signal. It might be a problem with the opamp, but I think there's also an internal passive offset adjustment and I'd try trimming that first and see if it goes away.


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Post by lacquer_monkey » Sun May 24, 2009 11:39 pm

Why don't you call John and ask him what he suggests? After all, he designed it and thus probably knows more about them than anyone else. His number is on his website.
uh... what??

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Post by John Hardy » Tue May 26, 2009 6:17 pm

Why don't you call John and ask him what he suggests?
What a revolutionary idea!

Or, I could answer the question here. The problem is either a dirty/rough pot, or there is a small DC voltage/current appearing at the wiper of the pot due to a small current (the input bias current) flowing from the inverting input of the 990C to the wiper of the pot.

All of my mic preamp products have "input bias current compensation" circuitry that is supposed to null the input bias current of the 990C op-amp (typically 2.2uA). If the current is sufficiently nulled, there will be little to no noise during adjustment of the pot.

The pot is fully sealed, so using a contact cleaner will not work.

Thank you.

John Hardy
847-864-8060
866-379-1450 (Toll-Free)

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