pot noise on clean op-amp boost circuit

[eddie_bowers]

I have a pretty simple op-amp boost circuit and it works great EXCEPT you hear a scratching noise when adjusting the gain.
The gain is controlled by a 100k pot in the negative feedback loop of the op-amp (inverting in this case).
I can always just use a fixed resistor, and then put a volume pot on the output, but i'm wondering if there is a more elegant solution AND if this is normal.

Thoughts?

-Eddie

[tiges_ tendres]

Perhaps try a small capacitor in the feedback loop.  I think a scratchy sound usually means DC on the pot.

[eddie_bowers]

That might be it. Any idea of a good value?  47pf or so?

[DDD]

Scratcing sounds may be pretty loud if you didn't connect wiper with one of the "side" legs of the pot. Make such a connection, and the scratch will be reduced.
Also, if the gain control will work "vice versa", just connect the wiper to another side leg.

[tiges_ tendres]

That might be it. Any idea of a good value?  47pf or so?

I haven't set up a booster quite like you're describing, but from previous experience with opamp based negative feedback circuits the capacitor does affect the frequency response.  I would try that value and see what happens.

[merlinb]

Hopefully you are using a FET input opamp, as these should have small DC offsets. Also, is your input cap-coupled?

[eddie_bowers]

Thanks everyone.

I'm using the OPA2134 (FET input) and the input is cap coupled.
I'm going to try the cap accross the feedback loop and tie down the other wiper (not that way now) and hopefully that will do the trick.

-Eddie

[eddie_bowers]

That fixed 90% of the pot noise (cap and other lug).  I can still hear it a little, but I think it's acceptable.

Thanks guys.

[Hides-His-Eyes]

Which fixed it, or did you do both at once?

[eddie_bowers]

I did both. (yeah, not very scientific)