How To Get Rid Of DC On Pot In This Circuit?

[Paul Marossy]

I'm building the Bulldog Speaker Cabinet Simulator again for use in my studio and it works OK except that when I turn the blend pot, I get scratching noises. I know it's DC, but I am not sure of where or how you can add coupling caps without affecting the frequency response of the circuit. I don't think I've made any mistakes anywhere.

Any ideas?

EDIT: If I connected the - leg of C7 to ground instead of Vref would this happen? I thought maybe that was a mistake on the schematic but now I think perhaps it wasn't...

[R.G.]

I'm building the Bulldog Speaker Cabinet Simulator again for use in my studio and it works OK except that when I turn the blend pot, I get scratching noises. I know it's DC, but I am not sure of where or how you can add coupling caps without affecting the frequency response of the circuit. I don't think I've made any mistakes anywhere.

Any ideas?

Insert a BFC between R8/C4 and the end of the Blend pot, R6. 2uF NP/bipolar ought to do it for all output loads more than about 10k ohms.

EDIT: If I connected the - leg of C7 to ground instead of Vref would this happen? I thought maybe that was a mistake on the schematic but now I think perhaps it wasn't...

No. That's irrelevant to this issue. The problem is that the top end of R6 is at 0Vdc and the bottom end of R6 is at Vref, held there by the bias and output levels of the opamps.

[Paul Marossy]

Thanks RG, I will give that a try.

EDIT: I added a 2.2uF bipolar cap between the junction of R8/C4 and the one leg of the blend pot and problem solved! Sure is nice to have you around here more R.G.