Boost/Attenuate

[Buffalo Tom]

Hi. Here is a schematic for Boost/Attenuate +/- 20dB with 0dB at 12 o' clock. The boost works but when attenuate there is a crackling noise along with the volume attenuation. How can I fix this? (Its not a bad pot.. must be something wrong in the schematic 

[EBK]

Its not a bad pot..

How do you know it isn't a bad pot?

[j_flanders]

crackling noise along with the volume attenuation.

Usually this means there is DC on the pot (because you lack a DC-blocking cap somewhere) but in this case with the dual power supply I don't know.
Is this a schematic you adapted for a pedal running on 9V (split power supply) or are you really running this from -12V/+12V supply?

Googling your schematic, I got your old thread over here:
https://www.diystompboxes.com/smfforum/index.php?topic=121873.0
which confirm my suspicions.

[Buffalo Tom]

crackling noise along with the volume attenuation.

Usually this means there is DC on the pot (because you lack a DC-blocking cap somewhere) but in this case with the dual power supply I don't know.
Is this a schematic you adapted for a pedal running on 9V (split power supply) or are you really running this from -12V/+12V supply?

Googling your schematic, I got your old thread over here:
https://www.diystompboxes.com/smfforum/index.php?topic=121873.0
which confirm my suspicions.

Thanks for your reply. Here is my power supply.

[Kipper4]

A series 100nf+ cap after OA1 might help. Is it on the breadboard?

[Buffalo Tom]

A series 100nf+ cap after OA1 might help. Is it on the breadboard?

Yes two breadboards at the moment. One for the power supply and one for the op amp circuit. Later on its going to be built in a 19 rack format.

[Kipper4]

Did you try a cap and see if the dc is still on the pot, adding crackle on turning.

[Buffalo Tom]

Did you try a cap and see if the dc is still on the pot, adding crackle on turning.

I hope your idea with the cap will fix it.. But unfortunately I cant try it until tomorrow morning. Will let you know how things are going tomorrow! Thanks   

[antonis]

Probably the cap will fix the issue but not the problem..
(which is the presence of DC on GND..)

[EBK]

All issues of DC on pots causing crackling can be fixed with a little paint. 

[Buffalo Tom]

Did you try a cap and see if the dc is still on the pot, adding crackle on turning.

The cap alone did not work, still noice along the attenuation.. But then I added a series 150 ohm resistor after OA2. Then all the crackle disappeared and both boost and attenuation is smooth without noice. I experimented some more and removed the cap after OA1 and the crackle did not come back. So it was the 150 ohm resistor after OA2 that did the trick alone.

I have also messed around with the power supply and tried different versions.. The boosted sound did not sound good so I started to think that maybe I have too much power. 24V in with +/- 12 virtual earth thing. The power supply posted here. I went more basic and just powered the circuit with "only" 12 volt. The boosted sound is now more "amp-like".. So this is where I am at the moment and Im happy with it. Thanks for your help... Im learning a lot.