Stacked Timmies oscillate/squeal

[lars-musik]

I have built a double pedal comprising a Timmy and a Jan Ray circuit. When both are stacked AND the JanRay Treble and Presence controls (VR102 and TR100) are above 12 o' clock there'ssome high pitched oscillation (like a high whistle - frequency adjustable by either pot).

Now the strange thing: This does not happen with every amp (I tried a VOX TB35 and VOX AC15, both played without oscillation. My Workbench-Capsim (ROG Condor) also squeals and an acquainted guitarist has these problems with his two amps (Hook Little Lenny and Redplate Magica).

Do you have any idea how to keep the treble content but make the squeal/oscillation go away? I already doubled the caps in the opamps feedback loops without luck.

Thanks!

Lars

Schematic

[rankot]

I had similar problem with RAT build, and solved it with voltage divider at output.

[lars-musik]

I had similar problem with RAT build, and solved it with voltage divider at output.

Something else than the output potentiometers that already are in place? Could you be more specific?

Thanks!

[rankot]

I have no idea what have happened - it was almost the same build as the previous one, but I decided to try with TLP222G optocouplers for true bypassing, instead of simple 3PDT. But this build had that squeek once volume pot is rotated more than 20%, so I had to replace it with A20k, and add 82k resistor in front of it, to form voltage divider at output. Maybe those TLP222G optos create negative feedback path? I really don't know.

[highwater]

Different amps (or even cables) means different loading impedance, and some (but not all) of those combinations may be capacitative enough to make the opamp angry. If that's the case, a little bit of series resistance usually makes the opamp happy again. It's also possible that the oscillation is a high-enough frequency that you just can't *hear* it with all the amps even though it's still there.

Try adding 100ohm resistors in series with C104 and C203 (hard to read the schematic - the ones between the opamp and the volume pot).

Try the same thing at the output of the first-stage opamps (*inside* the feedback loop).

What opamps are you using? Different opamps (even of the same part-number from the same manufacturer) have different ability to tolerate capacitive loads. I've had the worst luck with TL072s in this regard.

[lars-musik]

Hey Highwater, thanks for your input!

(hard to read the schematic - the ones between the opamp and the volume pot).

The "schematic" text above the image is clickable, linked to a larger version of the schematic. Here's the link:

https://postimg.org/image/f9iopq5yz/

Try adding 100ohm resistors in series with C104 and C203 (hard to read the schematic - the ones between the opamp and the volume pot).

I will try that!

I've had the worst luck with TL072s in this regard.

At the moment the opamps are 4559 and 4558. I'll replace them with different ones and see if this gets me somewhere.