Parallel looper Issues update

spacecommandant · Post by **spacecommandant** » 21 Jun 2022, 18:01

I was having issues with a parallel looper design and asked for advice a while back to help eliminate switch popping and circuit noise. The attached file [SCHEMATIC_Version1] shows a pop-free version; the switch popping finally disappeared after removing all pulldown resistors. The guitar signal was too quiet, though, and the circuit had a high noise floor. Although the schematic doesn't show it, I was using a voltage doubler.
The second file [SCHEMATIC_Version2] was a redesign using lower noise op amps and other changes. The switching itself remained the same. This version has switch popping issues.
The pulldown resistors at input/output/4PDT lug 4 were added later to try to eliminate the popping but didn't help. I tried lug 6 to Vref as in Version 1 but it didn't help. The LED being removed doesn't help either. I lowered all capacitors in the audio path to 100n to temporarily hear what that did when switching the 4PDT and SPDT(Tails) on and off. There was still a significant pop when either were turned to the on position.
I should note that I used an audio probe to determine that there is *almost* no pop going through the sends, mostly through the circuit output and output jack. I added a 100n capacitor after lug 2 (not shown) and this reduced the pop when switching the 4PDT to the off position.
If there's anything else worth trying, I'd appreciate any suggestions!

mauman · Post by **mauman** » 24 Jun 2022, 21:31

Random thoughts: Are you testing it with (1) other pedals in the send/return loops, or (2) nothing connected to send/return jacks, or (3) send A patched to return A, and send B patched to return B? Have you tried adding pulldown resistors to the send/return signal leads? Have you considered ventriloquism, wherein the amp or another pedal has a DC bias that makes this pedal seem to be the culprit? I have an amp that does that, every pedal connected to it pops, but none of the pedals pop with other amps.

mauman · Post by **mauman** » 24 Jun 2022, 22:16

One more thought: instead of doubling +9V to ~+18V and then dividing it, what about inverting +9V to -9V and using ground for your op amp reference? All the same to the op amps (18V between V+ and V- either way), same headroom, and it simplifies your power section.

And I just noticed you have both U3B and U4B biased to Vref in version 2, with DC continuity between them. Did you mean to split R30 and R31 with a cap?