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A Simple Pedalboard, Two Problems, and One Buffer Box Doing Double Duty
You finally finish wiring up a board. Everything's patched, the cables are tidy, and you start running through the effects. You hit the chorus switch and the loudest pop you've ever heard comes out of the amp.
This was supposed to be an easy one. It turned into two separate problems, both caused by the same pedal, and I learned a fair bit from it. Here's what happened and how we fixed it.
What the board was supposed to do
The customer wanted basic effects with two ways of getting an amp sound. One is a real tube amp (a Soldano SLO) run in 4 cable method. The other is a Friedman IR-X as an amp-in-a-box for going direct. Both needed to have pedals in their FX loops.
A JHS Switchback handles the choice. Loop one goes to the SLO's input, and the SLO's effects send comes back into the Switchback's loop one return through the patchbay. Loop two does the same job for the IR-X. A remote switch picks which one is live, and everything after that is shared.
The effects are a Digitech FreqOut, a Carl Martin Quattro, and a DOD Rat, plus a passive volume pedal. We put the volume after the preamps on purpose. The SLO is a high gain amp, so you can treat its preamp as one big overdrive. Put the volume pedal after it and you get all that clipping through the whole sweep. Put it before the amp input and the amp cleans up as you roll back, which is fine if that's what you want. It's just a different sound.
The pop on the chorus switch
The volume pedal sat in the Quattro's own effects loop. That's the send and return built into the Quattro for inserting other pedals, not to be confused with the amp loops.
The pop only showed up with the volume pedal in the quattro loop. Adam swapped every cable, then checked the guitar and the power supply. He pulled the volume pedal out and everything was fine. He plugged in an active volume pedal and it was still fine. Then he tried a different brand of passive volume pedal, and the pop came straight back.
The fix was a buffer after the passive volume pedal, before the Quattro's return. Adam ran the volume pedal into one side of an Output TX, which is our stereo buffer and summing box, and from there into the Quattro's return. Pop gone.
If you ever get a big pop from a passive volume pedal sitting last in another pedal's effects loop, try a buffered return first. It's a lot quicker than working through cables, power and a pedal you suspect is faulty.
The amps bleeding into each other
Problem two was sneakier. The plan was simple. The Quattro has a left and a right output, so one would feed the SLO's effects return and the other would feed the IR-X's return, which goes on to the XLR out. Most stereo pedals buffer and therefore electrically isolate their outputs, so the two amps shouldn't interact.
Adam was only testing the tube amp side, and he started hearing the IR-X's channel switching. It was faint, but it was there. The IR-X wasn't in the signal path at all.
He checked his cables, then checked for continuity between the two outputs. The tips were connected. The Quattro's "stereo" output is a passive split. It's one mono signal jumpered to two jacks, so whatever was connected to one output was connected to the other.
I don't think this is something you'd find in the manual. Manuals tell you to use these outputs for two amps and put pedals in the loop. They don't get granular enough to tell you the stereo outputs are a passive split. Sometimes you just have to find out.
Using the Output TX in the middle of the board
The Output TX is meant for the end of a pedalboard, and this one is nowhere near it. That's the strange spot I mentioned. Adam needed two isolated buffers, and the Output TX is two buffers in one box. One side already served the volume pedal. He used the other side to isolate the Quattro's output from the IR-X return. It wasn't what the box was designed for, but two separate buffers was exactly what the job needed.
What I'd take from this
Test every component in every context. If Adam had only tested the IR-X, or only the tube amp without touching the IR-X, we may never have found the bleed. We caught it in the shop. If the customer had caught it, we'd be shipping a full pedalboard back to ourselves and then out again, and that's a trust problem as much as a logistics one.
I also doubt this particular combination will come up often. It's the first time in almost 20 years either of us has had a Quattro on a build. But effects loops are common, and now we know the symptoms.
Happy building, Grant.
