I’ve tried a couple of times to power USB devices (notably my Keystep Pro) from my modular, and it always ends up throwing really annoying noise back into my system. Usually I just resigned myself to powering the devices externally, but my latest project is a polyphonic MIDI controlled synth based on a Raspberry Pi Zero that I’d like to integrate into my rack, so I can play some chords. Think Poly Cinematic on the cheap. I could just make this an external box to solve the noise problem, but I’d really like to have it in the rack. So far I’ve tried adding isolating capacitors, powering it via GPIO pins instead of the USB jack, disabling all wireless connectivity on the pi, and using a Raspberry Pi 3a’s built-in audio jack instead of a DAC shim on a Zero. None of these made a difference. In a desperate move I even tried to remove the ground from the audio connection, thinking that it must be some sort of ground loop that creates this noise, but that only made things worse. The only solution that’s worked so far was to power the Pi externally. The noise is really annoying, sounds like SMS back in the day. When I’m logged into the Pi from my notebook you can even hear clicks from typing, with no physical connection whatsoever between the two machines.
Any hint on what else I might try to get rid of this noise, or at least alleviate it, would be much appreciated!
It was indeed a ground loop problem. I had the KeyStep CV outputs plugged into the synth, the synth plugged into an audio interface, USB from the audio interface to a hub, and USB from the KeyStep to the hub. Inserting a ground loop noise isolator in the connection between the synth and the audio interface eliminated the noise.
Ok, so. I had everything working in prototype state, on breadboard. To reduce bulk I decided to rip open the isolator and solder everything instead of using jacks. Everything’s working fine, except now the noise is back. My new question is, what did I fuck up? I’m pretty sure it must be the isolator, I might’ve cut some traces or lifted a pad or something. But what could cause the noise to come back? Which connection did I cut, or make, to ruin the isolator?
EDIT: I didn’t check before, but now there’s an electrical connection between the two grounds. Left and right channels are not connected.
Just a quick update: I abandoned the project. None of the raspberries I have is powerful enough for what I want. If you play too many notes too quickly they drop the audio. I might try a 3B or a 4 if those weren’t wicked expensive right now, if you can even find them.
Should’ve stayed away from computers in the first place.