After sorting out the distortion issues with my single-supply opamp, I thought I was essentially finished with audio quality issues, at least until I started to pack these components closely in an enclosure. Boy was I wrong.
I've left the core of the mixer circuit breaded and active as I play with the display circuit, and I began to notice some noise. Then when I plugged the USB interface in, the noise got distractingly bad. It's definitely some sort of interference that's getting passed down the power or ground lines, and it absolutely has to go.
This has been a more difficult subject to research, mostly because the material I find is largely dedicated to high-frequency and RF circuits, not audio. One of the first successes was putting buffering capacitors (1uf and 0.1uf) on the voltage-dividing-buffering opamp, between the supply and ground. This immediately cleaned up all but the tiniest bit of already-tiny noise I was hearing without the USB attached. (That is mostly the noise from the microprocessor,) The USB noise is proving much harder to tackle. A trip to Fry's got me an assortment of chokes and ferrite rings, and I've been swapping them in and out on supply and ground from the USB bridge, with varying success. The biggest problem is that this sort of interference seems to be very hard to chase down; just waving my hand over the board makes a difference, and the inherent component-jostling that comes with swapping parts makes for fairly unrepeatable results. I can't tell if my efforts are making a difference, or if I'm just having a lucky low-interference moment...
I noticed that the USB cable I'm using doesn't have a choke on it. Since the noise I'm hearing seems to track the computer's activity at least as much as the arduino's, I'll see what a better cable can do.
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