I’m making progress on testing the latest board and I got it making some noise. But, it’s not all the noise that I was looking for. It’s more! Specifically, I made some layout choices that turn out to be quite poor. Let me save you from making the same mistake. Click through for the details.
The number one rule to follow for keeping audio signals from picking up noise is to keep them from away from noise sources. Noise sources in my synthesizer most importantly include serial communication signals which are used for talking to the digital potentiometers in the filter and the i/o expander used for the switches. Well, I happen to have crisscrossed these communication signals with the audio signals. When I first plugged it in and heard the sound, there was a distinct presence of noise. In fact, it was louder than the audio signal. Boo!
The number two rule for reducing noise is to keep a nicely unbroken ground plane underneath the audio traces. This rule ties into the first rule in that it prevents you from running noisy traces underneath the audio traces. It is impossible to keep an unbroken ground plane in a two layer design because traces have to run on both sides. I really messed it up though by running my noisy communication signals through the middle of the board, breaking up the ground plane and passing the noisy signals under the audio signals that should have been peacefully passing over a ground plane. See what I mean in the image below.