Grounding problems? Try this...

[Soundman1402]

I discovered that, after my first build of pres, I had a buzzing problem. After troubleshooting with my DMM, I found that the modules in question had no continuity between the gain switch and the chassis... unless I pressed hard with the probe. As I found out, the gain switch was perfectly centered in the hole in the faceplate, and so wasn't touching it.

Rather than jimmy the module to make connection, I used a jeweler's flatblade screwdriver to scrape some paint off the front of the faceplate, so that the nut would make electrical connection. After doing this, I measured 0.01 ohms between the switch and chassis. Good enough! I plugged it in and cranked the gain... Voila, no more buzz.