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Post subject: FM 15 DSP amp grounding issue
Posted: Fri Apr 04, 2008 6:39 pm
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I've got a FM15 DSP that I bought used, just to find out it wasn't working right.

Here's the short version:

I get TONS of noise pickup (electrical and RF) with known good guitars and known good cables, while plugged into a good, grounded socket. Touching the strings/amp chasis/cables makes gives me more noise.

Here's the long version:

Somewhere before the final amp it's getting saturated with interference and noise.

This was driving me crazy until one time I was poking around inside with an o-scope probe. I noticed that if I put the ground of the o-scope (also plugged into the same circuit as the amp) anywhere along the audio path, the interference would go away and the amp would work great.

A-ha! I must have a ground loop of some kind happening! Sadly, though, I don't understand audio amp theory enough to understand my symptoms enough to repair the amp yet.

- The guitar and cables and house wiring are fine, they work with other amps just great
- The problem doesn't seem to be with chasis ground at all - the noise issue is only mitigated when I ground anything along the incoming audio path through my o-scope.
- The 5v regulator seems to get warm, but I think that's probably because it feeds the final amp stage as well as the DSP board and both are busy amplifying lots of noise right now

If anyone has any suggestions for what I can try next, I'd appreciate it!


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