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Odd M5 Hum Problem

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I have a medium large pedal board with a few 'dirt' pedals, and which had a Line6 M5 for the rarely used effects such as rotating speaker. All the pedals use an isolated output 9v supply, and hum and noise is at very reasonable levels, all things considered. However, when I put the M5 in line (using its own independent power supply), I get an awful(!) hum from the gain pedals when they are engaged. If I disconnect the M5 power, the hum goes away (M5 still inline with the other pedals). I tried two different supplies for the M5, with no apparent difference in noise level, and the hum is there even with the M5 in idle/bypass mode - it just has to be powered up. I'm scratching my head wondering what might cause this. Any ideas?

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Ground loop.


I take it that all your other pedals are using one power supply ?


I could try and explain further, but I'd only be rewriting what i had read about 'em from Wikipedia. It's something to do with using two or more power supplies and floating earths.


Possible cures include a D.I. Box that can do a "Ground Lift" on the signal.


....And that is my knowledge on the subject exhausted. :)


I'll leave it for someone more knowledgeable from here.

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