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USB ground loop?

thefsb

Addicted to Fun and Learning
Joined
Nov 2, 2019
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I have a guitar multi-effects device with output connected to a MOTU M4 audio interface, which is connected to powered monitors and headphones. The M4 connects to my computer through a powered USB hub. The M4 does not have its own DC power supply but the FX device and USB hub each have their own supply. This much works fine.

The FX can optionally connect to the computer over USB to program it. But when I plug it in a nasty noise appears in the M4's output. It's not real loud but it's enough to be annoying and it sounds like computer noise with buzzing high frequency sounds that change frequently in character. The loudness and quality of the noise changes depending which USB port I connect the FX to. It seems the cable makes a difference too.

If I disconnect the FX from its PSU (it accepts USB power) then the noise gets much worse.

What to do?

In the diagram USB is green, low voltage DC is red, 110 VAC is purple, and the analog audio signals are black. The problem only arises when I connect the FX USB to computer or to hub.

FX topology.png
 
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