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There is a ground loop in your setup, likely caused by MIDI devices receiving MIDI without an opto-isolator in the circuit, or perhaps not designed properly to isolate the digital signals from the audio signal.
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In the MIDI standard, the MIDI receiving device should be designed with an opto-isolator to isolate the MIDI controller from the MIDI device. |
We are not able to comment on why the MIDI device is not designed with an opto-isolator, or why your MIDI device is leaking digital noise into its audio signal path. Check with the manufacturer on that.
This problem has nothing to do with the MIDI Controller. The MIDI Controller simply sends a MIDI signal out from its MIDI ports. In the MIDI standard, the receiving device needs to implement proper isolation to prevent ground loops.
Solution
Usually, cutting the ground in your TRS cable will might help break the ground loop and solve the noise issue.
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