I’ve been using my OP-1 as both a MIDI controller and as a sound generator, but when I plug both into my computer I get the high whining sound you hear at the beginning of this video: Ambient Video Entertainment Network - YouTube
like my good man @mistercharlie suggests
if u update to the new firmware which opens up USB AUDIO streaming
by making the OP1 an audio interface
u can also get AUDIO and MIDI over the USB without any need to traditional audio cables and such
this would also prolly eliminate your ground loop
nice! glad u got it sorted.
fwiw the USB isolator linked above is the same thing as the AUDIO one i linked.
just isolates the USB instead of the AUDIO
the results would be the same.
i have both and have used both with much success