The Title says it all :)

PS

I just noticed that it hums more in Performance than Voice, if that makes any difference to te overall problem.

Hi

Most likely an USB ground loop problem.
There will be several threads on the forum about it, here’re a couple:
http://www.motifator.com/index.php/forum/viewthread/478574/

http://www.motifator.com/index.php/forum/viewthread/479397/

Best of luck

Cheers
zpink

PS. No idea why it would be worse in performance mode though.

Hi zpink!

Are there any instructions on how to perform the usb cable surgery?

Funny enough, I failed to find a YouTube video showing someone doing this. Surprising since there are people showing more or less any basic task there?!

Not much to say really, apart from encouraging you to be careful since those wires in there are rather delicate!

I used a Stanley knife to gently cut around the cable, no more than maybe a millimetre in. Two cuts with maybe 15 mm between them, then one along the cable between the cuts.
Then remove the shield and pull that back. In the gap under where the shield used to be, you can most likely use your finger nails to get through the plastic there and expose the 3 wires.
Cut the black one and tape it all together again. Making sure that the shield stays apart!

Just be careful with those fine wires and you should be fine! ;-)

I followed your instructions (GREAT and PRECISE, btw :)), but now the bus powered Keystation I have at home doesn’t work (my MOXF8 is at the Rehearsal Hall), so I have no way to see if the surgery was successful, but I’m hoping/assuming that the cable can only be used with the MOXF8/other keyboards (that do not need powering).  :)

Ah, ok, nope, sorry, USB power will not work without the black ground lead.
I thought 5PinDIN mentioned that in one of the threads, but I may have missed that thread.

I didn’t think of mentioning that since you were talking about the cable between your MoXF and computer and the MoXF not being USB powered, sorry about that. :-(

All my gear is mains powered, the Mininova can be powered via USB, but I removed that option in my setup in favour of silence by cutting the ground also in its cable.

If you need something to be USB powered, you can go half-way and cut the shield but leave the black ground. It should still be quieter than with the shield in most cases where ground hum is an issue.