For those using the PSU + USB input on the DI and who want to get rid of USB power completely (and its noise), I found a quick and easy workaround not requiring cable surgery:
I've read on some websites that many DAC won't work without USB power, obviously. But we shouldn't be limited by that with the DI as it can be powered by its dedicated PSU.
I cutted small pieces of paper from a post it note, which I pushed in until I reached the bottom of the connector computer side, and held them both in place with some tape, letting only the data pins visible.
I plugged that and the DI was recognized by the PC when power via PSU. Switching to USB power at the back of the DI, it wasn't recognized anymore, so no USB power.
I had small pops in the sound so I think my paper insulation isn't very good and I must had brought bad contact with the data pins, but it proved that we can go this way if we want data only USB, it will definitely work with the DI.
I finally worked it out using rubber electrical insulation sticky tape, and that fixed the pops. Same deal, just cut small pieces of that tape and place it over the power pins of the USB plug computer side (that's both pin outward the inside of the plug).
I had the same power behaviour that with paper i.e. with power switch set on USB on the DI, the DI wasn't recognized, while with switch set on PSU, the DI worked. So no USB power any more going through the USB cable then.
After a short try, I don't find any difference in the sound after this mod though. It seems that the DI's USB isolation is already pretty good. Anyway, it couldn't be a bad mod, plus it was cheap and easy to make. I'll let it plugged like that.
Those who wish to try may let us know what they think about changes in sound if any.
Edited by Pacha - 2/4/13 at 2:41pm