Input ground loop hum only when p10 cables or XLR without phantom power are connected

Yeah, I've tried that, when I disconnect the power cord the hum vanishes...But if the problem is in the USB cable, why does the output sound perfect when there's nothing plugged into the Scarlett inputs? (I've tried switching the USB cable from the interface, and problem persists...haven't tried the galvanic isolation adapter)

One quick thing to check is unplug your USB input and see if the problem goes away. If so, you'll need the USB isolator from my blog page, or one like it.

Best,

Erik

Erik, I hear it through my headphones, I have no speakers...I do hear the hum when the cables alone are connected, although they're much louder when I plug instruments/mics

Yep, switching USB cables alone will not help you. There is a known problem with laptop power supplies and ground loops.  I can't answer your questions because I'm pretty confused and don't know all the details. However, the easiest, and least likely to interfere with your sound quality is isolate the USB line. If disconnecting the USB makes your problem vanish, that's what you need to isolate. It's not just a problem for audio. The device I list is actually for medical products, which suffer the same way.

Stereophile does all their USB DAC testing with a laptop  running from battery power. I think it's a boneheaded way to test DAC's, but that's their standard exactly because of this issue.


Best,


Erik

In fact, most of the time, ground loops are independent of cables (unless wired wrong).

A ground loop can't be fixed by noise filters or switching cables, or better shielding. You have to break the metallic contact in the loop.

Best,

Erik