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Very common with USB audio, it's the quality of the interface since the USB standard doesn't really call for the level of isolation the USB interface needs, hard to blame the PC since it's an RF nightmare inside the case.
I've got quite a few pro level interfaces but one of the nicest 'budget' ones was a Gustard. Simple USB to AES convertor but very good isolation. The Gustard DACs are a lot more money though.
Increasingly speakers use AES inputs anyway so I've found that USB->AES is a useful type of interface.
Airhead-
Interesting, I’ve not used a pro-level USB interface with this PC to be fair. I suspect a grounding issue with the case, as sending midi over usb will cause the issue to show up in the audio output of various bits of kit, even if that’s coming in on balanced analog cables (not using the cheap baked in usb audio interfaces).
Dramatic_Hammer
I have an issue with RFI interference with USB audio on my PC, which I can only assume is down to the PC case as it’s remained present through multiple powersupply, motherboard and audio interface changes! Fortunately FireWire solves the interface problem as it’s also electrically isolated (and old school midi, which is optically isolated).