After a good hour or two of fiddling this morning, I've finally narrowed it down.
If I plug the speakers directly into the motherboard's built-in sound card, there is no interference. If I plug the DAC into my work laptop instead of my home desktop, there is also no interference.
The logical conclusion is that the desktop computer, or some component inside, is generating interference over the USB bus that is not being filtered and is therefore passed through the USB cable, through the DAC, through the cabling, and into the speakers.
I tried other USB cables and other USB ports but no change. FWIW, the motherboard is only about 2 years old -- eVGA X58 FTW model. I would've expected a built-in audio chip to be far more noise-prone, but clearly that's not the case.
(for the record, my hearing is considered to be well-above average so it's very possible that most people don't even hear this noise that I do)
If I plug the speakers directly into the motherboard's built-in sound card, there is no interference. If I plug the DAC into my work laptop instead of my home desktop, there is also no interference.
The logical conclusion is that the desktop computer, or some component inside, is generating interference over the USB bus that is not being filtered and is therefore passed through the USB cable, through the DAC, through the cabling, and into the speakers.
I tried other USB cables and other USB ports but no change. FWIW, the motherboard is only about 2 years old -- eVGA X58 FTW model. I would've expected a built-in audio chip to be far more noise-prone, but clearly that's not the case.
(for the record, my hearing is considered to be well-above average so it's very possible that most people don't even hear this noise that I do)
