[tristor]

Which is why I consistently have told people to ensure that they pick a DAC which is powered independently if they intend to connect it via USB. Schitt Audio makes great stuff (it's what I have sitting on my desk right now) which is designed in that way, but there's no magic formula to beat physics when you physically power an audio device over a connection that is vulnerable to induced noise.

If you're trying to eliminate noise in your audio setup, the first and most important thing is having audio converted from digital to analog outside of the computer chassis itself (e.g. instead of a soundcard get a DAC). The second thing is to disconnect the power flows between the two systems (e.g. get a DAC which is separately powered). The third thing is to connect the DAC via a non-electrical connection so that the signal path is not vulnerable to noise in the environment between the two systems (e.g. use Toslink/optical and not USB/copper). The fourth thing is to condition the power input to DAC to remove transients (use an audio power conditioned, which does not need to be some grandiose thing, it's a bunch of capacitors).

Beyond that, there's not much you can do, after all there's EMI/RFI all of the time in the environment. If the DAC chassis is metallic and properly grounded, it should reject most, and the same should be true for the computer chassis, but there is always going to be /some/ incidental noise. As long as the noise floor is low enough that it's well below even quiet listening with amplification, you'll never hear it. But the default state of audio on most computer systems is pretty shit and people don't realize it, because they are mostly listening to Bluetooth earbuds (which at least provide no physical path for induced noise).