Power supply

Complains about noise caused by PSU’s are not uncommon.

I'm using a laptop with an Edirol UA-1Ex usb sound card, when my computer is plugged to AC I hear some noise on the speakers, Moreover when I do something cpu intensive tasks I hear more noises (while cpu is working hard). If the AC adapter is unplugged I don't hear any noises.
http://www.hydrogenaudio.org/forums/index.php?showtopic=66474&hl=

Buying a better (not necessary expensive) PSU solves these problems.

Some advice a linear PSU to drive the PC. Other doubt if this will help.

The Power System in a Motherboard is fairly complex. If you look closely, there are a bunch of local regulators and depending on the noise rejection and noise of the local regulators, it may not matter what happens in the PS. I've seen many posts where people change the PS in their audio equipment with mV noise figures to nV noise figures but the local regulator still generates mV noise and they claim better sound. But how can this be? the noise is dominated by the local regulator. The sound card may have its own local regulator too. For a meaningful measurement you will need to put a scope at the output pin of the local regulator of the sound card and then measure the noise (ripple) depending on processor load.

glt

An alternative, not uncommon, is to drive the sound card by a separate PSU to shield it from the dirt going on the power rails of the PC.
Invariably a linear PSU is recommended, a switched one is considered a audiophile crime.

Take for instance the EMU 0404 USB. Replace the power supply and the performance increase a ton. Same with the Squeeze box. But really both of these products should be applauded for the price performance ratio.
J. Gordon Rankin

If you have a laptop you can test the impact of the power supply.
Play a song with the mains connected and play it with the mains disconnected.
In the latter case it run on battery power only.

Now you have a clean DC and the switching power supply with all its HF content is inactive. Will this affect the sound?
It probably will, you can even measure it.