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=

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

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

I've done a lot of measuring and listening to different supplies with the Touch and other SB devices.

My major conclusion is that they have relatively little to do with the quality of power reaching the circuitry in side. They have far more to do with the noise injected back into the power line which can get picked up by other devices in your system.

The switching supplies almost all inject a certain amount of noise back into the line, but linear supplies can vary WILDLY as to how much noise gets injected into the line. It is NOT true that linears are noiseless, some will inject far more noise into the line than a good switcher.

John Swenson

Switching and Linear Power Supplies
The cost and weight of “magnetics”, that is inductors and transformers,
can be relatively high, particularly when the frequency at which they
operate must be low. Classically, a power supply operating at 50Hz
 (the mains supply in Europe for example) needs to be relatively large,
heavy and expensive. How much better power supplies could be if they
could run at a rate of 100’s of kHz. Indeed they would be better: the
faster that the circuit operates the less the size, the lower the cost of
the magnetics that it needs. Hence the commercial pressure to develop
a new kind of power supply: the switching power supply.
All new things win in the market place when they have just sufficient
functionality, and crucially, lower price. But if they can add a benefit
at the same time, then their adoption will be rapid. A switching power
supply manages all three and its additional functionality is the ability
to operate from 115V to 250V without user intervention.
In consumer electronics the art and manufacture of switching power
supplies is so advanced that rarely does a manufacturer bother to design
a new one. Stock switching power supplies in consumer electronics are
today a commodity item provided by the subcontractor: you may have
your product assembled in the Far East and the contractor will provide
his own low cost power supply for your product.
Switching power supplies have enabled low cost, good quality consumer
audio and, together with Class-D chips, a 7.1 surround sound home
theater may be purchased for less than the price of the early DVD
players.
Why, then, not use a Switching Power Supply every time? Sadly, the
very things that enable the switching supply to work, namely high
operating frequency, good efficiency and low cost magnetics, prevent its use in high precision, very low noise circuits. The high frequency aspect
radiates electro-magnetic noise in the vicinity and the high efficiency
means fast slew rates and high currents, which again contributes to
high emissions. The unit may be enclosed in a metal box, the IO may
be inductively snubbed, but experience shows that despite great efforts,
the noise will somehow break out and be detectable in the low noise
circuity.

Resonessence Labs Invicta user manual