First - the function of the USB cable depends on what component it is plugged into.
1. If the component DOES NOT utilize he power from the USB port, then all the cable is doing is passing data - better quality conductors will improve sound quality
2. If the component DOES use the power provided by the USB port then the cables is sending data and providing power.
In case #2, the best USB cable I have come across is this one...
http://www.ebay.ca/itm/Power-supply-and-USB-cable-separate-audio-signal-cable-/251589388236By having the power and signal cables separate there is no possibility of noise being induced between the signal conductors and power conductors, since they are separate cable runs
Using the Doukmall cable you can then experiment with different USB power supplies to get the very best SQ.
The better USB cables will employ carious cable geometries (i.e.how the conductors are positioned within the cable) to minimize the noise issue.
e.g.
- tightly twisting the signal conductors together will help minimize the noise
- ensuring there is some "space" between the signal and power conductors again assists in defeating noise.
If you have ever striped the outer insulation off a piece of CAT 5 and CAT6 cables you will see both of these methods have been employed in CAT 6 to improve the throughput.
Hope that helps