+1 to steakster and Rix. Since the USB ports of most server devices also provide output power for USB peripherals, they inherently introduce some noise from the music server's power supply into the USB output. That noise is nowhere near enough to effect the 0's and 1's, but if it is carried into the USB input of the DAC, it can effect the sound quality. If 1 millivolt of noise were to be introduce into the DAC's analog output, it would create a noise floor at -70dB.
Many higher end USB cables reduce the noise carried from the music server to the DAC's USB port. Also, using a LPS on the server instead of SMPS generates less power supply noise at the server's USB output. Converting USB into a format like AES/EBU or S/PDIF that does not carry power also reduces DAC noise. Even grounding the DAC circuit board (not the same as chassis) will helps to reduce DAC output noise in in some cases.
At the end of the day converting 0's to 1's in most modern audiophile DAC's should yield a good analog output signal. However, if even a small amount of analog noise is added to that signal output, there is a noticeable change in the noise floor through an audio system capable of 100dB or greater dynamic range. I know that some people and systems are less sensitive to low levels of analog noise, but I personally prefer my music with as low of a noise floor as possible...
Many higher end USB cables reduce the noise carried from the music server to the DAC's USB port. Also, using a LPS on the server instead of SMPS generates less power supply noise at the server's USB output. Converting USB into a format like AES/EBU or S/PDIF that does not carry power also reduces DAC noise. Even grounding the DAC circuit board (not the same as chassis) will helps to reduce DAC output noise in in some cases.
At the end of the day converting 0's to 1's in most modern audiophile DAC's should yield a good analog output signal. However, if even a small amount of analog noise is added to that signal output, there is a noticeable change in the noise floor through an audio system capable of 100dB or greater dynamic range. I know that some people and systems are less sensitive to low levels of analog noise, but I personally prefer my music with as low of a noise floor as possible...