Originally Posted by Walie
amb: Is there a noticable improvement when the AlienDAC is powered by external power compared to usb power?
I did some noise floor measurements via RMAA looped back into my M-Audio Firewire Audiophile, and at 16b/44.1k, there is practically no difference between USB power or an external 9V battery (both regulated to a measured 4.77V by the REG101UA onboard). At 16b/48k, however, the battery gives approximately 2dB lower noise floor through most of the audio band. Not a huge difference, and since these are all well below -100dB it's not going to be audible.
One thing I did observe, is that the noise floor is about 6dB worse when the board is uncased. Since I have very little RFI where I did the measurements, I think the extra noise is probably self-induced. Since the PCM2702 has the DAC and the analog output section built-in, and no provision for separate digital and analog grounds or other isolation between the two, there is probably noise crosstalk from the digital section into the analog section. By having the PCB encased in a grounded aluminum extrusion, the amount of radiated noise is partially absorbed.