It seems that there is a solution to the USB power noise problem.
iUSBPower http://ifi-audio.com/portfolio-view/micro-iusbpower/A 9V battery has 0.6uV noise and they claim 0.1uV !
I have not succeeded to lower USB noise any further and may be this is why... :
"To create a USB power supply that is ultra-quiet is an engineering feat in its own right."
Below is the PCB, nicely labeled:
All PCBs should be labeled like that I think. Neat.
I don't see how they solve the ground isolation they claim.
There is a switch for the ground management.
Maybe they are swapping the ground from the USB to the DC regulated 9V ground (?)
I think they are cheating and have not actually managed to filter the 5V USB power,
since they are using an external regulated clean 9V power supply. Maybe they are
using the 9V to power only the "Ultra Low Noise Super Regulator" chip.
Anyway, nice design but there is a small problem. It costs $230...
I think I will stick with the 9V battery.
0.6uV of noise, for free.
George.