This question is actually connected with this one: Getting 1.2V from USB
I've successfully connected my FIIO E3 headphones amplifier to USB (so that I don't have to use batteries). But I've notices few things:
1) I connect it to computer USB port. I get some strange noise - like humming and crackling. Actually, when I do something on my computer (e.g. I copy something to HDD), that noise gets louder.
2) If I connect it to the computer USB port with USB extension cord, I don't hear noise from 1), but I hear very loud white noise
3) Then I've tried to connect it to my iPhone charger (it gives 5V from 220V). It works perfectly (no humming or other noise).
So anyone got a clue what's happening ?