I just bought a Fiio E5 - it works fine while it is charging or disconnected from the charging power source - no significant noise, and it drives my headphones quite well.
But, when the E5 is fully charged and connected to the power source, the output has a very noticeable 140Hz hum (I checked the frequency against a tone generator), not 60Hz as one would expect from a US A/C line source. Most noticeable when the input is silent or low, but still audible even with the input at moderate levels.
I also tried it plugged into 3 separate power sources - a good-quality USB hub, a MacBook Pro (both connected to its charger, and running off its battery), and a standalone iPhone A/C->USB charger.
I've googled for this problem, and found some mentions of hum, but not this specific problem. Has anyone else run into this problem? If you haven't, can you test yours to make sure (easiest if there is no sound input into the E5, and see the note on testing below).
I plan to use this primarily at my desk, to power studio headphones, so I'd much rather be able to leave it plugged in, rather than unplug it when I want to use it.
Note for testing: even if it's fully charged when you connect it to the power source, it will take a few seconds or minutes to realize it's charged, and switch off charging (when powered on and plugged into power, the light changes from blue/red to blue only when it decides it's fully charged). I don't get the hum until it switches off charging mode.
Thanks!