Strange, I get this hissing with all chargers, even when charging from batteries, so as clean as DC gets. 5W apple, 10W apple, 12W belkin, 5W samsung, all the same.
From the FAQ in post #3 of the main mojo thread:
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is it normal to have the mojo make a hissing sound (ie the unit itself, audio output is fine) when plugged in to power? brand new unit but it makes this weird noise when its plugged in (both when off and on). the hissing dies down when i plug in the signal cable (unit still off), not sure if i should send my unit back.
It's normal - it is the charging regulator going into low power mode. Don't worry, there is nothing wrong.
Rob
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Further testing on the charger and cable compatibility. Chosen Anker PowerPort 5 for good measured performance and multi-port charging with Anker PowerLine due to good construction with braid + foil shielding. All are available at a reasonable price.
Case 1: Virtually silent, only heard very minor hiss when ear is pretty much on the unit
Case 2: Only noticeable hiss when you put your ears near the unit
Case 3: Loud whine lasting the only the first few seconds, faint charging noise after that.
Case 4: Loud whine, and it goes on for a few seconds and off for a few (voltage drop causing charging circuit to shut down)
Combo 1: Sony or Samsung charger + Sony or Samsung USB cable + extension cord = Case 4
Combo 2: Sony or Samsung charger + Any cable = Case 3
Combo 3: Apple 1A charger + Sony or Samsung USB cable = Case 2
Combo 4: Anker charger + Sony or Samsung USB cable = Case 2, slightly quieter than combo 3
Combo 5: Sony or Samsung charger + long 6ft Anker cable (same cable length as Combo 1) = Case 3
Combo 6: Apple 1A charger + Anker cable/Chord Mojo's bundled cable = Case 1
Combo 7: Anker charger + Anker cable/Chord Mojo's bundled cable = Case 1, slightly quieter than combo 6 directly compared, with no pattern to the noise (always the same loudness)
Anker PowerPort is tested at it's worse case scenario(unloaded), the ripple and spike measurements are better when the charger is fully loaded with devices. The Anker charger have noticeable more steady noise pattern than the Apple charger even with the best cable connected, the Apple charger's noise ripples in loudness and the Anker one is very steady at the same amplitude.
In short:
If you already have a Apple charger handy, just getting a quality USB cable like the one I've tested will yield noticeable gain, especially if you are using longer cables.
If you don't have an Apple charger, getting an Anker charger with their PowerLine cables will yield the best possible result without going to go with a lab bench linear power supply. Having a multiport desktop charager will also allow you to run shorter cables.
Link where I bought them:
Anker PowerPort 5:
http://www.amazon.co.uk/dp/B00VTI8K9K
Anker PowerLine:
http://www.amazon.co.uk/dp/B014H3GKZ4