I hope someone from Arturia can provide an answer for this. My Microfreak emits a loud annoying 60Hz ground hum/buzz when it is in the powered off state (no USB connected and using a nice gold plated 10ft monster cable). When I power on the Microfreak, I see the LCD panel first shows the "MicroFreak" splash screen for approx 2 seconds. During this 2 second period, the noise is still present. The LCD splash screen flickers once and the noise goes away. I first thought that whatever is suppressing the noise is software and not hardware based, or the noise gets suppressed when the MicroFreak firmware is initialized and there is some filtration for the incoming power. If I use a 3-prong adapter and lift the ground connection on the power supply, the noise level is much improved. Not completely gone, but much lower than before. Seems to me that when the power switch is off, the noise from the power supply bleeds over to the output jack. I find it kind of strange that you would even NEED a 3-prong adapter for a synth that runs on 12volt DC. I own 4 synthesizers that take DC adapters and not one of them has a 3-prong connection for the power supply.
The power supply model I have is Hon-Kwang Electric Co, Ltd. MODEL: HK-AY-120A100-DK. 12V @ 1.0A. A poor quality/design power supply seems to be at fault here but this is still a major annoyance for me and a show stopper.