FANKY wrote:Hmmm, if i touch some of the metal parts on the RPI the hum goes away... any thoughts?
Two things you could try:
1) Disconnect the shield connection to the microphone. (Do this in such a way that it can be reconnected if necessary.)
2) Provide a ground connection to the shield, either with the shield disconnected from the Pi, or with the shield reconnected thus grounding Pi 0V.
Mains hum is being picked up, probably on the microphone cable. The HDMI ground is enough to stop it, so a permanent ground should do the same. It might cause a ground loop, though, when the HDMI is connected. So the other approach is to try to disconnect the source of the hum. The microphone input should be fairly well balanced (wires routed identically, even if the electrical input is not balanced) so that is a less likely route. So the shield (which is currently floating, because Pi 0V is not grounded) is the most likely source. Isolating it may be enough. Isolating from the Pi 0V and grounding should preserve the screening of the mic connections, just in case the input is not balanced and the hum gets in that way, but will avoid a ground loop with HDMI.
Note: to be ultra safe, check that there is no AC or DC voltage present between the shield and ground before connecting. Use a 1K resistor for a temporary ground connection first, and check the voltage across that, otherwise the hum you are trying to remove may show as an AC voltage.