Hi everyone, I'm not an audio engineer or even sure what one is but I have a problem with a humm. It happens when ever I have some audio device connected to the wall outlet. Let me explain further and see if one of you gurus sees how to fix it or at least track down the problem: I have a Sony EX-3 video camera that can use batteries or a power connection via it's battery charger. On battery, no problem. Plug in the battery/charger-power for the camera and instantly, the hum appears. I just bought a PreSonus Blue DP V2 audio amplifier for use with some mics. It uses a wall wart DC power unit 12v 1amp for power. I use that into a Azden field mixer that runs on AA batteries. It goes into the EX-3. Using the Azden alone into the EX-3 when all are on battery, no problem. Connect the wall power to either unit and presto, the hum is back with a vengeance. More info: All cables are XLR Canare or Mogami, no computer hookups anywhere near. No power lines are crossing or near anything. There is a light dimmer on the circuit but it's off if that's a clue. I ran a power cable to an entirely different circuit and the problem was there. I use a Steinberg audio interface to the computer and I've used the same setup with the PreSonus amplifier and field mixer into the Steinberg and there is no hum. I run into the problem with only one input into the field mixer and that's from an AKG microphone. The hum also occurs with just the field mixer connected to the EX-3 and no PreSonus connection. I surmise it is the power connection and it's feeding through the DC power supplies and causing the problem ... except that using ad different circuit entirely does not change the problem. I'm stumped about how to track down the problem and what to use or do to get rid of it. At the moment, I'm thinking about an exorcist... Admittedly, it doesn't take a big problem to stump me... But I am hoping some of you experts can give me the knowledge to diagnose and fix the underlying problem. Many thanks for you help.