I have the same problem with my desktop. Mic trim doesn't fix it for me. I have noticed that I don't have the hum problem when connected to the laptop running on battery power. Seems like it would be a grounding issue. The desktop is plugged into one power strip into an outlet, the HD500 and amp are plugged into another power strip at a different outlet. I guess i'll look into the comment about the computer seeing the HD500 as an audio interface, which i'm pretty sure it is since that is how I had to set it up to record and playback from the computer, only I didn't have the amp in the loop when doing that. I hooked the computer speakers right to the HD500. The hum is like when you have your amp running and don't have a guitar plugged in, touch the tip of the cable and that's the hum I hear.