Thank you for your reply. I have disconnected everything else from the mixer, (all outputs/inputs and even grounds from turntables) and only getting sound via usb and internal mixer audio interface. So the only connections in the mixer are the send/returns, the usb and the power cable. Unfortunately the hum is still there when monitoring via the headphones. I do not have a different pedal and no second pair of cables. So i guess i will have to get some different cables, this time unbalanced and hope for the best..