I am helping a friend with a simple sound system for her new yoga studio. The instructors bring their own phones/mp3 players for their music during yoga, so we ran unbalanced 3.5mm over UTP (cat5e adapters for a longer run - about 75'). You turn up the amp that drives the speakers and we get a terrible noise. Not uncommon for unbalanced, and it was reduced with a ground loop isolator/transformer in line.
The problem is that the signal is clean and DEAD quiet when a phone is connected. When the cable is not plugged in to an active device, it has a ground loop hum.
Why is the signal clean when it's active, but when left open, is noisy? This isn't acceptable for her uses, so what can be done to solve the problem? Thanks!