So, we've got a hum in our sound system. It's 60hz and its associated harmonics. Our system is structured as such: the amplifiers and crossover are backstage, and all other equipment is in the booth. It's a mono system and there's only one send to the backstage amp rack. In the booth, the mixer's mono out goes to a digital graphi-Q unit, then through an analog EQ, and down the snake into the crossover. The hum is dependent on the dimmers, it's completely absent when they're at 0 or 100. This is odd because the dimmers get their power from a completely different place. They were added later than the facility was built, so they run off a separate three-phase feed. Things I've tried: Unplugging the send from either the crossover or the booth EQ's output stops the hum, so it's in that line. Any setting of the EQ has no effect, nor does disconnecting its input. Bypassing the crossover has no effect. Lifting the ground has no effect. Isolation with a transformer has no effect. Increasing the gain of the crossover or amplifier will affect the volume of the hum as with all other signal. Any suggestions as to where to proceed?