What causes 60-cycle hum sometimes when I try to hook up an on-stage guitar amp? Why doesn’t the Red-Eye normally need a Ground Lift Switch? How do you solve the problem?
You only run into problems with AC ground loops and 60-cycle hum when AC grounds from two different AC power circuits are connected together. Specifically, it the ground of AC power running the stage is connected to the ground of AC power running a venue’s sound system are connected together through any DI.
The Red-Eye is normally used to connect an isolated acoustic instrument to a venue’s mix board so there is no connection to any on-stage AC ground. In the normal situation, stage ground is not connected to the Red-Eye, so there is no problem.
With the Red-Eye connected to the venue’s sound system ground via an XLR cable, connecting a pedal board to the Red-Eye's Effects Loop is not normally a problem because pedal (wall-wart) power is isolated and there is no connection to stage AC power ground.
An example that could cause 60 Hz hum would be connecting a Red-Eye to a venue's mix board ground via the XLR connection and also to an on-stage guitar amp ground via a ¼-inch cable. The AC ground is different for each AC circuit and tying them together through a DI like the Red-Eye would induce 60 Hz hum in the grounded device connected with a single-ended ¼-inch cable, i.e. the guitar amp.
If you need to connect to an on-stage amp or some other AC-grounded device via the Red-Eye's Effects Out 1/4-inch connector and at the same time connect to a venue's remote mix board you will likely encounter 60-cycle hum. Some guitar amps have a ground-lift switch which solves the problem. If there is no ground lift on the guitar amp, you can use a ground-lift adapter to connect between the Red-Eye's XLR output and the cable to the mix board.
Here's a link to a popular XLR ground-lift adapter...
Others, as inexpensive as $10, are available through Amazon.
If you do use an XLR ground lift adapter, it will interrupt Phantom Power from the mix board and the Red-Eye will need to run on its internal 9-volt battery.