Kramer Electronics VM-1610 - Manual (Page 15)

Problem Remedy
Low frequency hum in
the output signal
60Hz hum (ground loop) is caused by a difference in the ground
potential of any two or more devices connected to your signal path. This
difference is compensated by passing that voltage difference through any
available interconnection, including your audio cables.
WARNING!
DO NOT DISCONNECT THE GROUND
FROM ANY PIECE OF AUDIO
EQUIPMENT IN YOUR SIGNAL PATH!
Check the following to remove ground loops:
1. Confirm that all interconnected equipment is connected to the same
phase of power, if possible.
2. Remove equipment connected to that phase that may introduce noise,
such as motors, generators, etc.
3. Disconnect all cables and reconnect them one at a time until the ground
loop reappears. Disconnect the affected cable and replace, or insert an
isolation device (opto isolator or transformer) in the signal path.