Some preamplifier designs are prone to developing internal ground loops when used in conjunction with stereo microphones such as the Royer SF-12. A ground loop manifests itself as unwanted noise, buzz or hum (usually 60Hz or 120 Hz). The problem may be more apparent with lower output microphones such as dynamics or passive ribbons because of the high gain required for normal operation.

Ground loops are brought on when the left and right transducer elements of a stereo microphone are plugged into two inputs of a stereo or multi-channel preamplifier. Stereo microphones usually have a multi-conductor cable that carries the two independent, balanced signals and then splits them to a pair of standard three-pin XLR outputs. This pair of three-pin connectors usually shares Pin-1 as ground. If the grounding scheme within the preamplifier is poorly designed (or the distances to internal ground are too great), a ground loop may develop.

You can perform a simple test to check for ground loops (preferably done with a pair of headphones to avoid feedback). Plug one side of the stereo microphone into either preamplifier input. Listen to the output of the preamp. All should be quiet except for the mic signal. Now plug the second side into the next preamplifier input. If a noise or buzz develops, you have a ground loop. The ground loop may be very slight or more pronounced, depending on the preamp. Battery powered preamps usually do not exhibit this problem, and neither do well designed, AC powered mic preamps.

The simple fix is to disconnect one of the microphone’s two Pin-1 ground connections. A better method is to make a small ground lift adapter from a male-female XLR barrel adapter. Switchcraft makes a nice one, and it takes less than five minutes to wire it up. Simply connect Pin-2-to-Pin-2, Pin-3 to Pin-3, and leave Pin-1 disconnected. Insert the adapter between one of the microphone’s outputs and the preamplifier input. Correcting the problem at the preamplifier would be preferable, but is often more difficult and/or expensive.