Stereo Microphones and Ground Loops
Some preamplifiers are prone to developing ground loops when used in conjunction with stereo
microphones such as the SF-24. Ground loops can develop in the preamplifier with any stereo
microphone regardless of type (i.e. condenser, dynamic, ribbon). A ground loop manifests itself
as unwanted buzz or hum at 60 Hz and/or harmonics of 60 Hz.
The condition is brought on when the left and right transducer elements are plugged into two
inputs of a stereo or multi-channel preamplifier. The two three-pin male XLR connectors of the
stereo microphone cable usually share Pin-1 as ground, so they are grounded to each other
through the cable set. If the grounding scheme within the preamplifier is poorly designed, or the
distances to internal ground are too great, a ground loop develops.
You can perform a simple test to check for this condition (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, line-
operated 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 fashioned from a male-
female XLR barrel adapter. Switchcraft makes a very 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.
Correcting the problem at the preamplifier is preferable, but is often more difficult and/or
expensive.
Equalization and Ribbon Microphones
One of the great strengths of ribbon microphones is how well they take EQ. Even with
substantial amounts of equalization, ribbons retain their natural, “real” quality. For example,
when a lead vocal is being performed on a ribbon microphone, you can boost the upper-end
frequencies to the point where the ribbon mic emulates the performance curve of a condenser
mic with excellent results. This is not to say that a ribbon microphone can replace a quality
condenser mic, but the EQ friendliness inherent in ribbon microphones does allow for an
enormous amount of flexibility.
The reason that ribbon mics take EQ so well is their inherent low self-noise, unusually smooth
frequency response characteristics and freedom from off-axis coloration. Dialing in high
amounts of equalization on condenser or dynamic microphones also means dialing in extra
amounts of the microphone's distortion products and self noise: garbage that contributes to an
unnatural, unpleasant sound. Because distortion and self-noise are almost nonexistent in ribbon
microphones, high levels of EQ can be used without adding harshness or excessive noise.
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