models do. The latter design could still produce unwanted distor-
tion due to overloading, even if the pad were used. Although this
is rarely an issue, we felt that it was important to cover the sub-
ject.
Since we’re on the subject of preamplifiers, we thought you might
find the following information on stereo microphones an insight-
ful addition
to the information presented on preamplifiers, per-
formance and selection.
Stereo Microphones and Ground Loops
Some preamplifier designs are prone to developing internal
ground loops when used in conjunction with stereo or multi-
channeled microphones, such as the Royer model SF-12. Al-
though this phenomenon is uncommon, ground loops can develop
in the preamplifier with any stereo microphone, regardless of the
type (i.e. condenser, dynamic, ribbon). A ground loop manifests
itself as unwanted noise, buzz or hum (usually 60 Hz or 120 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. Stereo microphones usually have a multi conductor
cable that carries the two independent 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, 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 in-
ternal ground are too great, a ground loop develops. The problem
may be more apparent with some low output microphones such as
dynamics or passive ribbons because of the high gain required for
efficient operation.
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 in-
put. Listen to the output of the preamp. All should be quiet ex-
cept for the mic signal. Now plug the second side into the next
preamplifier input. If a noise or buzz develops, you have a
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