Not quite mic preamp noise, but another source of noise has come to
my attention recently due to a client compaining about noisy mic
cables:
The "plug-in power" available on the mic jacks of many small
MiniDisc recorders can cause bad crackling noises at the mic cable
jack. This is due to small changes in resistance causing a shift in
the DC voltage drop, and can be especially problematic when using
the new right-angle mini-plug connectors as they can tend to rotate
a little if not tied down.
This can be solved by inserting a small electrolytic capacitor in
the XLR cable shell, with the negative lead soldered to pin 2 and
the positive lead connected to the wire that was previously on pin 2.
A Panasonic FC series (low ESR, good for audio) cap that works well
and fits in a Neutrik shell is available from Digikey, item # P11214-
ND. It is 27uF/25v, and measured with a Sharp MD-DR7-A MiniDisc
recorder introduces only a 0.3dB drop at 20 Hz and no measurable
distortion. Caution: Don't use this cap with an outboard 48volt
power supply that does not block the 48v volts from the output side.
48v self powered mics are no problem.
Thanks,
- Adam Liberman
Liberman Sound
www.libermansound.com
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