Microphone Rf Bypassing; Line Level Signals - Lectrosonics SMB Series Instruction Manual

Microphone RF Bypassing

When used on a wireless transmitter, the microphone
element is in the proximity of the RF coming from the
transmitter. The nature of electret microphones makes
them sensitive to RF, which can cause problems with
the microphone/transmitter compatibility. If the electret
microphone is not designed properly for use with wire-
less transmitters, it may be necessary to install a chip
capacitor in the mic capsule or connector to block the
RF from entering the electret capsule.
Some mics require RF protection to keep the radio
signal from affecting the capsule, even though the
transmitter input circuitry is already RF bypassed (see
schematic diagram).
If the mic is wired as directed, and you are having dif-
ficulty with squealing, high noise, or poor frequency
response, RF is likely to be the cause.
The best RF protection is accomplished by installing
RF bypass capacitors at the mic capsule. If this is not
possible, or if you are still having problems, capacitors
can be installed on the mic pins inside the TA5F con-
nector housing.
2 WIRE MIC
Preferred locations for bypass capacitors
SHIELD
AUDIO
CAPSULE
TA5F
CONNECTOR
Alternate locations for bypass capacitors
Install the capacitors as follows: Use 330 pF capaci-
tors. Capacitors are available from Lectrosonics. Please
specify the part number for the desired lead style.
Leaded capacitors: P/N 15117
Leadless capacitors: P/N SCC330P
All Lectrosonics lavaliere mics are already bypassed
and do not need any additional capacitors installed for
proper operation.
Rio Rancho, NM
3 WIRE MIC
SHIELD
AUDIO
BIAS
CAPSULE
TA5F
CONNECTOR
Super-Minature Belt Pack Transmitters

Line Level Signals

The normal hookup for line level signals is:
• Signal Hot to pin 5
• Signal Gnd to pin 1
• Pin 4 jumped to pin 1
This allows signal levels up to 3V RMS to be applied
without limiting.
If more headroom is needed, insert a 20 k resistor in
series with pin 5. Put this resistor inside the TA5F con-
nector to minimize noise pickup.
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