Microphone Rf Bypassing; Line Level Signals; Wiring Diagram For Mi39A Instrument Cable - Lectrosonics LMb 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
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 sig-
nal from affecting the capsule, even though the trans-
mitter input circuitry is already RF bypassed.
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 pos-
sible, or if you are still having problems, capacitors can
be installed on the mic pins inside the TA5F connec-
tor housing. Refer to the diagram below for the correct
locations of capacitors.
Use 330 pF capacitors. 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.
2-WIRE MIC
Capacitors next
to mic capsule
SHIELD
AUDIO
CAPSULE
TA5F
CONNECTOR
Rio Rancho, NM
3-WIRE MIC
SHIELD
AUDIO
BIAS
CAPSULE
TA5F
Capacitors in
CONNECTOR
TA5F connector
Digital Hybrid Wireless Belt-Pack Transmitter

Line Level Signals

The normal wiring 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.
Line Level
Normal Wiring
Line Level
More Headroom
(20 dB)

Wiring Diagram for MI39A Instrument Cable

The MI39ARA and MI39AST instrument cable assem-
blies allow an optimum match between musical instru-
ment pickups and Lectrosonics transmitters with 5-pin
input connectors.
The cables are wired to provide an extended low fre-
quency response with a roll-off at 35 Hz.
Note: This cable is prewired and cannot be
field modified. The cable is available in two
configurations, MI39ARA (right angle 1/4" plug)
and MI39AST (straight 1/4" plug).
SLEEVE
SHIELD
AUDIO
TIP
1/4 " PLUG
Preamp is located inside
the connector shell
See Fig. 8 on
previous page
PIN
SHIELD
1
Preamp
PWR
2
4
5
AUDIO
3 3
4
TA5F
5
PLUG
1
2
15