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 prob-
lems with microphone/transmitter compatibility. If the
electret microphone is not designed properly for use
with wireless transmitters, it may be necessary to in-
stall 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.
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, capaci-
tors can be installed on the mic pins inside the TA5F
connector 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:
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
P/N
3-WIRE MIC
SHIELD
AUDIO
BIAS
CAPSULE
Capacitors in
TA5F
CONNECTOR
TA5F connector
Digital Hybrid Wireless

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
connector 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 as-
semblies allow an optimum match between musical
instrument pickups and Lectrosonics transmitters
with 5-pin input connectors.
The cables are wired to provide an extended low
frequency 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
AUDIO
TIP
1/4 " PLUG
Preamp is located inside
the connector shell
®
UHF Belt Pack Transmitter
See Fig. 8 on
previous page
PIN
SHIELD SHIELD
1
Preamp
PWR
2
AUDIO
3
4
5
4 1
5
2
3
TA5F
PLUG
15