5
BLOCK DIAGRAM
FIGURE 11
CAPACITOR
CARTRIDGE
FET
IMPEDANCE
CONVERTER
LF
RESPONSE
FILTER
CLASS
A
COMPOUND
AMPLIFIER
RFI
FILTER
BALANCED
TRANSFORMER
REGULATOR/
REVERSE
VOLTAGE
PROTECT
CAPACITIVE
ATTENUATOR
CIRCUIT
DESCRIPTION
A
block diagram of the SM81 is shown in Figure 1
1. The ca
-
pacitor cartridge is followed by a switch-controlled capacitive
attenuator stage which provides for 10 dB attenuation at the
cartridge output. The signal is fed to a field-effect transistor
(FET)
impedance
conversion stage. The FET output drives an
active
low-frequency response (high-pass) filter controlled by a
three-position switch. The filter output from a compound tran-
sistor, Class A, emitter-follower amplifier is transformer-
coupled,
providing a balanced output to the RFI protection filter
at the microphone connector. An active, constant-current
power supply circuit regulates the phantom voltage, allowing
the
SM81 to operate over a very wide range of voltages. A
re
-
verse
voltage protection diode guards against miswired cables
and
equipment. The circuit contains five
semiconductors to pro
-
vide low noise, low distortion, wide frequency response and
ultra-reliable
operation over a very wide range of operating con
-
ditions.
SERVICING
Troubleshooting
Due to the high packing density and circuit complexity of
the
SM81, only basic servicing is recommended. The following
steps should be taken if problems arise:
1. Check
the power supply output voltage to the
microphone.
For the Shure PS1A, this should be 21.5 ± 1.5 Vdc open
circuit.
2. Check the voltage on microphone connector pins 2and 3
(at
back of connector; cable connector disassembled from
shell
but connected to microphone). The voltage at pins 2
and
3 with reference
to pin 1 should be between 10 and 48
Vdc.
3. If
more than one SM81 is available, interchange cartridge
assemblies
to determine whether the
cartridge or amplifier
is at fault.
Disassembly
The SM81 can be disassembled as follows:
1. Unscrew the grille and cartridge assembly (counterclock-
wise from top).
2. Lift the 10 dB attenuator actuator ring up and over the
screw threads.
3. Grasp
the center contact of the 10 dB attenuator switch as
-
sembly and lift upward to remove it.
4. Turn the slotted head screw near the plug counterclock-
wise (inward) as far as it will go. Using long-nose pliers,
grasp
one pin of the three-pin connector and withdraw the
RFI filter and plug element from the handle.
5. Remove
the low-frequency response switch
knob by plac
-
ing
a small screwdriver or tweezers
into the slot under the
ridge of the knob and lifting outward.
6. Remove
the small Phillips screw near
the plug. T
ap the car
-
tridge end of the handle
gently on a firm surface; the printed
circuit
board and support assembly will slide out of the car
-
tridoe end.
ARCHITECTS’
SPECIFICA
TION
The microphone shall be a condenser microphone with a
frequency
response of 20 to 20,000 Hz. It shall have a unidirec
-
tional pickup characteristic, with cancellation at the sides of 6
dB and a minimum cancellation at the rear of 15 dB at 1 kHz.
The
microphone
shall have a rated output impedance of 150
Ω
for
connection to microphone
inputs of 150 ohms or higher
. The
open circuit voltage shall be –65
dB (0.56 mV) (0 dB equals 1
volt per microbar).
The microphone shall contain a three-position low
frequency response switch and a lockable 10 dB attenuator
pad.
The overall dimensions shall be 212 mm (8-11/32 in.) in
length
by 23.5 mm (15/16 in.) in diameter
. The handle diameter
shall
be 20.1 mm (25/32 in.). The weight shall be
230 grams (8
oz).
The microphone shall be capable of being powered by a
phantom power supply with an output of 11 to 52 Vdc, or by a
mixer,
audio console or tape recorder capable of supplying 1
1 to
52 Vdc.
The microphone shall be a Shure Model SM81.
FURNISHED
ACCESSORIES
Swivel Adapter A57D.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 dB Attenuator Lock 34A830.
. . . . . . . . . . . . . . . . . . . . . . . .
Carrying/Storage Case 65A1797.
. . . . . . . . . . . . . . . . . . . . . .
Windscreen RK311
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OPTIONAL
ACCESSORIES
Pop-Filter Grille A81G.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Heavy-Duty Windscreen A81WS.
. . . . . . . . . . . . . . . . . . . . . .
Tripod Microphone Stand (4.3 m [14 ftl) S15.
. . . . . . . . . . . .
Stereo Microphone Adapter A27M.
. . . . . . . . . . . . . . . . . . . . .
Cable (7.6m [25ft]) C25F.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phantom Power Supply PS1A.
. . . . . . . . . . . . . . . . . . . . . . . .
Omnidirectional Cartridge R104A.
. . . . . . . . . . . . . . . . . . . . .
REPLACEMENT
P
ARTS
Cartridge and Grille Assembly
R104
.
. . . . . . . . . . . . . . . . . .
SERVICE STATEMENT
For
additional service
or parts information, please contact
Shure’s Service department at 1-800-516-2525. Outside the
United States, please contact your authorized Shure Service
Center.
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