Shure SM81 User manual

Model SM81 User Guide
©2005, Shure Incorporated
27C2916 (Rev. 6)
Printed in U.S.A.
MODEL SM81 UNIDIRECTIONAL
CONDENSER MICROPHONE
The Shure Model SM81 is a high-quality, unidirectional con-
denser microphone designed for studio recording, broadcasting,
and sound reinforcement. Its wide frequency response, low noise
characteristics, and low RF susceptibility have made it a standard
for applications involving acoustic instruments, especially guitar,
piano, and cymbals.
The SM81 is ruggedly constructed. It operates on phantom pow-
er and performs over a wide range of temperatures and humidity
conditions. It is furnished with a swivel adapter, attenuator-switch
lock, foam windscreen, and case for carrying and storage. Other
accessories are available.
Model SM81 Features
• 20 Hz to 20 kHz frequency response
• Flat response curve for accurate reproduction of sound
sources
• Low noise and high output clipping level
• Low distortion over a wide range of load impedances
• Cardioid polar pattern, uniform with frequency and sym-
metric about axis, providing maximum rejection and min-
imum coloration of off-axis sounds
• Low RF susceptibility
• Selectable low-frequency response: flat, 6 or 18 dB/oc-
tave rolloff
• 0 dB/10 dB lockable attenuator switch
• Phantom powering (DIN 45 596 voltages of 12 to 48 Vdc)
• Rugged steel construction for durability
• Field-usable over wide range of temperature and humidi-
ty conditions
SPECIFICATIONS
Type
Condenser (electret bias)
Frequency Response
20 to 20,000 Hz
TYPICAL FREQUENCY RESPONSE
FIGURE 1
Polar Pattern
Cardioid (unidirectional) response-uniform with frequency, sym-
metrical about axis
TYPICAL POLAR PATTERNS
FIGURE 2
Output Impedance
Rated at 150 Ω(85 Ωactual)
Recommended minimum load impedance: 800 Ω(May be used
with loads as low as 150 Ωwith reduced clipping level)
Output Configuration and Connector
Balanced, transformer-coupled output; male XLR connector
Sensitivity (at 1,000 Hz)
Open Circuit Voltage: . . . . . . . . . . . .–45 dBV/Pascal (5.6 mV)
(1 Pascal = 94 dB SPL)
Clipping Level (at 1,000 Hz)
800 ΩLoad: . . . . . . . . . . . . . . . . . . . . . . . . . . –4 dBV (0.63 V)
150 ΩLoad: . . . . . . . . . . . . . . . . . . . . . . . . . –15 dBV (0.18 V)
Total Harmonic Distortion
Less than 0.5% (131 dB SPL at 250 Hz into 800 Ωload)
20 200001000 1000050 100
9876543298765432
+10
0
–10
Hz
dB
-20
+20
1 METER
150°
120°
150°
120°
180°
30°
60°
90°
30°
60°
0
90°
–10 dB
–20 dB
–15 dB
–5 dB
150°
120°
150°
120°
180°
30°
60°
90°
30°
60°
0
90°
–10 dB
–20 dB
–15 dB
–5 dB
500 Hz
100 Hz
1000 Hz
5000 Hz
10000 Hz
2000 Hz
–5 dB
–10 dB
–15 dB
–20 dB
–5 dB
–10 dB
–15 dB
–20 dB

2
Maximum SPL (at 1,000 Hz)
800 Ωload: . . . . . . . . . . . . . . . . . . . . . 136 dB (attenuator at 0)
146 dB (attenuator at -10)
150 Ωload: . . . . . . . . . . . . . . . . . . . . . 128 dB (attenuator at 0)
138 dB (attenuator at -10)
Hum Pickup
-3 dB equivalent SPL in a 1 mOe field (60 Hz)
Self-Noise (equivalent sound pressure levels; measured with true
rms voltmeter)
16 dB typical, A-weighted
19 dB typical, weighted per DIN 45 405
Signal-to-Noise Ratio
78 dB (IEC 651)* at 94 dB SPL
*S/N ratio is difference between microphone output at 94 dB SPL and
microphone self-noise A-weighted.
Overvoltage and Reverse Polarity Protection
Max. external voltage applied to pins 2 and 3
with respect to pin 1: . . . . . . . . . . . . . . . . . . . . . . . . . +52 Vdc
Reverse polarity protection: . . . . . . . . . . . . . . . . 200 mA max.
(diode-clamped)
Polarity
Positive pressure on diaphragm produces positive voltage on
pin 2 relative to pin 3
Cartridge Capacitance
54 pF
Low Frequency Response Switch Positions
Flat; -6 dB/octave below 100 Hz; -18 dB/octave below 80 Hz
Attenuator Switch Positions (Lockable)
0 or -10 dB
Power
Supply Voltage:. . . . . . . . . 11 to 52 Vdc, positive, pins 2 and 3
Current Drain: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 mA max.
Environmental Conditions
Temperature
Storage: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –29° to 74° C
(–20° to 165° F)
Operating:. . . . . . . . . . . . . . . . . . . . . . . . . . . . –6.7° to 49° C
(20° to 120° F)
Humidity
Storage: . . . . . . . . . . . . . . 0–95% relative humidity at room
temperature (72° to 80° F, 22° to 27° C)
Case
Steel construction with vinyl metallic paint finish and stainless
steel screens
Dimensions
See Figure 3
Weight
Net: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 grams (8 oz)
Packaged: . . . . . . . . . . . . . . . . . . . . . . . 740 grams (1 lb 10 oz)
Certification
Eligible to bear CE marking. Conforms to European EMC Direc-
tive 89/336/EEC. Meets applicable tests and performance cri-
teria in European EMC Standard EN 55103 (1996) parts 1 and
2 for residential (E1) and light industrial (E2) environments.
OPERATION
Power
The SM81 requires phantom power. This may be supplied to the
microphone by a mixer, preamplifier or console with built-in phan-
tom power or from an external power supply (such as the Shure
model PS1A). Phantom power sources providing between 11 and
52 Vdc are suitable.
Use only high-quality cables because intermittent shorts be-
tween broken shield wires and balanced conductors will cause ex-
tremely large noise transients in the system. Avoid ground loops
due to grounded connector shells or the microphone case touching
other grounded metal objects. Follow generally accepted audio
grounding practices.
Impedance
A minimum load impedance of 800 Ωor greater should be used
for maximum signal handling and minimum distortion. The load
may be as low as 150 Ω, but a reduction in output clipping level will
result. It should be noted that the power supply itself may add load-
ing (3300 Ωin the Shure PS1A power supply) to the microphones.
PS1A Power Supply
When using the Shure PS1A to phantom power the SM81, con-
nect the microphone cable to the SM81 and to the MICROPHONE
connector of the power supply. The power supply uses the bal-
anced audio cable pair to carry the supply current to the micro-
phone and the cable shield as a ground return.
Connect the OUTPUT connector of the power supply to a
low-impedance microphone input of a mixer, audio console or tape
recorder. A second SM81 may be connected to the remaining pow-
er supply channel in a similar manner.
OVERALL DIMENSIONS
FIGURE 3
212 mm
20 mm
23.5 mm

3
Low-Frequency Response Filter
The SM81 has a three-position low-frequency response filter
controlled by a switch located on its handle. The switch is recessed
to avoid accidental movement, but may be easily moved with fin-
gertips. The user may select either flat response, low-frequency
rolloff of 6 dB per octave below 100 Hz, or low-frequency cutoff of
18 dB per octave below 80 Hz (see Figure 4). When close-miking
instruments or vocalists, an increase in low-frequency response
(proximity effect) takes place. Figure 4 illustrates this effect with the
switch in each of the three positions. Note that the low-frequency
response filter may be used to compensate for proximity effect or
reduce low-frequency noise from stage traffic and other sources.
PROXIMITY EFFECT AND COMPENSATION
FIGURE 4
Attenuator Switch
The SM81 has a switchable 10 dB capacitive attenuator to pre-
vent high sound pressure levels from overloading its internal elec-
tronics. The attenuator is engaged by rotating the actuator ring,
located directly below the grille assembly, until it reaches the “-10"
position. This reduces the output of the microphone by 10 dB and
increases the maximum sound pressure level at clipping by 10 dB.
There are no intermediate levels of attenuation available.
The attenuator ring may be locked in either the “0" or “-10" po-
sition as follows. Unscrew the grille and cartridge assembly by un-
screwing counter-clockwise from the top. Turn the actuator ring to
the “0" or “-10" position as desired. Insert the actuator ring lock
(small clear piece of plastic) in the area behind the actuator ring be-
tween the pin and the edge of the slot. This will prevent the ring
from turning. Replace the grill and cartridge assembly.
Mixer Overload
The SM81's output is about 15 dB higher than most dynamic mi-
crophones. At moderate to high SPLs, this additional output may
require attenuation (or padding). If no attenuation is available on
the preamplifier, mixer or console being used, a resistive attenua-
tor can be inserted between the microphone and the input. The
Shure Model A15AS Attenuator (15, 20, or 25 dB switch-select-
able) is specially designed for use with condenser microphones
such as the SM81. Alternately, the attenuator design shown in Fig-
ure 5 may be used. The resistors shown are 1/2-watt, 1% toler-
ance, and the circuit may be packaged in a Switchcraft S3FM
adapter housing. The circuit will provide15 dB of attenuation and
can be used between the SM81 and the PS1A (or other power sup-
ply), or between the PS1A and the mixer. Two of these circuits may
be used in series to provide 30 dB of attenuation. (Note that, due
to excessive loading, commercially available 150 Ωattenuators,
such as the Shure Model A15AS, are not recommended when two
are used in series.)
15 DB ATTENUATOR CIRCUIT
FIGURE 5
Wind Noise
The wide frequency response of the SM81 makes it sensitive to
wind, breath, and air currents from heating, ventilation and cooling
(HVAC) systems. The foam windscreen included with the SM81
can be used to reduce wind and breath noise, while the low-fre-
quency response filter can be used to reduce low-frequency room
noise caused by HVAC systems.
The Model A81G Pop Filter Grille attenuates breath popping
sounds when the microphone is close-talked, and permits its use
outdoors with minimal pickup of rushing and rumbling sounds. To
install the A81G, slip it over the SM81 until the inside of the A81G
touches the top of the microphone. Tighten the A81G by rotating
the knurled collar clockwise from the bottom. (Note: When remov-
ing the A81G, first loosen the knurled collar. Otherwise the car-
tridge will unscrew with the A81G.)
For outdoor use under very windy conditions, use the Model
A81WS large foam windscreen.
20 100050 100
+10
0
–10
Hz
dB
152.4 MM ( )
152.4 MM ( )
152.4 MM ( ) 12
3
1
2
3
412
412
215
OUTPUT TO
MIXER
INPUT FROM
MICROPHONE

4
BLOCK DIAGRAM
FIGURE 6
CIRCUIT DESCRIPTION
A block diagram of the SM81 is shown in Figure 6. The capacitor
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 con-
version stage. The FET output drives an active low-frequency re-
sponse (high-pass) filter controlled by a three-position switch. The
filter output from a compound transistor, 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 volt-
age, allowing the SM81 to operate over a very wide range of volt-
ages. A reverse voltage protection diode guards against miswired
cables and equipment.
TROUBLESHOOTING
If the SM81 fails to operate properly, verify that the microphone
is powered properly.
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 2 and 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 11 and 52 Vdc.
Due to its high packing density and circuit complexity, disassem-
bly of the SM81 is not recommended. Contact Shure's Service De-
partment if problems persist.
ARCHITECTS' SPECIFICATION
The microphone shall be a condenser microphone with a fre-
quency response of 20 to 20,000 Hz. It shall have a unidirectional
pickup characteristic, with cancellation at the sides of 6 dB and a
minimum cancellation at the rear of 15 dB at 1 kHz. The micro-
phone shall have a rated output impedance of 150 Ωfor connec-
tion 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-freq-uency
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 (8oz).
The microphone shall be capable of being powered by a phan-
tom power supply with an output of 11 to 52 Vdc, or by a mixer, au-
dio console or tape recorder capable of supplying 11 to 52 Vdc.
The microphone shall be a Shure Model SM81.
FURNISHED ACCESSORIES
Swivel Adapter........................................................A57F
10 dB Attenuator Lock ....................................... 34A830
Carrying/Storage Case ..................................... 65A1797
Windscreen..........................................................49A111
OPTIONAL ACCESSORIES
Pop-Filter Grille...................................................... A81G
Heavy-Duty Windscreen ..................................... A81WS
Tripod Microphone Stand (4.3 m [14 ft])..................S15A
Stereo Microphone Adapter ................................... A27M
Cable (7.6m [25ft]) ..................................................C25F
Phantom Power Supply ......................................... PS1A
REPLACEMENT PARTS
Cartridge and Grille Assembly ............................... R104
CAPACITOR
CARTRIDGE
FET IMPEDANCE
CONVERTER
LF RESPONSE
FILTER
CLASS A
COMPOUND
AMPLIFIER
RFI
FILTER
BALANCED
TRANSFORMER
REGULATOR/
REVERSE
VOLTAGE
PROTECT
CAPACITIVE
ATTENUATOR