Shure SM81 User Manual page 3

Low-Frequency Response Filter
The SM81 has a three-position low-frequency response fil-
ter controlled by a switch located on its handle. The switch is
recessed to avoid accidental movement, but may be easily
moved with fingertips. 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 1). 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.
152.4 MM (
+10
0
152.4 MM (
–10
20 50 100
PROXIMITY EFFECT AND COMPENSATION
Attenuator Switch
The SM81 has a switchable 10 dB capacitive attenuator to
prevent high sound pressure levels from overloading its internal
electronics. 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 attenua-
tion available.
The attenuator ring may be locked in either the "0" or "–10"
position as follows. Unscrew the grille and cartridge assembly
by unscrewing counter-clockwise from the top. Turn the actua-
tor ring to the "0" or "–10" position as desired. Insert the actuator
ring lock (small clear piece of plastic) in the area behind the ac-
tuator ring between the pin and the edge of the slot. This will
prevent the ring from turning. Replace the grill and cartridge as-
sembly.
Mixer Overload
The SM81's output is about 15 dB higher than most dynam-
ic microphones. At moderate to high SPLs, this additional out-
CAPACITOR
CARTRIDGE
CAPACITIVE
ATTENUATOR
)
152.4 MM ( )
)
1000
Hz
FIGURE 4
LF
FET
RESPONSE
IMPEDANCE
FILTER
CONVERTER
put may require attenuation (or padding). If no attenuation is
available on the preamplifier, mixer or console being used, a re-
sistive attenuator can be inserted between the microphone and
the input. The Shure Model A15AS Attenuator (15, 20, or 25 dB
switch-selectable) is specially designed for use with condenser
microphones such as the SM81. Alternately, the attenuator de-
sign shown in Figure 5 may be used. The resistors shown are
1/2-watt, 1% tolerance, 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 supply), 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, com-
mercially available 150 Ω attenuators, such as the Shure Model
A15AS, are not recommended when two are used in series.)
INPUT FROM
MICROPHONE
1
3
Wind Noise
The wide frequency response of the SM81 makes it sensi-
tive 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-frequency response filter can be used to reduce
low-frequency room noise caused by HVAC systems.
The Model A81G Pop Filter Grille attenuates breath pop-
ping 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 removing the A81G, first loosen the knurled collar.
Otherwise the cartridge will unscrew with the A81G.)
For outdoor use under very windy conditions, use the Mod-
el A81WS large foam windscreen.
BALANCED
TRANSFORMER
CLASS A
COMPOUND
AMPLIFIER
BLOCK DIAGRAM
FIGURE 6
3
412
2
215
412
15 DB ATTENUATOR CIRCUIT
FIGURE 5
RFI
FILTER
REGULATOR/
REVERSE
VOLTAGE
PROTECT
OUTPUT TO
MIXER
2 1
3