2
T
otal Harmonic Distortion
Less than 0.5% (131 dB SPL at 250 Hz into 800 Ω
load)
Maximum
SPL (at 1,000 Hz)
136 dB (attenuator at 0); 146 dB (attenuator at 10)
with 800 Ω
load
128 dB (attenuator at 0); 138 dB (attenuator at 10)
with 150 Ω
load
Hum
Pickup
–3 dB equivalent SPL in a 1 moe field (60 Hz)
Output
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
Overvoltage
and Reverse Polarity Protection
Max. External V
oltage Applied
to Pins 2 and 3 with Respect to Pin 1 + 52 Vdc.
. . . . . . .
Reverse Polarity
Protection 200 mA max. (diode-clamped).
. . . . . . . . . . . . .
Phasing
Positive pressure on diaphragm produces positive voltage
on pin 2 relative to pin 3
Cartridge
Capacitance
54 pF
LP
Response Switch
Flat,
–6 dB/octave below 100 Hz, –18 dB/octave below 80
Hz
Attenuator
Switch
0 or 10 dB (120 pF)
Power
Supply V
oltage
11 to 52 Vdc, positive pins 2 and 3.
. . . . . .
Current Drain 1.0 mA to 1.2 mA.
. . . . . . . . . . . . . . . . . . . . . .
Environmental Conditions
Relative Humidity 0–50%
– 29
to 74 C.
. . . . . . . . . . . . . . .
(–20 to 165 F)
Relative Humidity 0–95%
– 29
to 57 C.
. . . . . . . . . . . . . .
(–20 to 135 F)
Connector
Professional 3-pin audio**
Case
Steel
construction with metallic vinyl paint finish and stain
-
less steel screen
Dimensions
See Figure 3
OVERALL DIMENSIONS
FIGURE 3
Weight
Net 230 grams (8 oz).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Packaged 1.2 kg (2 lb 10 oz).
. . . . . . . . . . . . . . . . . . . . . . . . .
*S/N
ratio is dif
ference between microphone
output at 94 dB SPL and micro
-
phone
self-noise A-weighted.
**Designed
to mate with Cannon XL series,
Switchcraft A3 (Q.G.) series or
equivalent
connectors.
Certification
Conforms
to European Union directives, eligible to bear
CE
marking;
meets European Union EMC Immunity Requirements
(EN 50 082–1, 1992); RF radiated (IEC 801–3); ESD (IEC
801–2); EFT (IEC 801–4).
OPERATION
The SM81 is designed for phantom powering by Shure
Model
PS1A Power Supply
, or by virtually any microphone pow
-
er supply providing 12 to 48 Vdc phantom voltage.
Use only high-quality cables, as intermittent shorts be-
tween
broken shield wires and balanced
conductors will cause
extremely 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 ac-
cepted audio grounding practices.
Paralleling or “Y-ing” the SM81 with another microphone
(two microphones on the same input) is not recommended;
separate inputs are preferable. However, paralleling two
SM81’s may be accomplished with either a reduction in maxi-
mum
SPL and output level, or a reduction only in
output level if
the microphones are electrically isolated. With the micro-
phones
paralleled either before or
after a PS1A Power Supply
,
the
maximum SPL is reduced by approximately 10
dB and the
output level by 6 dB. The reduction in maximum SPL can be
avoided by using either two Shure A15AS Attenuators and a
Switchcraft 391Q43 Y-Adapter to isolate the microphones, or
an
isolation network as shown in Figure 4. The network
reduces
each microphone output level by 8 dB, while the A15AS re-
duces
the output level by 5 dB plus the attenuator
’
s 15, 20, or 25
dB (switch-selectable). The network or attenuators can be in-
serted between the power supply outputs and mixer input, or
between two microphones and a single power supply input.
Note
that a PS1A Power Supply can power two SM81’
s on each
input;
other power supplies should be checked to see if they can
supply
a minimum of 10 Vdc at each microphone when both mi
-
crophones are connected.
A
minimum load impedance of 800
Ω
or 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 loading (3300
Ω
in the Shure PS1A power supply) to
the microphones.
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