Shure SM91A User manual

Copyright 1993, Shure Brothers Inc.
27A2669(MC)
Printed in U.S.A.
GENERAL
The SM91A Microphone is designed specifically for
surface-mounted applications. It is a professional-qual -
ity permanently-biased condenser microphone with a
half-cardioid
pickup pattern (cardioid in the hemisphere
above the mounting surface.)
The S M91 A t ake s a dvantag e o f t h e w ell-know n p rinci-
ple that, at a barrier or boundary, sound pressure doubles
compared to its value if the boundary is removed. When
placed
s ufficientl y n ea r t h e b oundar y s urface , a m icrophone
has effectively 6 dB higher sensitivity and approximately 3
dB
g reater rejectio n o f rando m b ackgroun d n oise.
Because of its half-cardioid polar pattern, the
SM91A surface-mounted microphone discriminates
against sounds originating from the rear, suiting the
SM91A
for conditions where an omnidirectional pattern
makes other surface-mounted microphones impracti-
cal.
The intrinsic unidirectionality of the SM91A can be
a great benefit when it is desirable to isolate a particular
vocalist, instrument, or group from the rest of an en-
semble being recorded. Because of a cardioid pickup
pattern, no physically isolating barriers are required,
and
directionality is maintained to low frequencies.
The SM91A can be used for individual instrument
pickup,
e.g., mounted inside the lid of a grand piano or
on the floor next to a bass drum. Experimental place-
ment and critical listening will help to determine the best
location for any particular purpose or ef fect desired.
The meticulously optimized design of the SM91A in -
cludes a unique cartridge, developed at Shure. The re -
sult
is high sensitivity , exceptionally accurate sound re -
production over the entire audio frequency range, and
freedom from of
f-axis coloration comparable to the fin -
est unidirectional microphones.
The furnished ILP-1 preamplifier is a high-clipping
level
low-noise unit, phantom powered by 1 1 to 52 Vdc.
It has selectable gain of 0 or +10 dB, and switchable low-
frequency response, flat or low cut (rolled off at 12 dB/
octave). The low cut is particularly useful to minimize
pickup
of ambient low-frequency noise from sources like
heating or air-conditioner fans, for instance.
The SM91A consists of a small, rugged, surface-
mounted microphone finished in professional durable
matte-black
enamel, a 7.6 m (25 ft) small diameter two-
conductor shielded interconnecting cable with two
3-socket miniature Switchcraft connectors; and the
ILP-1
preamplifier assembly with standard 3-pin profes -
sional
audio connector output.
Features:
•Wide flat frequency response for faithful sound re-
production
across the audio spectrum
• High sensitivity , low self-noise
•V ery low distortion and high output clipping level
•Half-cardioid polar pattern minimizes pickup from
rear of microphone, permits aiming microphone,
e.g.,
toward performers and away from audience, or
toward singers and away from instruments
• Low susceptibility to RFI, electrostatic and electro -
magnetic hum
•Extremely rugged construction of both microphone
and preamplifier for outstanding reliability
•Switch-selectable 12 dB/octave low-frequency cut -
off permits tailoring response to suit conditions
• Switch selectable preamp gain of 0 or +10 dB
•Accepts p hanto m p owe r o f 11 t o 5 2 V dc , p ermit s u se
of
w id e v ariet y o f a mplifier s o r phanto m s upplies
•Standard 3-pin male XLR-type output connector on
preamp can be directly connected to any 3-pin fe -
male XLR-type phantom-powered input
•Low profile and matte black finish for unobtrusive
appearance on-camera or onstage; on floor, table,
ceiling, wall, or lectern
•Usable over very wide range of temperature and hu-
midity

2.
SPECIFICATIONS
Type
Cardioi
d c ondenser (electre t b ias) for surfac e m ounting
Frequency Response
20 to 20,000 Hz (see Figure 1)
TYPICAL FREQUENCY RESPONSE
(measured at 30
o
incidence to infinite surface)
FIGURE 1
Polar Pattern
Half-cardioid (cardioid in hemisphere above mounting
surface), uniform with frequency, symmetrical about
axis (see Figure 2)
TYPICAL POLAR P ATTERN
FIGURE 2
Output Impedance
Rated at 150 Ω(90 Ω actual)
Recommended minimum load impedance: 800 Ω
(May be used with loads as low as 150 Ω with reduced
clipping level)
Output Level (at 1,000 Hz, measured with sound source
at 30
o
incidence to flat surface)
Open Circuit V oltage –71.5 dB (0.27 mV) . . . . . . . . .
0 dB = 1 V /µbar
Preamplifier Output Clipping Level (at 1,000 Hz, less
than 0.1% THD)
GAIN
0 +10
800 Ω load 0 dBV –7.0 dBV . . . . . . . . . . . . . . .
(1.0 V) (0.45 V)
150 Ω load –13.0 dBV –21.0 dBV . . . . . . . . . . . . . .
(0.22 V) (0.09 V)
Maximum SPL (at 1,000 Hz, less than 1% THD)
800 Ω load
ILP-1 gain set at 0 dB 146.5 dB . . . . . . . . . . . . . .
ILP-1 gain set at +10 dB 136.5 dB . . . . . . . . . . .
150 Ω load
ILP-1 gain set at 0 dB 129.5 dB . . . . . . . . . . . . . .
ILP-1 gain set at +10 dB 1 18.5 dB . . . . . . . . . . .
Electromagnetic Hum Pickup (maximum)
–0.5 dB equivalent SPL in 1 mOe field (60 Hz)
Preamplifier Controls
Selectable Low-Frequency Response: Flat/Low Cut
(12
dB/octave rollof f below 80 Hz)
Selectable Gain: 0/+10 dB
Output Noise
25.5 dB SPL, A-weighted
32 dB SPL, C-weighted
28.5 dB SPL per DIN 45 405
Signal-to-Noise Ratio
68.5 dB re 94 dB SPL
Dynamic Range
121 dB (800 Ω load, gain set at 0 dB)
Phasing
Positive p ressur e on d iaphrag m p roduce s p ositive v olt-
age
o n p i n 2 r elativ e t o p in 3 o f p ream p o utpu t c onnector
Power
11
to 52 Vdc phantom (simplex) voltage (operational
down to 9 Vdc with reduced clipping level); current
drain 2.2 mA at 52 Vdc, 1.8 mA at 1 1 Vdc
Case
Microphone: Matte black enamel die-cast base and
perforated steel grille with replaceable or cleanable
fine
mesh screen and foam pad wind/dirt barrier
Preamplifier: Matte black enamel finished steel
Cable
7.6 m (25 ft) two-conductor shielded, small diameter ,
with Switchcraft Tini (Q.G.) 3-pin (female) connector
on
each end
Environmental Conditions
Operating Temperature: –18 to 57
o
C (0 to 135
oF)
Storage T emperature: –29 to 74
o
C (–20 to 168
oF)
Relative Humidity: 0 to 95%
Dimensions
See Figure 3
OVERALL DIMENSIONS
FIGURE 3

3.
Net W eight
Microphone: 280 g (9.9 oz) less cable
ILP-1 Preamplifier: 170 g (6 oz)
LOCATION
To maintain the flattest possible low-frequency re-
sponse
a n d t h e b es t r ejectio n o f r ando m b ackgroun d n oise,
choose a f la t s urfac e a s l arg e a s p ossibl e o n w hic h t o l ocate
th e S M91A . T h e s urfac e c a n b e a f loor , w all , c eilin g o r t able.
Too small a mounting surface causes a low-frequency
rolloff b eginning a t t h e f requenc y w hos e w avelengt h i s c om-
parable t o t h e s iz e o f t h e s urface . T h e r ollof f c ontinue s a t a
rate o f a bou t 3 dB p e r o ctav e u nti l i t r eache s a p latea u a p-
proximately 6 d B l owe r t han t h e m id - a n d h igh-frequenc y r e-
sponse. In a similar fashion, too small a mounting surface
decreases
t h e r ejectio n o f l ow-frequenc y b ackgroun d n oise.
MOUNTING
The SM91A can be permanently mounted to a lectern,
tabletop, floor, ceiling, or wall using two No. 6 screws lo-
cated 50.1 mm (2 in.) apart. The location of two keyhole
slots in the base of the microphone is marked on the
nameplate. Cut through the marked slots before sliding
the
base onto the screws.
MOUNTING CLAMPS, PREAMPLIFIER
FIGURE 4
The s upplied m ountin g c lamp s a r e i ntende d t o h ol d t he
preamplifier i n p lac e i n p ermanent i nstallation s ( se e F igure
3). U s e e ithe r o n e o r t w o c lamp s d ependin g o n l ocatio n a nd
application.
T o a voi d g roun d l oop s a n d h um , a voi d g round-
ing the microphone preamplifier housing to metal b uilding
structures. T h e I LP- 1 p reamplifie r c a n a lso b e p lugge d d i-
rectly
i nt o a f lus h m ounte d c onnector i f d esired.
INTERCONNECTING CABLE
One 7.6 m (25 ft) cable is supplied for connecting the
SM91A Microphone to the ILP-1 Preamplifier . T o retain ac -
cess to the controls located in the preamplifier , it is some -
times desirable for the units to be located a greater dis-
tance
apart. Up to 15 m (50 ft) of additional cable can be
used between the SM91A Microphone and ILP-1 Pream -
plifier
with no loss in response or output.
CLEANING
When the microphone is located in a dusty environ-
ment, periodic cleaning may be desirable. This can be
easily
accomplished by removing the Phillips-head screw
on the grille, and lifting of f the grille, the fine mesh screen,
and the foam pad. Clean the fine stainless steel mesh
screen
by washing it in soapy water. Dry it thoroughly , and
replace the foam pad, screen, and grille. Fasten firmly with
the Phillips screw .
DISASSEMBLING AND REASSEMBLING THE ILP-1
1. At the XLR-3 end of the preamplifier, turn the slotted
head setscrew fully inward (counterclockwise), and
use a long-nose pliers to withdraw the connector from
the case.
2. Unsolder the three lead wires from the XLR-3 board.
3. Remove all four Phillips-head screws from the ILP-1
case (three on the switch side, one on the back).
4. Grasp the end cap and withdraw the cap assembly , pc
board assembly , and connecting wires and jacks from
the case.
REASSEMBLY
1. Make sure the pc board is seated in the slot of the end -
cap and that no wires are pinched.
2. With the XLR-3 connecting wires foremost, feed the
endcap-board assembly into the case. Again, take
care not to pinch the wires connecting the board to the
endcap.
3. Rotate the assembly until the four holes for Phillips
screws line up with the holes in the case.
4. Replace the four Phillips screws.
5. Resolder the three lead wires to the back of the XLR-3
board as shown below .
RED
BLUE
BLACK
2
3
1
WIRING T O XLR-3 PC BOARD TERMINALS
FIGURE 5
6. Replace the XLR-3 board-connector assembly in the
case lining up the key in the connector with the slot in
the case. Take care not to engage the butterfly-
shaped
ground contact in the key slot as it will prevent
seating the connector properly .
7. Insert the connector fully in the case until the slotted
setscrew can be seen in the case hole; tighten the set -
screw firmly by turning it clockwise.

4.
PHILLIPS-HEAD SCREW
PERFORATED MET
AL
GRILLE
STAINLESS FINE-MESH
SCREEN
FOAM PAD
BASE
CARTRIDGE
CONNECTOR
MICROPHONE
P ARTS
FIGURE 6
To paint the base:
1. Remove the flat-head Phillips screw in the top center
of the perforated metal grille. Lift of f the grille, screen,
and foam pad.
2. Wrap the grille in thin plastic film, use a pointed instru -
ment to perforate an opening for the screw, and re-
place the wrapped grille on the microphone.
3. Replace and tighten the screw .
4. Carefully paint the microphone base and screw head
with the desired color. To avoid getting any paint on
the connector , either mask it thoroughly or connect a
mating plug to it.
5. Allow the paint to dry thoroughly .
To paint the metal grille:
1. Once again, remove the flat-head Phillips screw in the
top center of the plastic-wrapped perforated metal
grille. Lift of f the grille.
2. Remove the plastic wrap and paint the outside of the
grille. T ake care to use a thin layer only; do not fill the
perforated holes with paint.
3. Allow the paint to dry thoroughly .
4. Use fine sandpaper to remove any paint that may
have adhered to the outer edges of the grille and the
edging around the screw-hole.
IMPORTANT
For proper shielding it is extremely important to
maintain electrical continuity between the grille
and the metal base.
5. Before replacing the grille, check that the fine-mesh
screen is clean. Otherwise, wash it in soapy water.
Rinse and dry it thoroughly .
6. Replace the foam pad, mesh screen, and perforated
grille; and fasten securely with the Phillips screw .
REPLACEMENT P ARTS
Reference
Designation Part Number Description Commercial Alternate
Preamplifier
A1 90B4220 ILP-1 Preamplifier Assembly None
A2 90HZ2600 Pc Board Assembly None
P1 95A8077 Plug Assembly , Male, Mini Switchcraft, TB3M T ini
P2 90HV2600 XLR-3 M C onnector an d P c B oar d A ssembly None
MP1 66A264 Preamp Switch Cover None
MP2 80A476 Mounting Clamp All States 3/4-HNB
Microphone
A3 98A144 Microphone Assembly None
A4 R129 Cartridge and Impedance Converter None
MP3 53A1879B Grille None
MP4 37A147 Inner Screen None
W1 C107 Cable Assembly , 7.6 m (25 ft), T wo Conductor ,
Shielded, Small Diameter
None

5.
NOTES:
1 UNLESS OTHERWISE SPECIFIED, ALL RESISTORS ARE 1/8 WATT , 5%.
2 UNLESS OTHERWISE SPECIFIED, ALL CAPACITORS IN F, 10%, 50 V OR GREATER. ELECTROLYTIC CAPACITORS SHOWN IN F X VOLTS, 20%.
3 THE FOLLOWING SYMBOLS DENOTE:
PC BOARD GROUND CHASSIS GROUND.
.
GAIN
RED
BLUE
BLACK
D
S
G
CONDENSER
CARTRIDGE
GRN
OR
BROW
N
OR
CIRCUIT DIAGRAM
FIGURE 7