Overall Dimensions; Battery Operation; Circuit Description - Shure SM94 Mode D'emploi

Output Impedance
Rated at 150 Ω; 200 Ω actual
Recommended Minimum Load Impedance
800 Ω
Sensitivity (at 1,000 Hz)
Open Circuit Voltage: –49 dBV/Pa (3.5 mV) (phantom)
–50 dBV/Pa (3.2 mV) (battery) (1 Pa = 94dB SPL)
Output Clipping Level (1 kHz, 3% THD)
800–Ω Load . . –14 dBV (0.63 V) (phantom), 1% THD;
–23 dBV (0.71 V) (battery), 3% THD
Total Harmonic Distortion
< 0.25% (130 dB SPL at 250 Hz into 800 Ω load)
Maximum SPL
800 Ω Load: 141 dB (phantom), 123 dB (battery)
Hum Pickup
–3 dB equivalent SPL in a 1 millioersted field (60 Hz)
Output Noise (equivalent sound pressure levels;
measured with true rms voltmeter)
22 dB typical, A–weighted
25 dB typical, weighted per DIN 45 505
Dynamic Range
119 dB (phantom); 101 dB (battery) (maximum SPL,
2000–Ω load, to A–weighted noise level)
Signal–to–Noise Ratio
72 dB (IEC 651)* at 94 dB SPL
Overvoltage Protection
Maximum External Voltage Applied to Pins 2 and 3
with respect to Pin 1: +52Vdc
Reverse polarity protected to 75 Vdc
Polarity
Positive pressure on diaphragm produces positive volt-
age on pin 2 relative to pin 3
Cartridge Capacitance
24 pF
Power
Phantom Operation
Supply Voltage: 11 to 52 Vdc, (+) pins 2 and 3
Current Drain: 1.0 TO 1.2 mA max. at 52 Vdc

Battery Operation

Type: 1.5 V alkaline, AA size (NEDA 15A)
Life: Up to 5,000 hours with fresh battery
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 tem-
perature (72_to 80_ F, 22_ to 27_ C)
Connector
XLR professional audio 3–Pin
Case
Steel and brass construction with gray finish and stain-
less steel mesh grille
Dimensions
See Figure 3

OVERALL DIMENSIONS

Weight
Net . . . . . . . . . . . . . . . .
Packaged . . . . SM94–LC: 949.7 grams (2 Ib 1
Certification
Eligible to bear CE Marking. Conforms to European
EMC Directive 89/336/EEC. Meets applicable tests
and performance criteria in European Standard
EN55103 (1996) parts 1 and 2, for residential (E1) and
light industrial (E2) environments.
*S/N ratio is difference between microphone output at 94 dB SPL and
microphone self–noise A–weighted.
OPERATION
The SM94 is designed for phantom powering by a Shure
PS1A Power Supply, or by any microphone power supply
providing 12 to 48 Vdc phantom voltage, or by any micro-
phone mixer (such as the Shure SCM262 and SCM268)
with a phantom supply. In addition, the SM94 can be pow-
ered by a single 1.5–volt AA alkaline battery. The battery
also serves as a backup in case of phantom power failure.
A new alkaline battery will provide up to 5,000 hours of con-
tinuous microphone operation. Note that the microphone is
powered at all times during battery operation but there is no
battery drain during phantom–powered operation.
BATTERY OPERATION
Disconnect the microphone cable, and unscrew the
SM94 handle, turning counterclockwise (from top) until the
handle is free of the microphone body. Slide the handle
away from the grille, exposing the battery compartment. In-
sert a new battery (or replace the old battery), observing
the polarity marking in the compartment.
Slide the handle toward the grille, and tighten the handle
by turning it clockwise (from top).
WIND NOISE
Use the optional foam windscreen (A3WS) when using
the SM94 outdoors on a windy day, indoors near strong air
currents such as air–moving equipment, or as a vocal mi-
crophone.

CIRCUIT DESCRIPTION

A block diagram of the SM94 is shown in Figure 4. The
capacitor cartridge is followed by an impedance converter
stage. This stage has an active load to obtain good load–
driving capability with the low battery voltage of 1.5 volts.
The audio signal is transformer-coupled for a balanced
output to the microphone connector. An active power sup-
2
FIGURE 3
250 grams (8.8 oz) less battery
1
/ oz)
2