Battery Operation; Wind Noise; Circuit Description - Shure SM94 Manual Del Usuario

Output Impedance
Rated at 150 Ω; 200 Ω actual
Recommended Minimum Load Impedance
800 Ω
Sensitivity (at 1,000 Hz)
Open Circuit Voltage: –69 dB (0.35 mV) (phantom)
–70 dB (0.32 mV) (battery) (0 dB = 1 volt per microbar)
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
150–Ω Load . . –15 dBV (0.18 V) (phantom), 1% THD;
–31 dBV (28 mV) (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)
150 Ω Load: 135 dB (phantom), 120 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
Case
Steel and brass construction with gray finish and stain-
less steel mesh grille
Dimensions
See Figure 3
OVERALL DIMENSIONS
Weight
Net . . . . . . . . . . . . . . . . . .
Packaged . . . . . .
Specifications
Conforms to European Union directives, eligible to bear
CE marking; meets European Union EMC Immunity Re-
quirements (EN 50082–1: 1992).
*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 powered 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 continuous micro-
phone operation. Note that the microphone is powered at all
times during battery operation––the on–off switch only af-
fects the audio signal—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. Insert 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 cur-
rents such as air–moving equipment, or as a vocal micro-
phone.

CIRCUIT DESCRIPTION

A block diagram of the SM94 is shown in Figure 4. The ca-
pacitor 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.
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FIGURE 3
250 grams (8.8 oz) less battery
SM94–LC: 949.7 grams (2 Ib 1
1
/ oz)
2