Battery Operation; Circuit Description - Shure SM94 Guide Rapide

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
voltage 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
temperature (72° to 80° F, 22° to 27° C)
Connector
XLR professional audio 3-Pin
Case
Steel and brass construction with gray finish and
stainless steel mesh grille
Dimensions
See Figure 3
OVERALL DIMENSIONS
Weight
Net
Packaged SM94-LC: 949.7 grams (2 Ib 1 1/2 oz)
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 microphone 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 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. 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 currents such as
air-moving equipment, or as a vocal microphone.

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 supply circuit
2
FIGURE 3
250 grams (8.8 oz) less battery