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222 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
Fig. 45. Western Electric 640AA condenser microphone and RA-1095 amplifier. Frequency characteristic: Free-field response: see Figure 45A. Output level: Approximately 49.5 db below 1 volt (open circuit) per dyne/cm2 with 200 volts d.-c. polarizing potential. Output impedance: Essentially that due to its capacitance which is approximately 50 mmf. to 60 mmf. Operates into: Grid circuit of closely associated vacuum tube amplifier (such as Western Electric RA-1095 amplifier). Polarizing voltage: 220 volts d.-c. maximum from well regulated quiet supply. (Caution: polarizing voltage exceeding 220 volts should not be applied as higher voltages may dn mage the instrument.) Dimensions : Cylindrical shape approximately 1" diameter and 1" long. Weight: Approximately l1/^ ounces. Mounting: For optimum signal-to-noise the microphone should be closely associated with the first stage of amplification and preferably mounted in the structure containing this amplifier. External connection: The 640AA microphone is especially designed to mount on the RA-1095 amplifier. It has a spring mounted plunger and male base threads for providing connection to the amplifier. When associated with another type of amplifier, the microphone should be connected to the grid of the vacuum tube by means of a short, well-shielded, low capacitance lead to the center contact at rear of instrument. The cylindrical shell of the microphone should be connected to the grounded side of the grid circuit thereby serving as a shield for the inner components. Protection : Provided with a dust cap for each end of the cylinder when instrument is not in use.
RA-1095 Amplifier — Typical Electrical Characteristics Frequency characteristic: See curve, Figure 45A (microphone and amplifier in combination). Output level: Approximately -29.5 dbm with the 640 AA microphone in a sound field of 10 dynes/cm2 or 49.5 dbm in a 1 dyne/cm2 sound field. Signal-tonoise ratio: Approximately 40 db at an output level of -49.5 dbm (0-15,000 cyele band). Distortion: Less than one percent at -3 dbm output level. Operates from: 640 A A condenser microphone. Output impedance: Designed to be used with equipment having rated source impedance of 25-50 or 150-250 ohms. Power supply: Quiet sources required for both filament and plate power. (Heater1: 6.3 volts, 150 milliamperes, direct current. Plate: 220 volts maximum, 3 milliamperes, direct current. Caution: Plate voltages exceeded 220 volts should not be applied when the 640 type microphone is attached, as higher voltages may damage the microphone). Dimensions : Approx. 7%" long, 21/2// diameter. Weight: Approx. 1% pounds. External connections: Through 6 prong socket in base of amplifier. (Use Cannon 6 hole female plug P6-11). Mounting: Suspend from socket cord or use shock mounting hanger to fit user's microphone boom or other suspension mounting.