Altec 639 (early 1950’s)

A dual-element general purpose microphone combining ribbon and dynamic transducers for selectable directional patterns

Overview

The Altec 639 is a general purpose magnetic microphone made in the early 1950s by Altec. Bell Telephone Laboratories designed it originally, and Western Electric manufactured it. It came in two models, the 639A and 639B. The 639B is nicknamed "the birdcage" because of its grille. Both put a dynamic moving coil pressure element and a ribbon velocity actuated element in one housing, and a switch lets the user select or blend directional patterns. It was widely used in studio recording, broadcasting and remote pickups, especially in multi-microphone setups like orchestral recordings, where noise and feedback control were critical.

Specifications

Models	639A and 639B
Sensitivity (open circuit output level)	84 dB below 1 volt/dyne/cm², 64 dB below 1 volt/10 dynes/cm²
Power Output Level	–56 dBm for a sound pressure level (SPL) of 10 dynes per square centimeter, or –76 dBm for 1 dyne per square centimeter when terminated with a resistance equal to its internal impedance
Frequency Range	Substantially uniform from 40 to 10,000 Hertz
Signal-to-Noise Ratio	The signal for 10 dynes per square centimeter sound pressure is 78 dB above the thermal agitation noise generated within the microphone, and 58 dB above for 1 dyne per square centimeter
Directivity (639A)	Three patterns, C, D, and R, selectable through a three-position screwdriver-operated switch. At the angle of minimum response, the average discrimination with respect to 0° response is 20 dB over the range from 40 to 10,000 cycles per second
Directivity (639B)	Six patterns: R, D, C, 1, 2, and 3, selectable through a six-position screwdriver-operated switch. At the angle of minimum response, the average discrimination with respect to 0° response is 20 dB over the range from 40 to 10,000 cycles per second
Impedance	Average value is 40 ohms. Intended for use with equipment having a rated source impedance of 25 to 50 ohms
Dimensions	7½" high, 4⁷⁄₁₆" long, 3⁷⁄₁₆" wide
Weight	3¼ pounds
Settings/Microphone elements	Includes C, D, and R settings (C: Combination, D: Dynamic, R: Ribbon); ribbon and dynamic elements

Design

The Altec 639-type uses two independent transducer elements: a dynamic moving coil pressure element and a ribbon velocity actuated element. The moving coil element alone gives non-directional (omnidirectional) pickup. The ribbon element alone gives a figure-eight pattern. Combined through the selector switch, they produce a cardioid pattern. The 639A offers three selectable patterns - cardioid, bi-directional and omnidirectional. The 639B adds three more positions (1, 2, 3), which are variations of the cardioid pattern with increasing rear sensitivity. The pattern select switch and a Cannon (XLR-type) connector sit in the base. The housing is rugged, and a built-in two-stage windscreen supports use in varied environments.

Context

The Altec 639 was used extensively in wide-range sound recording, from motion picture soundtracks to magnetic tape. One source notes that the majority of such recordings used one or more of these microphones.