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 produced in the early 1950s by Altec, originally designed by Bell Telephone Laboratories and manufactured by Western Electric. It comes in two models—639A and 639B—with the 639B nicknamed "the birdcage" due to its distinctive grille. This microphone combines both a dynamic moving coil pressure element and a ribbon velocity actuated element in a single housing, allowing the user to select or blend directional patterns via a switch. 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 microphone uses two independent transducer elements: a dynamic moving coil pressure element and a ribbon velocity actuated element. The moving coil element alone provides non-directional (omnidirectional) pickup, while the ribbon element alone yields a figure-eight pattern. When combined via the selector switch, they produce a cardioid pattern. The 639A offers three selectable patterns—cardioid, bi-directional, and omnidirectional—while 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 are located in the microphone’s base. Its rugged housing and built-in two-stage windscreen support use in diverse environments.
Context
The Altec 639 was used extensively in wide-range sound recording from motion picture soundtracks to magnetic tape, with one source noting that the majority of such recordings employed one or more of these microphones. It is described as a legendary piece of audio history that captured the essence of sound across decades and offered unmatched versatility in capturing a wide range of frequencies.
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