Microphone 12 of 222

Altec M11 System

Amperite RA-1

Altec 639A

Manufacturer:

Altec

Model:

639A

Country of Manufacture:

United States

Microphone Type:

Other

Polar Pattern:

Multi-Pattern

Production Start Year:

1938

Production End Year:

1965

Rarity:

Microphone History:

MoMics View

Back in the early 1930s there was a major quest to develop cardioid microphones for the relatively new ‘talking picture’ movie industry. The first talkie was The Jazz Singer in 1927, which was recorded using a primitive condenser microphone alongside the Vitaphone sound-to-disk system. However, the earliest condenser and dynamic microphones were omnidirectional, whilst ribbon microphones pick up from front and rear and are naturally figure-of-8. The benefit of a cardioid pattern is that the microphone can be pointed at the actor and other sounds are largely excluded.

Western Electric invented a new microphone by combining both ribbon and dynamic elements in a system which became their new 639 ‘Birdcage’ model. The front of the ribbon is in phase with the dynamic capsule and adds to it, whereas the rear of the ribbon is reverse phase and cancels. Add these together in the right proportions and we get a cardioid microphone. In practice this works well enough in the mid range but veers away from cardioid at the high and low ends of the frequency range. To compensate, these microphones use a filter network to match the responses as well as possible.

The film set version of the 639 is the RA-1142 model, which featured a more rugged brass enclosure which is less prone to breaking than the 639’s cast housing*. It also featured a white cross on the rear which helped the microphone’s boom operator site the actor and focus the pickup on them. RCA’s KU2a and KU3a models also featured stripes on the rear, which is a sure sign that a microphone was designed for a move or television stage.

Western Electric’s 639 technology was licensed by ST&C who built the model 4033A in England, and to Altec in the USA, who continued to make the 639 after WE had ceased production. These models all had switches at the rear to select either ribbon or dynamic capsule alone, or both together, which made them some of the earliest multi pattern microphones. These mics were soon superseded by other simpler cardioid dynamic and ribbon mics.

* Our own 639 has clearly been dropped at some point in its lifetime and has its original top has been broken. Despite this and the amateur paint job, it sounds suprisingly good.

Technical Description:

From the Altec Catalog of 1965.

The 639 Microphone, originally developed by the Bell laboratories, has remained as a recognized standard in the recording, motion picture, and broadcast field for two decades. It consists of two independent elements, one dynamic and one velocity (ribbon), carefully phased and acoustically integrated to provide the finest poly-pattern microphone in current manufacture. The selective six-directional patterns of the 639B, ranging from cardioid through Figure 8, to omnidirectional, result in an instrument for universal application of unfailing quality capable of producing the constant standard results required in broadcast. motion picture and phonograph recording. Sometimes known as a "super cardioid", because of its high front to back pick-up ratio.

In the all-Important mid-range, the 639 Microphone has contributed its quality towards the "Oscars" awarded a number of motion pictures by the Academy of Motion Picture Arts and Sciences for outstanding sound.

Type: Dynamic (Moving Coil) & Ribbon (Velocity)
Pickup Pattern: 639A: Cardioid, Bi-directional, and Omni-directional; 639B: Cardioid, Bi-Directional, Omni-directional, plus three additional characteristics, each having full frontal sensitivity but with varying degrees of rear sensitivity.
Frequency Response: Uniform, 40 - 10,000 Hz
Output Impedance: 30 to 50 ohms
Sensitivity: - 52 dbm/10 dynes/ cm'
Hum: -120 db (Ref: 10 milliGauss)
Protection: 2-stage windscreen (built into housing)
Housing: Die-Cast Aluminum
Weight: 3.25 Ibs.
Finish: Dark Gray