RCA KU-3A Cardioid Ribbon Microphone #H-1380 (Vintage)

History

A ribbon mic works by suspending a thin piece of corrugated aluminum, (the 'ribbon'), between two poles of a magnet. The compression and rarefaction of air vibrates this ribbon in the magnetic field, which creates voltage changes that are run through an output transformer to the external amplifier of your choice. Like a dynamic mic, a ribbon requires no tubes, batteries or power supply of any kind. In the case of the RCA KU-3A, the chassis is built to absorb mechanical shocks, the output transformer is shielded against magnetic fields, and the capsule is resistant to changes in humidity and temperature.

From the original brochure:
"The RCA Type KU-3A Unidirectional Microphones were designed to meet the exacting requirements for sound-film recording at motion-picture studios. These microphones provide uniform and smooth reproduction over a wide audio range (suitable for dialog and music), and have the unidirectional pattern considered so desirable for sound-motion-picture applications. The exhibit a high sensitivity for sound originating in front. Rejection (cancellation) at the back is such as to provide a new low in output of unwanted signal. The light weight, small size, rugged construction, and attractive appearance of the units make them outstanding in their field.

These microphones are of the type in which the moving element is a single thin extremely light corrugated metallic ribbon. The ribbon is suspended in an air gap between the poles of a permanent magnet, so as to vibrate freely with a motion corresponding closely to the motion of the air particles of the sound wave striking it. The voltage generated as the ribbon cuts the magnetic lines of force is the electrical equivalent of the velocity of the particles of the sound waves. An acoustical labyrinth formed in the cylindrical section of the microphone below the magnet and pole-piece assembly is coupled to the air gap by a tubular connector. This connector is sealed to the rear of the air gap, and contains a small silk-covered opening in the plate at the rear. The cloth-covered opening is of suitable dimensions to provide a unidirectional response characteristic for the microphone. The ribbon and magnet assembly are enclosed in a cloth-lined perforated-metal grille assembly which provides protection against mechanical injury, dust, and to a certain extent wind. The hemispherical shell below the acoustical labyrinth covers an impedance-matching transformer with output taps for 30, 150, and 250 ohms impedance. A three-pin output receptacle (male) is mounted on the shell, and 180-degree rotation of the shell is possible as a means of changing the output receptacle location.

The microphones are finished in a flat two-tone umber gray. A three-fourths inch white reference stripe on the back of the grille assembly is provided as a “panning” guide to the boom man, to indicate the “dead” side of the microphone.

Sensitivity

The Type KU-3A Unidirectional Microphone has a higher order of sensitivity than other previously manufactured RCA film-recording microphones. The sensitivity is approximately 6 dB higher than that of the double-ribbon type of unidirectional microphone, and approximately 4 dB higher than that of current models of high-quality pressure and velocity microphones.

Directional Pattern

These microphones have a very uniform response for sound incident at the zero axis (at the front of the microphone), with the output decreasing slowly as the angle of the sound source with the zero axis increases. The pickup angle may vary approximately ± 50° from the zero axis with less than one dB difference in output, and approximately ± 90° before the output is down 6 dB over the frequency range normally reproduced in theaters. The broad solid angle of relatively equal response reduces and simplifies “panning,” and makes possible the coverage of practically any action with a single microphone."