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The RCA Type 50-A
Inductor Microphone (1933)

RCA 50-A

The Inductor Microphone is the result of continued re­search and development toward the production of a low-priced microphone, the characteristics of which are high quality, high sensitivity, freedom from shock excitation troubles, minimum response to wind effects, compactness and ruggedness.

RCA 50-A

This microphone is a pressure-operated microphone of the moving conductor type. A single 0.010-inch aluminum con­ductor is rigidly coupled to a diaphragm and located be­tween the poles of a permanent magnet with its length per­pendicular to the magnetic lines of force. The ends of the conductor are connected to a transformer which matches the impedance of a 250 or 50 ohm line. Sound waves reaching the diaphragm vibrate the conductor within the magnetic field set up by the magnet. The vibration of the conductor is in accordance with the sound vibrations and, oc­curring as it does within the magnetic field, sets up cor­responding alternating electric potentials across the pri­mary of the microphone transformer. These minute vol­tages are subsequently amplified to the power level required for broadcasting.

RCA 50-A

Quality of Response
The frequency response of the microphone is uniform over its useful operating range from 60 to 10,000 cycles. The variation of the frequency response characteristic with the direction of the incident sound is very similar to that of any other pressure operated microphone of comparable size, in that the response to the higher frequencies is attenuated as the angle between the direction of the incident sound and the plane of the diaphragm is decreased.

RCA 50-A

The microphone and microphone transformer are enclosed within a sturdy and attractive metal case on the back of which is mounted a recessed male connector for the attachment of the microphone cable. The microphone case is fitted with a perforated metal front cover which serves to protect the transmitter from mechanical injury and adverse wind effects. This assembly is pivoted in a fork, to which is attached a threaded flange mounting by means of which the microphone may be fastened to the top of a microphone stand. A suspension mounting (Type UP-4277) is also sup­plied with the microphone to permit the unit to be suspended overhead when desired.

RCA 50-A

Quoted from Radio Corporation of America Instructions for Inductor Microphone Type 50-A (MI-4030-A)
(MI-4030-B) (MI-4030-C), Publication IB-23801-3,
© Copyright 1934, RCA Victor Company, Inc.

RCA 50-A RCA 50-A RCA 50-A RCA 50-A RCA 50-A Marian Anderson, Leonard Bernstein

Marian Anderson rehearses with Leonard Bernstein at Lewisohn Stadium, New York, during June, 1947.


From Decibels and Microphones, published by the Central Technical Institute,
Kansas City, Missouri. Publication date unknown; likely during the late thirties.

Eleanor Roosevelt

Eleanor Roosevelt, seated at microphone at radio station WOL, Washington, D.C.,
presenting a “My People” program, devoted to African Americans.

Here are some gorgeous 50-A photos that were on eBay.

Here are four more eBay auction photos.
Winning bid was $1,276.51. Check out the yoke:

RCA Type 50-A RCA Type 50-A RCA Type 50-A RCA Type 50-A Billy Graham

Billy Graham

Upton Sinclair

Upton Beall Sinclair (1878-1968)

Harry S Truman

President Harry S Truman

Operating Instructions cover

Download the instructions for this mic.

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