The RCA Type 50-A
Inductor Microphone (1933)
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.
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.
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.
The microphone and the 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.
Quoted from the 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.
From Decibels and Microphones, published by the Central Technical Institute,
Kansas City, Missouri. Publication date unknown; likely during the late thirties.
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:
Upton Beall Sinclair (1878-1968)
President Harry S Truman
Download the instructions for this mic.