RCA 74-A

MoMics view

The 74-A features a yoke mount and large grille perforations giving it a closer resemblance to the more expensive and larger RCA 44BX model, than its predecessor, the MI-4010-A, or successor the 74B.

These microphones are quite rare and certainly much less common than the very popular 74-B model. The extant 74-A manual is dated 1936 . However the 74-B already appears in the RCA catalogue from the same year and the 74-A does not. This suggests that the 74-A was available only for a brief period of time, perhaps only a year, before being replaced by the newer model.

From the 1936 RCA instruction manual for the 74A:

"Instead of a "diaphragm" (in the commonly accepted meaning of the word), the velocity microphone contains a thin metallic ribbon suspended between the poles of a permanent magnet with its length perpendicular to, and its width in the plane of, the magnetic lines of force. The opposite ends of the ribbon are connected to a transformer which matches the impedance of the ribbon to a 250 or 50 ohm line. Sound waves reaching the ribbon vibrate it within the magnetic field set up by the magnet. The vibration of the ribbon is in exact accordance with the sound vibrations and, occurring as it does within the magnetic field, sets up corresponding alternating electric potentials across the primary of its associated transformer.

These minute voltages are subsequently amplified to the power level required for broadcasting. The microphone amplifier may be located remotely from the microphone unit when necessary or desirable.

Taken from the RCA technical description from 1936:

• Output Impedances: 50 and 250 Ohms
• Frequency Response: 70 Hz to 8,000 Hz.
• With an input sound pressure of 10 dynes per square centimeter perpendicular to the ribbon, the microphone delivers 635 microvolts across a 250-ohm load, equivalent to an output level of -69 dB (12.5 milliwatts reference) or -66 dB (6 milliwatts reference).
• On an open circuit basis, with an input of 1 dyne per square centimeter, the output is equivalent to -83 dB (12.5 milliwatts reference)

The RCA instruction manual for the 74A, from 1936

A comparison of the RCA Junior ribbon mics at the Xaudia Blog

The data sheet is availiable at the Coutant website.