Five Classic RCA Ribbon Microphones

by Mike Dorrough, KO6NM, and Gary Halverson, WA9MZU

Why Ribbon?
Considered by many as the most natural sounding microphone every made, ribbon mics were immediately embraced by the broadcast and recording industries. Not requiring any awkward power supply or batteries in their operation, the first commerically produced ribbon mircophones appeared in the early 1930's.

The ribbon microphone was also known as the velocity microphone and was the last of the four basic microhone types developed, following the dynamic, condensor, and carbon microphones.

The ribbons' natural sound can also be made to sound warm, big, and syrupy (Bing Crosby-like) when placed within two or three feet of the talent (generally, you can't close-talk a ribbon without having a greatly exaggerated bass characteristic). An adequate breath filter in front of the ribbon is also necessary to protect the delicate ribbon (NEVER blow into a ribbon mic).

The output level of the ribbon is nearly always lower than that of a dynamic microphone. For this reason, hum rejection and shielding are important considerations in ribbon microphone construction.

The active element is of course the ribbon, consisting of a very thin corrugated aluminum ribbon clamped under light tension and mounted between the poles of a strong magnet. The extremely low impedance of the ribbon (tyically on the order of 0.2 ohm) is fed directly into a step-up transformer to match low impedance lines (50, 250, or 600 ohms).

Although other companies at the time were producing ribbon microphones, RCA was top dog for several reasons. First, RCA had the sales channels in place, with a pipeline into the broadcast market. They also had huge advertising budgets. But perhaps a more compelling reason was that the research and development effort that went into these microphone produced a quality that was unmatched by the competition. At the time, Western Electric was the only other manufacturer that could successfully compete with RCA, however, their only ribbon microphone was being targeted into the motion picture industry.

Each of the following RCA ribbon microphones are high-fidelity types designed for broadcast applications that have become classics.


Granddaddy of the ribbon microphone the 77A is among the rarest of the RCA ribbons microphones. Designed by Dr. Harry F. Olsen, RCA's lifelong resident audio genius, during the late 20's and early 30's the 77A set the performance benchmarks for all RCA ribbons to follow for the next four decades.

It's rumored that prototypes actually existed in 1929 and 1930, however the 77A wasn't announced until 1932. It featured two vertical in-line ribbons and an acoustic labyrinth inside the case, which enabled it to be uni-directional.

The 77A is a huge microphone resembling a cannon shell with a large perforated windscreen on the top portion. It is mounted gimbelled at its center of gravity in a U-shaped fork.



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