Five Classic RCA Ribbon Microphones

by Mike Dorrough, KO6NM, and Gary Halverson, WA9MZU

BK5
Smaller than the 77, the BK-5 was designed primarily for the control room in AM, FM, and TV broadcast applications. It's frequency response was essentially flat from 50 to 15,000 cycles.

It's maximum pickup sensitivity lies on its major mechanical axis, and hence was called a uniaxial type microphone. A dual layer blast filter was mounted in the front of the ribbon to offer protection from extremely high sound pressure level noises.

Like the 77D or DX, the rear side of its ribbon was coupled to an acoustic labyrinth having phase shift openings. This labyrinth gave the BK-5 its unidirectional characteristic. A matching transformer and voice or music response switch were also mounted in the body of the microphone.

The BK-5 could be either mounted on a desk stand or hung from a boom on its special boom mounting adapter.

KU- 3/A
The KU-3A was a single-ribbon, high output boom-mounted microphone. Larger than the 77, it is somewhat similar in appearance except that the front side of its windscreen basket is flattened and trapazoidal in shape. The entire microphone is acoustically isolated from the boom by being floated in its yoke from a thick rubber band-like circular "suspender".

For years the KU-3 was regarded as the "standard of quality comparison in the major motion picture industry" according to the RCA literature. It was also popular with TV broadcasters in live studio programs where boom operation was necessary. Its excellent frequency response and output was very uniform over its frontal pickup angle of 90 degrees. This afforded broader tolerances in microphone handling on booms.

The KU-3A was also the most expensive ribbon microphone produced by RCA. Although not nearly as common as the 44 or 77, the KU-3A is still prized today as a premium microphone for professional recording where a ribbon is desired.

Ribbon Mics Today
Today, only a few microphone manufacturers offer ribbon models. For many collectors the RCA ribbon microphones are the cream of the crop, and accordingly are harder to find and more expensive than dynamic mics.

Because of their premium sound quality, collectors face competition from commerical studios and recording engineers also seeking these mics. Network and call-letter flags for these microphones are also highly sought after.

Common restoration items include costmetic recondition, re-ribboning, replacement of the shock mount rubber, and recabling. The re-ribboning process is said to be an art on the verge of extinction. For one thing, you can't breathe while mounting the fragile ribbon. Even the gentle force of a exhaling normally could easily destroy the delicate ribbon.

Like artifacts of an ancient civilization long gone, these gran dames of microphones have a secure place in history as the sirens of legends.