Five Classic RCA Ribbon Microphones

by Mike Dorrough, KO6NM, and Gary Halverson, WA9MZU

Why Ribbon?

Considered by many as the most natural-sounding microphone ever 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 commercially produced ribbon microphones appeared in the early 1930s.

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

The ribbon’s 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 (typically 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 is a high-fidelity type designed for broadcast applications that has become a classic.


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

It is 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 gimballed at its center of gravity in a U-shaped fork.

44A, 44B/BX

RCA 44-B

Successor to the 77A, the 44 could be considered a “cost-reduced” version. However, it was this cost reduction that catapulted the 44 to its legendary commercial success. Even today, some fifty years after its introduction, 44s can be found in broadcast and recording studios throughout the world.


The first of the 44 family was the 44A, a large microphone, although considerably smaller than the 77A. It used a large horseshoe magnet around the ribbon and featured a figure-8 pickup pattern.

The slightly larger 44B was introduced in about 1938. It also had a figure-8 pickup pattern. Both the 44B and the BX were bi-directional having figure-8 patterns. Its frequency response extended from 30 cycles to 15,000 cycles.

Within its case, the 44 was provided with “V” (voice) or “M” (music) jumper positions. When a lead was connected to the terminal marked “V,” a choke was connected in parallel with part of the transformer winding, which substantially attenuated the low frequency response.

The basic difference between the suffixes within the 44 family were:

The 44 BX was manufactured up to around 1955.

77B, C, D, DX

RCA 77B The 44 was replaced in the late 40s by a smaller, restyled version: the 77B. Resembling a large capsule, the 77B and its decendants, the 77C, 77D and DX have become the standard microphone icon known throughout the world. RCA 77-DX

Like its ancestor the 77A, the 77D and DX had an acoustic labyrinth in the body of the microphone. However, they were poly-directional microphones. Their poly-directional capability was accomplished by a tube connecting the back of the ribbon to the labyrinth that was slotted directly behind the ribbon and fitted with an adjustable shutter to close off portions of the opening. By positioning the shutter to completely close off the opening, a non-directional pickup could be obtained. When the opening was wide open, the pickup pattern was bi-directional. Positions in between produced other pickup patterns.

Unlike the 44, the instructions claimed the 77D could be “close-talked” when in the non-directional configuration as the low frequency response was not accentuated.

The low-frequency attenuation could also be attenuated by switching in a choke across the output of the microphone. A screwdriver-operated switch was provided at the bottom of the lower shell. Positions were marked M (music), V1, and V2 (voice).

Although the 77DX enjoyed a long commercially successful product life with a number of different letter suffix models preceding it, two basic versions of each type were offered: a satin chrome version (the radio broadcast model), and a non-reflective umber grey version (the TV model). Given a choice, collectors prefer the broadcast model due to its distinctive contrasts.

The basic differences between the 77D and 77DX models are that the 77DX had an improved magnet and transformer, which produced a little more output.

The 77 was discontinued around 1967 and replaced by the BK-11.


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

Its maximum pickup sensitivity lies on its major mechanical axis, and hence was called a uni-axial 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.


RCA KU-3A side 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 trapezoidal 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.” RCA KU-3A front

For years the KU-3A 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 commercial studios and recording engineers also seeking these mics. Network and call-letter flags for these microphones are also highly prized.

Common restoration items include cosmetic recondition, re-ribboning, replacement of the shock mount rubber, and re-cabling. 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 exhaling normally could easily destroy the delicate ribbon.

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

Reproduced here with the kind permission of Kay Dorrough,
granted on July 9, 2001.

Click here for more RCA microphone history.