The RCA Type 77-DX
poly-directional ribbon microphone

Hear the sound of this mic.

The umber gray television version.

New logo on the last of the 77 series.

Description
The RCA Type 77-DX Poly-directional Microphones MI-4045-F and MI-11006-C are high-fidelity micro­phones of the ribbon type which may easily be adjusted to obtain a variety of directional patterns. Since the MI-4045-F micro­phone is primarily intended for broadcast use, it is finished in satin chrome and a low-gloss umber-gray enamel. The MI-11006-C micro­phone is intended for television use and is therefore com­pletely finished in a low-gloss umber-gray enamel which eliminates glaring reflections usually seen on highly polished micro­phones.

Instead of being open on both sides as in the conventional velocity mi­crophone, the ribbon element in this micro­phone is coupled to an acoustic labyrinth which forms the body portion of the micro­phone. The tube connecting the back of the ribbon to the labyrinth is slotted directly behind the ribbon and fitted with an adjustable shutter to secure various areas of opening. When the opening is completely closed, the micro­phone operates as a non-directional pressure micro­phone; at the wide-open position the micro­phone becomes bi-directional. With the proper size opening the pattern becomes a cardioid by virtue of the phase shift which occurs. Openings smaller or larger than this critical size produce directional patterns with various sized rear lobes. Different amounts of low-frequency attenuation are obtained by a reactor shunting the output. These micro­phones are intended primarily for indoor use and if used outdoors may require some additional protection against the wind.

Directional Characteristics
The adjustable shutter over the slot in the tube leading to the acoustic labyrinth may be rotated by means of a screwdriver adjustment ex­tending through the rear screen flush with a designation plate. The choice of directional patterns makes possible a considerable degree of control of the ratio of direct sound to reverberant sound as well as the possible reduction of unwanted sound such as audience noise in a studio. The wide angle of pick-up provided by the cardioid pattern is useful in covering large groups with a single micro­phone. For close-talking applications the non-directional characteristic is of considerable value since the low-frequency response is not accentuated as in the case of a velocity micro­phone. Numerous other applications of the various direc­tional patterns as well as the different response curves will no doubt suggest themselves to the user.

Directional Pattern Selector—The plate is marked U, N and B, as symbols for the uni-directional, non-directional and bi-directional pat­terns. Three additional markings L-1, L-2 and L-3 are used as reference points for other directional patterns which may be obtained. Refer to Figure 4 for the patterns associated with each of the six symbols. Stops are provided on the continuously variable pattern selector at the six marked positions, although the shutter may be set at any intermediate position.

Differences in the 77-D and 77-DX pattern selector escutcheons.

The directional pattern selector shutter.

Frequency Response Settings—At the bottom of the lower shell is a screwdriver-operated selector marked M (music), V1 and V2 (voice). The voice positions connect a reactor across the entire secondary or part of the secondary of the output transformer, depending on the switch position. Refer to Figure 5 for the frequency-response characteristics of each setting. As can be seen from the curves, the reactor attenuates the low-frequency response. This is especially desirable when the micro­phone is less than three feet from the source of sound and the low-frequency response would otherwise be exaggerated.

Quoted from the Radio Corporation of America Industrial Electronic Products Broadcast Audio Equipment
Instructions for the Type 77-DX Poly-directional micro­phone, MI-4045-F and MI-11006-C, circa 1955.

Download the 77-DX Catalog B Sheet.
Note: 5.6 megabyte high-resolution PDF.

Download the 77-DX Instruction Book.
Note: 5.7 megabyte eight-page PDF.

Height comparison of the 77-C and the 77-DX.

See the RCA Type 77-B1 specification sheet.

See the RCA Types 77-B and 77-C1 microphones.

The RCA Type 77-C1
RCA introduced the 77 series of all-purpose ribbon micro­phones in 1932 (designed in 1929) to provide a uni-directional pick-up pattern and wide frequency response. The designer was Harry F. Olson. The early 77 series featured two ribbon micro­phones in series to create a uni-directional pattern. A switch provided operation of either mic individually or both in series. A labyrinth tube behind the pressure mic’s ribbon directed the air movement into sound absorbing material. When using only the pressure mic, the pattern was non-directional. When using only the velocity mic, the pattern was bi-directional. Later, the 77-D series utilized a single ribbon and inserted a moveable shutter assembly between the ribbon and the labyrinth tube which allowed various directional patterns to be realized. Also in the D series, a reactor could be placed into the signal path to reduce low frequency response and compensate for close talking. Frequency response with the 77-C series depended on the directional pattern selected but 50 Hz to 10 kHz was typical. The 77-C1 was made only in 1940 and was replaced by the 77-D, which evolved into the 77-DX.

A historical note—RCA microphone restoration expert Jerry Silvia says: “The C is the rarest of the chrome/black series as they were destroyed by the repair center when they were returned for repair and replaced with the C1 at no charge. They were a failed attempt to offer an external directional pattern selector switch.”

“Reveille with Beverly”
Click photo to learn more about Ms. Hay.

Hoagy Carmichael

Norman Corwin narrating One World Flight series, 1947.
Click photo to learn more about Mr. Corwin.

Edward R. Murrow
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“Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach only from one end of the bar to the other.” —Ed Murrow

Download the data sheet for the Type 77-B1

Download the data sheets for the Type 77-C

Download the data sheet for the Type 77-C1