Mic Archive
RCA 77-DX
Photo: Radio Corporation of America, from its instructions 5, p. 1.

Radio Corporation of America

RCA 77-DX

Passive ribbon, polydirectional (shutter on an acoustic labyrinth: unidirectional, bidirectional, nondirectional and three in-between settings), broadcast and television microphone

Approximate Some readings for this mic are uncertain; see the graphs.

Type
Ribbon
Pattern
Cardioid / figure-8 / omni
Sensitivity
−55 dBV/Pa computed
Impedance
250 Ω
Rated range
50 – 15k Hz

Frequency response

Angle
Distance

Frequency response: from the instructions, p. 9. "OPEN CIRCUIT RESPONSE OF A TYPICAL TYPE 77DX MICROPHONE", panel "UNIDIRECTIONAL POSITION", curve M. Distance, room and smoothing not stated; the book says sources more than three feet away need no low-frequency attenuation. Approximate reading error up to about 1 dB.

Dashed: outside the rated range, or simulated (other angles and distances). Hover or tap the graphs for values.

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All specs

From the instructions
Capsulecorrugated aluminium foil ribbon, 2 in long x 0.054 in wide, in an Alnico V magnet circuit; the back of the ribbon opens into a tube to an acoustic labyrinth, with an adjustable shutter over a slot behind the ribbon. Ribbon size from the parts list ("Ribbon, microphone, corrigated aluminum foil, 2" lg. x .054 wide"); labyrinth and shutter from p. 5; Alnico V from the 1955 catalogue, p. 11.p. 12
Sensitivity−55 dBV/Pa, unidirectional, open circuit; also −52 dBV/Pa, bidirectional (Gm -144 db); also −58 dBV/Pa, nondirectional (Gm -150 db). Not stated in volts. Computed from the RETMA ratings "Uni-directional (U) -53 dbm, GM -147db", "Bi-directional (B) -50 dbm, GM -144db", "Non-directional (N) -56 dbm, GM -150db" (all output connections) with the 1955 catalogue's formula, Gm = 20 log(E/P) - 10 log 150 - 50 dB, P in dynes/cm² (catalogue p. 6): -55.2, -52.2 and -58.2 dBV/Pa. L-1, L-2 and L-3 are "between -50 and -53 dbm".p. 5
Output impedance250 ohm, as shipped; 30 or 150 ohm by changing connections. "250 ohms may be changed to 30 or 150 ohms"; reconnected on the transformer terminal board (p. 6, figure 3).
Poweringnone (passive). No power supply in the technical data; ribbon and output transformer only.
Low cutM, V1, V2; screwdriver selector in the base: a reactor across all or part of the transformer secondary. M (music) for sources more than three feet away; V1 down to one foot and V2 for shorter distances, in the uni- and bi-directional patterns. "If the non-directional characteristic is used, no low-frequency attenuation should be required even for very small distances." The selector is described on p. 6.p. 7
Connectornone: 30 ft captive 3-conductor shielded cable, no plug. Printed "MI-43-c"; the later book (IB-24884-5, p. 1) and the catalog sheet say MI-43-D, the 1955 catalogue MI-43-B.p. 5
Weight1361 g, without cable. "Less cable - 3 pounds"; "Total - 4-1/2 pounds" with the cable.
Dimensions292 × 95 × 64 mm. Length x width x depth: "11-1/2 inches", "3-3/4 inches", "2-1/2 inches". The 1955 catalogue (p. 12) gives 2 1/4" deep.
FinishTV gray and satin chrome (MI-4045-F); TV gray, low gloss (MI-11006-C)
Effective output levelBi-directional -50 dbm (Gm -144 db); uni-directional -53 dbm (Gm -147 db); non-directional -56 dbm (Gm -150 db); L-1, L-2, L-3 between -50 and -53 dbm; sound pressure 10 dynes/cm²
Recommended load impedanceUnloaded input transformer
Hum pickup-128 dbm, hum field 1 x 10^-3 gauss
Pattern positionsU, N, B, L-1, L-2, L-3 (six stops on a continuously variable shutter)p. 6
Mounting1/2-inch pipe thread; cushion mounting and forkp. 5
ModelsMI-4045-F (satin chrome and umber-gray, broadcast), MI-11006-C (low-gloss umber-gray, television)
PhasingRed cable lead electrically positive for positive pressure on the frontp. 6
TransformerTapped for 30, 150 and 250 ohms, turns ratio 1:23.5 ±2%p. 12
Voice reactorIron core, 91 ohms at 50 cps (0.5 V), d.c. resistance 28 ohms
CautionDo not test continuity with a circuit checker without a resistor of at least 50,000 ohms in seriesp. 7
From the catalog sheet
Rated frequency range50 Hz to 15 kHz. "Frequency Response 50-15,000 cps". The 1955 catalogue table (p. 6) gives 50-15,000 too; the instruction book gives no range.p. 2
From the catalog 1955
Price (1955)$150.00 (MI-4045-F and MI-11006-C)p. 156

Documents

Copies kept by Mic Archive, because manufacturers move and drop their documents. Each is © Radio Corporation of America, unchanged, with its original address.

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