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.
Power None (passive). Switch phantom power off before you plug it in: a faulty cable can damage the ribbon. Low output: it needs a lot of preamp gain.
- Type
- Ribbon
- Pattern
- Switchable, 3
- Sensitivity
- −55 dBV/Pa computed
- Rated range
- 50 – 15k Hz
Proximity effect: the bass it gains up close
From the instructions, p. 9. Approximate
"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.
In practice
All specs
| From the instructions | ||
|---|---|---|
| Capsule | corrugated 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 ribbonRibbon 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/Paunidirectional, 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 impedance | 250 ohmas 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). | |
| Powering | none (passive)No power supply in the technical data; ribbon and output transformer only. | |
| Low cut | M, V1, V2; screwdriver selector in the base: a reactor across all or part of the transformer secondaryM (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 |
| Connector | none: 30 ft captive 3-conductor shielded cable, no plugPrinted "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 |
| Weight | 1361 gwithout cable. "Less cable - 3 pounds"; "Total - 4-1/2 pounds" with the cable. | |
| Dimensions | 292 × 95 × 64 mmLength 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. | |
| Finish | TV gray and satin chrome (MI-4045-F); TV gray, low gloss (MI-11006-C) | |
| Effective output level | Bi-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 impedance | Unloaded input transformer | |
| Hum pickup | -128 dbm, hum field 1 x 10^-3 gauss | |
| Pattern positions | U, N, B, L-1, L-2, L-3 (six stops on a continuously variable shutter) | p. 6 |
| Mounting | 1/2-inch pipe thread; cushion mounting and fork | p. 5 |
| Models | MI-4045-F (satin chrome and umber-gray, broadcast), MI-11006-C (low-gloss umber-gray, television) | |
| Phasing | Red cable lead electrically positive for positive pressure on the front | p. 6 |
| Transformer | Tapped for 30, 150 and 250 ohms, turns ratio 1:23.5 ±2% | p. 12 |
| Voice reactor | Iron core, 91 ohms at 50 cps (0.5 V), d.c. resistance 28 ohms | |
| Caution | Do not test continuity with a circuit checker without a resistor of at least 50,000 ohms in series | p. 7 |
| From the catalog sheet | ||
| Rated frequency range | 50 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 | ||
| Price1955 | $150.00 (MI-4045-F and MI-11006-C) | p. 156 |
Documents
- Instructions, Type 77-DX Polydirectional Microphone, MI-4045-F and MI-11006-CIB-24884-2
- Instructions, Type 77-DX Polydirectional Microphone, MI-4045-F and MI-11006-CIB-24884-5
- Polydirectional Microphone Type 77-DXcatalog sheet B.1009
- RCA Audio Equipment, Second Edition (1955), with prices
Copies kept by Mic Archive (manufacturers move and drop their documents). Each is © Radio Corporation of America, unchanged, with its original address.