Mic Archive
RCA 44-BX
Photo: Radio Corporation of America, from its instructions p1, p. 1.

Radio Corporation of America

RCA 44-BX

Passive ribbon (velocity), figure-8, broadcast studio microphone

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

Type
Ribbon
Pattern
Figure-8
Sensitivity
−54 dBV/Pa computed
Impedance
250 Ω
Rated range
50 – 15k Hz

Frequency response

Angle
Distance

Frequency response: from the instructions, p. 1. "Figure 4 - Frequency Response", "RELATIVE RESPONSE IN DB". The bulletin: "The frequency response to plane sound waves for the M, V1 and V2 positions is shown in figure 4. Approximately plane sound waves are obtained at the microphone when the source is more than three feet away." Room and smoothing not stated. 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 p1
Output impedance250 ohm, as shipped; 30 or 150 ohm by changing connections. "30, 150 or 250 ohms (connected for 250 ohms when shipped)".p. 1
Poweringnone (passive). No power supply in the technical data; the catalogue (p. 7) describes the voltage as generated in the ribbon itself.
Connectornone: 30 ft captive 3-conductor shielded cable (MI-43B), no plug
Weight3912 g, unpacked. "8 pound 10 ounces (unpacked)"; the catalogue (p. 8) says 8¼ lbs including mountings.
Dimensions305 × 117 × 79 mm. Length x width x depth: "12 inches (including mounting)", "4-5/8 inches", "3-1/8 inches". The catalogue (p. 8) gives 12" x 4 3/4" x 3 3/8".
FinishTV gray and satin chrome
Effective output level-55 dBm at 1000 cycles (sound pressure 10 dynes/cm²); Gm = -149 db (RTMA rating)
Recommended load impedanceUnloaded input transformer
Hum pickup-115 dbm (terminal-board link in M position), hum field 1 x 10^-3 gauss
Mounting1/2-inch straight pipe thread; yoke and shock (cushion) mounting
ModelsMI-4027-H, -J and -K (bulletin); MI-4027-J (1955 catalogue)
From the catalog
Capsulethin corrugated metallic ribbon supported at the ends between the pole pieces of a magnetic circuit; impedance-matching transformer and compensating reactor in the base. The parts diagram in the bulletin (figure 8) lists "RIBBON #20935" and a magnetic screen.p. 7
Sensitivity−54 dBV/Pa, open circuit, 250 ohm connection; also −57 dBV/Pa, 150 ohm connection (Gm -149 dB); also −58 dBV/Pa, 30 ohm connection (Gm -150 dB). Not stated in volts. Computed from the RETMA rating "250 Ohm Output Impedance -146 db" with the catalogue's own formula (p. 6): Gm = 20 log(E/P) - 10 log 150 - 50 dB, P in dynes/cm²; E/P = 1.94 mV per Pa, -54.2 dBV/Pa. The bulletin (p. 1) gives Gm = -149 db without naming the connection.p. 8
Rated frequency range50 Hz to 15 kHz. "Frequency Response 50-15,000 cycles"; the bulletin (p. 1) says "essentially flat frequency response (50 to 15,000 cycles)".
RETMA rating Gm (catalogue)-150 db (30 ohm), -149 db (150 ohm), -146 db (250 ohm); effective output level -54 dbm
Price (1955)$129.00p. 156
From the instructions
Low cutM, V1, V2; jumper on the terminal board: a reactor across all or part of the transformer. MI-4027-H and -J have M, V1 and V2; the MI-4027-K has only M and V (V = V1). M for sources at least three feet away; V1 down to 12 inches; V2 down to seven inches.p. 2
PhasingRed lead electrically positive for positive pressure on the front; "Rotating a velocity microphone 180 degrees reverses the phase."p. 3
Voice-music connection"It is not intended that the voice-music connections be changed between numbers on a program."p. 2
TransformerPart #58871 (H and J), #55273 (K)p. 5

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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