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.
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
- Figure-8
- Sensitivity
- −54 dBV/Pa computed
- Rated range
- 50 – 15k Hz
Instruments sources recommend it for most
Proximity effect: the bass it gains up close
From the instructions, p. 1. Approximate
"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.
In practice
All specs
| From the instructions p1 | ||
|---|---|---|
| Output impedance | 250 ohmas shipped; 30 or 150 ohm by changing connections. "30, 150 or 250 ohms (connected for 250 ohms when shipped)". | p. 1 |
| Powering | none (passive)No power supply in the technical data; the catalogue (p. 7) describes the voltage as generated in the ribbon itself. | |
| Connector | none: 30 ft captive 3-conductor shielded cable (MI-43B), no plug | |
| Weight | 3912 gunpacked. "8 pound 10 ounces (unpacked)"; the catalogue (p. 8) says 8¼ lbs including mountings. | |
| Dimensions | 305 × 117 × 79 mmLength 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". | |
| Finish | TV gray and satin chrome | |
| Effective output level | -55 dBm at 1000 cycles (sound pressure 10 dynes/cm²); Gm = -149 db (RTMA rating) | |
| Recommended load impedance | Unloaded input transformer | |
| Hum pickup | -115 dbm (terminal-board link in M position), hum field 1 x 10^-3 gauss | |
| Mounting | 1/2-inch straight pipe thread; yoke and shock (cushion) mounting | |
| Models | MI-4027-H, -J and -K (bulletin); MI-4027-J (1955 catalogue) | |
| From the catalog | ||
| Capsule | thin corrugated metallic ribbon supported at the ends between the pole pieces of a magnetic circuit; impedance-matching transformer and compensating reactor in the baseThe parts diagram in the bulletin (figure 8) lists "RIBBON #20935" and a magnetic screen. | p. 7 |
| Sensitivity | −54 dBV/Paopen 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 range | 50 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 Gmcatalogue | -150 db (30 ohm), -149 db (150 ohm), -146 db (250 ohm); effective output level -54 dbm | |
| Price1955 | $129.00 | p. 156 |
| From the instructions | ||
| Low cut | M, V1, V2; jumper on the terminal board: a reactor across all or part of the transformerMI-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 |
| Phasing | Red 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 |
| Transformer | Part #58871 (H and J), #55273 (K) | p. 5 |
Documents
- Instructions for Velocity Microphone Type 44-BX, MI-4027-H, -J, -K (IB-24267-5), page 1
- Instructions for Velocity Microphone Type 44-BX (IB-24267-5), figures 3 to 8
- 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.
Recommended for
By recording guides, engineers and mic makers, for this mic or an earlier version of it.