RCA MI-4887
A magnetic cutter head that promised ±2 dB from 50 to 10,000 cycles - and asked you to rebuild your whole monitoring chain to hear it.
Overview
The MI-4887 is a high-fidelity magnetic recording head. It is a direct replacement for the standard cutting head furnished with RCA's 72-C recorder, and NBC's San Francisco recording room credited it with dragging its operation out of the reference-work era. It is not a playback cartridge, and it is not a consumer item. It is a lathe cutter, the transducer that physically carves the groove. RCA's own documentation describes it as a bandpass mechanical network terminated in a dry mechanical resistance material, with a balanced-type armature centered by an adjustable tempered steel spring and riding on knife-edge bearings. Pole pieces are nicaloi.
The headline claim is the response figure. The standard head in the 72-C covered 60 to 6,000 cycles. The MI-4887 is specified within ±2 dB from 50 to 10,000 cycles, and RCA states that each unit is held within close frequency limits, departing from an ideal response curve by no more than 2 decibels across that span. Heads were matched for sensitivity within 2 dB at 1,000 cycles. Those are manufacturer figures, and they carry the usual caveat. Performance data was taken at the time of manufacture by optical measurement of stylus tip motion for constant input, which deliberately excludes transfer and needle losses. RCA's own notes call those losses "rather severe at high frequencies at low record surface speeds." So the ±2 dB is a statement about the head, not about what survives to a playback stylus.
Most of the MI-4887's documentation is really about the rest of the chain. RCA is blunt that the head should never be connected directly to an amplifier. A compensator pack ships with it to nullify head inductance and present a nearly constant impedance. The recommended hookup puts ten ohms in series with the cutter, bypassed with 0.5 microfarad. That series resistance has a floor: RCA warns that below 7.5 ohms total the frequency response is seriously affected. The compensator's adjustment exists only to equalize sensitivity between multiple heads. Critical operators were told they could nudge the series resistance in steps of no more than 2.5 ohms to compensate for manufacturing tolerance - up on the hot heads, down on the weak ones. In most cases, RCA says, no adjustment is needed. Average impedance of head and compensator together is fifteen ohms, varying a few ohms across 50 to 10,000 cycles, which RCA judged not enough to cause objectionable mismatching.
RCA recommends an independent monitor system, a bridging amplifier and loudspeaker, rather than monitoring off the cutter amplifier's output. It gives two reasons. One is impedance interaction between cutter and speaker when both hang off a common output. The other is that cutter amplifiers are normally compensated at high frequencies for recording work. The argument is that a limited-range monitor will hide distortion that a wide-range playback channel will expose, and the operator will blame the recording when the fault lies upstream in a long line or a bad studio feed.
Stylus choice gets similar treatment. RCA recommends sapphire for everything except unimportant tests. It argues that the moderate resharpening charge brings cost per minute to equal or below steel cutting points, and that sharp, unchipped sapphires produce a groove surface quieter by 6 dB or more than steel. The company also notes that improvements in blank formulation had by then produced a lacquer that cuts freely, wears well, and ages stably. Waxing or lubrication at time of cutting or afterward was no longer required, though storage in metal containers was still recommended.
RCA's recommended associated equipment shows the intended installation. For ordinary work, an amplifier with uniform response to 10,000 cycles and eight watts undistorted output is called sufficient. For master discs that must match commercial transcription loudness, RCA wanted 17 watts to handle the high peak voltages. The suggested playback reference chain was specific: a 70-C (MI-4871) transcription turntable with an MI-4856 or MI-4875 diamond point reproducer, and a 64-B (MI-4400) broadcast monitor speaker with MI-11209 amplifier.
The strongest endorsement is not from RCA at all. In the pages of the A.T.E. Journal, NBC San Francisco engineer J. A. O'Neil wrote that the arrival of RCA Type MI-4887 cutting heads and an ND-45 A.A.G.C. amplifier marked a turning point for a facility that had been running single-channel with four discs changed in twenty to thirty seconds and personnel "on the verge of talking to themselves." The heads were followed in rapid succession by Presto 8A tables, more ND-45s, ND-46 drivers, and crossover filter specifications, leading to a rebuilt control and selection system. Audio (February 1949, p.15) later traced the MI-4887's lineage, describing it as the disk recording head originated by H. J. Hasbrouck and noting its excellent measured characteristics.
These are 1941-vintage magnetic cutters with knife-edge armature bearings and a tempered steel centering spring, and they were serviced, matched, and compensated as part of a system. A bare head with no compensator pack is only half the device. RCA's own literature ties the whole setup to a temperature-controlled room, with response and sensitivity standardized at 72° F and drifting no more than 3 dB between 65° F and 80° F.
The MI-4887 is best treated as a component of the 72-C ecosystem. It appears in RCA's accessory list alongside the MI-4894/4913-2 automatic equalizer, the MI-4916 Orthacoustic recording filter, the MI-4876 outside-in lead screw, and a range of steel and sapphire styli - MI-4879-A, MI-4878-B, and MI-4842 (70°). The head was also adopted well beyond RCA's own lathes. The A.T.E. Journal account of the San Francisco installation shows MI-4887 heads running in Presto 6D channels with ND-45 faders and ND-46 drivers. Prices and current market activity are not documented here.
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