Nakamichi OMS-50II (circa 1986)
The Nakamichi OMS-50II is a CD player from around 1986. It was a cost-reduced version of the company's higher-end OMS-70II, which got the same player minus the remote control and headphone output. Price at introduction was ¥208,000. Almost everything written about it is technical narrative, not review coverage, and nearly all of that traces to a single Japanese database source. So the record has gaps: no production years beyond the circa-1986 price reference, no dimensions, no weight, and no full spec sheet.
The one problem this machine was built around is glitches during D/A conversion. Nakamichi's answer was a "glitch free" conversion system with no sample-and-hold circuit at all, on the reasoning that analog switches, capacitors, and their internal resistance degrade sound quality. The signal path ran a 16-bit digital signal through a 4-times oversampling digital filter (44.1 kHz up to 176.4 kHz, pushing unwanted noise down to 156.4 kHz). From there it went through a photocoupler, which kept servo-system noise out of the analog stage via ground loops. The dual 16-bit D/A converters used a Nichrome thin-film ladder resistor network. A digital circuit controlled each bit's switches so they operated simultaneously, which kept glitch occurrence near zero.
The analog side got the same treatment. A third-order linear phase Bessel filter with a 30 kHz cutoff was built discretely from a two-stage DC amplifier. It reached a 0.0003% distortion rate with minimal negative feedback. Left and right channels were laid out perfectly symmetrically, each powered by independent regulators. The regulators were discrete transistor designs with no feedback loop, fed by separate transformer windings for the analog and digital sections. Three layers of pure copper bus bars sat between the D/A converter and the analog filter as shielding. The parts list is long: metal film resistors, mica capacitors, Cerafine capacitors, non-inductive capacitors, brass cap resistors for the NFB positions, and Hi-gm dual FETs for differential inputs. Gold-plated pin jacks were soldered directly to the analog board to avoid wiring degradation.
The mechanics got just as much thought. The disk drive mechanism was attached directly to a zinc alloy die-cast chassis for rigidity. It was independent of the main base and loading mechanism, and it floated from the main chassis on springs and damping material. A magnet chuck system secured the disk, and a unique centering mechanism prevented read errors from disk eccentricity. The servo circuit used a newly developed digital signal processing LSI that could hold focus through a scratch until the RF signal recovered.
Known specs and details:
- D/A converter distortion: 0.0015% or less
- Digital filter: 16-bit, 4-times oversampling
- Analog filter: third-order linear phase Bessel, 30 kHz cutoff, discrete construction
- Laser pickup: 3-beam / astigmatic
- Display: time / total time / remaining time
- Original price: ¥208,000 (circa 1986); a later reference suggests ¥190,000 (circa 1989)
Spare parts exist for a drive belt and a replacement laser pickup, which implies the usual age-related service needs. A service manual exists. No period magazine reviews of this specific model have surfaced, and there are no verified current market prices. Don't confuse it with the separate Nakamichi OMS-50 or OMS-70 II models. The OMS-50II is the simplified sibling of the OMS-70II.
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