Sony CDP-M51 (1990)

The Sony CDP-M51 is a compact CD player launched in 1990 at ¥24,800. It's a "midi" sized component, about 35.5 cm wide, and it sat at the budget-to-mid end of Sony's lineup, nowhere near the ES flagships. The interesting part is the D/A conversion. Sony gave the M51 its newly developed "pulse" 4D/A converter. It switched two current values at high speed (45 mega pulses per second) to express the music waveform directly, instead of using conventional multi-bit conversion. The system combined a high-density linear converter, a 45-bit noise-shaping digital filter, and a pulse D/A stage. Sony claimed that stage completely eliminated differential non-linear distortion, glitch, and zero-cross distortion.

The M51 also got a few thoughtful engineering touches for its class. A "direct digital sink" generated the master clock on the same chip as the pulse D/A converter, which Sony said eliminated jitter in principle. The servo circuit was the third-generation "S Servo III", with a digital CLV servo controlling spindle rotation at 10-bit resolution. The RF amplifier was integrated into the pickup section for lower noise. The digital signal processing LSI was Sony's fourth-generation design, with 32 kbit of RAM built in and a strict 6-fold error correction method. The laser pickup is a KSS-240A, and the DAC chip is a CXD2552Q.

The front panel has a 20-track music calendar display and a headphone jack with its own volume control. A wireless remote (RM-D491) came in the box.

Documentation on the M51 is thin. No formal reviews or detailed owner discussions have surfaced, and even the service manual is scarce (a 25-page PDF circulates via independent manual sites). The classifieds do show that the CD tray belt is a known wear item, since replacement square belts are sold specifically for this model. They also show that the optical laser lens pickup is a serviceable part, shared with the CDP-M69 and CDP-M79. Used units appear now and then on European and Asian classified sites, typically priced around €50-60 or the equivalent in local currency. One Malaysian listing rated a unit 8/10 condition but noted the remote was missing.

Known specs:

- Disc format: CD (12 cm and 8 cm)

- Frequency response: 2 Hz–20 kHz ±0.5 dB

- THD: 0.003% or less (EIAJ)

- S/N: 100 dB or more (EIAJ); dynamic range: 98 dB or more

- Channel separation: 97 dB

- Wow and flutter: below measurement limit (±0.001% W. peak)

- Output: 2 Vrms (fixed); headphone: 10 mW into 32 Ω

- Power consumption: 10 W

- Dimensions: 355 × 95 × 310 mm (W×H×D); 3.0 kg