Marantz Model 250M (1975–1978)

Name: Marantz MODEL 250M
Brand: Marantz

The 250M is a stereo power amplifier - a dedicated main amp with no preamp section of its own - and it sits high in Marantz's 1970s lineup. HiFi-Stereo (July 1971, p.21) announced the Model 250 as the successor to the Model 16 with increased power, noting its two large front-panel VU meters and a muting circuit to suppress the turn-on surge. There's a dating wrinkle. That notice is for the Model 250, not the 250M, and it appeared in 1971, several years before the 250M's commonly cited 1975-1978 production window. Whether the 250M is a revised 250 or a differently marketed variant isn't something the surviving record settles.

The circuit is settled, because a factory service manual survives. The 250M is an all-silicon solid-state design running class AB. A unity-gain complementary feedback input stage feeds a differential amplifier, an inverter, complementary pre-drivers and drivers, and a four-transistor output stage per channel. Output current limiting is handled by a sensing network that clamps the drive signal on excessive current peaks. Overall voltage gain is set at 21 V/V. A time-delay relay circuit handles power-up, and the amplifier runs on ±58.5 V rails derived from an 86 V AC secondary.

The service manual's performance figures are the most reliable numbers on record. Where they conflict with the general spec table, the manual wins:

Rated output: 126 W per channel continuous average, both channels driven, into 8 Ω, 20 Hz–20 kHz
THD: 0.1%; intermodulation distortion below 0.1%
Frequency response: +0/−1.5 dB from 2 Hz to 100 kHz, ±0.1 dB from 20 Hz to 20 kHz
Input sensitivity: 1.51 V for rated power; input impedance 100 kΩ
Damping factor: greater than 100 into 8 Ω
Total noise: better than 106 dB below rated power into 8 Ω
Power requirements: 120 V AC, 500 W, 50/60 Hz (field-convertible to 100, 220, and 240 V via transformer taps)

The front panel carries a VU meter with a three-position, 10 dB-per-step range pad, so levels can be read across a wide span. The 250M was also sold with optional accessories: a mahogany wood cabinet (WC-2N) or a rack-mount adapter (RA-2). The unit is field-convertible between line voltages, which is why surviving examples turn up configured for different markets.

Documentation beyond the service manual is thin. No period lab review of the 250M specifically has surfaced, and owner impressions in the record are sparse and unverified. The amplifier is sometimes described as US-built, though that comes from seller listings rather than a manufacturer statement. It was a high-power, well-built main amp, and when one appears today it tends to appear restored and priced accordingly.