Marantz MODEL 1150 (1975)

At 15kg, it lands on the rack like a declaration—this is serious engineering, built when amplifiers were meant to last.

Overview

The Marantz Model 1150 is a solid-state integrated amplifier released in 1975. It represents a transitional moment in Marantz’s design philosophy—bridging the lush tube-influenced voicing of earlier models with the precision of fully transistorized circuitry. Weighing in at 15kg and measuring 390 x 146 x 315 mm, it occupies space with authority, its mass hinting at the substantial power supply and robust internal layout. Originally priced at ¥125,000, it was positioned as a premium component in an era when high-fidelity gear was both investment and heirloom.

Owners report a machine built for longevity, with over-engineered power regulation and meticulous signal path design. The amplifier delivers 80W per channel into 8Ω (20 Hz to 20 kHz, both channels driven), with a power bandwidth stretching from 7 Hz to 45 kHz—indicating strong control at the frequency extremes. Distortion is tightly managed: total harmonic distortion remains under 0.1% across the audible band, while the preamplifier section achieves an impressive 0.05%. These figures weren’t just marketing—they reflect a design obsessed with linearity and stability.

Unlike many contemporaries, the Model 1150 employs direct coupling throughout all push-pull stages, eliminating coupling capacitors that can degrade over time and compromise signal integrity. The output stage uses an inverted Darlington configuration, known for high gain and low output impedance, contributing to a damping factor of 45 or more into 8Ω. This means it can exert real control over difficult speaker loads, a trait still valued by collectors who drive vintage transmission-line or high-sensitivity designs.

Specifications

Key Features

Direct-Coupled Push-Pull Drive with Inverted Darlington Output

Every stage of the push-pull drive is direct coupled—a design choice that eliminates phase shift and transient distortion introduced by coupling capacitors. The output stage uses an inverted Darlington configuration, which provides high current gain and excellent thermal stability. This topology reduces crossover distortion and improves linearity, especially at low signal levels. Combined with the amplifier’s high damping factor, it delivers tight, articulate bass response even with complex speaker loads.

56,000 μF Tandem Power Supply Capacitor

The power supply uses a 56,000 μF large-capacity filter condenser arranged in an internal tandem configuration with minimal capacitance deviation between positive and negative rails. This balance reduces ripple and improves stability, particularly in the high-frequency range where power supply imbalance can introduce distortion. The result is a clean, stable voltage supply that supports dynamic transients without sag or compression.

Three-Stage Direct-Coupled NF Equalizer Amplifier

The phono stage employs a three-stage, direct-coupled negative feedback (NF) circuit with differential input amplification. This design includes a differential transistor pair, constant current source, high-gain amplifier stage, and stabilized dual power supply. It allows compatibility with a wide range of moving magnet cartridges without overloading, while 100% DC feedback enhances stability and low-frequency linearity. The phono input sensitivity is 1.8mV with an input impedance of 47k Ω, standard for MM cartridges of the era.

Low-Row-Cage Coupling Capacitors in Preamp Section

To preserve signal integrity, a low-row-cage type coupling capacitor is used in the preamplifier section. These capacitors are selected for superior dielectric linearity and low distortion, minimizing phase anomalies and preserving transient detail. While not as exotic as Teflon or polystyrene types, their use here reflects a deliberate effort to maintain fidelity in critical signal paths.

Darlington-Connected Tone Control Circuit with Bypass

The tone amplifier section uses four transistors per channel in a two-stage Darlington configuration, enabling precise control over bass, mid, and treble (±10 dB, ±6 dB, ±10 dB respectively). A turnover switch shifts the influence of tone controls to below 250 Hz (bass) or above 4 kHz (treble), allowing surgical adjustments. Most critically, when the tone switch is disengaged, the signal bypasses the tone circuit entirely and routes directly to the pre-out, preserving flat frequency response—a feature audiophiles still appreciate.

Active Filter Circuitry (High, Low, Subsonic)

The Model 1150 includes active high-pass (30 Hz), low-pass (9 kHz), and subsonic (10 Hz) filters. Engaging any filter connects it to the amplifier’s feedback loop, producing a 12 dB/octave roll-off. The subsonic filter is particularly useful for eliminating turntable rumble and warped record-induced excursions, protecting speakers and improving clarity. These are not passive roll-offs but active servo-assisted circuits, ensuring minimal interaction with the main signal path.

PEP (Pure Electronic Protection) and Speaker Relay Delay

A PEP (Pure Electronic Protection Device) circuit guards against excessive current and voltage due to short circuits or thermal runaway. It includes temperature compensation for reliable operation under extreme conditions. A built-in speaker protection relay delays connection for approximately four seconds at power-on, preventing thumps and protecting drivers. The relay also disconnects speakers if fault conditions are detected, a feature that has likely saved countless vintage tweeters.

Front-Panel Dubbing Jack with Auto-Switching

A front-panel dubbing jack allows easy connection of portable recorders without accessing rear panels. Plugging into the Dubbing In jack automatically disconnects Tape2 In, while Dubbing Out and Tape2 Out are connected in parallel—enabling simultaneous recording to three tape decks. Any tape source can be monitored via the front panel switch, a thoughtful touch for users managing multiple recorders in an analog setup.

Collectibility & Value

Market data is sparse, but units have appeared in private sales around $300 USD for unrecapped, working condition examples. Restored units appear on platforms like Reverb, often priced higher depending on service quality. No official spare parts or service bulletins are documented, but the circuit design is conventional enough that competent technicians can source replacements. Common maintenance concerns include capacitor aging—owners frequently note whether a unit has been recapped—and the need to inspect and clean input/output jacks and ground connections to prevent noise and channel dropouts.

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Related Models

• Marantz 3600 (1976-1977)
• Marantz MODEL 112 (1971-1973)
• Marantz MODEL 115 (1975-1977)
• Marantz MODEL 260 (1976-1978)
• Marantz MODEL 28 (1971-1973)
• Akai AM-2850 (1975)
• Akai AP-206 (1975)
• Nakamichi BX-1 (1985)
• Acoustic Research research-ar-17 (1978)
• Acoustic Research AR-19 (1994)