Marantz MODEL 1150 (1975)

Name: Marantz MODEL 1150
Brand: Marantz

At 15kg, it is a heavy amplifier to lift onto a rack. 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 sits between the tube-influenced voicing of earlier Marantz models and the precision of fully transistorized circuitry. It weighs 15kg and measures 390 x 146 x 315 mm, and most of that mass is the power supply and internal layout. It was originally priced at ¥125,000, a premium price 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 careful signal path design. The amplifier delivers 80W per channel into 8Ω (20 Hz to 20 kHz, both channels driven), with a power bandwidth of 7 Hz to 45 kHz. That points to strong control at the frequency extremes. Total harmonic distortion stays under 0.1% across the audible band, and the preamplifier section reaches 0.05%. These figures weren't just marketing - they reflect a design built around linearity and stability.

Unlike many contemporaries, the Model 1150 uses direct coupling throughout all push-pull stages. That removes coupling capacitors, which can degrade over time and compromise signal integrity. The output stage is an inverted Darlington configuration, known for high gain and low output impedance, and it gives a damping factor of 45 or more into 8Ω. That means it can keep a firm grip on difficult speaker loads, which collectors still value if they drive vintage transmission-line or high-sensitivity designs.

Specifications

Manufacturer	Marantz
Product Type	Pre-main amplifier
Year of Production	1975
Rated Output	80W + 80W (8 Ω, 20 Hz to 20 kHz, both channel drive)
Frequency Characteristic	20 Hz to 20 kHz ± 1.0 dB
Total Harmonic Distortion Rate	Not more than 0.1% (20 Hz to 20 kHz)
Total Harmonic Distortion Factor (Preamplifier Part)	0.05%
Intermodulation Distortion Factor	Not more than 0.15% (20 Hz to 20 kHz)
Cross Modulation Distortion Factor	0.15% or Less (20 Hz to 20 kHz)
Cross Modulation Distortion Factor (Preamplifier Part)	0.05%
Power Bandwidth	7 Hz to 45 kHz
Damping Factor	45 or More (8 Ω Load)
Gain	Phono → preamplifier output: 58 dB; Phono → Recording output: 40 dB; High level → preamplifier output: 18 dB
Input Impedance	Phono: 47k Ω; High level: 60k Ω
Input Sensitivity	Phono: 2mV; Aux/High Level: 150mV
Phono Input Equivalent Noise	1.0 μV (Input Conversion Value)
Dynamic Range (Phono)	106dB
Output Impedance (Pre out)	470 Ω
Tone Control	Bass: ±10 dB; Mid: ±6 dB; Treble: ±10 dB
Filter	High: 9kHz; Low: 30Hz; Subsonic: 10 Hz
Power	100 VAC, 50Hz/60Hz
Power Consumption	250W
External Dimensions	Width 390 x Height 146 x Depth 315 mm
Weight	15kg

Key Features

Direct-Coupled Push-Pull Drive with Inverted Darlington Output

Every stage of the push-pull drive is direct coupled. This eliminates the phase shift and transient distortion that coupling capacitors introduce. The output stage is an inverted Darlington configuration, which gives high current gain and excellent thermal stability. This topology reduces crossover distortion and improves linearity, especially at low signal levels. Combined with the high damping factor, it gives tight, articulate bass 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 the 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 supply stays clean and stable, and it supports dynamic transients without sag or compression.

Three-Stage Direct-Coupled NF Equalizer Amplifier

The phono stage uses a three-stage, direct-coupled negative feedback (NF) circuit with differential input amplification. It includes a differential transistor pair, a constant current source, a high-gain amplifier stage and a stabilized dual power supply. It works with a wide range of moving magnet cartridges without overloading, and 100% DC feedback improves stability and low-frequency linearity. 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, the preamplifier section uses a low-row-cage type coupling capacitor. These capacitors are selected for good dielectric linearity and low distortion, which minimizes phase anomalies and keeps transient detail. They are not as exotic as Teflon or polystyrene types, but they show a deliberate effort to maintain fidelity in the 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. It gives control over bass, mid and treble (±10 dB, ±6 dB, ±10 dB respectively). A turnover switch moves the tone controls to below 250 Hz (bass) or above 4 kHz (treble), for finer adjustments. When the tone switch is disengaged, the signal bypasses the tone circuit entirely and goes straight to the pre-out, which keeps the frequency response flat. Audiophiles still appreciate that.

Active Filter Circuitry (High, Low, Subsonic)

The Model 1150 has 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 useful for removing turntable rumble and warped-record excursions, which protects speakers and improves clarity. These are not passive roll-offs but active servo-assisted circuits, so they interact minimally 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 from 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, which prevents thumps and protects the drivers. The relay also disconnects the speakers if fault conditions are detected.

Front-Panel Dubbing Jack with Auto-Switching

A front-panel dubbing jack allows easy connection of portable recorders without reaching the rear panel. Plugging into the Dubbing In jack automatically disconnects Tape2 In, while Dubbing Out and Tape2 Out are connected in parallel. That allows simultaneous recording to three tape decks. Any tape source can be monitored with the front panel switch.

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.