Marantz Model 240 (early 1970s)
Eleven-point-eight kilograms of Chatsworth-built solid-state muscle, with no power switch on the front and a reputation for blowing up if you feed it the wrong transistors.
Overview
The Model 240 is a stereo power amplifier. No tuner, no preamp, no tone controls, just a chassis, a transformer, and two channels of transistor output built by the Marantz Company, a subsidiary of Superscope Incorporated, at Chatsworth, California 91311. The service manual names the address itself. This was American-built gear from the era when Superscope was pushing Marantz into the serious audiophile market, and the 240 was the entry point to that push.
For a footprint of 362 × 156 × 241 mm and 11.8 kg, you get a rated 125 watts per channel into 8 ohms, 150 watts per channel into 4 ohms, and 64 watts per channel into 16 ohms, all measured at rated output across 20 Hz to 20 kHz. These aren't peak or "music power" numbers. They're continuous average output with both channels driven. The Japanese-market documentation for the 240 notes that typical production units delivered more, roughly 280 watts total into 8 ohms and 340 into 4.
The distortion ratings are conservative on paper. Total harmonic distortion is rated at 0.1% or less from 20 Hz to 20 kHz at rated output, with typical units measuring around 0.05%. Cross-modulation distortion carries the same 0.1% figure across any two frequencies in the audio band. Frequency response runs from 2 Hz to 100 kHz within +0 and −1.5 dB, and the IHF power bandwidth spans 5 Hz to 45 kHz.
Input sensitivity is 1.5V for full output into a 100kΩ load. That means the 240 wants a line-level source with real voltage swing, a proper preamp and not a passive attenuator bolted to a CD player. Damping factor is rated at 100 or more into 8 ohms, and total noise is better than 106 dB below rated power into 8 ohms. Power consumption is 480W. The mains configuration is 100 VAC at 50/60 Hz, with 220V available on special order. The original Japanese-market price was ¥195,000, reportedly around 1972.
Key Features
Differential Amplification with Pure Complementary Symmetry
The 240 uses a differential input stage and a push-pull output stage with complementary transistors, the same NPN/PNP pairing on both halves of the waveform. The Marantz documentation says plainly that transistors were selected as "perfect pairs" during manufacturing, and that the basic circuit itself was adjusted so the amplifier is perfectly balanced in DC. Matching matters here. When the two halves of the output stage aren't matched, you get DC offset at the speaker terminals, asymmetric clipping, and the kind of slow degradation that kills tweeters over years.
All-Stage Direct-Coupled OCL Circuit
There are no coupling capacitors between stages. The 240 is a direct-coupled, output-capacitor-less design from input to speaker terminals, which removes the phase shifts and low-frequency rolloff that coupling caps introduce. The result is bass that goes all the way down without the gradual sag of a capacitor-coupled design. It also means the amplifier has no protection against DC faults at the output. That is why the service manual is so emphatic about checking every driver and output transistor after you find a single shorted semiconductor. The driver and output stages are direct coupled to each other, so one failure can cascade.
Class AB Bias for Low-Level Linearity
The design notes are candid about the problem Marantz was solving. High-power transistor amplifiers running close to class B at low levels produced notching distortion, a switching artifact around the crossover point. Marantz supplied sufficient idling current to run the 240 in class AB, which trades some efficiency for linearity at low output. The documentation calls this achieving "bare characteristics that can be used without feedback." That means the open-loop linearity was good enough that the amplifier wasn't relying entirely on negative feedback to clean itself up.
Dual 20,000μF Power Supply Capacitors
Two 60V 20,000μF smoothing capacitors sit in the power supply, a serious reservoir for an amplifier of this output class. They let the 240 deliver its rated power on sustained bass transients without the rail voltage sagging. They're also the first thing a restorer should look at. Electrolytics of this vintage are well past their design life, and a dried-out pair will make the amplifier sound thin and compressed long before it actually fails.
Separate Circuit Boards per Stage
Instead of one large board with everything crowded together, the 240 uses separate boards for each functional block, with a thick ground line running between them. The stated purpose is avoiding interference between the first and last stages, keeping the low-level input signal physically separated from the high-current output section. It also makes servicing easier. A failed output stage can be worked on without disturbing the input board, and the ground bus is generous enough to keep hum down.
No Front-Panel Power Switch
The 240 doesn't have one. It's designed to be switched on and off by an external source, typically a preamp with switched outlets or a separate power controller. That means less clutter on the front panel and one fewer mechanical failure point. It also means you can't just walk up and turn the amp on. It has to be part of a system with coordinated power sequencing, or you'll get a turn-on thump through the speakers every time.
Current-Sensing Protection Network
Output current regulation is handled by a sensing network built around resistors R531 and R532, whose voltage drop is monitored by current sensors Q516 and Q517. When output current peaks get excessive, those sensors generate peak-limiting signals that feed back to Q505 and Q506, which disable the pre-drivers and cap the output current at a safe level. It's a soft-clamp approach rather than a hard shutdown. The amplifier limits itself instead of cutting out, which is part of why the 240 can be pushed hard without immediately destroying itself, provided the transistors are within spec.
Historical Context
The 240 was built in the early 1970s, when Marantz was operating as a Superscope subsidiary out of Chatsworth, California. The service manual's introduction is addressed to authorized warranty stations, which puts the 240 in the era of dealer-network support and factory-issued service bulletins. The manual even notes that supplementary bulletins would be issued as field problems were identified.
The period literature places the 240 within the Marantz line: it offered the same power, craftsmanship, and endurance as the Marantz 250, but without the 250's two front-panel meters. A contemporary notice in Audio (December 1972, p.14) listed the Model 240 stereo power amplifier alongside other new components of the season, placing it in the early-1970s separates market.
The 240 was the meterless version of the same amplifier, a deliberate cost reduction that kept the electrical design intact. It is also documented as the intended partner for the Marantz Stereo Model 33 preamp. At least one dealer listing describes the pair as "made for one another" and refuses to separate them. The original Japanese-market price of ¥195,000, reportedly around 1972, puts it in the upper-middle tier of the era's separates rather than the budget end.
Collectibility & Value
Collectors have mixed feelings about the 240. Sellers and owners describe the sound as lush, warm, and punchy in the bass, but the amp is also notorious for blowing up if the transistors don't meet certain specifications. Because of that sensitivity to component tolerances, original, unmodified units are desirable since they haven't been reworked with unknown substitutes. Many surviving examples have already been through restoration. Provenance and parts correctness matter more than cosmetics.
Service documentation is available, including scans of the original manual, and enthusiast forums have active discussions about restoration kits and replacement transistors. A repair video documenting a full refurbishment of a unit with a broken right channel is in circulation. The service manual's troubleshooting section lists open AC input circuits, failed output transistors, shorted bias assemblies, and open rectifiers as the usual suspects. It also warns that a single shorted semiconductor in the direct-coupled driver/output chain can take out its neighbors, so anyone buying a "tested, works" unit should ask what was actually checked.
Market data from eBay completed sales shows a wide spread. The high over the tracked period was $1,291, the median $754, the low $365, and the most recent sale $795, with roughly two units moving per month. Over the past twelve months, nine completed sales landed between $482 and $830 with a median of $754. Current listings run higher: one at $650, one at $900, and one at $1,095 for a unit described as having original parts. A dealer offering a 240 paired with a Model 33 preamp lists the pair at a regular price of $1,899 with a sale price of $1,450. The volume is thin enough that individual sales move the numbers, so treat any single data point as directional rather than definitive.
Market Value
┌──────────────────────────────────────────────────────────────────────────┐ │ MARANTZ MODEL 240 Nov 2026 │ ├──────────────────────────────────────────────────────────────────────────┤ │ High $1,291 Latest $795 │ │ Median $754 Trend — │ │ Low $365 Volume ~2/mo │ ├──────────────────────────────────────────────────────────────────────────┤ │ $2,000 ┤ │ │ │ │ │ │ │ │ $1,271 ┤ . │ │ │ │ │ │ . .│ │ │. . . . │ │ $300 ┤ . │ │ ├─────────────────────────────────────────────────────────────────│ │ vol │▄▄ █ ▄ ▄ ▄ ▄ ▄│ │ └─────────────────────────────────────────────────────────────────│ │ 2026-03 2026-05 2026-07│ └──────────────────────────────────────────────────────────────────────────┘
eBay completed sales. Excludes accessories, parts, and non-working units. Hover points for details.
eBay Listings
Recently sold on eBay: $482–$830 (median $754, based on 9 completed sales, past 12 months)
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