Accuphase C-230 Preamplifier

Name: Accuphase C-230
Brand: Accuphase

A control center that made input capacitors obsolete and put MOS FETs in the output stage for the first time anywhere.

Audio (October 1981, p.46) surveyed the preamplifier field of the period and placed the C-230 alongside the SAE X-1P, Quad 44, Mark Levinson ML-7, and Adcom GFP-1.

Overview

The interesting parts of the C-230 are buried in the circuit description in the manual, not on the front panel. It's a stereo control center, built by Accuphase Laboratory, Inc. in Japan. The company's own documentation calls it a high-gain control center that aggregates the brand's audio technology of the period. Two things stand out: a built-in head amplifier for direct connection of moving-coil cartridges, and a full-fledged headphone amplifier rather than a tap off the line stage. The manual calls the C-230 a control center "of the highest order," which is marketing, but the circuit description underneath it is unusually specific about what was actually done.

The core of the design is a complementary-symmetry, Class-A push-pull driven DC amplifier circuit running through every stage - from the head amplifier input all the way to the final output, and even in the headphone amplifier. Accuphase's stated reasoning is that this topology has the best inherent linearity, lowest distortion, and highest stability. Building it into every stage, not just the line amp, is the extravagant part. The DC amplifier system has a concrete consequence: it eliminates DC blocking capacitors from the negative feedback loop and all coupling capacitors in the signal path. That makes it a true Input Capacitor-Less design. It sounds like spec-sheet talk until you read the servo circuit description.

Physical dimensions are quoted at 445 x 128 x 370mm (W x H x D). Mains voltage varies by market, so units turn up wired for 220V and for 240V depending on where they were sold. Made in Japan. The C-230's production years aren't documented in the sources available here. A Pinterest pin caption associates the model with May 1979, but that's a pin caption and not a confirmed date, so treat it as a hint.

Key Features

MOS FETs in the Output Stage

Accuphase's manual makes a specific and checkable claim: the C-230's equalizer and head amplifier outputs employ MOS FETs, and it was the first time anywhere that the device had been used in a control center. The stated reason is that MOS FETs are voltage-controlled, so they don't exhibit notching distortion - the crossover artifact that plagues conventional bipolar output stages at low levels. Accuphase says the choice followed extensive listening tests conducted over a long period, which is what every manufacturer says, but the technical argument behind it is sound. The equalizer and high-level amplifier output stages are built around devices that behave differently from the bipolar transistors used almost everywhere else in 1979-80 preamplifiers.

DC Servo Control in the High-Level Amplifier

Conventional high-level amplifiers combine the tone control function by varying capacitance and resistance in the negative feedback loop. In that arrangement even a small DC current at the output can produce a pulse-type shock noise when switching. The usual fix is a DC blocking capacitor, but that turns the amplifier into something that is no longer a DC amplifier, and the capacitor colors the sound. The C-230 uses a DC Servo Control system (patent pending, per the manual) to make the blocking capacitor unnecessary. It holds DC drift to practically zero, so the unit operates as a pure DC amplifier even with tone control engaged. The servo detects output DC drift, amplifies it, and uses it to control the constant-current load of the input buffer device. It does not directly control the signal path stage, which is why Accuphase argues it doesn't affect sound reproduction. A separate feedback-cancelling circuit built around two transistors and two diodes prevents capacitive feedback from the buffer device without a compensating capacitor, which would otherwise alter the sound.

Exclusive Voltage-Regulated Supplies for Every Unit Amplifier

Every unit amplifier in the C-230 carries its own dedicated voltage-regulated power supply, mounted on the respective unit amplifier printed circuit board to keep circuit impedance to a minimum. Accuphase's own framing is that the ideal solution would be to position a regulated supply directly at the base of each amplifying device, and this is essentially that. The manual attributes the C-230's stability of characteristics to this approach, calling it an "almost ideal, extravagant" design. The regulation is distributed rather than centralized, along with the separate power supply and star grounding noted in the design documentation.

Built-In Moving-Coil Head Amplifier

The C-230 can be connected directly to moving-coil cartridges without an outboard step-up. The head amplifier is a complementary-symmetry push-pull design using the latest low-noise transistors. They were selected because the signals handled at MC level are extremely low and the goal was a signal-to-noise ratio close to the theoretical limit. Two disc inputs are provided, and one of them carries an input impedance matching selector offering 100 ohms, 47k ohms, 82k ohms, and 150k ohms. An owner thread on a German forum describes someone bringing in a C.230 specifically to take measurements on the MM phono section. A single classified listing quotes distortion at 0.005%, though that figure is single-source and should be treated accordingly.

Class-A Headphone Amplifier

The headphone output isn't a resistor off the line stage. It's a completely independent Class-A amplifier with direct-coupled, complementary-symmetry push-pull circuitry, the same topology used through the rest of the unit. Accuphase describes it as ideal for monitoring program sources to check fidelity. In effect it's a second small power amplifier inside the chassis.

Tone Control and Turnover Switching

Tone control on the C-230 is more flexible than the usual bass/treble knobs. A turnover selector switch lets you pick 200 Hz for bass and either 2 kHz or 7 kHz for treble. The 200 Hz and 7 kHz settings are the ones Accuphase recommends for smooth control across the widest range. A 10-step rotary switch gives accurate 10-step tonal variation and also handles on/off switching of the tone circuit. The loudness compensator is a three-position switch rather than a single curve: COMP 1 gives +6 dB at 50 Hz, COMP 2 gives +9 dB at 50 Hz, and COMP 3 gives +10 dB at 50 Hz plus +6 dB at 20 kHz, with those values specified at a volume setting of -30 dB. A 17 Hz, -12 dB/octave subsonic filter handles the low-end cleanup, and a -20 dB attenuator is on hand for level matching.

Tape and System Flexibility

Two tape recorders can be connected, with an independent copy switch for dubbing from one machine to the other while a different program source plays. The tape output can be cut with an independent switch, which also prevents the distortion that occurs when a tape deck's power is off and its input circuit loads down the control center. Two phonos and two outputs mean the C-230 can run two power amplifier systems, and owners paired it with the Accuphase P-260 power amp. The manual also documents a New IHF-IM distortion measurement at 15V output with 19 kHz and 20 kHz signals fed at a 50:50 ratio, where no intermodulation products were detected within the 0.0027% measuring limit.

Historical Context

The C-230 builds on the C-200. The source documentation is explicit that it takes the C-200's foundation and elevates the phono stage. The C-200 dates to August 1973 and the C-200S to September 1977 according to a collector's board that catalogues the family, which puts the C-230 later in that line. The C-230's own production dates aren't confirmed in the available sources. A Pinterest pin caption associates it with May 1979, but that's not authoritative enough to state as fact.

Accuphase itself is described in the period literature as one of the large companies in the Hi-Fi industry, focused on quality and building products aimed toward the future. The C-230's manual is written by engineers who expected the reader to follow a servo circuit description, not a brochure. The pairing with the P-260 power amplifier was the standard configuration, and it's the combination that shows up most often in the used market today. The C-230's tone control versatility is one of its special features in addition to all the standard functions available in the highest grade Accuphase C-240 control center.

Collectibility & Value

The C-230 trades on the Japanese secondary market through Yahoo Auction proxies. Both japex.ru and samurayushka.ru catalogue it, and japex.ru lists it alongside the C-220, suggesting both models circulate in similar volume. In Europe, one unit was offered on audio-markt.de as a second-hand transistor preamp, advert 8423100997, with the listing closing in April 2022. A Gumtree Australia listing priced a C-230 at $1,800 AUD, and a Vietnamese dealer page shows a C-230/P-260 pre/power pair at 40,000,000₫. That price covers both units and can't be attributed to the preamp alone.

Buyers should watch for one trap: the eBay listings that surface under a C-230 search are frequently for the C-2300, a completely different and much newer model. Those listings run $5,980 to $6,500 and have nothing to do with the C-230's market value. No common failure modes, repair notes, or maintenance guidance for the C-230 turned up in the available sources, so there's no reliable service history to report. Given the age and the discrete-component, capacitor-heavy construction, a technician's inspection before purchase is the sensible move, but that's general advice, not a documented pattern for this model.