Accuphase P-266 Power Amplifier (1983)

At 20kg and with a triple push-pull MOS FET output stage, the P-266 is a heavyweight performer built for precision and power—switchable into pure Class A with the flick of a switch.

Overview

The Accuphase P-266 is a stereo power amplifier introduced in March 1983 by Accuphase Laboratory, Inc., positioned as an improved flagship model in the company’s lineup. Priced at ¥220,000 at launch, it was engineered to deliver both brute force and surgical accuracy, leveraging Power MOS FET technology in a triple push-pull configuration. Owners report a machine built to last, with dual-mono power rectification, a robust 20kg chassis, and a design philosophy that prioritizes stability and low distortion. While it shares circuit elements with earlier models like the P-300X and M-100—particularly in the cascode-connected pre-drive stage—the P-266 stands apart with its unique bias switching system and operational flexibility.

What sets the P-266 apart is its ability to switch between Class A and Class AB operation. This isn’t a half-measure; in full Class A mode, it delivers up to 55W per channel into 4Ω, with distortion dropping to just 0.01% (4Ω load) across the audible band. The switch engages via an optocoupler-based bias circuit, a design choice that isolates control logic from high-current paths and reportedly improves long-term reliability. For those demanding even more power, the amplifier supports bridged monaural operation, delivering 400W into 8Ω in normal mode and 110W in Class A—figures that were formidable in 1983 and still command respect today.

Despite its sophistication, the P-266 isn’t without known flaws. A design oversight in the original layout connects the sources of complementary output MOS FET pairs directly via copper traces, effectively bypassing the individual 0.47Ω source resistors (R34–R39). This undermines current balancing and is widely suspected to contribute to premature output transistor failure. Repair logs suggest cutting these traces as a standard refurbishment step—a mod so common it’s practically expected in working units today.

Specifications

Triple Push-Pull MOS FET Output Stage

The P-266’s output stage relies on a triple push-pull configuration of Power MOS FETs, a design that was cutting-edge in 1983. This arrangement spreads thermal and electrical load across multiple devices, contributing to the amplifier’s ability to deliver 200W into 4Ω with less than 0.01% THD. The MOS FETs are driven by a cascode-connected pre-drive stage, a topology inherited from the P-300X and M-100, known for its wide bandwidth and stability under dynamic loads. However, the original PCB layout contains a critical flaw: the sources of the N-channel and P-channel output pairs are directly connected by copper traces, shorting out the 0.47Ω source resistors intended to balance current flow. This oversight can lead to thermal runaway and catastrophic failure of the output transistors. Repairers routinely cut these traces to restore proper function—a mod so prevalent it’s considered standard practice for any unit undergoing restoration.

Switchable Class A / Class AB Operation

One of the P-266’s most distinctive features is its ability to switch between full Class A and Class AB operation. In Class A mode, the amplifier draws 150W at idle and delivers up to 55W per channel into 4Ω, with distortion dropping to 0.01% (4Ω) and below. The transition is managed by a bias switching circuit incorporating an optocoupler, which isolates the control signal from the high-current output stage and ensures clean, chatter-free operation. According to available documentation, Q22 remains on in Class B mode but switches off in Class A, altering the bias path accordingly. While one listing claims 40W per channel in Class A, the manufacturer-sourced specification from audio-database.com confirms 30W into 8Ω, making the higher figure questionable. Regardless, the ability to run in pure Class A—even at moderate power—was a rare luxury in 1983 and remains a hallmark of Accuphase’s engineering ambition.

Direct-Coupled FET Input with DC Servo

The P-266 eliminates the input coupling capacitor, opting instead for a direct-coupled FET input stage. This design avoids phase shifts and transient distortions associated with capacitors while improving low-frequency response. To manage DC offset, the amplifier employs a DC servo amplifier that applies feedback to stabilize drift. This combination ensures clean signal transfer from source to output while maintaining long-term stability. The input stage is further protected by a capacitance multiplier circuit—centered on transistor Q18 (originally a 2SD667 BJT with a 560Ω base resistor)—which filters power supply noise before it reaches sensitive front-end components. A potential replacement for Q18 is the BC639, a higher-gain transistor sometimes used in repairs.

Bridged Monaural Operation

The P-266 includes a bridge drive switch, allowing it to operate in monaural mode with doubled voltage swing. In this configuration, it delivers 400W into 8Ω under normal operation and 110W in Class A. The gain increases to 33.7 dB, and input sensitivity drops to 1.1V for full output. Damping factor is halved to 60, reflecting the change in output impedance, but remains high by contemporary standards. The bridge mode is useful for driving difficult subwoofer loads or high-efficiency horn systems that benefit from massive dynamic headroom. The separate rectifying circuits for left and right channels—each installed on its own PCB—help maintain channel separation and reduce crosstalk in both stereo and bridged modes.

Output Relay and Speaker Protection

The amplifier uses an output relay to engage the speaker terminals only after power-up stabilization, protecting speakers from turn-on thumps and DC faults. This relay also serves as the switching mechanism for the speaker selection feature. The protection circuitry is tied to the DC servo and bias monitoring systems, with fault conditions likely triggering relay disconnection. However, the reliability of the STV-4H bias diode (D11) has been called into question; similar VD-series diodes were known to develop intermittent connections, which could destabilize bias and destroy output transistors. Owners are advised to inspect or preemptively replace this component during servicing.

Collectibility & Value

The Accuphase P-266 is regarded as a high-end vintage amplifier with a loyal following, though units often require restoration due to known design and component vulnerabilities. The most common failure mode involves burnt-out output transistors, likely exacerbated by the flawed source resistor layout. The STV-4H bias diode (D11) is another point of concern, with repairers advised to monitor or replace it proactively. Troubleshooting guides recommend checking capacitors C36/C37 for leakage or shorting, as well as measuring leakage current in C11 and C15. A restoration log hosted at amp8.com provides detailed guidance for refurbishing non-functional units. While no formal market valuation data is available from the fact sheet, a used P-266 was listed for CA$1,450 on Canuck Audio Mart in 2025, and a P-266 paired with a C-222 preamp sold for €3,595 on audio-markt.de, suggesting strong demand among collectors.

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