Accuphase P-266 Power Amplifier (1983)
The P-266 weighs 20kg and has a triple push-pull MOS FET output stage. It can be switched into pure Class A with one switch.
Overview
The Accuphase P-266 is a stereo power amplifier introduced in March 1983 by Accuphase Laboratory, Inc., as an improved flagship model in the company's lineup. It cost ¥220,000 at launch. It uses Power MOS FET technology in a triple push-pull configuration. Owners report a machine built to last, with dual-mono power rectification, a 20kg chassis, and a design that favors stability and low distortion. It shares circuit elements with earlier models like the P-300X and M-100, particularly the cascode-connected pre-drive stage. The bias switching system and the operational flexibility are its own.
The P-266 can switch between Class A and Class AB operation. In full Class A mode it delivers up to 55W per channel into 4Ω, with distortion down to 0.01% (4Ω load) across the audible band. The switch works through an optocoupler-based bias circuit. That isolates the control logic from the high-current paths and reportedly improves long-term reliability. For more power, the amplifier supports bridged monaural operation, delivering 400W into 8Ω in normal mode and 110W in Class A. Those were big numbers in 1983.
The P-266 has a known flaw. In the original layout, the sources of the complementary output MOS FET pairs are tied together directly by copper traces. That bypasses the individual 0.47Ω source resistors (R34 - R39). Current balancing suffers, and the fault is widely suspected of contributing to early output transistor failure. Repair logs suggest cutting these traces as a standard refurbishment step. It is so common that working units are practically expected to have it done.
Specifications
| Model | P-266 |
| Manufacturer | Accuphase (Accuphase Laboratory, Inc.) |
| Type | Stereo power amplifier |
| Production year | Released in March 1983 |
| Original price | ¥ 220,000 |
| Continuous average power (20 Hz to 20 kHz, Distortion Factor 0.01%) | Stereo specification Normal: 200W/ch (4 Ω), 130W/ch (8 Ω), 65W/ch (16 Ω) |
| Continuous average power (20 Hz to 20 kHz, Distortion Factor 0.01%) | Stereo specification Class-A: 55W/ch (4 Ω), 30W/ch (8 Ω), 18W/ch (16 Ω) |
| Continuous average power (20 Hz to 20 kHz, Distortion Factor 0.01%) | Monaural specification (bridge connection) Normal: 400W (8 Ω), 180W (16 Ω) |
| Continuous average power (20 Hz to 20 kHz, Distortion Factor 0.01%) | Monaural specification (bridge connection) Class-A: 110W (8 Ω), 55W (16 Ω) |
| Total harmonic distortion rate (20 Hz to 20 kHz 0.25W ~ continuous average output) | Stereo: 0.01% (4 Ω Load), 0.005% (8 Ω ~ 16 Ω Load) |
| Total harmonic distortion rate (20 Hz to 20 kHz 0.25W ~ continuous average output) | Monaural specification (bridge connection): 0.01% (8 Ω Load), 0.005% (16 Ω Load) |
| IM Distortion Factor (IHF) | 0.003% |
| Frequency characteristic | 20 Hz ~ 20 kHz + 0 -0.2 dB (Continuous Average Output, Level Control MAX) |
| Frequency characteristic | 0.5 Hz ~ 300 kHz + 0 -3dB (at 1W Output, Level Control MAX) |
| Frequency characteristic | 0.5 Hz ~ 150 kHz + 0 -3dB (Level Control -6dB at 1W Output) |
| Gain | Stereo: 27.8 dB; Monaural specification: 33.7 dB |
| Load impedance | Stereo specification: 2 Ω ~ 16 Ω; Monaural specification: 4 Ω ~ 16 Ω |
| Damping factor | Stereo specification: 120; Monaural specification: 60 |
| Input Sensitivity / Impedance (at 8 Ω load) | Stereo: 1.3V/20k Ω (at continuous average output), 0.12V/20k Ω (IHF at 1W output) |
| Input Sensitivity / Impedance (at 8 Ω load) | Monaural specification: 1.1V/20k Ω (at continuous average output), 0.06V/20k Ω (IHF at 1W output) |
| S/N (A correction) | Stereo: 120 dB (Input Short, Continuous Average Output), 100 dB (Input 1k Ω, 1W output, IHF) |
| S/N (A correction) | Monaural specification: 110 dB (Input Short, Continuous Average Output), 90 dB (Input 1k Ω, 1W output, IHF) |
| Stereo headphone | Low output impedance type |
| Subsonic filter | 17 Hz, -12dB/oct. |
| Output meter | Logarithmic compression peak indication type, -40dB to + 3 dB and output direct reading scale; With Peak Hold Switching |
| Semiconductor used | Transistor: 34; FET: 18; IC: 7; Diode: 68 Pieces; Opto-coupler: 2 pcs |
| Power supply voltage | AC100V/117V/220V/240V, 50Hz/60Hz |
| Power consumption | Normal: 120W (no input), 500W (at 8 Ω load rated output); Class-A: 150W (no input), 190W (at 8 Ω load rated output) |
| External dimensions | Width 445x Height 160 (Including Legs) x Depth 373 mm |
| Weight | 20kg |
| Sold Separately Wood Cabinet | A-8 (¥ 16,000); Wood Cabinet dimensions: Width 466x Height 190x Depth 385 mm |
Triple Push-Pull MOS FET Output Stage
The P-266's output stage uses a triple push-pull configuration of Power MOS FETs, which was cutting-edge in 1983. This spreads thermal and electrical load across multiple devices, and the amplifier delivers 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 and known for wide bandwidth and stability under dynamic loads. The original PCB layout has a critical flaw, though. The sources of the N-channel and P-channel output pairs are connected directly by copper traces, which shorts out the 0.47Ω source resistors meant to balance current flow. This can lead to thermal runaway and failure of the output transistors. Repairers routinely cut these traces, and it is considered standard practice for any unit being restored.
Switchable Class A / Class AB Operation
One of the P-266's most distinctive features is the 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 at 0.01% (4Ω) and below. A bias switching circuit with an optocoupler handles the transition. It isolates the control signal from the high-current output stage and keeps the switching clean and chatter-free. According to available documentation, Q22 stays on in Class B mode but switches off in Class A, which changes the bias path. One listing claims 40W per channel in Class A, but the manufacturer-sourced specification from audio-database.com confirms 30W into 8Ω, so the higher figure is questionable. Pure Class A, even at moderate power, was a rare feature in 1983.
Direct-Coupled FET Input with DC Servo
The P-266 has no input coupling capacitor. It uses a direct-coupled FET input stage instead. This avoids the phase shifts and transient distortions associated with capacitors and improves low-frequency response. To handle DC offset, a DC servo amplifier applies feedback to stabilize drift. A capacitance multiplier circuit further protects the input stage. It is centered on transistor Q18 (originally a 2SD667 BJT with a 560Ω base resistor) and filters power supply noise before it reaches the 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 has a bridge drive switch that puts it 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 that is still high by contemporary standards. Bridge mode suits difficult subwoofer loads or high-efficiency horn systems that can use the dynamic headroom. The left and right channels have separate rectifying circuits, each on its own PCB. These help maintain channel separation and reduce crosstalk in both stereo and bridged modes.
Output Relay and Speaker Protection
An output relay connects the speaker terminals only after power-up has stabilized, which protects the speakers from turn-on thumps and DC faults. The same relay does the switching for the speaker selection feature. The protection circuitry is tied to the DC servo and bias monitoring systems, and fault conditions likely trigger the relay to disconnect. The STV-4H bias diode (D11) has been called into question, however. 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 part during servicing.
Collectibility & Value
The Accuphase P-266 is regarded as a high-end vintage amplifier with a loyal following, though units often need restoration because of known design and component weaknesses. The most common failure is burnt-out output transistors, likely made worse by the flawed source resistor layout. The STV-4H bias diode (D11) is another concern, and repairers advise monitoring or replacing it proactively. Troubleshooting guides recommend checking capacitors C36/C37 for leakage or shorting, and measuring leakage current in C11 and C15. A restoration log hosted at amp8.com gives detailed guidance for refurbishing non-functional units. 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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Service Manuals, Schematics & Catalogs
- Catalog — archive.org
- Catalog (1992) — archive.org