Referencing the Accuphase A75 circuit, a compact pure Class A MOSFET power amplifier A75S.

Why build this circuit?

The A-75 is the successor to the classic A-70. It incorporates Accuphase's new technologies and improvements to certain circuit sections, along with upgraded components. This high-end stereo power amplifier uses MOSFETs and operates in pure Class A, delivering a rated output of 480W×2 @ 1Ω, 240W×2 @ 2Ω, 120W×2 @ 4Ω, and 60W×2 @ 8Ω. Each channel uses ten balanced "power MOSFET" amplifier modules. Fully balanced amplification offers better symmetry, lower distortion, higher stability, and stronger drive capability compared to conventional amplification. The A75 continues many excellent designs while introducing major upgrades in several key technologies, including an enhanced MCS+ circuit (an upgraded version of MCS), a front-end buffer amplifier circuit, an improved multi-stage parallel circuit design, and a current feedback design to ensure linear response at all levels. Parallel connection of amplifier unit circuits reduces noise, and a DC stabilizer power supply is integrated into the driver stage and MCS+ circuit to improve the S/N ratio and sound purity. To ensure sufficient and stable power supply, the power section uses an 800VA high-efficiency large transformer combined with two 67,000µF "large water tank" filter capacitors, providing ample and stable power. Low dielectric constant, low loss Teflon substrates and gold-plating on key parts such as input terminals enhance sound purity. Extra-large speaker output terminals are convenient for connecting audiophile-grade speaker cables. The front panel uses a full digital display showing the amplifier's operating mode, with a DSP digital circuit calculating and displaying output power and status.

The A75, a masterpiece from Accuphase's peak period, received wide recognition and praise after its release and is one of Accuphase's most representative works. The sound, tuned with pure Class A and MOSFETs—among many other high-quality elements—breaks the bias some audiophiles held against Japanese amplifiers. Its tone is rich, mid-to-high frequency details are brilliant and precise, imaging is accurate, low frequencies are solid and powerful, and the overall transition is smooth and natural. The strong driving power is enough to handle any speaker claimed to be difficult to drive, and even ordinary speakers can sound like floor-standing models. Through this amplifier, I also understood why many high-end enthusiasts insist on high-power amplifiers.

After completing a 1:1 direct copy of the machine, we realized that the cost of the direct copy version might not satisfy most users. After careful analysis, we found that typical HiFi speakers rarely go below 4Ω, and using ten output transistor pairs per channel is somewhat wasteful. After multiple tests, we feel that using four output pairs per channel already provides very good driving power and low-frequency control. When driving 4Ω and 8Ω loads, there is almost no audible difference compared to the 1:1 direct copy. By preserving the circuit structure while only reducing the number of output transistors, we can significantly lower costs. The power transformer uses a single 800W toroidal transformer, and the circuit operating voltage remains the same as the original. The output stage uses four pairs (8 MOSFETs) per channel of the same K1530/J201 MOSFETs as the original. The filter capacitor bank uses two 67,000µF units. The bottom includes a soft-start circuit and an input signal board. The overall structure and function are consistent with the direct copy version.

Output power: 60W×2 pure Class A @ 8Ω
      120W×2 pure Class A @ 4Ω
Frequency response: 10Hz – 40kHz
Distortion: 0.001%
Load range: 4 – 8Ω
Net weight: 28kg (packaging approx. 31kg; total weight of sets may vary, extra shipping fee applies)
Dimensions: 430mm (W) × 170mm (H) × 409mm (D) (excluding front/rear handles)
Included accessories: One power cord (default Chinese standard)