Accuphase A-75 Class-A Stereo Power Amplifier

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Yokohama, Japan | 2018 – The A-75 is a new addition to the highly acclaimed Accuphase lineup of quality Stereo Power Amplifiers.

Instrumentation amplifier topology using discrete semiconductors realizes ideal gain allocation. Balanced Remote Sensing optimizes efficiency of negative feedback. 10-parallel push-pull configuration in power amplification stage. The overall result is outstanding S/N ratio and amazingly high damping factor. Furthermore, constant-voltage drive handles even drastic speaker impedance fluctuations with ease. Power MOS-FETs and other carefully selected components and materials make this Pure Class A Stereo Power Amplifier a sheer delight to experience.

High S/N ratio results in sparkling clarity

The A-75 uses a balanced instrumentation amplifier circuit configu- ration throughout all amplification stages, minimizing susceptibility to noise and ensuring outstanding characteristics. The signal input stage is configured with discrete semiconductors as a push-pull circuit. This makes it possible to allocate a high gain of 22 dB (factor of approx. 12.6) to this section and conversely keep the gain of the power amplification section to a low 6 dB (approx. x2.0). As a result, even large signal amplitudes are conveyed accurately from the signal input stage to the power amplification stage, and noise components processed in the amplification stage are kept low. Compared to the previous model, this results in a noise reduction of 11%. S/N ratio is rated at an astonishing 122 dB (at max gain setting), which manifests itself in superb sonic clarity.

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High damping factor realizes ideal speaker drive

To correctly track the dynamic changes in loudspeaker impedance and drive the speakers with ideal characteristics, the power amplifier output impedance needs to be kept as low as possible, which in turn means an increase in the damping factor. To achieve this aim, the A-75 not only positions the negative feedback sensing point close to the speaker terminals, it also obtains sensing information from the ground line at the same time. This method is called Balanced Remote Sensing. The aim is to minimize output impedance and therefore achieve a significant increase of the damping factor. Furthermore, ten power MOS-FETs with excellent current capacity are used in a parallel configuration, and MOS-FET switches that eliminate all mechanical contacts guarantee outstanding long-term reliability. The edgewise coil with large cross-section area, Hall element for current detection, and many other sophisticated features further contribute to reduced output impedance. The end result is a damping factor rating of 1,000 which represents a 25% improvement over the previous model.

Output power characteristics suitable for constant voltage drive

A massive toroidal power transformer and large filtering capacitors (100,000 μF x 2) in the power supply ensure that plenty of power is available for handling high amplitude input signals, and large heat sinks efficiently dissipate any excess thermal energy. As a result, the power amplification stage is rated for a continuous 60 watts into 8 ohms, 120 watts into 4 ohms, 240 watts into 2 ohms and 480 watts into 1 ohm (music signals). This linear power progression demonstrates that constant voltage drive has been successfully realized. The maximum output rating is 131 watts into 8 ohms, 232 watts into 4 ohms, 370 watts into 2 ohms and 547 watts into 1 ohm (music signals). Although a Pure Class A Stereo Power Amplifier, the A-75 can easily drive even large and demanding speakers.

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Advanced Features

■ Current feedback principle in signal input stage and power amplification stage assures excellent phase characteristics in high range.
■ MCS+ topology improves S/N ratio and THD characteristics in voltage amplification stage.
■ Printed circuit board of power amplification stage made from glass cloth fluorocarbon resin.
■ Digital power meter showing output power levels and bar graph meter showing output voltage with indication range to –50 dB, both with on/off switching capability.
■ Speaker output short-circuit monitoring func- tion ensures complete operation safety.
■ Two sets of large speaker terminals accept spade lugs and also enable easy bi-wiring connection.
■ Operation mode switch supports bi-amping and bridged connection.
■ 4-stage gain selector. ■ Phase selector for balanced inputs.

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