HITACHI HA-8700 (1979) | Power Mos-Fet

 

The Hitachi HA-8700 MOS-FET pre-main amplifier is a classic example of late-1970s Japanese high-end vintage hi-fi engineering. Designed using technology derived from the Hitachi HMA-9500, this DC-coupled MOS-FET amplifier combines wide-bandwidth power amplification, advanced phono equalizer circuitry, and dual-mono power supply design—features that made it a standout vintage audiophile amplifier. Designed for demanding audiophile systems, the amplifier combines high-speed MOSFET power devices, a fully DC-coupled signal path, and dual-mono power supply architecture to deliver accurate, transparent sound reproduction across the entire audible spectrum. At the heart of the amplifier is a newly developed Power MOSFET power amplifier stage. Compared with conventional bipolar transistor amplifiers, MOSFET devices provide significantly faster switching performance and wider frequency response. Key performance characteristics include: Ultra-fast switching speed: 0.05 µs | Wide frequency bandwidth: 1 MHz (-2 dB) | High power gain: 85 dB at 100 kHz. This extremely wide bandwidth enables stable negative feedback at very high frequencies, reducing distortion and improving overall amplifier linearity. The fast switching capability also ensures stable push-pull operation even in Class-B output configurations, resulting in excellent transient response and accurate musical reproduction. To fully exploit the performance advantages of MOSFET output devices, the amplifier employs a three-stage power amplifier topology with minimal phase shift. This advanced circuit design ensures stable operation across the entire frequency range while preserving signal integrity and transient accuracy. Low-frequency performance is further enhanced through: DC amplifier power supply architecture | Independent left and right power supplies | Large-capacity high-grade capacitors. This design provides excellent dynamic headroom, bass control, and channel separation, essential for high-fidelity audio reproduction. The main amplifier gain has been increased from 29 dB to 45 dB, allowing the amplifier to deliver higher output capability and improved dynamic performance. A dedicated Main Direct switch allows the power amplifier stage to be connected directly to the equalizer amplifier output, bypassing intermediate circuitry. This shorter signal path reduces signal loss and improves clarity, transparency, and detail retrieval. When the Main Direct function is disabled, the amplifier operates with reduced gain to ensure optimal compatibility with external sources. The entire signal chain—comprising the head amplifier, phono equalizer amplifier, preamplifier, and main power amplifier—uses a complete DC amplifier configuration. By eliminating coupling capacitors both at the input stages and within the negative feedback (NFB) loops, the design achieves: Improved phase accuracy | Faster transient response | Extended low-frequency performance | Greater midrange transparency and high-frequency clarity. This fully DC-coupled amplifier topology is a hallmark of advanced vintage audiophile designs and contributes significantly to the amplifier’s natural and precise sound reproduction.

The amplifier features a high-stability dual-mono power supply, using separate left and right power transformers and large-capacity filter capacitors. This architecture significantly improves channel separation, dynamic stability, and power delivery. To further prevent interference between circuits, dedicated regulated power supplies are used for the tone amplifier and head amplifier stages, isolating small-signal amplification from the main power section. Both Speaker Output A and B terminals use direct output connections, allowing the signal to reach the speaker terminals without passing through the speaker selector switch. This design reduces contact resistance and preserves signal purity and damping performance. The power supply and main amplifier sections are built on a 70-micron copper PCB, twice the thickness of standard circuit boards. This heavy-duty board design reduces impedance between the power supply rails and the output stage, improving current delivery, amplifier stability, and overall audio performance. Specifications: 90W + 90W (8ohm, both channel driven) | THD: 0,002% (8ohm 1kHz) | Frequency characteristic DC-100kHz | Damping factor 80 (1kHz 8ohm) | Dimensions (W/H/D) 435x165x420 mm | Weight 20kg

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