YAMAHA A-8 (1981)

The A-8 integrated amplifier from Yamaha features a linear transfer circuit supported by X-power supply and a preamplifier section using a pure current servo amplifier. The basic circuit configuration is simple : DC equalizer amplifier with new pure current servo amplifier system → high-gain DC main amplifier with linear transfer circuit. The main amplifier circuit is a two stage differential amplifier with a dual FET differential input as the first stage. All the circuits are cascade connected, so that low distortion rate characteristics are obtained up to large amplitude and high frequency. In the power stage, the linear transfer circuit developed for the separate power amplifier B-5 is adopted. The bias voltage is fixed, and each operating point of the power transistors connected in parallel is shifted so that the composite characteristic approaches the square characteristic and the linear total transfer characteristic is achieved. Smooth current waveform and low distortion factor characteristics with little crossover distortion are obtained from minute output to large output. An X-power supply is used for the power supply section, which uses an AC control element, TRIAC, and a reference voltage comparator. It controls the conduction phase angle of the voltage waveform applied to the primary side of the power transformer, and supplies power at high speed with the precision required according to the dynamic range of music. As a result, the output stage of the main amplifier section can be set to a constant voltage, which was impossible with conventional power supplies. At the same time, the low temperature of the amplifier with good regulation can be obtained. For the frame structure of the amplifier, an assembly method similar to the power transistor mounting structure is adopted, and insulation points and conduction points are clarified to realize a loop-less structure. The chassis surface is plated with copper plated with copper and the surface layer is a conductive layer that does not show magnetic properties, thereby reducing the sound quality degradation caused by iron. In addition, high-purity copper foil with a thickness of 35 μ m is used for the electric shield material that does not require strength, and the bottom cover is made of plastic to prevent eddy current loss. The Chemi-Con in the  power supply section, which has a significant effect on sound quality, uses a capacitor in a plastic case, an improved version of the one used in the A-9. The Chemi-Con in other signal paths also uses a plastic case. In addition, tantalum nitride  resistors are used for NFB circuits and other circuits with high element sensitivity. These resistors have excellent voltage distortion factor, resistance value stability, and low power supply noise. There are also used quality parts that have been developed and carefully selected, such as polypropylene capacitors, Mylar capacitors, and mica capacitors.

Technical data:

Power rating (20 Hz to 20 kHz, 0.003%) 150W + 150W (8 Ω)

Power Bandwidth (75W, 0.002%) 10 Hz ~ 100 kHz (8 Ω)

Damping factor 200 (8 Ω, 1 kHz)

Rated power consumption 240W

External dimension Width 435x Height 144x Depth 422 mm

Weight 12.5kg