This class A amplifier circuit requires a preamp as it hasn’t got much gain. It requires big heat sinks and a large transformer and a great power supply and careful wiring, but in the end it is extremely simple and it sounds very good. The zener diode rejects any ripple coming from the power supply, But you still only want a ripple of 10mV max. The ripple reaching the input is amplified, so the zener gets rid of that, but whatever ripple there is will still reach the power stage.
Many early amplifiers operated in Class A, but as output powers rose above 10W the problems of heat dissipation and power supply design caused most manufacturers to turn to the simpler, more efficient Class B arrangements and to put up with the resulting drop in perceived output quality. Why Class A ? Because , when biased to class A, the transistors are always turned on, always ready to respond instantaneously to an input signal. Class B and Class AB output stages require a microsecond or more to turn on. The Class A operation permits cleaner operation under the high-current slewing conditions that occur when transient audio signal are fed difficult loads. His amplifier is basically simple, as can be seen from the block diagram.
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