Microprocessor-controlled bias adjustment in a thermionic valve audio amplifier

摘要

The grid bias for the thermionic valves in a push-pull audio amplifier output stage is adjusted by a microprocessor in dependence on the cathode currents as sensed by small resistors. The quiescent valve currents can thus be stabilised without large power-wasting self-biasing cathode resistors and their bypass capacitors. The quiescent cathode currents may be reduced when the input audio level is low. The grid, anode, or heater voltages may be adjusted to match the outputs of the valves. Total harmonic distortion (THD), valve temperature, and amplifier hum also may be measured.