A multi-loop, self-balancing power amplifier

A multi-loop, push-pull power amplifier of exceptional characteristics is described. It employs special circuits to maintain accurate push-pull signal balance throughout and to hold the static or steady-state dc cathode currents of the output tubes equal. A pair of 807 tubes are used in class AB2to yield 65w average power output at less than 1 per cent intermodulation distortion with 30 db of over-all negative feedback. Using local positive voltage feedback in addition, the intermodulation distortion is 0.1 per cent at 45w and less than 0.2 per cent at 60w. At full power output, the -0.5 db points occur at 19.8 cps and 22.4 kcps. The rise time of the amplifier is 3 µs, and its transient response and recovery from overload approach the ideal. There are no peaks at the ends of the response curve. A noise level of -106 db referred to 60w output is attained. Extensive measurements of amplifier characteristics under various conditions are described.