An Analysis of Distortion in Class B Audio Amplifiers

The more important defects of class B audio-frequency amplifiers are classified and examined in detail. A general method of determining harmonic components introduced by curvature and asymmetry of the combined plate current curves of a pair of tubes is given, with a tabulation of results in a practical case. The main body of the paper deals with an accurate quantitative determination of the harmonic components introduced in the output by the effects of irregular grid currents on regulation of the driver amplifier, and the characteristics of practical coupling transformers. The effects of loading the driver amplifier are analyzed, and it is shown that a class A driver has minimum regulation when unloaded, and a class B driver has minimum regulation when loaded to the limit of anode dissipation. Class A and class B drivers are compared, and a general figure of merit for any driving system is proposed. It is shown that distortion due to grid currents in the output tubes can be reduced as far as desired by proper design, the only limit is economic. Methods of impedance correction and removal of reactance effects in coupling transformers by the addition of capacitors are discussed. Building out the transformer reactances into filter sections results in a wider transmitted band, more uniform transmission in the band, suppression of components contributing to adjacent channel interference, and lowered nonlinear distortion because of improved uniformity of load impedance.