Williamson amplifier

A Williamson amplifier refers to a type of vacuum tube (valve) amplifier whose circuit design uses the same principles as a design published by D.T.N. Williamson. Williamson proposed the standard which became generally accepted as the target figure for high-quality audio power amplifiers, for less than 0.1% total harmonic distortion at full rated power output.

In April and May 1947, the British magazine Wireless World published a pair of articles by D.T.N. Williamson, under the title "Design for a High-quality Amplifier." The design, which became known as the "Williamson Amplifier", was an early example of high fidelity in an audio amplifier (the commercial LEAK "Point One" Type 15 amplifier of 1945 pre-dates it). The design became widely known for the notably high quality of its audio reproduction, and many Williamson amplifiers were built, both for own use and for sale; follow-up articles were published, with a slightly revised design

The simplicity of the design shows in the circuit diagram. The overall negative feedback is clearly seen to be applied by the resistor R25 from the secondary of the output transformer to the cathode of the input V1. While the output transformer is represented simply by the appropriate circuit symbol, it is a component which must be designed with great care.

Printed circuit boards (PCBs) were rarely used at the time (they were less necessary than they became for complex semiconductor circuits, due to the relative simplicity of circuits and the use of large socketed components); point-to-point wiring was used.

The Williamson amplifier was of symmetric push-pull design and used negative feedback and a specially designed output transformer to produce lower levels of distortion than previous designs.

The design used triodes as phase inverters and drivers. The original output stages used triode-connected KT66 tetrodes, although a 6L6, with slightly lower output, could be used.

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