Williamson Amplifier - Description

Description

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.

A notable characteristic of the design was the use of a negative feedback loop enclosing the whole amplifier, including the output transformer, rather than separate feedback around individual stages. This approach to feedback requires care to be taken to avoid excessive phase-shift around the feedback loop (the transformer being a particular problem), to avoid the feedback becoming positive at any frequency, which would cause undesirable oscillation.

Earlier designs used transformers to couple the output signal from one stage to the next. A transformer with a centre-tapped secondary was used as a simple method to drive the push-pull output valves in anti-phase from the previous single-ended stage.

Signal transformers are a source of distortion; if the transformers are not included in the feedback loop, this distortion is not corrected. They also produce large, frequency-dependent phase shifts at higher frequencies, making feedback loops including the transformers problematical.

Williamson eliminated all transformers except the output transformer (unavoidable for matching the high valve impedance to the low loudspeaker impedance) by using capacitor coupling between stages, and a split-load valve voltage amplifier stage ("phase splitter"), again capacitor-coupled, to provide antiphase signal inputs to the symmetrical push-pull output stages. There was thus a single signal transformer in the circuit; with careful design of the transformer and the feedback network significant negative feedback could be used before the onset of instability. A 1993 book by Linsley Hood discusses the characteristics of the Williamson amplifier design and its performance compared to earlier and later designs.

The Williamson design demonstrates the maturity that tube amplification had reached by the late 1940s. Other than the British Mullard 5-10 circuit and David Hafler's Dynaco ST-70, there was little improvement in the fundamentals. One improvement that was widely used was an output transformer with additional taps allowing use of Blumlein's ultra-linear technique, which allowed the output stage, before applying feedback, to operate at a power intermediate between that available from a triode and the significantly higher power from a pentode or beam tetrode, with distortion better than either.

Even two decades later, the Williamson amplifier's performance was a standard of comparison for innovative successor developments, including well-known semiconductor audio amplifiers such as Linsley Hood's transistor "Simple Class-A Amplifier", published in 1969, with explicit discussion of Williamson's criterion.