Williamson amplifier

A Williamson amplifier refers to a type of vacuum tube (valve) amplifier whose circuit design is similar to that originally published by D.T.N. Williamson.

Explanation

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."^[1] The design, later referred to as the "Williamson Amplifier" was, perhaps, one of the first examples of a high fidelity audio amplifier (although the LEAK "Point One" Type 15 amplifier of 1945 pre-dates it). The 1947 articles aroused a level of interest that stimulated the publication of an updated version in 1949 (see references below). The detailed descriptions inspired many builders to construct copies, both for their own use, as well as for sale, and the design became widely known for the notably high quality of its audio reproduction.

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 output stages were triode-connected KT66 tetrodes, although a 6L6, with slightly lower output, could also be used.

A notable characteristic of the design was the use of a negative feedback loop enclosing the whole amplifier, including the output transformer—in contrast to the use of a number of separate smaller feedback loops around individual stages of the circuit. Care had to be taken to avoid excessive phase-shift around the feedback loop, to avoid the feedback becoming positive at any frequency, which would cause oscillation.

Earlier designs used transformers to couple the output signal of one stage to the next. In particular, it was very easy to use a transformer with a centre-tapped secondary to produce from the single-ended signal output by the earlier voltage amplification stages a pair of signals in antiphase to drive the two output valves.

Signal transformers are a source of distortion; if the transformers are not included in the feedback loop, this distortion is not corrected. However, transformers produce large phase shifts at higher frequencies, which makes the use of feedback loops including the transformers, or overall feedback, problematical.

Williamson avoided all transformers except the output transformer by using a split-load valve voltage amplifier stage ("phase splitter") to provide antiphase signal inputs to the symmetrical push-pull output stages. This permitted significant amounts of negative feedback to be used without instability. (See Linsley Hood (1993)^[2] for discussion of characteristics of the Williamson amplifier design and its performance compared to earlier and later designs.)

The Williamson design demonstrates the maturity that tube amplification reached in the late 1940's. Other than the British Mullard 5-10 circuit and David Hafler's Dynaco ST-70, there was little improvement in the fundamentals.

Even two decades later, the Williamson amplifier's performance became a standard of comparison for innovative successor developments, including well-known semiconductor audio amplifiers (see for example Linsley Hood (1969)^[3]).

See also

• Audio amplifier
• Valve audio amplifier
• Negative feedback amplifier

References

1. ^ D.T.N. Willamson, The Williamson Amplifier - A collection of articles reprinted from Wireless world, (Reprint 1994). Original articles appeared in Wireless World for April and May 1947, and (with an updated version) for August 1949 at pp.282-287 (with additions in October 1949 at p.365, and November 1949 at p.423).
2. ^ John Linsley Hood, "The Art of Linear Electronics", (publ. Butterworth-Heinemann, Oxford, 1993) esp. ch.9, pages 144-175.
3. ^ John Linsley Hood, "Simple Class-A Amplifier", Wireless World for April 1969, pp.148-153.