The Williamson amplifier is a four-stage, push-pull, Class A triode-output valve audio power amplifier designed by David Theodore Nelson Williamson during World War II. The original circuit, published in 1947 and addressed to the worldwide do it yourself community, set the standard of high fidelity sound reproduction and served as a benchmark or reference amplifier design throughout the 1950s. The original circuit was copied by hundreds of thousands amateurs worldwide. It was an absolute favourite on the DIY scene of the 1950s, and in the beginning of the decade also dominated British and North American markets for factory-assembled amplifiers.

The Williamson circuit was based on the 1934 Wireless World Quality Amplifier by Walter Cocking, with an additional error amplifier stage and a global negative feedback loop. Deep feedback, triode-connected KT66 power tetrodes, conservative choice of standing currents, and the use of wide-bandwidth output transformer all contributed to the performance of the Williamson. It had a modest output power rating of 15 Watts but surpassed all contemporary designs in having very low harmonic distortion and intermodulation, flat frequency response throughout the audible frequency range, and effective damping of loudspeaker resonances. The 0.1% distortion figure of the Williamson amplifier became the criterion for high fidelity performance that remains valid in the 21st century.

The Williamson amplifier was sensitive to selection and matching of passive components and valves, and prone to unwanted oscillations at infrasonic and ultrasonic frequencies. Enclosing four valve stages and an output transformer in a negative feedback loop was a severe test of design, resulting in a very narrow phase margin or, quite often, no margin at all. Attempts to improve stability of the Williamson could not fix this fundamental flaw. For this reason, and due to high costs of required quality components, manufacturers soon abandoned the Williamson circuit in favour of inherently more stable, cheaper and efficient three-stage, ultralinear or pentode-output designs.

In 1925 Edward Kellogg published the first comprehensive theory of audio power amplifier design. Kellogg proposed that the permissible level of harmonic distortion can reach 5%, provided that distortion rises smoothly rather than abruptly, and that it generates only low-order harmonics. Kellogg's work became the de facto industry standard of the interwar period, when most amplifiers were employed in cinemas. Early sound film and public address requirements were low, and customers were content with crude but efficient and affordable transformer-coupled, class B amplifiers. The best theatre amplifiers, built by Western Electric around their 300A and 300B power triodes, far exceeded the average level but were expensive and rare.

By the middle of the 1930s Western Electric and RCA improved performance of their experimental audio equipment to a level approaching modern understanding of high fidelity, but none of these systems could be commercialized yet. They lacked sound sources of matching quality. Industry leaders of the 1930s agreed that the improvement of commercial amplifiers and loudspeakers would make sense only after the introduction of new physical media surpassing low-quality AM broadcasting and shellac records. The Great Depression, World War II and the post-war television boomconsecutively delayed this goal. Development of commercial audio equipment came to a standstill; the few enthusiasts seeking higher level of fidelity had to literally do it themselves. American DIYers experimented with novel beam tetrodes. Australians preferred traditional push-pull circuits built around directly-heated triodes and complex, expensive interstage transformers.

British school of thought led by Walter Cocking of Wireless World leaned to push-pull, class A, RC-coupled triode output stages. RC coupling, as opposed to transformer coupling, argued Cocking, extended the amplifier's bandwidth beyond the required minimum of 10 kHz and improved its transient response. Tetrodes and pentodes were undesirable due to higher harmonic distortion and higher output impedance that failed to control fundamental resonance of the loudspeaker. Cocking wrote that Kellogg's 5% distortion limit was too high for quality amplification, and outlined a different set of requirements - the first definition of high fidelity. Instead of Kellogg's single figure of merit (harmonic distortion), Cocking set three simultaneous targets - low frequency distortion, low harmonic distortion, and low phase distortion. In 1934 Cocking published his first Quality Amplifier design - a two-stage, RC-coupled triode class A amplifier that achieved no more than 2–3% maximum distortion without using feedback. Feedback appeared in his 1943 Wartime Quality Amplifier, built around American 6V6 beam tetrodes; however, both the input stage and the output transformer were placed outside the feedback loop. Cocking's Quality Amplifier family became the foundation of post-war British and Australian audio industry, including the Williamson amplifier.

In 1943, in the middle of World War II, twenty-year-old Scotsman Theo Williamson failed mathematics exam and was discharged from the University of Edinburgh. Theo was not physically fit for military service, so instead the authorities drafted him for mandatory civilian work at Marconi-Osram Valve. In April 1944 Williamson transferred from production line to Applications Laboratory of the company, where he had enough free time for his own DIY projects. Management did not object, and by the end of 1944 Williamson had conceived, built and tested the amplifier that would soon be known as the Williamson amplifier. Another wartime projects, a novel magnetic cartridge, would be commercialized in 1948 as the Ferranti ribbon pickup.

Following Cocking's ideas, Williamson devised a different, much stricter set of fidelity requirements: