companies adapted the circuit to locally available components, and soon began importing "premium" British valves and transformers, thus launching the market for British hi-fi in the United States. By the end of 1949 the Williamson amplifier became a universally recognized reference design, and a starting point for all valve designs employing global feedback. The spread of DIY construction and the abundance of publications addressed to the amateurs had a solid economic reason: factory-made electronics of the 1940s were too expensive. The industry has not yet reorganized for mass production of affordable consumer products. Home construction of valve electronics was relatively simple and promised considerable savings. The number of home-made Williamson amplifiers is estimated at least in hundreds of thousands; they absolutely dominated the DIY scene in English-speaking countries. Stereo has not been commercialized yet; almost all surviving Williamson amplifiers are monaural. Each one differs in minor details, assembly quality is usually inferior to factory-made models. In the 21st century these monaural amplifiers are commonly sold at online auctions, but finding a matching pair is almost impossible. Small-scale factory production in the United Kingdom began in February 1948; first big manufacturer, Rogers, announced production in October 1948. In the early 1950s the Williamson amplifier dominated factory production in both the United Kingdom and the United States; John Frieborn of Radio-Electronics wrote in 1953 that "since Williamson published the first description of his High-Quality Audio Amplifier, other audio designers had two apparent choices, beating him [Williamson] or joining him." Design features Specifications Tube complement, 1947 version: 4x L63 (each equivalent to 6J5), 2x KT66, 1x U52 directly-heated rectifier. The 1949 version also provided for the use of 6SN7 or B65 double triodes, and replaced rectifier with the 53KU indirectly-heated type; Output power and maximum distortion: 15W RMS at no more than 0.1% THD; Intermodulation: not specified (Williamson did not have the necessary test equipment); Frequency range: 10-20000Hz at ±0.2dB; 3-60000Hz at ±3dB; Phase shift within 10-20000Hz: "never exceeds a few degrees" at the extremes of audio spectrum; Noise and hum: -85 dBbelow maximum output, almost entirely consisting of mains frequency hum. Topology The Williamson amplifier is a four-stage, push-pull, class A triode valve amplifier built around a high quality, wideband output transformer. Its second (concertina-type phase splitter, V1B), third (driver, V2A and V2B) and fourth (output, V3 and V4) stages follow Cocking's Quality Amplifier circuit. The added first stage (V1A) is a