Peter Walker and Frank McIntosh. In September 1952 Williamson and Walker (then business partners in the development of the Quad Electrostatic Loudspeaker) agreed that the ultralinear stage was, indeed, preferable in mass production. Williamson gradually stepped aside from audio engineering. He made his living by designing milling machines and flexible manufacturing systems, which later earned him election to the Royal Society, and never considered audio design a serious occupation for himself. In 1956 most production amplifiers in North America followed the Ultralinear Williamson template, but in the next few years it was retired, too. The new three-stage reference design combined phase splitter and driver functions in one valve, and thus cost proportionally less than four-stage amplifiers. Hafler's Dynaco Stereo 70, which followed this topology, became the most produced valve amplifier in history. North American consumer market was flooded with millions of similar, almost identical amplifiers and receivers claiming 25 to 20W per channel, as well as clones of less powerful British designs like the Mullard 5-10. Advertisements claimed that these models performed as well as the original Williamson, with higher output power and with guaranteed stability. The customers could not verify these claims, and had to rely to listening tests, hearsay and expert advice. The problem was partially addressed by the concept of subjective listening, advanced by Hafler and Keroes back in 1951: "Excellent measurements are a necessary but not a sufficient condition for the quality of sound. The listening test is one of most importance... the most stringent test of all". By the end of the 1960s subjectivist approach was adopted by the audiophiles and marketing people, who eagerly forgot about the objective principles devised by Williamson in the 1940s. Objectively, many deep-feedback valve designs of the 1950s matched or exceeded the 0.1% distortion rating of the Williamson amplifier, but none could significantly improve on this figure. Williamson had found that valve amplifier performance was limited mostly by the output transformer. Transistor amplifiers did not have this limitation, and yet it took around 15 years to bring their performance to the level attained by Williamson in 1947. Comments Ferrari Pier Paolo - LEAK Audio Hi-Fi - Sandit Editore -Italy (2014) . Ferrari Pier Paolo - 1947: l'anno di Mr. Williamson - Sandit Editore - Italy (2013) . Notes References Sources ; also reprinted as A collection of articles from the late 1940s and early 1950s, including: Vacuum tubes Valve