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. Design targets Following Cocking's ideas, Williamson devised a different, much stricter set of fidelity requirements: Negligible non-linear distortion (sum of harmonic distortion and intermodulation products) up to the maximum rated output, at all audible frequencies from 10 to 20000 Hz; Linear frequency response and constant output power at all audible frequencies; Negligible phase shift within the audible frequency range; Good transient response which, in addition to above frequency and phase requirements, demands perfectly constant gain when handling complex waveforms and transients; Low output impedance and, inversely, high damping factor. At the very least, output impedance of an amplifier must be lower than the loudspeaker impedance; Output power of 15–20W for reproduction of orchestral music via a dynamic loudspeaker, or for a horn loudspeaker. Williamson reviewed contemporary amplifier configurations, and, just like Cocking, settled on a low distortion push-pull, class A, triode output stage. Unlike Cocking, Williamson believed that such a stage can deliver high fidelity sound only when the amplifier is governed by 20–30dB deep negative feedback loop (and thus the complete amplifier must have 20–30dB higher open loop gain to compensate the effect of feedback). Deep feedback inevitably causes sudden, harsh onset of distortion at overload but Williamson was content with this flaw. He argued that it is a price worth paying for an improvement in linearity at medium and high power levels. On the contrary, wrote Williamson, slow but steady rise of distortion to 3–5%, as advocated by Kellogg, is distinctly unwanted in a high fidelity system. Prototypes and tests Valve complement of the original Williamson amplifier was determined by scarce supply in wartime Britain. The two suitable and available output valves were either the PX25 triode, or a triode-connected KT66 beam tetrode. Williamson initially used the PX25, an already obsolete directly-heated triode introduced in 1932. In his second prototype, Williamson used the more efficient KT66, which became the valve of choice in post-war period. Powered from +500 V power supply, the KT66 prototype delivered 20 Watts at no more than 0.1% distortion. A less