Leak made his fame primarily with the TL/12 triple-loop feedback amplifier that was released in December 1948.
But the H. J. Leak company was established well before that - in 1934. Prior to this, Leak had worked with sound systems in the cinema industry - with Gaumont British Cinemas where he learnt electronics, and also as a radio service technician. From the information available, the H.J. Leak company initially produced audio amplifiers under sub-contract to other companies.
See also the Leak Timeline available at this web-site.
The exact products the company produced between 1934 and 1946 are not known. Although it seems likely that Leak may have continued to build amplifiers for Gaumont British Cinemas (a Gaumont amplifier installed some time after the War in the Dendy Theatre, Brighton, Australia, had a circuit basically the same as that used in the Point-One range of amplifiers produced by Leak). A review of advertisments in the technical press from 1934 onwards, shows that one of the first post-war advertisements was placed in "Wireless World" in February 1946. It is also reported that Leak placed an advertisement for a 13 Watt P.A. amplifier in Wireless World during1938 (although a search so far has failed to locate it.)
The 1946 Wireless World advertisement announces the arrival of a product that appears forgotten these days - it was the "Type 15" audio power amplifier. The amplifiers claim to fame was the then astonishingly low total harmonic distortion - at just 0.1 per cent for 15 Watts output. Other interesting details were:
The "Type 15" preceeded the well-known TL/12, but it was the amplifier that set the performance standard adopted by Leak for all its amplifiers, and also initiated a famous trade mark - the "Point One" range of amplifiers, so named of course for the low distortion figures achieved. The companies own advertising material makes mention of these performance standards being achieved in June 1945 by the designing of the original "Point One" series - presumably this is also referring to the "Type 15".
Circuit details of the Type 15 are not available, but would be of much interest, and would be included on this page if they become available. The amplifier had 2xKT66's output valves, three octal-based signal valves (possibly Osram single triode L63's), an octal-based pentode for the first stage, and one rectifier.
The Leak Type 15 amplifer preceeded the Williamson, which was first published in Wireless World in April and May 1947. The Williamson was capable of very similar technical performance to the Leak Type 15, but as many manufacturers made the Williamson available as a kit, it meant that many audio enthusiasts could afford such superlative performance. The Williamson amplifier, like the Leak Type 15 is a four-stage circuit.
In December 1948, Leak released the amplifier that gave them a reputation for high performance at resoanble cost, with excellence in manufacturing standards. This was the Leak TL/12 - the schematic is attached.
The amplifier uses a pair of KT66 valves in the output stage that are connected in a triode configuration with cathode bias. The first stage is a pentode voltage gain stage, and the second stage a long-tailed pair phase splitter and driver. This configuration was the basis for all Leak power amplifier circuits that would follow for the next 20 years (except of course for the later transistor amplifiers).
To achieve the distortion figure of 0.1%, the main feedback loop encompasses all three stages and the output transformer. The loop gain is 20 (ie: 26dB) so the distortion present in the amplifier without feedback would be 2%. In addition Leak published stability margins for the TL/12 emphasising their claim that the application of heavy amounts of feedback cannot be done without due consideration to the stability of the circuit.
The TL/12 was very well constructed, and featured a power supply with no electrolytic capacitors. The oil filled paper capacitors used instead were costly but have a much longer life. Wiring standards were impeccable, and all transformers were finished for tropical operation.
As part of the promotion for this amplifier, Leak produced a 28-page booklet titled "Leak 'Point One' Amplifiers" which fully referenced classic technical publications of the period. This booklet has been used as a reference for much of this Web page.
See also the TL/12 50'th Anniversary Web Page at this web-site.