In 1951, The QUAD amplifier, successor to the QA12/P found favour amongst early audio enthusiasts. It was with this model that the 'variable high frequency filter' was introduced, and it has been a 'must' ever since. It established the format for domestic high-fidelity equipment consisting of a pre-amplifier with controls necessary for the selection of programmes, and a separate power amplifier.
Music listeners did not take long to realise the virtues of Acoustical's products and to appreciate that the acronym QUAD, derived from 'Quality Unit Amplified Domestic' was much easier to pronounce and remember than Acoustical. The company also developed the cathode coupling of the output transformer which had been used for many years in their P.A. amplifiers. This was later taken up in the USA in a rather retrograde form and called 'ultra-linear'. The reputation of Quad is founded on the Quad II which was introduced in 1953. Its matching QCII Pre-Amplifier featured push buttons to select inputs and alternative record equalisation characteristics. Shortly after the appearance of the Quad II, all contract work on P.A. amplifiers was discontinued in order to concentrate efforts on the QUAD line. In order to demonstrate the reproduction quality of hi-fi equipment, Quad, in co-operation with the late Gilbert Briggs of Wharfedale, held a series of public concerts in London's Royal Festival Hall as well as in other halls in Britain and the USA. The live concert, using a variety of works and instruments, was instantly compared with recordings of the same performance. This helped considerably to promote a wide appreciation of just how good quality reproduction could be.
The full range electrostatic loudspeaker, the ESL-57, was first demonstrated in London to an invited audience of leading audio engineers in 1956, and marketed the following year. This was the result of a long-standing appreciation of the basic advantages of the electrostatic principle and about three years of concentrated, non-stop, often day and night research into materials and techniques, all carried out in considerable secrecy at Huntingdon. Although electrostatics had long been recognised as providing the answer to most of the basic problems of loudspeaker design, the limitations in earlier days to materials such as animal intestines for diaphragms and the problems of handling very high voltages in domestic equipment rendered the whole idea impracticable. With the introduction of the moving coil loudspeaker, electrostatics were largely abandoned until the development of suitable materials for light diaphragms which then made it possible to manufacture them.
The fact that the ESL-57 remained in production, virtually unchanged for 28 years, is a credit to the thought and care invested in the design and development of this product, which was truly years ahead of its time. It became the standard by which all others were judged. Braun, in Germany, were the first foreign firm to manufacture the loudspeaker under license, followed by KLH and Acoustech in the USA. Acoustech's were much larger units than the Quad, being similar to the pre-production model which was then too big to sell in any quantity, particularly with stereo on the horizon!
The introduction of stereo records and broadcasts resulted in the Quad 22 Control Unit and the Quad Stereo Decoder. A stereo decoder was added to the Quad FM Tuner to bring the BBC stereo broadcasts into the home.
