It must therefore have occurred to every circuit designer that it should be a simple matter to take a portion of the output, compare it with the input to derive an error signal.

Walker was also attributed with the famous hifi quote "the perfect amplifier is a straight wire with gain" with the implication being that nothing would be added, and nothing taken away from the signal, just a bigger version of the same thing at one end.

In 1995, QUAD Electroacoustics Ltd was bought by Verity Group plc, joining its existing brands of Wharfedale and Mission.

[1] In September 1997 the company changed ownership again as Verity Group sold off businesses to finance its development of flat panel loudspeakers.

With Wharfedale it became part of the Chinese International Audio Group under the management of Bernard and Michael Chang.

In 2003, a book was commissioned "QUAD:The Closest Approach" which offered a history of the company from its creation to that point.

This unit's sound quality reproduction was high compared with other products on the market at the time, and was thus adopted for use by the BBC.

Subsequent electrostatic models, the 988/989, the 2805/2905 and then the 2812/2912 were successive refinements of the ESL-63 design, featuring increased power handling and output levels, more sophisticated overload protection, and greater structural rigidity.

Following the Verity acquisition, Quad developed and market a range of conventional electrodynamic loudspeakers alongside its electrostatic line, available in both passive and active (i.e. featuring in-box amplification) configurations.