Quad 405 in Production
The Quad 405 employs the "feed forward" technique. A feed-forward amplifier is one in which samples of the input and output signals of a main amplifier are adjusted in amplitude and algebraically combined to produce a sample of the error introduced into the output signal by distortion and noise in the main amplifier. Sound pretty mundane in this modern digital world, but was actually a leap forward in the 1970's. Practically speaking, it meant that the manufacturer did not have to employ large output devices to cope with every load condition come what may. The "come what may" approach is valid, but does have the disadvantage that the customer would have to pay for some every large output devices and the power supply to go with them. The AHB2 amplifier in current production is an example of a modern amp that implements feed forward in combination with a switching power supply to obtain light weight and high quality output.