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Quad II

Quad II
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Quad 303

The outstanding performance of the Quad 303 is largely due to the unique output circuit developed by Quad. The circuit employs symmetrical triples, greatly reducing distortion and rendering the quiescent current independent of output transistor temperature. This development enables the designers to reduce distortion to any desired degree without sacrificing stability, and to maintain this excellence under all dynamic conditions. To put this in perspective, the lowest theoretical distortion present on the very best tape and disc will still be some hundred times greater than that of the 303. Such a level is, of course, completely inaudible no matter how refined the test. The use of triples permits simple and effective control of the output current and since the voltage is also controlled, the output transistors are completely protected and must always operate within their ratings with the obvious advantages of greatly improved reliability under all conditions, without the use of elaborate and expensive circuitry. The amplifier cannot be upset even when overdriven by an asymmetrical signal or by shorting or disconnecting the loudspeakers.

Quad 303
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Quad 405

The Quad 405 power amplifier is intended for use with loudspeakers of less than normal efficiency. Great care has been taken to ensure that the high power output of the 405 does not increase the risk of damage to the loudspeaker. Although direct coupled, the d.c. offset voltage is confined to negligible values even' under gross overload. A fixed high pass filter is incorporated to prevent spurious subsonic signals from reaching the loudspeaker. The Quad 405 uses a current dumping output circuit, a Quad invention which eliminates many of the problems associated with transistor amplifiers. The basis of current dumping is 'feed forward error correction' first proposed by Harold Black in 1928, in which a derived error signal is added to the output signal of an amplifier in exactly the right amount to cancel the distortion produced by the amplifier. While it is relatively easy to add error current to signal current without Interaction, as in Black's invention, it is very much more difficult to do so in voltage terms. Loudspeakers require a stiff source and so 'feed forward' has found no successful application in audio amplifiers. Weight 9kg.

Quad 405
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Quad 606

Quad 606
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