Quad 405 Power Amplifier

Peter Walker founded the Acoustical Manufacturing Co. in 1936, and since then all Quad products have displayed originality in design, and over the years, made a major contribution to the improvement of sound quality. What we have here is a 405 power amplifier that is in good cosmetic condition for its age and is fully working. It comes suitably packaged, with manual and as always, our standard 3 month warranty.
Quad Say….
The external design continues the Quad tradition of practical simplicity which has won prizes and world-wide acclaim, on previous models. A combination of mouldings and extrusions is used to produce a unit which is strong and pleasing both to the eye and the touch.
The internal appearance is unmistakably Quad and reflects concern with economy of manufacture and ease of service.
The Quad 405 “current dumping” amplifier is designed primarily for use in high quality domestic installations with loudspeakers of less than normal efficiency but is equally suitable for professional use. 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.
What Quad has done – not quite as easy as it sounds ¬ is to produce a current controlled amplifier, apply a special type of feed forward current error correction, and then apply overall voltage feedback lo convert the whole to a voltage amplifier. There is in effect, both a low powered very high-quality amplifier and a high-powered heavy-duty amplifier, the current dumpers. The low power amplifier controls the loudspeakers at all times, calling upon the high-power section to provide most of the muscle. The small amplifier is so arranged – it carries the error signal – that provided the large power transistors get within the target area of the required output current, it will fill in the remainder accurately and completely. The result is that the quality is solely dependent upon the performance of the low powered high-quality amplifier together with four passive components. When correctly designed, all distortion in the output stage is reduced to zero. The advantages of this technique are that it is possible to produce an amplifier of very high performance without using carefully matched relatively fragile output devices.
There are no crossover biasing problems, no alignment or adjustment is required to obtain optimum performance, and nothing can go out of alignment during life. In the event of component failure, replacement can be effected and performance restored without realignment.
The Quad 405 is normally used with the Quad control unit, but other signal sources can be readily accommodated. Since the amplifier has no controls, it may be mounted out of sight in a well-ventilated cupboard.
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 dc 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.
In the unlikely event of component failure, the output to the loudspeakers is instantaneously interrupted by a clamp circuit.
Not all loudspeakers can safely accept the full power output of the Quad 405, and the amplifier is provided with a power limiter which is fitted internally for use in such cases. The Quad electrostatic loudspeake1 can be used safely with the power limiter in.
The Quad 405 has won both a Design Council Award 1976 and The Queen’s Award for Technological Achievement 1978.
Note: Quad electrostatic loudspeakers prior to serial no. 16800 must be modified for use with the Quad 405 amplifier.
Specifications:
Power output: 100 W into 8Ω (stereo)
Frequency Response: 20Hz to 20kHz
Total Harmonic Distortion: 0.05%
Input Sensitivity: 0.5V
Signal to Noise Ratio: 95dB
Speaker Load Impedance: 4Ω to 16Ω
Dimensions: 340.5 (W) x 195 (D) x 115 (H) mm
Weight: 9kg

For a vintage review from Gramophone magazine way back in 1976 please click here