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EXPIRED - FOR SALE: Quad 405 Stereo Amplifier

Item #649376438
Info: Quad 405 Stereo Amplifier

Asking Price:
CAD $390.00
Payment method: Cash
Condition: 8 - Very good (?)
Date Posted: Sep 02, 17 9:17am (PST)
Edited: Sep 02, 17 9:17am
About Seller: Same
Kitchener, ON
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Quad 405 Stereo Amplifier
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Technical Specification
Power output: 100 watts per channel into 8Ω (stereo)
Frequency response:

-1db at 20Hz
-0.5db at 20kHz
-3dB at 50kHz
Total harmonic distortion: < 0.01%
Hum and Noise:
‘A’ weighted -96dB ref full power
Unweighted -93dB ref full power (15.7kHz measurement bandwidth)
Left/Right Channel Crosstalk:
80db @ 100Hz
70dB @ 1kHz
60dB @10kHz
Input sensitivity: 0.5V for 100 watts into 8 ohm load
Speaker load : 4 to 16 ohms (nominal)
Signal to noise ratio: 95dB
Speaker load impedance: 4Ω to 16Ω
Dimensions: 115 x 340.5 x 195mm


In 1936, Peter J. Walker formed a company, in London, initially called S.P. Fidelity Sound Systems, then later, after a move to Huntington (near Cambridge UK), renamed to the Acoustical Manufacturing Co. Ltd., finally becoming best known for the Quad brand (from an acronym ‘Quality Unit Amplified Domestic’.)
After building a reputation for designing quality valve hi-fi amplifiers and electrostatic loudspeakers for the hi-end domestic market, Peter Walker applied a little known audio circuit technique called feed-forward error correction (first patented way back in 1929 by Harold Black) and designed the ‘current dumping’ Quad 405 power amplifier, an innovation still used in Quad power amplifiers to the present day.
From Peter’s December 1975 technical paper in the ‘Wireless World’ magazine,
‘An audio power amplifier is required to produce an output signal that differs from the input signal in magnitude only.  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. It is then only necessary to amplify the error signal and add it to the output in the correct amplitude and phase to cancel completely the distortion of the primary amplifier.’
Peter put this principle into practice using two amplifiers per channel instead of one.
The first stage ‘error’ amplifier is low powered but very high quality. The second amplifier is high powered, but of lesser audio high quality.  (It’s a lot more difficult to achieve very low distortion in high powered amplifier stages).  Peter designed a way to compare the high powered output with the original audio input and derive the required error correction signal which is then injected into the audio path, in such a way that the high power audio output achieves a very low distortion figure, even at very high power levels.
This innovative product earned Quad the Queen’s Award for Technological Achievement in 1978.
In the late ‘70’s and ‘80’s, if you were seriously into hi-fi and had the spending power, then a Quad 405 would be sitting in your listening room.  I had the desire, but sadly not the spending power!
Whilst the Quad 405 was most common in home hi-fi set ups, I came across many examples of them and their brethren in the BBC during the 1980’s.


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