Quad 405-2

With this article I intend to open the discussion about audio power amplifiers. My last passion about the world of microcontroller makes me to let a little bit behind the old and forever passion, probably for most of you, for audio amplifiers.

Theory about this sector can be found in all the internet corners, so here I will put into discussion a couple of schematics and designs which impressed me so far. If some of you come with something new or if it's seen from other angles, let me know, I’m open for discussions and if I will consider the information as valuable one, I'll put it here.

Quad 405-2 a piece of art.

For those who doesn't have any idea about what does it mean quad or 405, I will initiate whit a little introduction:

The company called Quad Electroacoustic founded by Peter J. Walker in 1936 in London is a British manufacturer of hi-fi equipment, based in Huntingdon, England.

The company initially produced only public address equipment but after the war they began to produce equipment designed for use in the home as a result of the rising demand for high quality domestic sound reproduction.

The name "QUAD" is an acronym for "Quality Unit Amplifier Domestic", used to describe the QUAD amplifier. In 1983, when having become known for their QUAD range of products, the Acoustical Manufacturing Co. Ltd changed its name to QUAD Electroacoustic Ltd.

The famous Quad “Current Dumping” amplifier launched in the mid 1970’s, and featured feed forward error correction that effectively removed the class B cross over distortion that plagues this type of output circuit structure.

The design caused a stir in the audio industry at the time of its release, and was the subject of a patent and a paper at an AES convention a year earlier. Although the 0.005% midband distortion placed it firmly in the upper echelons of measured performance in the industry at the time, some reviewers failed to warm to the sound. Nevertheless, this was a landmark design that creatively solved the cross over distortion problem.

Productions:

Current Dumping Power Amplifiers

Quad 405 – 1975 to 1982 – 64,000 units

Quad 405–2 1982 to 1993 – 100,000 units

Quad 405-2 is one of the most known audio power amplifier for its audio performance.

Quad 405-2 Front/Rear

Quad 405-2 Inside view

Quad405-2 PCB-Channel

Technical Specifications

Power output: 100 watts per channel into 8Ω (stereo)

Frequency response:

1.      1dB at 20Hz

2.      0.5dB at 20kHz

3.      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:

1.      80dB @ 100Hz

2.      70dB @ 1kHz

3.      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

Weight: 9kg

MODIFICATIONS

Throughout time a lot of electronic engineers and hi-fi audio hobbyist around the world, start to modify this audio power amplifier from redesigning the electronic schematic, paying attention from quality of components and in/out connectors to the percentage of Ag consistency from the soldering material, at the end to succeed improvements of sound response.

Below I will post a series of electronic schematics starting with the original one and ending with those who has the most significant modifications. All of them are fully functional.