Douglas Self’s 8 Distortions and a Few More
(pictured above is the hifisonix 180 Watt per channel e-Amp)
No one had codified all the major audio amplifier distortion mechanisms until Douglas Self published the first edition of his ‘Audio Power Amplifier Design Handbook’ or APAD’ as it has become known, in 1996. He used a standard Lin* topology amplifier and then proceeded to show in a logical and easily understood manner, how all of the major distortion mechanisms in audio amplifiers could be reduced to ‘vanishingly low levels’ i.e. parts per million levels. In an industry awash with voodoo engineering claims, he was thus careful not claim his amplifier was the best, but merely that in terms of the whole signal chain from studio microphone to the consumers loudspeaker in a domestic environment, the power amplifier contributed the least distortion. He thus termed his resultant design the ‘blameless’ amplifier. 8 key distortion mechanisms were identified in his book and in the short article you can download below, I’ve described each one, how much distortion it can contribute, and how it can be negated with good design.
*Lin amplifier topology is named after Harry Lin, a Bell Research Labs scientist who in the late 1950’s first proposed the now standard three stage Voltage Feedback Topology (VFA) amplifier: Input voltage to current stage, integrator and the output stage buffer. All VFA amplifiers are derived from this basic topology.