The story started more than quarter century ago when I practically started working with first opamps and comparators and my friend George Pilko in other room tried to resolve uncertainty in the class-AB power amplifier transition zone at low signals. We had a lot of discussion which gradually shove us to turn to primary source of information - in our case there were publications of Matti Otala, Walt Jung and John Curl. Matti Otala was the first who woke up sine-wave-minded engineer approach, and Walt Jung and John Curl extended his ideas and made them popular among analog IC designers and discrete audio amplifiers.
The Otala approach was criticized many times, that suggested criteria were too stringent for audio, that his DIM test didn't represent real music, signal, etc. I would like to say that all these gentlemen have the same approach - extended quality. Once some critical condition is found the unit should be operated well below this critical condition, with a substantial margin, to be on a safe side. The engineering practice is such thing that nobody knows what parameter would be critical tomorrow e.g. one can say that frequency response in phase linear systems can be measured at -3dB, and that is fine, but later it appears that 45 degree phase shift can be audible, thus the units which were design with -0.5dB in mind also had much lower phase shift within the band. The same is with Otala criteria, later he showed that substantially below it phase intermodulation distortion can be generated. Works by Matti Otala, Walt Jung and John Curl taught me to design the unit in several dimensions, having all possible variables in mind and never stick to one parameter (or small set) as a measure of quality.
I would like to present here almost full compilation of John Curl postings on the net during last three years. I tried to organize them in logical manner, later the exact references and omitted pictures will be added and some pictures will be redrawn. I would like to make this as the next digital generation arrived without any knowledge what's going on in analog (audio) circuitry.
So, ladies and gentlemen, "Words of wisdom" by John Curl
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