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1st cycle distortion mainly measures bandwidth

Posted by jcox (A) on August 13, 2004 at 08:55:03

In Reply to: EW 'Class A imagineering' 1st cycle distortion posted by toxicport.e on August 12, 2004 at 21:43:08:

1st cycle distortion mainly measures bandwidth

GM’s “first cycle distortion” is confusing to engineers too, the sudden switch on from a flat line to the sine wave is actually a very high bandwidth signal, lots of higher frequency content is involved in the leading edge discontinuity of the 1st cycle

GM seems to count any deviation of the output from the input as distortion in his 1st cycle distortion, while most engineers would draw distinctions between a linear filter’s frequency/transient response deviations and added nonlinear distortion products, without further refinement GM’s 1st cycle distortion concept is mainly a measure of amplifier bandwidth – one that makes MHz audio amplifier bandwidths look necessary

As I recall, having assumed a high bandwidth test signal the earlier articles seem to be a confusingly low level heuristic flavored exploration of output impedance/disturbance rejection of a dominant pole compensated feedback amplifier with respect to the high bandwidth signal

The discussion seems to assume the dominant pole is local to the VAS and the slow Cdom-Miller integrator then controls a class B EF output stage with constant global negative feedback factor – this feedback loop’s limitations are well understood by engineers in the form of Bode’s sensitivity analysis with nonlinearities modeled as added disturbance inputs

I haven’t thought too much about the loudspeaker/crossover network interaction with the commonly used Zobel series inductor but again his discussion is confused by the constant mixing of “back EMF” with crossover filter and amplifier impedance, it would be much simplified by recognizing back EMF as simply a view of a complex impedance, nonlinear effects and other phenomena are again usually modeled as added disturbance sources, allowing the whole analysis to move forward in a single language of network analysis

Sorry if the above is still confusing but we’re desperate here at propeller-head for a subject with some technical meat

GM did try to convey some complicated concepts in a simplified form and I respect his effort but the presentation leaves me with questions about the assumptions, depth of his understanding and the utility of the “explanations” so far; I eagerly await amplifier schematics to see what he’s really getting at

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