There has been a great deal of material in the literature in recent years on transient intermodulation distortion (TIM) as a major distortion mechanism in audio amplifiers, particularly IC op amps. A detailed study of high level high frequency performance of op amps involving over 100 different device samples reveals the true distortion mechanism to be slew induced distortion (SID), with TIM actually being only one particular manifestation of SID. The study demonstrates a direct correlation between device slewing rate and THD, two tone IM, and TIM test results, as well as listening tests. The results allow not only predictable electrical and audible results of feedback amplifiers based on slew rate behaviour, but also dispel several popular myths involving open loop bandwidth and feedback factors as design criteria. Some major implications of this study are a new slew rate criteria for high quality audio circuit performance, the nature of various op amp slewing behaviour patterns, the audible nature of SID as correlated to actual slew rate, and the necessity for industry recognition of slew rate in both equipment specs and testing methods.
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