Linear amplifiers and preamplifiers for audio applications have a well-known intrinsic dynamic limitation known as slew-rate (SR) and produce distortion when the output signal slope (SS) tends to equal or exceed this limit. A great deal has been written on this subject over the last few years, but it is only recently that a clear and definitive analysis has been formulated. In a series of papers - Jung, Stephens, and Todd - have been able to determine the relevant parameters for the formation of TIM distortion. It is a widespread belief that the use of large amounts of feedback is the cause of TIM distortion. Only recently has it been concluded that large amounts of feedback do not increase the possibility of TIM if the amplifier has a sufficiently high slew rate. In particular, for SS lower than SR, an increase in the feedback factor reduces the TIM, while for lower values of SR increasing the feedback increases the TIM, hence the TIM is under control for values of the ratio SS/SR less than unit. The SR of a hi-fi audio amplifier is therefore an essential design parameter, whereas the small signal parameters, the feedback factor, and the open-loop frequency response, are per se irrelevant as far as dynamic distortion is concerned. This paper describes the "IS-TIM" measurement system and the results of experimental and simulation work on monolithic preamplifiers and power amplifiers for hi-fi.
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