Phase intermodulation (PIM) distortion in audio amplifiers, in particular that which may arise from the use of negative feedback, is considered. PIM theory is discusseed and the linking of PIM to amplitude intermodulation (AIM, or SMPTE IM) distortion is illustrated. It is noted that amplifiers without negative feedback are also likely to produce PIM. Coherent SMPTE IM instrumentation capable of resolving less than 2 ns of equivalent time dispersion and measurement results from several different real amplifiers are presented. The results show that PIM is not a problem in contemporary amplifiers and suggest that negative feedback reduces total PIM in most cases.
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