This set of articles was written in the early 1980’s by Bob Cordell, a few years after Matti Otala (RIP) identified TIM (Transient Inter-Modulation) as the probable cause of sonic imperfections in high feedback solid state amplifiers in the 1970’s.
Otala’s paper sent shock waves through the amplifier design community but sadly it ended up ‘subjectivizing’ amplifier design: instead of it being treated as an entirely science based activity, it subsequently became corrupted with folklore, voodoo engineering, snake oil and plain misinformation that demonized feedback as sonically bad and the cause of the ‘solid state sound’ and TIM which of course it is not. The result is many hundreds of commercial designs that are sub-optimal through lack of it, or through the incorrect application of it. Unfortunately, the problem still persists – you can pick up an amplifier brochure, or go on a commercial website where the vendor proudly boasts about their ‘zero feedback’ products that feature high distortion, load dependent frequency responses and the inability to drive some loudspeakers.
Bob Cordell’s exposé below delves into the finer details of TIM and lays out clearly and succinctly how to avoid it without having to discard feedback which is without doubt one of the most useful tools available to any amplifier designer.