12-12-11: Atmasphere
The 'TIM' amps of the late 70s and early 80s proved to be a dead end. What they proved was that you can go overboard chasing specs to the detriment of the final product.
There was a THD (total harmonic distortion) spec war starting in 1976 and continuing into the '80s, but I never heard of a TIM war. Effective Jan. 1, 1976, the FTC set a federal standard in amplifier measurement to put an end to the confusing power ratings claimed by using different measurement standards. There was RMS, EIA peak power, and there was IPP (instantaneous peak power), and often a bandwidth and variation tolerance was not specified. The FTC rules starting in 1976 started requiring a 1-hour warm-up at a steady 1/3 of maximum power, followed by power testing. The resulting power specs had to be for continuous RMS power over a specified bandwidth (e.g., 20-20KHz) into a specified resistance (e.g., 8 ohms), a frequency fluctuation tolerance (e.g., +0, -2 dB), and a THD distortion rating (e.g., 0.5%).
This prompted a THD war among lower quality amps. It was easy enough to lower the THD measurement by adding more negative feedback to the amp circuit. Although it made the amp measure better per FTC requirements, it altered the slew rate and limited the amp's ability to perform wide voltage swings. Also, the FTC rule only required testing into a resistor of a specific value. This resulted in some bad-sounding amps that had excessive negative feedback and low current designs optimized to measure well into a resistive load (an 8-ohm resistor) rather than into a reactive load (a loudspeaker). It got to where some receiver designers stopped listening to their products altogether and shipped their designs as soon as they met the bench test spec--which may have been set by marketing. The result was a generation of mid-fi electronics that sounded sterile, flat, harsh, and uninvolving, like all those bogus department store rack systems of the '80s.
If anything, TIM was first described by Finnish electronics engineer, consultant, and professor Matti Otala as an unpleasantly audible byproduct of too much negative feedback, a dissonance that went undetected by the steady state measuring methods of the '70s.
Designing to reduce TIM helps make an amp sound better and is still an important parameter in high end amp design today. See
this recent review of a $12.5K pair of Electrocompaniet monoblocks. Also see this
recent interview with Tim de Paravicini, particularly
page 3, where Tim discusses the challenges and effects of TIM in amplifiers, particularly solid state ones where the negative feedback loops slow down the slew rate and allow transient overload.
