Oh yes, Allied, Lafayette, Radio Shack, and others had amps and other products made for them also.
Why amps, pre-amps, integrated amps???
OK, having thusfar asked questions on this forum that have exposed me to the odd raised eyebrow and snicker for my gross audio ignorance, I shall go farther still, and venture to ask: What, exactly, are amps, pre-amps, and integrated amps??. More to the point, what, exactly, is their purpose; what do they do? And why do pre-amps and amps still exist comfortably in the audio market when you can get them combined as an integrated amp?? I just don't get it. Would much appreciate your learned revelations - after, of course, you've finished with your hoots, knee-slaps, and cat-calls.
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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. |
Yes, excessive loop feedback has been a problem in the past! I concur with the THD wars, but amps built for low TIM back in the early 80s or thereabouts definitely got a bad reputation. It does not surprise me at all that progress has been made in that department. I suspect that we should also be paying attention to transient forms of harmonic distortion as well. |
Well, audio is certainly a balancing act and if you pursue one spec at the expense of others the overall sound will suffer. Still, taking TIM into account (a balanced approach) probably had a lot to do with why there were more good-sounding SS amps from the '80s than from the '70s. Another guy who was very hip to this early on was Bob Carver. It probably helps that he's a physicist and not an electrical engineer. He had few preconceived notions and had his own way of tracking down and solving problems. I was just re-reading Absolute Sound's article about 10 most influential amps, and in its writeup of the Phase Linear 700, it mentions that Carver had noticed that tube amps were capable of far wider voltage swings than typical SS amps, so he designed the Phase 700 to make bigger swings like the tube amps. Excessive negative feedback narrows these voltage swings. That may be why Carver never joined the THD wars; his Phase Linear and Carver amps typically claimed .5% THD while his competition was trying to get below .1%. |
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