The amplifier that brought high-fidelity stereo to the masses in the USA. Marantz, Harmon Kardon Citation, and McIntosh amplifiers may have been better, but the Stereo 70, with 300,000 unit sales, was 35Wpc of power to the people.
Not the first transistor amplifier, but arguably the first to show that transistor amplifiers could sound as good as, or indeed better than, tube amplifiers. Even today, it is startling how good the 303 sounds, if played within its power limits. Even in the late 60s, a transistor amplifier could, as the 303 showed, sound really wonderful.
Carver the man and the company would produce better amplifiers later and more powerful ones, too, but the Phase Linear 700 and the more popular 400 were the amplifiers that put high power on the map. After their 350 and 200 watts per channel, there was no going back to things like 25 watts—or there should not have been, anyway. 6 Audio Research D series and Conrad Johnson Premiere series: Arguably, none of these were really all that interesting as circuit design in principle, but they put tube amplifiers back in the center of things in American High End in the 1970’s and ‘80’s, for better or worse.
Hafler’s line of MOSFET amps begun in the 1970s, though very popular, did not quite dominate the American scene as had Dynaco. But Hafler could prove by his differential input-output test that the XL-280 was close to perfect and probably did not worry much that high end was largely preferring tubes, which were demonstrably less accurate by comparison.
Carver Sunfire, Lightstar, and A Series
Carver the man and the company, separate by that time, shared the remarkable power-supply technology that gave these compact and not very heavy amplifiers enormous power. The Sunfire Signature could produce 2500 watt pulses! And the Carver A Series, designed by Jim Croft, had the all but unique feature of no output network, for flat response into any load at all. Capabilities beyond the wildest dreams of decades past.
Digital amplification in some form or another was bound to show up eventually and there were earlier tries at something like “digital” amplification. The Tact Millennium was the first digital amplifier to realize the full sonic promise of keeping the signal in digital form almost up to the last instant. The future had arrived, or at least one version of the future.
Not around long enough yet to have had much influence, but give it time. This amplifier offers a unique mode of operation and not just the promise but the reality of an all but unprecedented ability to drive difficult loads playing demanding music with uncompromised sound quality. Analog amplification lives!
The David-Hafler-designed ST-70 not only sold in huge numbers, it became the blueprint (literally and figuratively) for generations of mid-powered tubed amplifiers. The circuit sounded so good that many of today’s tubed amplifiers are based on the ST-70’s fundamental topology. Perhaps the best-selling high-performance amplifier of all time.
Nelson Pass’ first power amplifier under the Threshold name (1975), the 800A signaled to the world that Pass was a designer of uncommon talent and original thinking. The 800A introduced active biasing that allowed the 800A to deliver far more Class A power than conventionally biased Class A/B amplifiers of similar size and heat dissipation. The triple-series, triple-parallel output stage was also groundbreaking. The 800A was the first product in a lifetime of creative approaches to amplifier design that continues to this day in Pass Labs’ products. I could have chosen several other Pass designs that would be worthy of this list.