Audio Research D150
The ARC D150 represented a turning point in tubed amplifier design at its 1975 introduction. Although it followed in the footsteps of the groundbreaking 76A, the D150 took the idea of high resolution (and high power) in a tubed design to another level. The D150 proved that transparency, resolution, and low coloration were not the exclusive province of solid-state amplifiers.
The KSA-50 is significant more for being the first commercial product from then-newcomer Dan D’Agostino and Krell Industries. It established the “dreadnought” build-quality of massive power supplies, lots of output devices relative to the output power, and the ability to drive the world’s most challenging loudspeaker loads. The KSA-50 was the antecedent of a long string of great power amplifiers from Krell, and the product that forced other manufacturers to step up their games with regard to bottom-end slam and bulletproof construction.
Phase Linear 700
Although not nearly as popular as the smaller 400, the 700’s claim to fame is that it was the first truly high-powered amplifier. Remember that when the 350Wpc Phase Linear 700 was introduced in 1970, 50Wpc was considered a powerhouse. Bob Carver’s 700 paved the way for the modern era of high-powered amplifiers, both tubed and solid-state. This trend toward high-powered amplifiers gave loudspeaker designers more latitude in their designs, unleashing the wave of loudspeaker innovation that began a few years later.
Sold as a kit or fully assembled ($299/$429 in 1980, if memory serves), the DH-200 represented the confluence of great sound (thanks in part to a wonderful MOSFET output stage), high reliability, and a dirt-cheap price. The DH-200 made a high-end powerhouse amplifier affordable to many.
This unassuming little integrated amplifier brought true high-end sound to a mass-market price, and in the process, exposed a whole generation of music lovers to high-end audio. The 3020 was simply the “go-to” amplifier for an entry-level high-end system. There are many more audiophiles today because of the 3020.
Audio Research Reference 600/610T
The forerunner of the 610T, the massive Reference 600 (34 tubes per side) not only set a new standard in timbral purity and realism, it opened my eyes to the virtues of tube regulation (6550’s as the series-pass elements). Completely devoid of grain and grit, the Reference 600 was revelatory in its seductive liquidity. The Reference 600 combined this exquisite delicacy with iron-fisted dynamic authority.
Although not the first amplifier to accept digital signals and directly convert PCM audio data to the pulse-width-modulated signal that drives a switching output stage (that was the Tact Millennium), the M2 is more significant in many ways. It sounds far better than any previous switching amplifier, accepts high-resolution digital audio, is affordable, fundamentally changes audio-system architecture, and is the forerunner of what is likely to be a long line of future NAD products based on the topology.
BAlabo BP-1 Mk.II
The Bridge Audio Laboratories BP-1 Mk.II hasn’t stood the test of time as others on this list have, but it’s surely worthy of inclusion by virtue of its sonic merits, which are singular in my experience. The BP-1 Mk.II’s core triumph is combining high resolution of musical information with an utterly relaxed and unfatiguing sense of ease. This amplifier doesn’t force you to choose between resolution and musicality.
Futterman H3 OTL
Julius Futterman was the first to realize the dream of a commercial output-transformerless tube amplifier, albeit for a brief period. Its life span was extended for a few more years by New York Audio Labs.