Yet another amp from Colorado! This is the third amp to come my way from that state (the others being the Spectrascan and the Rowland Research), and at this point, after living with it for a couple of months, I'm quite comfortable in declaring the Boulder 500 to be one helluvan amp.
Eleven years ago, Threshold Corporation entered the high-end audio market with the first amplifier ever to use sliding bias (footnote 1) in its output stages. Some 10 years later, Threshold spawned another innovation: their so-called Stasis circuitry, which yielded the S-series amplifiers. The SA-1 and its lower-powered sister SA-2 are the latest from Threshold, and are the first Threshold amps to abandon sliding bias for straight class-A operation. Both use the Stasis circuit.
Several issues back, I mentioned a major "new wave" of power amplifiers coming along: the Adcom 555, the New York Audio Labs transistor-tube hybrids, and the latest Krells, for example. They demonstrate that major audible improvements are still possible in something as well-explored as the power amplifier. Not only that, some of these products demonstrate that superior performance can be combined with relatively low price.
Some readers may feel that it is pushing poetic license beyond reasonable limits to call the Krell KSA-50 a "new-wave" amplifier. It has, after all, been around for several years. The Krell KSA-50 is new-wave enough, however, to be an incredibly stiff class-A design, rather than a pseudo–class-A circuit, and its 50 watts per channel are supported by enough of a power supply to drive an arc welder. You get about 70 watts of RMS power with 8 ohms, 150 watts with 4 ohms, and sufficient watts into 2 ohms to threaten my load resistors. There is almost enough power to drive a pair of Apogee Scintillas at their ohm setting—though I'd prefer at least the Krell KSA-100.
When I first heard the Eagle 2 at the 1985 Winter CES I knew this amplifier was a winner. I was eager for a chance to get my hands on it, but I also knew that J. Gordon Holt was champing at the bit to do the same. So it came as both a surprise and a delight when ye Gracious Editor gave me first crack at the Eagle 2. I wasn't disappointed; the little Eagle more than lived up to expectations. It's not the best power amplifier I've ever heard, but it's damn good. It is, in fact, better than its big brother, the Eagle 7A, in significant ways; in view of the 2's reasonable price, that's saying a lot.
I am reluctant to call any given transistor power amplifier a "best buy" or "breakthrough." From my talks with designers and other audiophiles, it is clear that the state of the art in power amplifiers is about to change. From where I stand, the Adcom GFA-555 is the first sample of this new wave. It is so clearly superior to past amplifiers in the low- to mid-priced range—not to mention most amplifiers two to three times its pric—that I can unhesitatingly recommend it for even the most demanding high end system.
James Bongiorno, the engineer behind the Sumo Andromeda, has enjoyed a long and colorful career as an audio amplifier designer. He has cast himself at times as an enfant terrible, exploding at audio critics and running scandalous advertisements (footnote 1). His best-known amplifier is the Ampzilla, produced by Great American Sound, but he also designed the Dyna 400. Currently Jim is living on a boat and serving as part-time consultant to the Sumo Company.
I must admit that even before I connected up this amplifier I was put off by the accompanying literature. B&K makes some persuasive points about the validity (or rather the lack thereof) of some traditional amplifier tests, but the literature was so loaded with flagrant grammaticides, syntactical ineptitudes, and outright errors that I could not help but wonder if the same lack of concern had gone into the product itself (eg, the term "infrasonic" is used throughout to mean "ultrasonic"). Good copy editors aren't that hard to find; B&K should have found one.
This is something we don't see too often: an entirely new approach to power amplifier design. As Quad points out in its literature for the 405, class-A operation of transistors provides the lowest distortion, but drastically limits the amount of power an output transistor can deliver without overheating. (Most transistor amps use class-AB output operation, in which each of a pair of power transistors handles part of each signal cycle and shuts down during the other part. Imperfect synchronism between the two halves causes the familiar "crossover distortion," which accounts for most solid-state sound. In class-A operation, each output transistor draws current though the entirety of each signal cycle, eliminating the crossover transition but doubling the amount of time current is drawn, and thus tending to cause the transistor to heat up more.)