Convergent Audio Technology SL-1 preamplifier Jack English 1992
The Convergent Audio Technology (CAT) story is actually the story of one man—Ken Stevens. He is an electrical engineer/applied mathematician with two degrees from the University of Rochester. While UR did not have an audio program per se, electrical engineering offered Stevens the best alternative. While a student, he worked on various amplifier and preamplifier designs, all of which were, predictably, solid-state, using transistors and op-amp chips. His first real design was something simply called the "FET preamp."
After graduation, Ken worked for more than a decade as an engineer with first Stromberg-Carlson, then Taylor Instruments, while continuing to refine the FET preamp. He eventually met a colleague who accepted an invitation to audition the FET. The friend's system consisted of an Audio Research SP-3 and D-76 with a Linn Sondek/SME/Denon front end driving Quads. They listened and finally substituted the FET for the SP-3 (or SP-3A). Rather than bowling over his friend, it was Stevens who was, in his own words, "simply thunderstruck." The FET, with its pedigree of measured purity, came in a poor sonic second.
Stevens accepted the differences he heard, but rather than abandon everything he'd worked so hard on for so long, he tried to further improve the FET by making it perform even better in every measurable way. The sonic results, as you might have guessed, were the same. The oft-praised ARC preamp was simply better. Reluctantly, Stevens began studying and working with tubes, starting with the venerable 12AX7. Although tubes had been covered in his academic program as items of historical interest, he discovered that they actually had some wonderful characteristics (eg, impressive linearity) that weren't fully appreciated.
Having decided to learn still more about tubes, Stevens purchased and built a Dynaco PAS-3X. While the FET was clearly superior at the frequency extremes, the 3X was superior in many other regards. He tried various modifications of the PAS circuit which led to improvements at both frequency extremes. No matter what he tried with the FET, the PAS remained superior in the other areas.
Next came a series of experiments with cathode followers. That led to further investigations of other types of tubes, and additional circuit designs. Contrary to popular belief, the CAT's predecessor was a 12-tube monster preamp which was never sold. Frustrations with this design and a long gestation period ultimately yielded the refined 10-tube circuit that remains at the heart of every SL-1 series preamp.
Once the design of the original SL-1 was finalized, Stevens began selling it directly to consumers. The primary competition, then as now, came from Audio Research and Conrad-Johnson. Nonetheless, many audiophiles came to know the CAT preamp. A review by Tom Miiller appeared in The Abso!ute Sound; another review by Anthony H. Cordesman appeared in Stereophile (footnote 1). Armed with these first positive reviews, Stevens began to build a dealer network. Like most small audio companies, the one-man CAT operation had its share of teething problems: uneven distribution, suspect reliability, and a constantly evolving product. Filling orders often seemed to take forever, repairs took longer, updates were offered only sporadically, and no two SL-1s sounded exactly alike. During this difficult growing period, CAT had as many detractors as admirers; a much larger segment of the audiophile population wasn't even aware of the company's existence.
About four years ago, CAT's dealer network began to stabilize, repairs became more standardized, and product delivery began to become more predictable. At about this time, the SL-1 also made a big leap forward in performance. Audiophiles noticed, and the underground press began buzzing about the CAT after a very favorable review by Michael Gindi in Sounds Like... (footnote 2). Oddly, the model designation remained the same, which caused confusion—two samples of the SL-1 were often two very different preamps. The Mk.II, or final version of the SL-1 Reference (which was one of four CATs I've owned), was very reliable, easy on tubes, and sonically splendid.
Equally important are the changes that have occurred behind the scenes. A separate production arrangement now has the capacity to supply preamps in quantity. This had never been true of CAT products in the past, when Stevens himself built each one. The new production arrangement has had an added benefit of improved build quality, although rumors of reliability problems continue to plague the preamp.
In early 1992, the Reference Mk.II was replaced by the Signature. One of the many things Stevens heard that had improved the performance of his design was George Tice's Power Block. Based on what the Power Block did for the preamp, the SL-1's own power supply was radically redesigned, and now includes an isolation transformer. As a result, CAT now suggests not using any external power-line conditioning with the preamp.
A second insight came from the internal suspension of Andy Payor's Rockport turntable, which led directly to the bright blue, viscoelastic, hockey-puck-like feet found on all Signature preamps. These well-damped feet are said to have a very low 13Hz resonant frequency and act to decouple the preamp from vibrations coming up through the cabinet from the floor.
The most significant change that led to the Signature was a major breakthrough in chassis construction. The primary design criterion had been the minimization of vibrations and resonance in the chassis. Most owners of earlier CATs had tried all sorts of things to control resonances. I'd used AudioQuest tube dampers, VPI bricks, and Goldmund cones, all with the CAT sitting on a marble slab on my heavily braced wooden rack.
Stevens was fully aware of these tweaks and their sonic benefits. To minimize resonances, the Signature chassis uses constrained mode damping much like the turntable platter introduced a number of years ago by SOTA. Chassis panels are constructed of layers of various materials of different physical properties, a form of construction often found in loudspeaker cabinets. These dramatically heavier panels (the Signature weighs 54 lbs) act to damp the impacts of airborne vibration.
Footnote 1: The Abso!ute Sound #38, Summer 1985; Stereophile Vol.9 No.7, November 1986.—Jack English
Footnote 2: Sounds Like... #4, 1989.—Jack English