Convergent Audio Technology SL-1 preamplifier:
Jack English 1992 part 2
No CAT story would be complete without a discussion of tubes. Stevens continues to measure and audition every type of tube he can get his hands on, over time finding his favorites. This may sound odd to those familiar with the preamp's evolution—different SL-1s often appear to use different tubes. In reality, though, the SL-1's tube complement has remained relatively constant. Tubes have generally come from the same sources via different suppliers, hence the variety of suppliers' logos on the tubes themselves. For the last eight years, all of the 12AU7s have come from the same German manufacturer. The 12AX7s continue to come from Yugoslavia, and the 6DJ8s are all from Russia. The only major tube change for the Signature is the substitution of Russian 6922s for 6DJ8s.

Stevens is obsessed with the sonic performances of the tubes used in the SL-1. He firmly believes that the entire sonic character of the product can change dramatically with different tubes. Having tried, with very mixed results, numerous tubes in the various CATs I've owned, I've learned to rely on his tube choices.

The CAT preamps are, have been, and will always be single-ended, not balanced. Stevens is adamant on this point. While recognizing the value of balanced designs for common-mode noise rejection, he feels that there are many other factors to consider, chief among them the complexity and cost of the additional circuitry required. As an alternative, he has elected to use higher-quality transformers coupled with an insistence on floating (ie, not grounding) equipment, especially the preamp. Such an arrangement, often called pseudo- or quasi-balanced, provides much of the benefit offered by fully balanced circuits, as described by Richard Marsh in the February 1988 issue of Audio.

Operation of the Signature is straightforward, even basic compared with that of other state-of-the-art products. There is no remote control. The volume control is stepped, although that perfect level is often somewhere between the available settings (especially with high-output sources such as the Theta processors). Like other designers (eg, Conrad-Johnson), Stevens feels volume pots are the source of too many problems, such as distortion due to resistance characteristics. On the positive side, there are: a stepped balance control; three inputs (two line-level, and a phono stage that accepts all but the very lowest-output moving-coils; cartridge loading is done externally with supplied RCA plugs); a tape loop; a mute switch; and two sets of outputs.

I used an extensive array of equipment for this review, including: Benz-Micro Ruby and Koetsu Pro IV cartridges; Versa Dynamics Model 1.0 'table/tonearm; Magnan Type Vi, XLO, and Cardas phono cables and interconnects; Theta Data transport; Altis, Aural Symphonics, and NBS digital cables; Mark Levinson No.30 DAC; Audio Research Classic 150, Metaxas Solitaire, and Sonic Frontiers SFS-80 amplifiers; Audio Research, XLO, and Cardas speaker cables; ProAc Response Three, Sonus Faber Amator, and Martin-Logan CLS IIZ speakers; the Kinergetics SW-800 subwoofer system; and dual Tice Power Blocks and Titans (the last were not used for the preamp itself).

Sound (purrrrr)
The Signature is the latest and greatest development in the never-ending evolution of tube circuitry. By that I mean that the CAT's essence was (and is) indeed the sound of vacuum tubes. The SL-1 has built upon the foundation of well-known tube circuit strengths: harmonic richness, midbass bloom, and expansive soundstaging. This latest version represents a significant refinement of the evolving splendor of tube design. All of the superlative strengths of good tube sound were present and accounted for. In addition, significant progress was evident in every area of notorious tube-design weakness: performance at both frequency extremes, reduction of noise, and increased resolution of inner detail.

The Signature had four superlative inner strengths woven into a tapestry of major strengths. The first of these, carried over from the Mk.II, was a stunning timbral realism embedded within a wonderfully flat and extended tonal balance. The second inner strength was a superb rendition of the subtlest dynamic shadings within a wonderfully wide overall dynamic voice. A third strength, which contributed mightily to the CAT's dynamic capability, was a noise floor significantly lower than that of previous units. (Both the improved dynamic range and lower noise floor are major improvements over the Mk.II.) The final inner strength was the magical ability to create breathtakingly lifelike three-dimensional images upon and within a thoroughly believable soundstage surrounded with air and space. CATs have had this wonderful spatial ability for a number of years; the Signature has taken it a step further.