Convergent Audio Technology SL-1 preamplifier Robert Deutsch 1995
To compare with the BAT VK-5, I had on hand two preamps considered to be among the best: the Sonic Frontiers SFL-2 and the Convergent Audio Technology SL-1 Signature (footnote 1). The Signature, Stereophile's 1993 Product of the Year, has served for three years as the primary reference preamp in my system; Jack English and Jonathan Scull also use it in this reference capacity. My unit has been updated twice, most recently when I was about halfway though the listening for the BAT reviews. The modifications in the latest update are fairly substantial: additional noise filtering in the power supply; provision of a new, much heavier bottom plate; a new power-supply-to-preamp cable; and some tweaks about which Ken Stevens (Mr. CAT) declines to go into detail.
The effects of the update (footnote 2) were quite significant: the SL-1 Signature is now quieter (both phono and line-stage), and has even more of a "see-through" quality. CAT owners are strongly urged to get their unit updated.
Because the CAT SL-1 Signature is a single-ended design, comparison with the BAT VK-5 was not completely straightforward. Although the VK-5 can be used balanced or unbalanced, the balanced aspect is such a major part of the design that it wouldn't seem fair to use it in the single-ended mode; ie, with a single-ended connection between the VK-5 and the power amplifier (in this case, the VK-60). But should the BAT have the potential advantage of using the Sonic Frontiers SFD-2 Mk.II's balanced outputs? To keep the playing field level, I decided to make the comparisons using the SFD-2 Mk.II's unbalanced output (footnote 3).
So, how does the BAT VK-5 compare with the improved version of what Jack English in his original review (Vol.15 No.12) called "the finest preamplifier available today?"
The VK-5 and the SL-1 Signature differ in many design aspects, but their sonic similarities are much greater than their differences. In fact, I had a great deal of difficulty getting a handle on exactly how they differed. Several times, just as I thought I had identified a difference, switching back to the other preamp (at matched volume) made me doubt my initial conclusion. Both preamps had the kind of open, airy sound that invites listening. Both preamps were superb in the communication of dynamics, macro and micro. Harmonic accuracy was excellent with both. On some material, the VK-5's midbass was very slightly fuller, which gave string bass more weight and presence. (That was in the context of my system, in my room. In some other systems and other rooms, the slightly leaner presentation of the CAT might well be preferable.)
With both preamps the soundstage was supremely wide and deep, extending, with the right recordings, way past the listening-room walls. At times, I felt that the VK-5 presented voices as being somewhat more up-front, sibilants being a touch more prominent—or maybe the SL-1 Signature was a bit laid-back, softening sibilants.
Overall, the results were just too close to call. CAT owners don't have to start thinking about placing a "For Sale" ad in "Audiomart," but anyone considering the purchase of a preamp in this price range—especially those who need only a line-stage and are attracted by the balanced feature—must listen to the BAT VK-5. It's that good.—Robert Deutsch
Footnote 1: For these comparisons, voltages at the amplifier output were set to be within ±0.1dB. I still think that, except in the case of instantaneous comparisons, precise level-matching is less important than some people claim—if judgments of sound quality are primarily determined by level, then anyone wanting to improve the sound of his/her system could just turn up the level by a dB or so—but the VK-5's continuous volume control allowed precise level-matching, so why not?—Robert Deutsch
Footnote 2: My CAT was also supplied with new tubes. The old tubes had only about six months' use, with no audible indication of aging—but, obviously, I can't be absolutely sure about the role of the update vs tube replacement in producing the sonic differences.—Robert Deutsch
Footnote 3: The SL-1 Signature was used without a power-line conditioner, as per Convergent Audio Technology's recommendation.—Robert Deutsch