Convergent Audio Technology SL-1 preamplifier Robert Deutsch 1996

Robert Deutsch auditioned the SL-1 Signature mk.II in December 1996 (Vol.19 No.12):

Conventional audio marketing wisdom has it that a company producing only a single product cannot succeed in the marketplace. Dealers want a line of products, varying in price and features, so that they can offer something for every potential customer. It's hard to build much of a following if you have only a single product, and an expensive one at that.

That is, unless your name is Ken Stevens and your product is the Convergent Audio Technology SL-1. Since its introduction in 1985, the preamplifier generally referred to as "the CAT" has established itself as the reference preamp in high-end audio. When Jack English reviewed it back in December 1992 (Stereophile, Vol.15 No.12, p.152), he was so enthused that he called it "the finest preamplifier available today"—inviting objections from people who pointed out that it's impossible to prove this statement unless one has actually heard every single preamplifier currently on the market.

Be that as it may, the CAT has the status of being owned by more audio reviewers than any other preamplifier. Ken Stevens has been content to keep his company small, concentrating on implementing evolutionary improvements to the design (footnote 1). Some of the changes/modifications were deemed too minor to justify a change in model designation; the formal model changes were in 1987 (SL-1 Reference), 1989 (SL-1 Reference Mk.II), 1991 (SL-1 Signature), and late 1995 (SL-1 Signature Mk.II). I bought my SL-1 Signature in 1993 and have had it updated twice since then, to the latest to Mk.II standards.

What is it? The CAT SL-1 Signature Mk.II falls into what has become a somewhat rare product category: a tube preamplifier with built-in phono section. In this digital age, most high-end preamps are line-only, the phono section being a separate—usually expensive—component. Although the CAT is not inexpensive, a top line-stage preamp plus separate phono section from any of its leading competitors would end up costing more. The CAT's phono section is integral rather than being an add-on, and has sufficient gain for low-output (but not super-low-output) moving-coils. I use an AudioQuest AQ7000nsx, which has an output of 0.3mV, and while I wouldn't describe the combination as dead quiet, the level of noise is low enough not to be intrusive.

Bucking another current trend, the CAT is single-ended rather than balanced. Ken Stevens doesn't believe that balanced circuits are inherently superior, but he admits to having built a few balanced-output versions of the CAT on special order. The power supply is in a separate box, connected to the preamp proper with a hard-wired, very stiff cable. The choice of cable and the decision to hard-wire rather than use the more convenient plug-in connectors were made on sonic grounds.

Features are minimalist, but not unduly so. There are two line-level inputs, one phono input, two main outputs, and a set of tape inputs/outputs. The Mute switch works by putting a short across the output rather than shorting the input. There's a single volume control and a balance control. Volume and balance adjustment are stepped, a design that uses discrete resistors rather than the more common—and more distortion-prone—potentiometer. The steps are too big to allow for matched-level comparisons, but this is not a problem except for reviewers. (I deal with this problem by the over/under "bracketing" of levels, and also by using other reference preamps when I want to match volume to ±0.1dB.)

In the early days, the CAT had a reputation for somewhat suspect reliability. These concerns faded when Ken Stevens established his own production facility rather than contracting out. In the three years I've had my unit it has been trouble-free, except for the bulb that illuminates the Power button, which has burned out twice.

What's changed? The SL-1 Signature Mk.II retails for $5950—a $1000 increase over the price of the SL-1 Signature. One would think there'd be some fairly substantial changes in materials and construction to justify such a price hike, and indeed there are. Here's the list of changes—courtesy Ken Stevens—that distinguish the latest SL-Signature Mk.II from the SL-1 Signature:
• Improved circuit-board construction: low-loss circuit-board material; thicker board; capacitors mounted with damping compound to reduce vibration
• Stronger main chassis: rigid beam welded inside bottom cover; thicker metal; U-shaped bracket for mounting circuit board (was L-shaped)
• New audio wire
• Visco-elastic shock mounts used to isolate transformer vibrations
• New power-supply cable is shielded and constructed with better wire and insulation
• New power-supply board: 10 times more audio supply capacitance; filter on audio supply to reduce input noise; filter for main regulator; buffer transistor greatly reduces current draw from the voltage reference (in the main preamp); nearly double capacitance in the three filament supplies; filtering of noise from filament regulators
• Thicker faceplate, without rackmount slots

Generally, these changes can be described as affecting the power supply, reducing noise, and improving the mechanical construction. What has remained the same is the basic circuitry, including the complement of tubes. Ken Stevens says that he's always evaluating various mods/tweaks, incorporating them into the design only when he's sure that the result is an improvement rather than just a change in sonic flavoring.

How does it sound? The SL-1 Signature I bought in 1993 was a superb-sounding preamp, and I had no reason to think that it needed improving. As I haven't had both older and newer units on hand at the same time, my comments are not based on A/B comparisons, but I have no hesitation in describing the changes brought about by the updates as significant improvements.

Most noticeable was the reduction of noise, especially when playing LPs. Music emerged from a quieter background, with a greater "suddenness" of transients. As there has been no change in the phono section circuitry, this effect is probably due to the improvements in the power supply. The line section was pretty quiet to begin with; the improvement here, while worthwhile, was less noticeable.

One of the things that originally attracted me to the SL-1 Signature was that it had the best aspects of "tube" sound—smoothness, harmonic accuracy, three-dimensionality—but without the rolloff at the frequency extremes that characterizes most tube designs. With the change to Mk.II status, both bass and treble have been improved: bass seemed tighter, better defined; treble had even greater sparkle, and a more airy quality. Never a slowpoke in matters of rhythm and pace, the latest SL-1 Signature Mk. II was even quicker.

Should I upgrade? If your CAT is an SL-1 Signature, and if there isn't something else in your system that more desperately needs attention, then by all means get in touch with Convergent Audio Technology to arrange for an update to Mk.II status. The cost is $695 or $995, depending on the serial number of your unit. (Some interim changes were incorporated into late SL-1 Signatures.) If you own a different preamp, one that you have reason to believe is not giving you the performance you want, then you should definitely listen to the current CAT. It very well might be "the finest preamplifier available today."—Robert Deutsch

Footnote 1: Convergent Audio Technology's JL-1 power amplifier is now in limited production.—Robert Deutsch

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