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Equipment Review
June 2007

Convergent Audio Technology SL1 Ultimate Mk 2 Preamplifier

by Marc Mickelson

"...a mature, established product of uncommon sonic sophistication."

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Review Summary
Sound "The SL1 Ultimate Mk 2 excels at two seemingly disparate things: explicating the internal life of music -- the fine details, texture and micro shifts in volume that are often the hardest things to get right -- and capturing the essence of large-scale music, its rhythmic flow and grandeur." "There is no spotlighting of detail, just a unified presentation in which the musical elements are all accounted for, not obscured or emphasized." "Through the midrange, the CAT preamp is beautifully balanced -- neither tart nor sweet," and bass is "supple, agile."
Features "The SL1 Ultimate Mk 2, like its forebears, has all-tube audio circuitry and a solid-state power supply with a built-in isolation transformer." It "uses the same five tubes in its line and optional phono stages: a matched pair of 6922s, a single 6922, and a matched pair of 12AX7s." "The SL1 Ultimate Mk 2 is a single-ended-only preamp. Ken believes that a properly designed and implemented single-ended circuit can sound better than a balanced circuit because there are fewer parts."
Use "The SL1 Ultimate has high voltage gain, 26dB, which can cause very audible hiss when it's paired with an amplifier that also has high voltage gain and a sensitive speaker." "Tubes were a particular bugaboo for CAT preamps of yore, but the tubes that Ken is using now seem to be very quiet and stable."
Value "If you can live with its foibles, you'll save yourself thousands of dollars on the way to owning the heart of a first-class audio system."

Better known by its acronym -- CAT -- Convergent Audio Technology is fast approaching an improbable two and a half decades in business. Why "improbable"? For more than the first half of its existence, CAT manufactured only one product, the SL1 preamp, and it seemed to be in constant flux. There have been Reference, Signature and now Ultimate versions of the SL1, with Mk 1, 2 and 3 tacked on for good measure. About a decade ago, CAT began producing amplifiers that followed a similar pattern, with standard and identical-looking Signature versions that often caused confusion in consumers' minds. CAT is also the only audio manufacturer of any appreciable size without an Internet presence. "People used to tell me I was dumb," CAT president Ken Stevens told me regarding his company's nonexistent website. "Now I'm so dumb I'm cool."

Despite an ever-changing product line and an unconventional marketing strategy, CAT survived, and thrived, in the volatile world of high-end audio for one important reason: Its preamps and amps had a reputation for sounding terrific, and audiophiles will tolerate a lot if terrific sound is the end product. Shortly after I purchased a brand-new SL1 Signature Mk 1 preamp back in 1995, the Mk 2 was available. I had what seemed like endless problems with tubes spitting and sputtering, and I had to find just the right amp to mate with it (more on this later), but I actually contemplated having the upgrades done to my SL1, something the technicians at CAT do only during the slow summer months, because I loved the preamp's sound. Even after selling my preamp, I followed what was happening at CAT, always thinking that someday I'd buy another SL1 -- upgrades be damned!

Although the company is not named after him, Ken Stevens is Convergent Audio Technology. He's the driving creative force, and the person to blame for the mercurial CAT product line. Stevens earned two degrees at the University of Rochester and has worked as an electrical engineer ever since. In the mid-1980s, he transitioned out of the corporate world and into high-end audio, which had always been an interest. After creating a few non-commercial products, including a 20-tube monster preamp, Stevens finally settled on the initial design of the SL1. He sold his first preamps directly to consumers, who spread its acclaim by word of mouth. Reviews in The Abso!ute Sound and Stereophile then put CAT on the audio map, and dealers started to call. Along the way, Keith Jarrett bought an SL1 (Ken trades him new tubes for CDs every few years) and so did over 5000 tube-lovin' audiophiles.

The SL1 Ultimate Mk 2 is a two-piece preamp -- main chassis and separate power supply -- with an attached umbilical connecting things. This configuration, along with the preamp's 64-pound weight, makes it a difficult piece to place on your equipment rack. The main chassis (19"W x 5 1/4"H x 12 1/4"D) and power supply (12 1/4"W x 4"H x 6"D) are too wide to fit on a single shelf, so you'll either need to allot two shelves or put the power supply on the floor, which is what I did. The umbilical is long enough to allow this, even if the main chassis is on the top shelf of most racks.

The SL1 Ultimate Mk 2, like its forebears, has all-tube audio circuitry and a solid-state power supply with a built-in isolation transformer. The audio circuit and transformer are Ken Stevens designs, and Ken even makes his own nonelectrolytic capacitors. Why go to such trouble when there are many sources for capacitors? "I want caps that sound like water," Ken told me, and only his caps do. He has encyclopedic knowledge of different passive parts; he can tell you how different caps and resistors sound, and why he has adopted or rejected each. "I want my products to sound utterly neutral," he says, and presumably some parts cut it and others don't.

The SL1 Ultimate Mk 2 is substantial -- both the main unit and power supply feel like solid masses. This is due to their steel chassis and the liberal use of what Ken Stevens calls a "resistive damping compound" internally -- the blue stuff you'll see adhered to much of the main chassis's internal surface area. The steel and damping compound were chosen because they control resonance, something CAT takes to heroic lengths. Circuit boards are made of G200, a material that's 12 times more expensive than the substrate used for many of the circuit boards in other high-end products. The SL1 Ultimate Mk 2 uses the same five tubes in its line and optional phono stages: a matched pair of 6922s, a single 6922, and a matched pair of 12AX7s. As I've mentioned, tubes were a particular bugaboo for CAT preamps of yore, but the tubes that Ken is using now seem to be very quiet and stable. I didn't hear a discouraging sound from the SL1 Ultimate Mk 2 while it was here.

The SL1 Ultimate Mk 2 has no multi-pole input selector. Ken prefers to use high-grade toggle switches to choose among the four inputs (two line, one phono or line, and one tape) because they sound better. In this spirit, the balance control is a series of resistors that provide 0.5dB steps over a 5dB range. "Customers often think the balance control doesn't work," says Ken. This is due to those half-dB steps, which can be hard to detect. The volume control is a stepped attenuator that's manufactured in-house. A Mute switch finishes off the front-panel niceties, although Ken prefers to switch inputs to mute the sound -- a good idea. Cardas RCA jacks are used throughout -- the SL1 Ultimate Mk 2 is a single-ended-only preamp. Ken believes that a properly designed and implemented single-ended circuit can sound better than a balanced circuit because there are fewer parts -- emphasis on "properly designed and implemented," of course.

My SL1 Signature Mk 1 cost $4950 USD when I bought it in 1995, and it had held steady at that price since 1990. A new Ultimate Mk 2 costs $6950 today. I don't know about you, but a $2000 jump in price over the course of 17 years seems like peanuts to me, especially in light of the circuit advancements that have surely been made. The all-tube MC/MM phono stage adds $1000 to the price, and many CAT owners consider it the Cracker Jack prize. Some digital-only customers will order SL1s with the phono stage because it increases resale value. Another option is a special CD-taper volume control specifically for those of us who don't play LPs. This overcomes the high output of some CD players by having less overall gain than the standard volume control. Your final option is a satiny silver or black faceplate.

Setting up

The main chassis of the SL1 Ultimate Mk 2 took a middle shelf of a Silent Running Audio Craz 4 Reference rack and was surrounded by tube and solid-state products of equally high pedigree. Digital gear consisted of an Audio Research Reference CD7 CD player, an Ayre C-5xe universal player, Zanden 5000S and 2000P DAC and CD transport, and Esoteric P-03 and D-03 DAC and CD/SACD transport. Amplifiers were Lamm ML2.1 and M1.2 Reference monoblocks along with Conrad-Johnson Premier 350 and CAT JL2 Signature Mk 2 stereo amps. A balanced-only Audio Research Reference 110 stereo amp had to sit it out with the single-ended-only CAT preamp. The Lamm amps rested on Silent Running Audio Ohio Class XL Plus2 platforms, while the digital gear and preamps had shelves of their own on the SRA rack, except for the Zanden separates, which sound best on a pair of Harmonic Resolution Systems platforms.

Speakers were Wilson Audio Sophia 2s, WATT/Puppy 8s and MAXX 2s, with a pair of Verity Audio Lohengrin IIs arriving as I was working on this review. Interconnects and speaker cables alternated between Shunyata Research Antares Helix and Orion Helix, and Crystal Cable Ultra. Power cords were Shunyata Research Anaconda Helix and Python Helix in both Vx and Alpha variations, and Essential Sound Products The Essence Reference.

Because the SL1 Ultimate Mk 2 has its own isolation transformer in its power supply, Ken Stevens cautions against the use of power conditioners, yet he does recommend certain brands of power cords, including those from Essential Sound Products, whose The Essence Reference sounded so good with the SL1 Ultimate Mk 2 that I used it for most of my listening, even with pricier cords here for the picking. A Shunyata Research Hydra V-Ray or ESP The Essence Reference power distributor cleansed AC from the wall for everything but the CAT preamp and amp, which went straight into the wall per Ken's decree.

Like the VTL TL-7.5 I reviewed a while back, the SL1 Ultimate has rather high voltage gain -- 26dB. Voltage gain is an indication of how much an amp or preamp is amplifying its own noise, and when tubes are employed, there is generally some audible noise. Voltage gain is expressed as a number that represents a multiplied value (20, as in 20 times) or in decibels. A certain voltage-gain figure isn't inherently good or bad, but high voltage gain from both a preamp and amplifier can create background hiss, especially if your speakers are sensitive.

Of the amps I had on hand, only the Lamm ML2.1 and CAT JL2 Signature Mk 2 were suitable for use with the SL1 Ultimate Mk 2. The Lamm M1.2 Reference, with its 32dB of voltage gain, produced noticeable hiss with the CAT preamp. When he visited to install his products, Ken Stevens remarked that the hiss produced with the SL1 Ultimate Mk 2 and Conrad-Johnson Premier 350, which has almost 35dB of voltage gain, was "not too bad." It would have been completely unacceptable to me. As a guesstimate, if your amp has 25dB of voltage gain or less and your speakers are 90dB sensitive or lower, you should be OK with the SL1 Ultimate Mk 2. As gain and sensitivity rise, however, the outcome will be based on your tolerance for background hiss.

If you had asked me how essential remote control was when I bought my CAT preamp a decade ago, I would have said "Not at all." There were few remote-controlled preamps on the market then, and the CAT wasn't one of them. It has stayed that way, mainly because Ken Stevens can't build the preamp he wants and incorporate remote control. I was cured of remote-control apathy -- first by the VTL TL-7.5 and then the Audio Research Reference 3. I now value being able to adjust volume and especially switch between CD players I'm comparing from my listening chair.

Even so, I found the transition to the SL1 Ultimate Mk 2 to be painless. While its volume control can be a little coarse with sensitive speakers like the MAXX 2s and Lohengrin IIs, not always allowing for ideal adjustment of the music's loudness, I found it easy to set it and forget it instead of using the remote to adjust the volume on a whim. I can easily understand and appreciate the allure of remote control, but sound quality comes first, and it's the thing you'll notice first about the SL1 Ultimate Mk 2.

Ultimate listening machine

I am by no means a car enthusiast, but I imagine that listening to the SL1 Ultimate Mk 2 is rather like driving a hot new sports car on a highway where you can open it up. The sound of the CAT preamp roars with sonic horsepower when the music requires it, yet it purrs like a well-tuned Porsche at a normal cruising speed. You know you're using something extraordinary, and you want to relish every minute with such a high-performance creation.

OK, enough waxing metaphorical. In sonic terms, the SL1 Ultimate Mk 2 excels at two seemingly disparate things: explicating the internal life of music -- the fine details, texture and micro shifts in volume that are often the hardest things to get right -- and capturing the essence of large-scale music, its rhythmic flow and grandeur. As you might be able to guess, the SL1 Ultimate Mk 2 doesn't favor one type of music over another. Acoustic or electrified, solo instrument or full orchestra, the CAT preamp conveys the music's character with exacting realism, but it never sounds clinical in the process. The SL1 Ultimate Mk 2's high fidelity and high refinement combine to produce a very enticing sonic outcome.

Two jazz labels -- ECM and MaxJazz -- have house sounds that are very different. ECM recordings emphasize space over the musicians' bodily presence, while MaxJazz albums sound more up front and physical. I own many CDs from both and have grown to appreciate their distinctly different sonics, probably because the music is always interesting. The SL1 Ultimate Mk 2 easily captured the airy ambience on Goodbye [ECM 1904] from pianist Bobo Stenson, bassist Anders Jormin and drummer Paul Motian, and the sprightly presence of Bruce Barth's Live at the Village Vanguard [MaxJazz MXJ205], which features Barth on piano, Ugonna Okegwo on bass and Al Foster on drums. The SL1 Ultimate Mk 2 conveyed the differences in sound in a way that never unduly highlighted them or undermined the differences in the music itself -- progressive jazz for Goodbye versus a more classic approach on Live at the Village Vanguard. This is what neutrality is about -- passing along and not revising -- and the CAT preamp does it as a matter of course. And when I tossed on something big, like the frenzied multi-piece ensemble on Don Byron's Bug Music [Nonesuch 79438-2], the SL1 Ultimate Mk 2 unraveled its essence without stress or strain. "Just right it is," as Goldilocks would say with some help from Yoda.

Music comes alive because of the abundant small gestures embedded in it, and the CAT preamp does a fine job of not only exposing these but also keeping them in context. There was no spotlighting of detail, just a unified presentation in which the musical elements were all accounted for, not obscured or emphasized. While slogging through the dollar bin at a local used-CD store, I found a copy of the soundtrack to the movie Love at Large [Virgin 2-91359], an intricate and dusky Alan Rudolph film. On it there are versions of the movie's main theme played on solo guitar and solo harp. Steve Martin used to say that you can't play a sad song on a banjo; well, you can't make ugly music with a harp. Recorded harp always sounds luminous and beautiful, but the pluck of the strings is often lost, tangled with the decay of the notes. Live harp has distinct attack and decay, and on the solo-harp piece from the Love at Large soundtrack, the SL1 Ultimate Mk 2 conveyed this perfectly along with the instrument's inherent beauty. Audio minutiae, yes, but also important to reproducing a complex instrument properly.

The SL1 Ultimate Mk 2's tubes are obvious only if you're used to the rigid, angular sound of some solid-state preamps, which is to say that it lacks tubey fluff and mush. Transients like the drum strikes near the beginning of "America" from the Bill Charlap Trio's Somewhere: The Songs of Leonard Bernstein [Blue Note 94807] were literally startling because of their quick concussion and piercing snap. "Whoa! That's a transient," say my listening notes.

Through the midrange, the CAT preamp is beautifully balanced -- neither tart nor sweet. When Lucinda Williams sings on West [Lost Highway 0602498583487], the slight dryness of the recording and characteristic nasality of her voice are intact. Extra midrange fullness can be pleasing with some recordings, but it is a deviation from neutrality and not Ken Stevens' thing. The SL1 Ultimate Mk 2 remains honest, but never displays the ashen colorlessness or brittle harmonics that are signs of brutal honesty.

While you don't get the whip-crack force of the bass from Einstein's The Tube preamp with the SL1 Ultimate Mk 2, you do get some of the most supple, agile low frequencies from any preamp, tube or solid state, that I've heard. Musical detail is abundant. "Jump" from the Bill Charlap Trio CD first made me think that the piano was going lower than I had ever heard recorded, until the SL1 Ultimate Mk 2 revealed what was really going on: bassist Peter Washington was plucking a string just as Charlap was hitting a left-hand note. There was plenty of low-frequency ambience spreading throughout my listening room, which made the differentiation of the piano and bass all the more remarkable.

Though it's what we reviewers do, I feel somewhat sheepish about breaking the SL1 Ultimate Mk 2 into its constituent sonic parts, so integrated is its sound. It is a product whose sonic performance is above any reasonable criticism, even as one preamp or another may do one thing or another better. It's like an Olympic decathlete: very good at every event and exceptional at a few. When you consider its brick-like build quality and fair price in relation to that of the cream-of-crop preamps against which it competes, the SL1 Ultimate Mk 2 is an audio champion.

Reference 3 vs. SL-1 Ultimate Mk 2

The Audio Research Reference 3 ($9995) may be at the apex of its popularity right now, two and a half years after its introduction. While no one component can claim critical and consumer dominance, the Reference 3 is certainly among a handful of line-stage preamps that can assert a primary position. It has received rave reviews all over the world, including my own here on SoundStage!, after which I bought the review sample. It's a "Reference" in my listening room to be sure.

Outwardly, the CAT SL1 Ultimate Mk 2 and ARC Reference 3 are diametrically different preamps, even though both are tube designs. The SL1 Ultimate Mk 2 is a two-piece preamp with its balance control and tape monitor being the only user-convenience features. The Reference 3 is a single-box, remote-controlled preamp with a large, interesting-looking vacuum-fluorescent display on its face. The CAT preamp is heavy and placement takes some consideration, while the ARC preamp is large but easy to place.

All of that is trivial compared to how these two preamps sound. However, there is an issue regarding direct sonic comparison: The SL1 Ultimate Mk 2 doesn't do balanced, which is the way in which the Reference 3 sounds its very best. Luckily, the Reference 3 does have single-ended inputs and outputs, and it sounds nearly as billowy and dynamically fluid single ended as it does balanced. It would have been impossible to disregard how much better a digital source like the ARC Reference CD7 or Ayre C-5xe sounds balanced, so I did this Macintosh-to-Granny Smith preamp comparison with the single-ended-only Zanden 5000S DAC connected to its matching 2000P transport, which together remain my favorite digital source, especially after some upgrades that I'll write about soon.

I praised the Reference 3 for its "dynamic coherence," its ability to go from soft to loud "with proper gradation" and without changing its essential nature. I also noted the state-of-the-art width of its soundstage, likening it to seeing the Grand Canyon, which commands your view in a way that even the best pictures of it simply cannot capture. All of this remains valid as I include the SL1 Ultimate Mk 2 in the discussion. It ramps up dynamically just as well as the Reference 3, which still has the slightly wider soundstage. More audio minutiae. Where the sound of these two preamps has its greatest divergence is in terms of the overall weight of the presentation and tonal saturation, and here the CAT preamp has the slightest edge. Throughout its range, the SL1 Ultimate Mk 2 presents instrumental lines with greater heft and a tonal palette that seems to have a few more colors. This is not a great difference, mind you, but it does make for music that charms more readily.

Both preamps avoid a soft or tubey presentation, both have powerful low frequencies, both present detail in a musically consonant way, and both are made by companies well known for their preamps. Which one is better for your system? There are many "ifs" to consider. If you don't need balanced inputs or outputs, and if you can live without remote control, and if you have an appropriate amplifier and speakers, the SL-1 Ultimate Mk 2 will be among the finest preamps you can buy, and it will likely cost less than any of its rivals. With the Zanden DAC and transport as a source, the CAT JL2 Signature Mk 2 amp and Verity Audio Lohengrin II speakers, it made unforgettable music. If you want to hear the most from your balanced CD player or DAC, or you have a high-gain amp and sensitive speakers, look to the Reference 3.

End notes

The CAT SL1 Ultimate Mk 2, perhaps this model's tenth official version, is a knockout line-stage preamp. Though its user features, if you can call them that, are throwback, the preamp's sound is thoroughly modern -- and complete. It does the big and little things with equal acuity, sounding forever real in the process. Space, bass detail, attack and decay -- the CAT preamp captured and conveyed it all amidst a corporeality that never drenched the music in excessive tonal color or squeezed the dynamic life out of it. What I appreciated most, though, was that none of these traits dominated, so the presentation lived up to Ken Stevens' goal of utter neutrality.

Once an underground darling, the latest CAT SL1 is a mature, established product of uncommon sonic sophistication. If you can live with its foibles, you'll save yourself thousands of dollars on the way to owning the heart of a first-class audio system. It's proof that the highest performance doesn't have to cost the highest price.

...Marc Mickelson

Convergent Audio Technology SL1 Ultimate Mk 2 Preamplifier
$6950 USD; phono stage adds $1000.
Warranty: Three years parts and labor.

Convergent Audio Technology
85 High Tech Drive
Rush, NY 14543
Phone: (585) 359-2700

E-mail: catamps@aol.com

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