Review: Convergent Audio SL-1 mkIII Tube preamp

Category: Preamps

This is a review/listening report of the Convergent Audio Technologies SL1 Signature Mk III tube preamplifier with an on-board phono section. This is a pre-owned unit that I own for 10 months now. Over these 10 months I have listened to many records & CDs using both the power amplifier section of my older Harmon/Kardon integrated amplifier and using my recently acquired Symphonic Line RG4 mono-block power amplifiers. I have tried to highlight what I think are the characteristics of the CAT SL1 through these listening test but before I commence with the review, a little about the remaining audio system & a bit about my listening room. WARNING: This review is long!

Listening room details:-

My audio room is located in my converted attic area & is quite asymmetrical in shape - the listening portion is 17’X19’ with a 9’ ceiling. There is also a knook which is 11’X6’ with an 8’ ceiling. So, speaker placement is quite important otherwise the left speaker might not have an interacting side wall whereas the right speaker will. I have experimented with speaker placement per the Cardas website but I found that the math did not work out for my room owing to the knook. I adapted his calculations for a modified speaker placement wherein the distance to the rear wall is less than the distance to the side wall but found that this did not work for me - soundstage was constricted tightly between the 2 speakers compared to my present position wherein they are separated 7.5’ center-center & 62” from the rear wall. I must also add that the vertical wall is really vertical for about 6’ & then it slopes towards the ceiling.

CAT SL1 Signature MkIII details:-

Convergent Audio Technologies is a small company founded & run by Ken Stevens in the Rochester, NY area. I’ve had a few lengthy conversations (each lasting 90-120 minutes) with Ken before I bought the CAT SL1 wherein I was trying to get some information on the design philosophy of the CAT preamp. The conversations were very enlightening & I realized that I was speaking to someone who had a background in electrical/RF/power electronics. Ken came across as a colourful & outspoken member of the audio community & he voiced his thoughts & opinion with some passion (screw diplomacy!).
The unit that I own is black in colour& it is a 2-chassis design. The total weight is somewhere in the 65-70lbs region & it is built like a tank. The power supply unit is much smaller than the preamp unit & houses 1 EI transfomer, large capacitors, some rectification electronics, a fuse & a power on switch. The unit is said to consume 1A of AC power which is provided by the stock power cord. I fiddled just a little bit with power cords & surprisingly the stock power has sounded the best so far. Ken informed me that toroidal transformers are great for solid-state electronics but wreak havoc with the sound when used with tube electronics hence the EI transformer. A small digression – I noticed that YBA & Audio Refinement also heavily use this type of transformers in their CDP, amps & preamps. The power supply is “hard-wired” to the preamp unit with a 6’ stiff umbilical type power cord. During the conversation, Ken said that he felt this was the best to ensure a good, solid power connection without giving the user a chance to un/intentionally mess things up. The preamp unit has 1 large PC board that has sockets for 10 vacuum tubes – 6 (Sovtek) 6922 & 4 (Electro-Harmonix) 12AX7s. Per Ken & Marty (the chief/lead engineer at CAT), the design is optimized (i.e. best sound) for Sovtek 6922 & E.I. 12AX7 – my combination is just a bit different. The PC board is very neatly laid out & there is no clutter in the layout of the electronics. Except for the tubes none of the other parts has any information about them regarding their value, their material type or polarity etc. I guess Ken is guarding his secret very carefully! Wire connections from the PCB to the RCA connectors are kept short using an internally designed & manufactured wire. I was informed that they used to use Cardas wire but changed from the Mk III version onwards. The unit has only female RCA connectors & allows just 2 line components + 1 tape deck + 1 turntable. On the reverse side of both the bottom & top plates there is some blue material (I don’t know its composition) that is used to dampen vibrations. I found this same material in the power supply section as well. I also found a heavy bottom plate under the PCB utilized for the same reason, I guess. Thus, most of the weight of the CAT SL1 comes from these heavy vibration dampening plates – in tube gear microphonics can be a serious issue. The front plate is very simple to the point of being austere – mechanical switches to select between line 1 & 2, another switch to select between line inputs & phono input, a 3rd switch for “tape”or “source” a 4th for “record” or “normal” & a 5th for “mute” or “operate”. There is a (bright) red LED that comes on after about 3 minutes after the unit powered up indicating that the unit is ready to play. There is one rotary knob each for balance & volume. The underside has 4 rubber feet, which Ken informed me, were sufficient to isolate the unit along with all that dampening material. He also told me, for best results, to plug the unit directly into the wall & not thru a power conditioner of any sort. The unit looks austere but I like its simple lines – it calls very attention to itself & this philosophy transcends into even its sound quality.

Listening Tests:-

I have a fairly wide palette of music – classic rock, soft rock, instrumental jazz, male & female jazz vocals, western classical, blues & an itsy-bitsy amount of opera music. I have played many tracks of both LPs & CDs to list here from all the above listed music genres & some of my favourites have been played several times.

The CAT phono preamp section is the best that I’ve heard so far. I must also admit that my experience in phono preamp sections is very limited. The other two that I can compare it to is the one from McCormack & the one in my HK integrated amp. The McCormack unit was connected in an all-McCormack rig using a DNA-125 & a ClearAudio Champion with a Benz Micro cartridge (model # forgotten!). The sound was very nice but it is my opinion that the CAT phono stage has a much fuller sound compared to the McCormack stage, which sounded thin. Compared to the HK phono stage, the CAT is light years ahead – the comparison is not even fair. The CAT phono stage is very, very hard to overdrive because the upper limit on input voltage is a whopping 200mV! I don’t think that they make any cartridge that will output that sort of (line stage) voltage. The CAT phono stage uses 5 vacuum tubes – 2 input 6922s, 2 gain12AX7s & 1 output 6922. It has 47dB of gain. The best part is that the output of the phono stage is routed into the input of the line stage, which has 26dB gain. So the total gain for the phono section is 73dB! This was done so that both MM & low output MC cartridges could be used. – the design is a compromise between dedicated MM & MC stages. However, do not let the word “compromise” fool you in any way – the phono stage is absolutely dynamite! So far it has provided me with an output that is smooth (no listening fatigue as I spun LP after LP), dynamic (the sound was “integrated” in that all the instruments were in their resp. places) & high in resolution (it picked out soft notes & the loud notes with equal ease & with the correct SPL). The CAT is a master at bass both in the phono & line stages – plenty of bass power making you think that this is surely a solid-state preamp, excellent bass extension going all the way down to 40Hz (my speaker limit), tight bass without the wooliness/fuzziness/defocus that some tube gear is known for & fast (without the bass lingering past is welcome point).

I won’t belabour the characteristics of the line stage because it has virtually the same qualities as that of the phono stage (remember that the phono output is fed into the line stage) & I say this after listening to many, many CDs. The CAT has the ability to let the listener hear whatever is fed to it without compression or alternation or colouration. It seems to fulfill Ken Stevens’ desire it being an accurate playback piece of electronics – it calls very little attention to itself. The CAT is also quite unforgiving in that if you play a bad recording, the CAT will not hide this fact. I think that it operates at such a high level of sophistication of sound that there are but a handful of preamps that rival it.

For completeness, since every coin has 2 sides, I have read, from previous CAT SL1 owners, that the Hovland HP100 preamp has better high frequency extension than the CAT – one owner likened the CAT sound to a photo taken at 12-noon whereas the Hovland sound was likened to a photo taken at 5pm. Unfortuantely, I have not heard the Hovland thus far & cannot comment on it.

Reliability-wise the CAT has proved to be great so far. I had read reports (on Audioreview.com) from previous owners that the Mk I version “ate” tubes. This is certainly no the case (so far!) for the Mk III version. The previous owner had replaced the tubes before selling & I have not needed to mess with them so far.

In conclusion, I’m a delighted owner of a CAT SL1 Signature Mk III preamp & there is very little that I can fault the CAT SL1 for sound-wise. When it plays music, I get insight (I can hear the texture of the music better than almost any other preamp), I get better transparency, better articulation & better finesse throughout the audio spectrum, I get better neutrality (instruments & vocals represented more life-like than virtually any other preamp) & I get to sit in the 1st row of a concert hall/theater/stadium & enjoy the artists perform in front of me.

Associated gear
The associated equipment:-

· Music Hall MMF5 TT with the stock Goldring 1012GX MM cartridge.
· Harmon/Kardon HD7525 CDP - this is my trusty-rusty CDP which has an 18-bit built-in DAC, analog & digital outputs (of which I’m using the analog L&R outputs) & a captive power cord. I have DH (large) Cones under the CDP chassis. Even though the unit is 7 yrs old, I feel that it has great sound. So far I have not been tempted to upgrade it.
· CAT SL1 Signature Mk III tube preamplifier – subject of this review.
· Symphonic Line RG4 Mk III mono-block power amplifiers – reviewed on this site in the amplifier (SS) category.
· B&W DM 604S2 3-way, 4-driver floor standing speakers in black ash, which was the only colour available when I bought them (much to my wife’s dismay!). Speakers are on 3 brass spikes each.
· Interconnects & cables used: 1m Groneberg TS Premium between TT & pre, pre & power. Tara Labs RSC Master Gen II between CDP & pre. Tara Labs Prism biwire speaker cables. In my opinion, the B&Ws are known to be on the bright side so I have tried to limit the brightness by avoiding silver speaker cables. ESP Essence power cords, for the power amps, connected to the Juice Bar & a Silver Audio Power Burst connecting the PS Audio Juice Bar to the wall outlet.
· Lovan Soverign 2+2 audio rack.
· 1 Richard Gray Power Co. 400S into which the CDP & TT are plugged & 1 PS Audio Juice Bar into which the pre & power are plugged. Note that the Juice Bar does not have any filtering of any sort – just like I want it.

Similar products
I’m comparing the CAT SL1 Sig. to other preamps such as the preamp section in my HK integrated, the McCormack RLD1, the Audio Refinement Pre, Theta Digital Cassanova (the smaller unit between Cassanova & Casablanca. If I wrote this incorrectly please read it as the smaller of the 2), B&K Pre 30, Krell preamp (forget the model # but this unit was connected to a Krell SACD player, Krell FPB 600 & Martin Logan Statement speakers), CJ PV14, the latest Lexicon preamp-processor & the stock Rogue 66.