April 2008Audio Space Reference Two Preamplifier
by Ken Choi
Even though I usually favor a simple tumbler of house plonk, I enjoy reading the occasional wine review. Unlike audio reviews, wine reviews are near-ubiquitous, appearing as they do in the weekend newspapers and even on the all-news radio station I listen to during my daily commute. Like audio reviews, an elaborate, descriptive vocabulary has evolved that allows the reader to savor secondhand the subject under review assuming, in this instance, that his palate is intimate with the flavors of gooseberry and buttery oak. Interestingly, while they probably exist, Ive not seen reviews of bottled waters, despite the wide choices available, with some of these in "fine" restaurants and costing more than the wine I usually order. Variously sourced or processed, waters clearly differ in taste, but putting these differences into words would present an onerous task, at least to me.
In theory, the preamplifier is audios drinking water, but some are more like wine. The preamp ideally serves as a totally transparent conduit, allowing the source signals to pass largely unadulterated on their way to the amp-speaker unit. But in practice we all know that a preamp can exert subtle or not-so-subtle and pervasive effects on the systems overall sound, which is why so many are available using diverse technologies to provide the critical first stage of gain.
The $9900 USD Reference Two is Hong Kong-based Audio Spaces entry into the ultra-high-end preamplifier sweepstakes. Released at the beginning of 2007, it is the second product in Audio Spaces Reference series heralded by the introduction of the Reference One monoblock power amplifiers, which I reviewed earlier. Built and voiced to standards aimed at competing in the upper end of the worldwide marketplace, the Reference series has now grown to include the Reference Three mono amplifiers and the Reference 3.1 integrated amplifier, both of which offer the user a choice of output tubes.
The Reference Two is a full-function tube preamp that features preservation of the signal in a fully balanced fashion from input to output. A pair of 6N9P high-gain octal tubes do duty in an input/driver stage, and a pair of 300B tubes per channel serves in an output stage. Another pair of 6N9Ps act as cathode followers, allowing for driving long cable runs and solid-state amplifiers with low input impedance. The phono stage, which is not balanced, uses a pair of more pedestrian 12AX7s for moving-magnet gain. Instead of requiring step-up transformers, moving-coil signals are processed by 6922s in a gain stage that features adjustable input impedance. Another 6922 is used as an output buffer to maintain signal strength. As with the Reference One amplifiers, all tubes are culled from the cream of Shuguangs crop. The Reference Two is the worlds only truly balanced, 300B-based line driver.
The preamps output impedance is less than 2000 ohms. The line stage provides 18dB of gain, and the moving-magnet phono stage 37dB. Moving-coil gain is an additional 20dB, for a total of 75dB. Two additional 1.5dB steps of moving-coil gain are user selectable via a front-panel knob. A front-panel knob also selects loading for moving-coil cartridges.
The Reference Two is a top-shelf component in the literal sense due to its size (16 3/4"W W x 10"D × 9"H) and semblance to a power amplifier. Its 46-pound weight is largely accounted for by a massive onboard power supply that is subject to solid-state rectification. The Reference Two shares the same dual-layer stainless-steel chassis construction that the Reference One amps use. It also features similar dense, aluminum side rails with integral spiked footers that serve more here to quell vibrations than to dissipate heat. Originally designed "floating sockets" for the 300B tubes make use of a proprietary rubber compound to further absorb vibrations and reduce the microphonics that these tubes are notoriously prone to.
A velvety manual volume control based on a four-channel ALPS potentiometer takes center stage on the front panel. In addition to the phono input, the Reference Two accepts two line-level inputs, with one of these being single-ended only. When asked about the relative paucity of inputs, the affable Canadian distributor of Audio Space products, Bernard Li, explained that one set of fully balanced line inputs occupies four times the termination sectors of a mechanical input selector that single-ended inputs would require. Audio Space decided that they would stay with a mechanical selector rather than employ an electronic input selector that could accommodate more inputs but potentially degrade the sound. The Reference Two does provide both single-ended and balanced outputs to the amp. Additional features include a rumble-cut switch for the phono stage and a phase-inversion switch at the output.
The Reference Two was used in place of my longstanding reference, the Convergent Audio Technology SL-1 Ultimate Mk 1. It took signals from an original EMM Labs CDSD and DCC2 as well as an Oracle Delphi V turntable with SME V tonearm and Koetsu Onyx cartridge. I used the Reference Two with Nagra VPA tube monoblocks, a solid-state Audio Research 100.2 stereo amp, and Audio Spaces own Reference One monoblocks. Speakers were Wilson Audio Watt/Puppy 7s.
All amps were driven via six meters of Purist Audio Designs Venustas interconnect. Due to logistical problems (i.e., a lack of appropriate cabling) I could not run my system fully balanced. However, there were no problems driving any of the amps through the six-meter interconnects. The Reference Two was powered through a Shunyata Hydra Model-8 via an Essential Sound Products The Essence power cord, which is what I usually use with the CAT preamp.
What it does
The sonic impact that the Reference Twos line stage made in my system was immediately apparent and consistent irrespective of the amps with which it was partnered. I did most of my auditioning with the Audio Research 100.2 stereo amplifier, however, as this amplifier made it easiest to identify the preamps character and also because the combination just sounded so darn good.
The overriding impression I had of the Reference Two was that it allowed the music to breathe with utter ease and naturalness. It created a remarkably open and spacious sound. The soundstage expanded in all dimensions but did not seem exaggerated. Depth and layering were especially impressive, with instrumental and vocal images rendered with increased separation and enhanced spatial resolution.
Although much of the music is electronic and the sound is heavily processed, albeit in a very pleasing way, several tracks on Jennifer Warnes The Well [Cisco SCD2034] served to illustrate the Reference Twos character. Listen to the choir that appears near the end of "Patriots Dream." Played through the Reference Two, the sound here engulfed me, the voices swelled and soared. This was the stuff of goose bumps. The trumpet on "Invitation to the Blues" and the piano on "The Well" appeared to be given more of their own space and were presented with added body within these spaces. The overall result was a heightened sense of realism.
Of course, it is really Warnes beautiful voice that is the focus of this album, and the Reference Two reproduced this with a kind of clarity that is a little difficult to describe. A varying amount of reverb was added to her voice, and the production notes indicate that some of this was applied digitally and at other times acoustically by singing in an echo chamber. The Reference Two resolved this reverberation so well that I felt that I could discern how the echo in Warnes' voice was generated from song to song. While we seldom consciously listen for reverberation, the Reference Twos ability to resolve this correlated with the production of a more enveloping soundfield. Decay of instrumental and vocal sounds was also enhanced, which lead to a more textured and dimensional rendering of all images.
For me, these attributes are particularly appreciable on minimally processed recordings of acoustic instruments. Apollo [North Star CD2390] features local Toronto musicians Daniel and Carey Domb performing music for cello accompanied by guitar. There was no echo added here apart from the natural ambience of the church where the recording was made, which the Reference Two reproduced very nicely. The sound of the cello was rendered with convincing body and purity of tone. The decay produced as the guitarist gently tapped the body of her instrument in time with the music on "50 Variations on a Theme of Corelli" was beautifully captured.
While the spaciousness of the sound and enhanced texture and dimensionality of images were what the Reference Two most obviously brought to the sonic table, there was really very little that it took away. It did not bleach or darken the overall tone, and instrumental timbres were not only presented faithfully but also instilled with vibrancy. The music appeared to remain true to the source without significant editorializing or artifact. Dont let those 300Bs fool you into thinking there was anything SET-like about this preamps sound. Dynamics were lifelike without being exaggerated. There was satisfying extension at both frequency extremes, although the midrange was where the preamp excelled.
However, all of this applied to the line stage. In contrast, the phono stage proved a little disappointing. While there were no issues with noise and gain was more than adequate, the soundstage seemed to collapse when I listened to Joni Mitchells Blue [Reprise MS 2038] and the late Michael Hedges Aerial Boundaries [Windham Hill Records WH-1032]. Mitchells vocals sounded somewhat recessed and two-dimensional. Turning up the volume knob did nothing to ameliorate the situation. Several cuts on Aerial Boundaries feature startling dynamics and, like The Well, generously processed echo. Unfortunately these were attenuated and truncated when played through the Reference Twos phono stage. Without having heard the line stage first, the performance of this phono stage could be considered acceptable although not terribly involving. Knowing what the line stage was capable of, however, elevated the standards by which the phono stage needed to be judged, and I found the disparity between the performance of the two more than a little surprising. Perhaps something was amiss with the review sample. I should note that these albums sounded spectacular when played through a modest Bel Canto PHONO1 phono stage plugged into the Reference Twos line input.
I did have one operational issue with the Reference One: Tube microphonics were evident and a distraction at times. However, if my experience with the Reference One amplifiers is any indication, the ingenious folks at Audio Space will come up with a solution to this problem.
I have owned and thoroughly enjoyed my CAT SL1 Ultimate Mk 1 preamp ($5950 when still available) for over five years now. I am enamored of its ability to resolve musical detail and nuance combined with its utterly neutral presentation, and it has a seriously good phono stage that's equal to separate units that cost almost as much as the preamp itself.
When the CAT preamp was reinstalled in place of the Audio Space Reference Two, the changes in the sound were subtle but at the same time obvious. The CAT preamp's line stage provided for a drier, cleaner presentation lacking some of the harmonic richness that the Reference Two offered up. However, the CAT is about dynamics, control and extension, and while the Audio Space preamp was not left wanting in these areas, it is really more about spaciousness and musical flow. The Jennifer Warnes CD demonstrated this ably, as the Reference Two had the indescribable clarity that really brought Warnes' voice to life. In terms of their phono reproduction, the SL1 Ultimate Mk 1 clearly bettered the Reference two in terms openness and three-dimensionality.
If these preamps were the stock with which long-simmering stews were made, the CAT-based dish would allow for a heightened appreciation of the individual ingredients that went into the mix. The Audio Space-based ragout would allow all the individual flavors to optimally meld together into a wonderful-tasting whole. Both are very satisfying.
Water to wine
Even with my reservations about its phono stage and microphonics, the Audio Space Reference Two set a new standard in preamplification for me -- or rather line-stage performance. A few seconds after first installing it, I made a mental note that I was listening to something very special, and that impression remained after a lengthy evaluation. Listening to it was as if a small dam had broken that allowed my line-level sources to flood the room with previously pent-up sound in which I was happily awash.
In fact, the Reference Two so impressed me that it caused me to reevaluate my longstanding relationship with the CAT SL1 Ultimate. Ive yet to purchase any review item, and that string remains unbroken. The phono stage is very important to me, and the Reference 2 comes up short here. Still, I would urge anyone shopping for a world-class preamp to use with digital sources or a separate phono stage to give the Reference Two a serious listen, because its line stage is seriously good.
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