Albert Von schweikert’s design goals for the UniField two, the middle model in VSA’s Studio Signature Series, were rather simple. Many of his customers asked for a small speaker that would work well in both small and large rooms with virtually any amplifier. Specific performance criteria were a large sweet spot, and enough bass output to simulate a larger floorstander. From a distance the UniField Two appears to be a two-driver system, and may well pass for a conventional stand-mounted two-way design. However, up close it is apparent that the upper 7-inch woofer is actually a coaxial design by SEAS of Norway with a 1-inch fabric dome tweeter nestled over the pole piece. So that would make the UniField a three-way design, right? Well, not exactly. The catch is that the upper woofer, featuring a PP/TPX polymer cone, is allowed to work into the deep bass while being augmented below about 80Hz by a 7-inch aluminum coned woofer. The end result is typically referred to a 2.5-way design, basically a two-way with a subwoofer. As Von Schweikert aptly points out, an important advantage of such a design is the elimination of the mid/woofer’s high-pass crossover network, and hence no capacitors in series with the critical midband. There has been much ado in recent years over the sound of capacitors with the cost of exotic types easily exceeding that of typical drivers. But it’s fair to say that the best-sounding capacitor is no capacitor at all. Instead of capacitive-coupling, the mid/woofer’s bass excursion is controlled by sealing it into a small internal chamber densely packed with Acousta-Stuf polyfill.
Coaxial drivers are rare birds in audiophile designs. Tannoy is justly famous for its dual-concentric driver and more recently KEF has made waves with its Uni-Q driver. A coaxial’s primary goal is to align the acoustic centers of the tweeter and midrange, “forcing” them to behave as a single driver so as to emulate the performance of a point source of sound. The payoff is vastly smoother off-axis performance relative to a conventional driver layout and thus a wider sweet spot. And as I’ll detail shortly, the SEAS coaxial is indeed capable of remarkable imaging due in great measure to its time-aligned wavelaunch. However, the practical engineering problem all along has always been the tweeter design, the traditional knock against coaxial tweeters being that they don’t measure very well. The fact that the coaxial tweeter is horn-loaded by the midrange-woofer cone makes it difficult to obtain a smooth frequency response. In fact, Von Schweikert considers the SEAS coaxial tweeter to be a bit of an enigma. He is well aware of its frequency-response imperfection, and that it lacks any type of fabric impregnation or fully pistonic motion, and yet despite all that, he finds it to sound wonderful. I was, at least initially, less enamored of this tweeter and can confirm that its on-axis frequency response is not particularly pretty due to a significant response dip in the lower treble and an excess of extreme treble.
On the matching factory stands, the speakers were at first toed in toward the listening seat, but I discovered rather quickly that I wasn’t happy listening to the tweeter head on. Since off-axis measurements showed a much more natural balance, I nixed the toe-in idea, pointing the cabinets straight ahead. This placed the tweeters at an angle of about 25-degrees relative to the listening seat and gave me the sort of balance I was after: a smoother lower treble partnered by a naturally rolled-off extreme treble. It should be noted that I’m not a fan of in-your-face treble and much prefer a middle of the hall presentation. With the speakers optimally set up, it became clear that the coaxial principle was working to perfection. Even without any toe-in, the resultant sweet spot was massive—no need to place your head in a vise to enjoy a stupendous stereo experience. When it came to imaging, the UniField delivered the goodies. I was most impressed by its exceptional focus and transparency, making it easy to resolve spatial outlines and subtle image shifts within the confines of a spacious soundstage. Resolution of massed voices was superb, allowing me to follow the ebb and flow of a particular voice in a chorus. After all of the preliminary experimentation, my view of the treble range crystallized sufficiently to pronounce it musical enough to enjoy, though it understandably lacked the transient finesse and purity of ribbons and electrostatic types.
The midrange driver turned out to be a winner, sounding smooth yet detailed, and manifesting a purity of tone which most cone mids would die for. The UniField dug into a complex mix with confidence. In particular, its resolution of artificial reverb launch and decay was scary good. Timbre fidelity was excellent even when scaling the full female soprano range. My only minor criticism had to do with slightly coarse upper-midrange textures, most obvious on violin overtones. This turns out to be the transition region between the midrange and tweeter, the crossover being at 2.2kHz.
The UniField Series uses curved side walls to minimize internal standing waves. In addition, much effort has gone into making the cabinets acoustically inert, which is all about minimizing cabinet-wall vibration. And that means reducing wall flexure by using thicker and stiffer materials. VSA’s elegant solution is based on the concept of constrained-layer damping but also strives to eliminate energy transfer from the drivers to the baffle by decoupling them with a 6mm-thick viscoelastic clay-polymer gasket. VSA’s triple-wall laminate combines three materials of different resonant signatures. The outer layer is MDF which is bonded to a layer of synthetic stone, fabricated from crushed gravel, minerals, and resin binder. The inner layer is hard felt, which is absorptive of sound energy. The total wall thickness is an impressive 2.5 inches and is the type of construction currently deployed in every Von Schweikert Mk2 speaker system. This approach is the antithesis of the British, so called BBC-style, thin-wall speaker-cabinet designs. The notion of “tuning the cabinet to the orchestra” comes to mind, and while it could be argued that some cabinet resonances are less objectionable to the ear and might in fact be consonant with the music, the end result is inevitably a sonic coloration. After auditioning the UniField Mk2, it seems to me that VSA’s approach of minimizing all cabinet resonances is the correct one in that it clearly raises the bar in terms of achievable bass precision.
The cabinet is vented which suggests a bass-reflex loading, and that is in fact the case, the box tuning being 38Hz. But this turns out to be no ordinary bass-reflex design. Four internal chambers filled with Acousta-Stuf define a mini-labyrinth, which significantly dampens the twin impedance peaks produced by a typical bass-reflex design. The measured minimum impedance was 3.8 ohms at 100Hz and the impedance magnitude was nearly single-peaked and fairly flat, varying by only a factor of two in the deep bass. VSA refers to this bass tuning as a hybrid reflex/transmission line. I like the idea of contouring the overall Q of the speaker system using acoustic damping. Although conceptually it is still a long ways from a classic transmission-line bass loading, this tuning works in practice by nudging bass performance toward that of an aperiodic sealed box.
In-room bass extension measured flat to about 38Hz. It was a case of bass reach coupled with a surprising degree of punch. Most stand-mounted speakers are not well suited for reproduction of symphonic music. The UniField Two proved to be a welcome exception. It possessed sufficient bass heft and dynamic prowess for realistic reproduction of an orchestra’s power range. The bass balance, usually an issue for stand-mounted speakers, was shifted toward the midbass. There was considerably more midbass energy relative to the upper bass which manifested as a slight emphasis of an upright bass’ body tone. Choice of partnering amplifier became an important issue. The UniField boogied superbly when matched with high-damping-factor solid-state amplifiers. It also scored highly in bass precision, a performance benchmark undoubtedly attributable to the combination of an acoustically inert cabinet and an aluminum-coned woofer. However, bass resolution suffered substantially when the UniField was driven by a low damping factor tube amplifier. Lacking amplifier control, the bass range took on a distinctly tubby character. The good news is that there is no shortage of high-damping-factor solid-state amplifiers in the 100 to 200Wpc range.
In the bass range, the UniField Two Mk2 offers impressive performance for a small box speaker. As such, it competes effectively with British stand-mounts from Spendor and Harbeth. Think greater rhythmic precision and bass heft. Its coaxial technology bestows upon it exceptional image focus. In fact, if you like mini-monitor soundstaging, you’ll love the UniField Two. Optimally set up, it will reward you with a steady stream of musical delight. And that’s what it’s all about.
SPECS & PRICING
Frequency response: 32Hz–25kHz (-6dB), 40Hz–20kHz (+/-2dB)
Nominal impedance: 4 ohms
Sensitivity: 88dB 1W/1 meter, anechoic
Power rating: Up to 200 wpc
Weight: 51 lbs.
Dimensions: 10″ x 17″ x 14″
Price: $7995 (stands included)
Von Schweikert Audio
1040-A Northgate St.
Riverside, CA 92507
GAS Ampzilla II amplifier modified by Mike Bettinger of GAS Audio, Lamm Audio M1.2 Reference and Bob Carver Cherry 180 monoblock amplifiers; Apple Mac BookPro running Sonic Studio’s Amarra Version 2.6 software, EAR DACute DAC, April Music Eximus DP1 DAC/ Pre and Stello U3 digital data converter; Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Clearaudio da Vinci V2 phono cartridge; Pass Labs XP-25 phono stage and XP-30 line stage; FMS Nexus-2, Wire World, and Kimber KCAG interconnects; Acoustic Zen Hologram II speaker cable; Sound Application power line conditioners
By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.More articles from this editor