ULTRA AUDIO -- Archived Article

November 1, 2004

Unison Research S6 Integrated Amplifier

It is not uncommon for the audiophile to keep a cherished component when, in a temporary lapse of judgment, that component is superseded in their system by the latest and greatest. Or perhaps an unneeded but long-coveted or "classic" piece might become available at the right price. What, then, to do with the superfluous gear? Well, if you’re anything like me, you build a second system. With time, second systems themselves become susceptible to upgrading. Given time enough and money, one can end up with a system in each room of the house. The fun never ends!

By the nature of their provenance, secondary systems tend to be simpler than their primary counterparts. While the main system may serve as a vehicle for endless tweaking and analytical listening, the second system’s usual function is to simply make music. As the second system is typically housed in a multifunctional space, domestic acceptance is a prime consideration. Which brings me to the Unison Research S6, an Italian-made, integrated tube amplifier that has been at the heart of my second system for the past six months.

Unison Research, along with its sister company, Opera Loudspeakers, is headquartered in northern Italy as Advanced Research In Audio (A.R.I.A.). Unison is perhaps best known on these shores for their Unico hybrid integrated amplifier, widely regarded as offering tremendous value. The Unico line has been expanded to include a number of sleek-looking electronics (including a CD player and tuner), most of which incorporate tubes somewhere along the signal path. But the company’s foundation was built on pure tube electronics, which they have manufactured since 1987. Presently, Unison offers consumers ample choice, producing amplifier models based on the KT88, EL34, and 845 output tubes in a variety of different configurations.

The S6 integrated ($2995 USD) uses three Electro Harmonix EL34 power tubes per channel. Operating in class-A, single-ended mode, the output stage produces a robust 35Wpc. Unison engineers have applied 14dB of negative feedback to produce what they feel is optimal sound. The preamplifier stage uses one 12AU7 (Philips JAN 5814As are supplied) per channel. Four single-ended, line-level inputs are available, and a tape loop is accessible via a toggle switch on the front panel.

When I unpacked the S6, the first thing I noticed was that it is considerably deeper than it is wide. Its rubber footers measure some 14" front to back, barely allowing the amp to sit level on a standard-sized shelf in an old Target rack. The S6 is also surprisingly hefty: 56.2 pounds, most of that weight in the rear, where reside the beefy output and power transformers. In addition to the inputs, the rear panel sports high-quality, insulated speaker binding posts with provision for output impedances of 4 and 8 ohms. A 15A IEC power-cord socket lies beneath the power switch at the back. The front panel is dominated by two large, finely machined, stainless-steel knobs for volume control and source selection.

A hallmark of the Unison aesthetic is the use of wood accents. Although the S6 is the least "woodsy" of the line, the incorporation of a small cherrywood insert at the front corner transforms the design from just another black box into living-room furniture. Well, almost. The S6 is also unique in that it is the only Unison amp that comes with an integral tube cage. This is an attractive, hinged, chrome affair that I have yet to determine how to remove completely. A palm-sized, wooden remote control has but two buttons, for increasing and decreasing the volume. As the remote functions via radio frequency, it is nondirectional and can be operated at long range and through the walls of adjoining rooms.


The S6 is plug-and-play easy, with no manual tube biasing necessary. I placed it on the living-room floor and connected it to two Sonus Faber Guarneri Homage speakers via 6m of concealed Nordost Flatline Gold Mk.II speaker cable from the S6’s 4-ohm outputs. The source was Unison’s own Unico CD player, via Cardas Hexlink 5 interconnect. After some 20 hours of burn-in, the S6 was ready to show its stuff.


The S6 produced a full-bodied, voluptuous sound in a front-hall perspective that I never felt sounded too forward. It was a midrange champ, rendering the voices of my usual suspects, such as Holly Cole and Diana Krall, with tremendous texture and presence. Well-produced, older recordings were also presented wonderfully. A case in point was Ella Fitzgerald’s Fine and Mellow [CD, Pablo 2310-829], which richly captures her fully matured pipes running the gamut from coo to growl while swinging all the way. This Grammy-winning album is basically a jam session featuring Ella and notable sidemen Zoot Sims, Clark Terry, and Harry Edison, among others. Individual instruments were precisely placed on a generous soundstage. I sometimes use "The Man I Love" to assess pace, rhythm, and timing, as the tune builds to a climax -- the S6 did not disappoint.

Brass and woodwinds were presented with vivid tonal colors. While definitely playing to the warmer side of neutral when compared to my reference Nagra VPA tube monoblocks, the S6 was not really euphonic in the way of, say, a Conrad-Johnson MV-50, the only other EL34-based amp I recall having had in-house -- some 20 years ago. As one might surmise, this system reproduced strings beautifully. Jordi Savall’s soundtrack for the film Tous les Matins du Monde [CD, Auvidis/Travelling K 4640] features 17th-century French music for the bass viol (viola da gamba). The sound of the solo viol is haunting, and the Unison reproduced the instrument’s sonorous harmonic overtones, as well as the recording acoustic, in breathtaking fashion. Detail freaks will revel in listening to Savall’s stertorous breathing as he vigorously applies bow to strings.

Rachel Podger’s reading of Vivaldi’s La Stravaganza [CD, Channel Classics CCS SA 19503] is another disc that has been in heavy rotation of late. Massed strings had the appropriate bite and sheen, but never sounded strident in this wonderfully energetic performance. Several movements (e.g., the Allegro from the A-minor concerto) lend themselves to high-volume listening; the Unison handled the ebb and flow of this highly dynamic material with aplomb, showing no sign of strain, at least when paired with the 88dB-sensitive, 8-ohm-nominal-impedance Sonus Faber Guarneris.

Although the expectation of a magical midrange was fulfilled, other attributes of the S6 came as more of a surprise. By a significant margin, its bass output was subjectively fuller and more extended than that produced by the Krell KAV-300R receiver, which preceded the Unison in this system. While the bass may not have been as taut as that produced by the little Krell, it was clearly more tuneful and articulate, which went a long way in helping to convey the rhythm and timing of music. On the Bill Charlap Trio’s All Through the Night [CD, Criss 1153 CD], Peter Washington’s bass lines on the title track induced involuntary head-bobbing and foot-tapping, while Charlap’s forcefully struck piano notes were presented with attack and decay characteristics not unlike those of the real thing. Similarly, highs were naturally extended and airy, with no sense of treble rolloff.

The S6 sounded considerably more ballsy than its 35Wpc rating would suggest. Nevertheless, on complex orchestral material such as the opening Invocation, from Prokofiev’s Scythian Suite [CD, Mercury Living Presence 432 753-2], the music can appear a touch confused and less resolute than when played back on my reference system. In fact, I inserted the S6 into the main rig to see how it would fare driving the Wilson Audio Sophias. It performed admirably indeed, yielding the same grainless purity of sound as it did with the Guarneris. There was a little less clarity of detail and some limitation in macrodynamics when compared to the CAT Ultimate preamp and Nagra VPA monoblocks I usually use here, but as this combination costs more than eight times as much as the S6, the comparison was not completely fair. What was impressive was that the S6’s basic musicality shone through. System matching is important; while the Sophias’ full potential was clearly not realized with the Unison, it is hard to imagine a better mate for the Sonus Faber Guarneris, with which I preferred the sound of the Unison over that of the much costlier Nagras.

Some listening was done with my long-term reference digital source, the Sony SCD-1 SACD/CD player and an old Rotel RCD-955AX, the latter taking pride of place in my wife’s office system. The S6’s performance characteristics remained consistent regardless of the source, but real synergy was apparent when it was used with the Unico CD.

I picked some tiny nits. The S6 ran extremely warm; placement on a top shelf or floor is mandatory. The location of the main power switch at the rear corner is awkward; again, attention to placement and dressing of speaker cables is necessary for ease of access. Although I felt no need for a subwoofer in my system, a subwoofer output might be useful for some.


In the S6, the folks at Unison Research have succeeded in engineering a product that displays all the musicality of tubes with none of the traditional drawbacks, such as flabby bass or attenuated highs. The fit’n’finish is top-notch, and my unit has operated flawlessly now for six months. Current-production replacement tubes are readily available and relatively inexpensive -- the S6 would be an ideal amp for the tube-phobic who have relatively efficient speakers and a penchant for smaller-scale acoustic music. Nor will the S6 disappoint tube aficionados, who could probably reach into their stashes of arcane NOS tubes to further fine-tune its sound.

Most of all, the Unison S6 is suited to music lovers, irrespective of technological bent. Will it alter my decades-old, bank-account-draining habit of equipment swapping and tweaking? Probably not, as that’s what I like to do. But were I to be banished to the proverbial desert island, I’d make sure to take the Unison S6 out of the second system and bring it along with me.

…Ken Choi

Unison Research S6 Integrated Amplifier
Price: $2995 USD.
Warranty: Three years parts and labor.

A.R.I.A. Advanced Research In Audio srl
Via Canova, 19/b
36073 Cereda di Comedo Vicentino, Italy
Phone: (39) 0445-446387
Fax: (39) 0445-459399

E-mail: aria.unison@iol.it
Website: www.unisonresearch.com

North American distributor:
VMAX Services
P.O. Box 8, 1217 Greene Avenue
Montreal, Quebec, Canada H3Z 2T1
Phone: (800) 771-8279
Fax: (514) 931-8891

E-mail: info@vmaxservices.com
Website: www.vmax-services.com

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