Unison Research SR1 Tube Amp Reviewed

Published On: January 11, 2009
Last Updated on: October 31, 2020
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Unison Research SR1 Tube Amp Reviewed

This tube-transistor hybrid packs both a retro-wood look with a modern industrial design to deliver from a company known more for single ended tube amps, a sound that is worth of this amp's good looks.

Unison Research SR1 Tube Amp Reviewed

By Author: Home Theater Review
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Think 'Unison Research' and, if the brand is known to you, you'll picture groovy wood and single-ended triodes. Trouble is, the new century has been celebrated by the firm with a change of the company structure, new partners and a revised view of the market, so that image must be altered. While S.E.T.s remain the core business, radical new lines are planned which should create an appeal for the products beyond the hardcore stereo tube crowd. The SR1 reviewed here is but a hint of what Unison Research has in store, and it encapsulates the new thinking. For the SR1 is a tube/transistor hybrid...

Additional Resources
• Read audiophile power amp reviews from the likes of Krell, Mark Levinson, Audio Research, Linn, Naim, VAC, VTL, NuForce, Pass Labs and many others here.
• Read about tubes on the Audiophile blog, AudiophileReview.com.
• Want to read audiophile stereo preamp reviews? We have dozens from brands like ARC, Krell, Classé and many more.
• In the market for audiophile loudspeakers? Here are over 100 reviews from brands like Wilson Audio, THIEL, MartinLogan, Bowers & Wilkins, PSB, Vandersten, Magnepan and many more.

Personnel in Italy told me it was best not to use the Italian word for hybrid - 'ibrido' - as it has a connotation at street level relating to sexual ambiguity. Er, oops. For audiophiles everywhere else, 'hybrid' means a mixture of solid-state and valves, typically with transistors serving as the output stages - precisely what constitutes the SR1's layout. The SR1 features a pure triode input stage followed by second triode driver stage with active bias control; the two matched ECC82 double triodes which constitute the valve element are positioned proudly at the front of the amplifier in the classic Unison Research manner. The output stage? A complementary pair of power MOSFETs, symmetrical and operating in what is called Dynamic Class A mode.

Along with the aforementioned evolutionary changes to Unison Research, the company has collaborated with Prof. Leopoldo Rossetto of the Applied Electronics Department of the University of Padua, hence a circuit which seems like such a departure from their norm. The most obvious manifestations are the modular internal construction, sophisticated protection circuitry, an unusual type of remote volume control, assorted warning lights and more. The result is a deliciously modern amplifier masquerading as a be-tubed anachronism. And all is revealed just by switching on.

Competition and Comparison
You can compare the Unison Research SR1 against other tube amplifiers by reading our reviews for the Beam Echo tube amplifier and the EAR Paravicini M100A/312 amplifier.  You can also find more information on different products by visiting our Amplifier section.

First, you operate the rocker at the back. A light on the front panel above the source/monitor toggle glows orange for the valve pre-heating 'warm-up' period, then green for 'fully operational'. Another LED which glows green to indicate reception of the remote's signal, while LEDs to the extreme left and right indicate over-driving. A red flashing indicates overheating, and active protection shuts down the amp, which automatically restarts the SR1 when the temperature lowers sufficiently. Full-on red indicates that the mains input is too high or low, a fuse needs replacing, or the input signal is in overload. And if it comes on simply because you're working it too hard (when THD exceeds 1 percent), you simply reduce the volume and the light extinguishes.

I'm taking all of that on faith because I couldn't get this amp to misbehave, despite it being the first to leave the possession of Unison Research. Even with speakers which could take it, like the Wilson WATT Puppy 6, I couldn't get the LEDs to flicker because the output level was more than I could bear. Suffice to say that despite the review sample being 220V in a room typically graced with 245V from the mains, the amp never shut down, nor blow one of its fuses.

It should be pointed out that, confidence-inspiring safety trickery aside, there are no relays nor fuses in the signal path. And everything must be described as generous or 'overkill', including the Unison Research-designed and in-house-constructed power transformers and power supply capacitors, the robust and fully-gilded speaker terminals and phono sockets, massive heat-sinks and the rugged case. Aaah, the case!

Yes, gang, this amp is as gorgeous as you'd expect of anything Italian. The basic unit is black, with a pair of sculpted, solid hardwood accents (organic lacquering, natch) adding that touch of style and luxury. True, Unison Research could release the SR1 as a strictly utilitarian unit without the wood, or the curved stainless steel heat reflector, but then it wouldn't be a Unison Research amp. The wood flanks the ECC82s at the top, and the large, stainless steel volume and source select rotaries at the front, butting up to the heat sinks at the back. Mounted at the very rear are the on-off rocker and the IEC mains input, the speaker terminals, and sockets for tape in/out and four line-levels sources.

Along with an 80W/ch rating, the SR1's stated specification includes an output impedance of 8 ohms, a generous bandwidth of 2Hz-70kHz (-1dB), and an input impedance of 47kOhms/50pF. The damping factor is 50, the feedback a mere 8dB, and power consumption is 290W at full output power. Physically, the SR1 is a genuinely compact 270x450x150mm (WDH), the weight a substantial 15kg.

Oh, almost forget: the teensy remote. It's no larger than it needs to be (meaning: small enough to get lost) and it sports only two buttons, volume up and volume down. It drives a motorised volume control and triggers an LED so you can see as well as hear the operation. But the deft touch is that it's not an IR remote. Instead, it uses radio frequencies, with the added benefit of the user not having to aim it at the SR1; it even worked from the next room or outside the building. Whether or not it opens your garage door, I can't say. But no planes crashed into my back garden, my neighbour's radio-controlled model helicopter stayed in his shed, and my wife never complained about interference while listening to Radio 2. What it beg, though, is a nice wooden sleeve...

I should tell you that the SR1 will sell for only £1250. Yes, this is a crowded market sector, just above mid-fi and smack in the middle of what I would describe as 'first upgrade' level, and the market is bursting with all-tube amps at this price. Moreover, there are other Italian valve and transistor amplifiers in this sector, if you buy according to country of origin. But what the SR1 positively oozes are qualities which don't appear to be available in a single package (though the small Pathos comes close). In particular, it's the valve sound-plus-transistor reliability marriage which makes this so enticing, the price, size, security features, remote control volume and sexy styling being mere bonuses.

Read much more on Page 2

Unisonresearch-SR-1Review.gifFed with the Marantz CD-12 and Krell KAV300cd CD players and the SME10/SME Series V arm/Lyra Lydian/Musical Fidelity X-LP analogue front-end, the SR1 was used to drive an assortment of speakers. To test its limits, I used the aforementioned Wilsons and the new Quad ESL 989s (but not the Apogee Scintillas) and - to ensure that the review included something more likely to be used with the SR1 for both price and 'conventionality' - my treasured Quad 77-10L two-way boxes. You already know that the amplifier never shut down, smoked nor misbehaved. What was so delightful was that it worked beautifully with such different speaker types.

It soon emerged, though, that the SR1 is finicky about cables and even valve types. Because the SR1 was so freshly baked, I experimented with the tubes to eliminate a little valve whoosh, with varying degrees of success. I suspect that this will be sorted by the time the amp reaches the shops, and must state that it was only audible with ear next to speaker. But the cables? This amplifier likes its speaker wire thick and coppery, and it has an affinity for silver or silver-over-copper interconnects. Assorted Kimbers and Siltechs worked beautifully for those duties, yet the best sound from the mains came via the ring circuit wired with Transparent. Go figure. Whatever seeming confusion this might cause, it also attests to the nature of the amp as a finely-tuned device which will allow you to experiment; at the same time, you can treat it as a 'set and forget' device because the cable issues only ever concern truly fastidious listeners.

After a long run-in period, given the newness of the unit, it emerged that the amp requires in day-to-day use a mere 15min from cold to reach its optimum performance, the gains after that being small. And yet it was immediately apparent that the amplifier offers thoroughbred tube sound despite its schizoid lineage. Normally, hybrids exhibit characteristics of both sounds in varying degrees; usually, it's a case of tube mids and transistor extremes because that's what most designers assume are the virtues of the two technologies. (The old Radford TT100 was a notable exception, with a solid-state front-end and valve output stages.) Unison Research either deliberately tuned the SR1 to sound like its all-valve siblings, or it happened to find some magical MOSFETs which actually emulate tubes all the way.

Whatever the types of music, the sound was rich and warm in the vintage valve manner. The bass was gloriously extended, enough to provide a work-out for both the Wilsons and the Quads, but a slight lack of control at the very bottom distinguishes it from, say, the Musical Fidelity Nu-Vista 300 - the other hybrid I have to hand and one which calls to mind the words 'hammer' and 'sledge'. Keep in mind that at no time during my listening tests did the SR1 ever reach let alone come near to its limits; the bass I'm describing is its normal sound. And regular readers will know that it's the sort I prefer, as I find the oppressive thwacking beloved of Ibiza habitués to be one of the modern world's most unnatural curses.

Why you would buy and then grow to adore the SR1 are its mid and treble regions. As you'd expect, the SR1 seemed a match made in heaven for The Persuasions' pure a capella with its myriad textures, crystal-clear female voices like those of the Judds and mainly acoustic material. Particularly beneficial in every case was a sense of air and openness, the SR1 being particularly deft at creating a large space and separating the individual elements within it. For an amplifier with such a reasonable price tag, it boasts the kind of detail retrieval and dynamic finesse which wouldn't shame a 3500 package.

What it won't do is pretend that it's a monster. Despite my failure to trip its security devices, and despite its ability to go painfully loud, you can hear the onset of clipping or mild distortion well before the lights might start to flash. Although the Wilsons are easy to drive to normal levels, they demand Krell-like power reserves if you want to fill a large room at high levels. In such situations, the SR1 then appears to be what it is: an affordable, medium power integrated amplifier.

But this is not a back-handed compliment, because it's not just affordable, medium power integrated amplifier. Rather, it's a highly sophisticated, ultra-modern device for valve crazies who don't want to jump every time their 300Bs or 845s spit or fart. I suppose, then, that it is to fire-breathing SETs what a Lotus Elise is to a Lamborghini Diablo: 99 percent of the thrills, but only one percent of the aggro.


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