Audio Research • Reference 250 Mono Amplifiers

A shade of wonderful.

by Marc Mickelson | July 27, 2012

nleashed on audiophiles in 2010, the Reference Anniversary was Audio Research's most ambitious preamp ever and the only limited-production component the company made in its 40+ years in existence. So it will come as no surprise that the Reference Anniversary also represents a fulcrum point for the company, as the Reference products that were PRA (post Reference Anniversary) were rather different in design and sound from even the products that immediately preceded them.

Among the things learned from the Reference Anniversary was not just the importance of the power supply, but the sonic benefits of an overbuilt power supply. The Reference Anniversary's was housed in a second full-size chassis and featured twice the energy storage of any previous Audio Research preamp -- and more than Audio Research's VS115, a 115Wpc stereo amplifier.

So it should come as no surprise that the power supply of the Reference 250 mono amp is enormous, but it is not just a case of more being better. Its 900 joules of energy storage represent a 15% increase over the Reference 210, the amp the Reference 250 replaced. It also features the unique circuit used in the Reference Anniversary's power supply, in which a 6H30 drives a 6550C, normally an amplifier output tube.

However, the greatest change comes in the form of the Reference 250's output tubes. The matched 6550Cs of the Reference 210 have been replaced with KT120s, a new-breed power tetrode whose plate dissipation, 60 watts, is 50% greater than that of the 6550C, so it can deliver greater power while remaining extremely reliable and having a longer life.

Memories of the KT90's glassy sound had me concerned that Audio Research had gone in a bad direction with the KT120, a tube I had never heard in use before visiting Audio Research a year ago to hear the Reference 250s in action. But that visit revealed many things about the Reference 250s, including that glassiness was definitely not part of their wide-ranging sound.

Also new to the Reference 250 -- although very old news in terms of Audio Research amps -- is a front-mounted analog meter, which replaces the vacuum-fluorescent display of the Reference 210. This indicates power while the amp is playing music (although you can turn it off), but more important is its use in biasing the amp, a procedure that's performed along with the amp's front-mounted pushbuttons and recessed adjustment pots. We audiophiles like meters; they remind us of the nuts-and-bolts side of our avocation. I mostly kept the Reference 250s' meters turned off, as I found the indicators' bobbing (when I listened with eyes open) to be distracting. You can also turn on/off the meter's integral light via a pushbutton, as well as adjust the speed of the amp's rear-exhaust fan. The fan was redesigned to lower its ambient noise, a complete success. Although it runs at all times, on its default low setting, it was undetectable from my listening seat eleven feet away.

What is detectable, especially during the summer here in Arizona, is the heat that a pair of Reference 250s emit, which raised the temperature in my large listening room a few degrees after the amps were running for a couple of hours. I didn't consider this, along with the AC power needed to produce that heat, an important issue, but your tolerance may be different. I wouldn't say the Reference 250s are extreme in this regard (the Reference 750s probably are). Additional heat is one of the prices you pay for having tube monoblocks.

Audio Research amps use negative feedback, 8.8dB in the case of the Reference 250. The use of feedback in an amplifier's circuit is controversial, with some makers (Lamm, Atma-Sphere, Conrad-Johnson, to name just a few) considering it a sin of commission. On the other hand, there are electrical advantages to be yielded, including the lowering of output impedance and increase of damping factor, both of which make the amplifier more suitable for a wide range of speakers. The only amplifier I've heard with the ability to add and subtract feedback is the Lamm ML3, and its sound definitely suffers from the addition -- in that circuit. However, like most things in high-end audio, it’s not what you do but how you do it that matters, and a carefully executed feedback loop can improve the sound of the right amplifier.

The input of the fully balanced Reference 250 is via XLR only, with output taps marked for 4-, 8- and 16-ohm loads. While the Wilson speakers I used with the amps are specified as 4-ohm loads, they sounded best from the 8-ohm taps, even if the 4-ohm were more electrically appropriate. This has also been the case with other Audio Research amps and Wilson speakers. The amp's power-cord inlet has a 20A connector. Audio Research supplies a very good cord with all of its products, although aftermarket alternatives can elevate performance noticeably. I had very good results with both low- and/or high-priced cords from Nordost, Essential Sound Products and Shunyata Research, the differences coming down to sonic preference more than one being universally better than the others. Even if you decide to avoid complication and use the cord supplied with the amps, you won't be sonically cheated.

One complication I did experience -- all for the better -- was in the form of a box of cables and power products that arrived near the end of the listening. To reacquaint me with its products as prelude to an upcoming review, Nordost shipped a full "loom" (love that word) of Frey 2 cables along with some of its Quantum power products. I spent the better part of a morning recabling my system -- beginning with the power cords, moving to interconnects and finally adding speaker cables. The cables brought out levels of transient speed and spatial dimension that were beneficial to realizing the amps' full potential, but they also rammed home once again the benefits of using the same cable right through the system.

I was even more impressed -- to the point of astonishment -- with the Quantum products. While we list these in our reviews as "power conditioners," they are really both more and less than this. The QB8 and QB4 do distribute power to the components, but with the goals of providing maximum AC throughput and a single grounding point. The Qx2 and Qx4, on the other hand, are, in Nordost's words, "field generators," whose proximity to electronics produces their effect.

As this isn't a review of the Quantum products, I won't go into greater detail, except to say that they work, deepening the view into the soundstage, and each recording, considerably and in a way that's consonant with bringing out the best in electronics -- and speakers, with which they can also be used. All of this was especially welcome with the Reference 250s, as you'll read. I have a difficult time wrapping my head around the explanation of how the Qx4 and Qx8 do what they do, and for this reason I regret that they do anything at all. It's products like these that make non-audiophiles roll their eyes -- or worse -- at our supposed folly. However, don't audition them unless you are willing to buy them -- and suffer the incredulous stares of friends and family to whom you try to explain their operation. I repeat -- they work.

Gettin' down to it

ost Audio Research products have a built-in gratification-delay system that allows you to consider what you're going to hear once everything is powered up and playing: the tubes are shipped uninstalled. With a preamp, you can be up and running fifteen minutes after opening the box (the Reference Anniversary, and its two chassis as well as extra tubes, notwithstanding). With a pair of mono amps, it's more like an hour, considering all of the tubes that need to be unpacked and installed. As I finished inserting the tubes in the sockets of the review amps, a well-traveled pair of demonstrators, I thought back to that first encounter with the Reference 250s last summer. I wrote a blog about the visit, so I won't restate my earlier impressions here, except to say that the amps sent to me for review had big expectations to fulfill, which happened to be of their own making.

The Reference 250s took all of this in stride. From the start and throughout my listening, they conveyed unshakable composure and boundless capability. They never seemed taxed by any kind of music in any format, and the more I listened, dutifully taking notes, the more complete their sonic personality became.

With so many amps, a ratio exists between resolution and image density. Amps that sound highly resolving, often conveying space especially well, can lack the presence and solidity that other amps display as a matter of course -- and pay for with less perceived detail and a limited sense of space. Space is a hallmark of Audio Research electronics, the soundstage seeming to expand in all directions, the outermost reaches, which are even more remote, fully illuminated. The Reference 250s were beyond accomplished here, portraying music with a vastness limited only by the recordings. But they also conveyed tone with a deep palette of colors consistent with the variation inherent in live music. This gave images a saturated quality that made them sound especially solid and present. Electronics normally push ultimate performance ahead in baby steps; sometimes there are larger steps forward in one direction balanced by equal steps backward in others. But the Reference 250s were truly special in their expansion of the sonic horizon in so many directions all at once.

The title is an inelegant mouthful: The Modern Jazz Quartet at Music Inn, Guest Artist: Sonny Rollins [Atlantic 1299]. The stereo version of this well-known LP provided insight into the Reference 250s' way with space and tone. Musically, this LP is stellar, although the title leads you to think that Rollins sits in with MJQ for the entire set, when, in truth, they're only together for two cuts. But what a pair: "Bags' Groove" and "Night in Tunisia." The soundstage is enormous in all dimensions, but messy. Rollins wavers between each channel, although he's mostly on the right side. Cymbals are center, while the drums are slightly left, which is disorienting. I'll give the engineer the benefit of the doubt and say that the sound here underscores the difficulty of recording music live.

The Reference 250s never made lemonade out of the recording's lemons, revealing the spatial discontinuities but also carrying the proceedings on the basis of tonality, which again was widely hued, giving Rollins' sax along with John Lewis's piano a physicality that made following their instruments that much easier. And when everything was just right, the Reference 250s made that fact plain. As the Music Matters series of 45rpm reissues (thankfully) continues, the sound of these special LPs has improved in ways that the Reference 250s revealed as a matter of course. Music Matters is on a summer hiatus right now, but the final thirteen new titles will begin shipping in September. Among them, don't miss The Golden Eight [Blue Note/Music Matter BST 84092], a remarkable large-ensemble session and one of the rarest Blue Note titles extant (until this reissue begins shipping, it's only available on original LP and Japanese CD). Here again was immense space along with body and texture. The Reference 250s revealed it all while keeping the fabric of the music intact.

Much of this came via the midrange, which was palpable and dimensional as well as articulate and nuanced. The KT120s came nowhere close to syrupy over-warmth or hard-edged glare, the mids displaying a SET-like character in the very best ways. Voices both human and instrumental were material in the extreme, though not due to a bloated, tubey quality but rather an inherent complexity that consisted of that wide tonal palette, quickness into and out of each note, dynamic alacrity, and, for lack of a better term, sweep -- the pulling together of everything that happens instead of parsing it all. In this way, the Reference 250s resisted analysis; hearing them was the very best explanation of their sound.

Amongst audiophiles, Patricia Barber has become a prisoner of the amazing sound of her recordings, with some listeners recognizing that her music is unique and others rejecting it because it's so often used as demo material. If you are familiar with Café Blue, either on LP [Premonition Records 90760-1] or SACD [Premonition Records/Mobile Fidelity UDSACD 2002], you have a fingerprint of the Reference 250s' midrange, its presence, sheer resolving power and, again, sweep. But it doesn't take only the best-made recordings to reveal this. A particularly good garage-sale find of mine was You Better Believe Me [Argo 750], a collaboration between R&B singer Jean DuShon and the Ramsey Lewis Trio. The pent-up power of DuShon's voice was revealed in bursts throughout the album, and like Barber's, it was both physical and agile, even airy, through the Reference 250s. Some extra weight wouldn't be unwelcome here, but none was given. The amps differentiated the qualities of the recordings and singers all the better because their strengths never became excesses, and the music was better served for it.

The bass of the Reference 250s was also a music-serving strength where one usually doesn't exist with tube electronics. This didn't come via solid-state-like depth and slam, for instance, but rather through a set of more naturally rendered qualities, including bloom and heft. Acoustic bass remained supple, electric bass controlled, weighty and fast off the mark, the whole region portrayed seemingly without limitation. In this way, the Reference 250s are a natural mate for the Reference Anniversary, another tube product for which the lowest reaches are an asset. For this reason -- and many others -- it was thrilling to hear these two products together. There was synergy in the traditional sense of a tight working relationship, but the ultimate advantage was that the amps extended the preamp's performance all the way to the speakers. Recordings I heard anew with the Reference Anniversary were even fresher with the Reference 250s in the system.

I've loved the Jayhawks' Hollywood Town Hall on CD since its release in the early 1990s. If you're into classic electrified Neil Young and haven't heard this album, you are in for a real discovery. The original vinyl has for years been very pricey, so the CD [Def American 26829], with its very good sound, has been a welcome alternative. However, the album was re-released on vinyl last year [American 88697], and that record practically begs to be turned up to 11, the combination of the Reference Anniversary and Reference 250s portraying this dense, electrified musical mass as an animated whole, even as instrumental lines were a cinch to follow, no matter how intertwined they were. Lesser electronics can blur the distinctions, creating more of a wall than a tapestry of sound. Not so with the Reference Anniversary and Reference 250s, with which the music was simply there -- available for any amount of analysis or none at all.

The dynamic boundaries of the Reference 250s will depend on your speakers -- as, in fact, is the case with any amplifier. I first used the amps with Wilson MAXX 3s, which the amps drove authoritatively. Among their merits, Wilson speakers are very good at both the micro and macro ends of the dynamic scale, and with the Reference 250s fronting them, details near the threshold of the noise floor were easily discerned, and the amps scaled deftly -- another stock trait of Audio Research Reference electronics -- to crashing levels. These things were only intensified when the MAXX 3s gave way to Alexandria XLFs, which display lower self-noise and greater dynamic range, perhaps due to their higher sensitivity. They built on what was happening with the MAXX 3s. While it seemed for all of my listening like the amps had endless power reserves, when used with the MAXX 3s, they could show signs of compression when I played them far beyond my tolerance for listening. Thus, perhaps with less sensitive speakers and a larger room, someone could tax them beyond their power limits, which may be why the Reference 750s exist. But this was discerned through high-volume testing, not normal listening, during which the amps showed no signs of strain.

It probably seems from all of this description that the Reference 250s basically accomplish everything sonically, and that really is the case. However, "everything" is not a constant when it comes to audio components; one that does "everything" better than all of its competition doesn't exist. Rather, "everything" is a statement of across-the-board proficiency, a defining totality of performance that encompasses broad qualities we all value: very wide bandwidth and dynamic range, high resolution at both high and low levels, a varied way with tone, a realistic perspective, the ability to convey the detail on each recording in a musically consonant way.

Earlier in the year, I similarly praised the Ypsilon Aelius monoblocks ($36,000 per pair), calling attention to their "sum total," as I referred to it in my review. While this may imply that they and the Reference 250s sound very similar, that's really not the case, as the ingredients comprising "everything" are in different proportions with these two stellar amps, creating distinctly different presentations. Among products that manage to do "everything," each will do some bits better than others. "Better" is where personal preference comes in -- and why it's possible for two amps that do "everything" to sound completely different.

The Aeliuses are hybrids that have a tube-based input stage (with a single C3g) and a MOSFET output stage, and they use no feedback. Their most immediately recognizable trait is an utter lack of background noise. This gives the Aeliuses a special resolving power at the micro end of the dynamic scale, as the noise floor, even with that lone tube in the circuit, is simply not an impediment to their performance. While the sound of the Aeliuses possessed "overwhelming clarity. . .[and] supreme retrieval of musical detail, transient definition, and front-and-center tonal neutrality" it also displayed "an inherent robustness and density," which kept the amps from sounding tonally threadbare, as so much of their completely solid-state competition can. Here is the first instance of "everything" being different. As good as the Reference 250s are at unearthing minute detail, the Ypsilon amps are the reigning champs in this regard. But this comes with a lighter kind of physicality and more narrow tonal palette, two areas in which the Reference 250s excel. If you listen to Hollywood Town Hall with both amps, you'll discover that the Aeliuses delineate and unravel the band's dense instrumentation, while the References 250s allow this same sort of analysis but don't put it front and center. Their presentation unites instead of separates.

These tendencies apply to the midrange and bass as well, the Ypsilon amps sounding more skewed toward the treble, which gives the music an airier feel, while the Audio Research amps, lacking no air, have a more even, coherent presentation top to bottom. In terms of perspective, the Ypsilon amps are more forward, the Audio Research monoblocks midhall -- about perfect, as far as I'm concerned. In the bass, again the Ypsilon amps sound lighter but also faster and possessing a greater sense of slam than the heftier-, much bloomier-sounding Reference 250s.

In revealing these differences, I don't really capture the character of either amp. All of what I say about them is true, but mostly in relation to each other. In a qualitative sense, they are peers, as they are grouped with precious few other amps that are as complete. However, this raises an important question for me -- and for you, if you are considering these amps for purchase: which one do I prefer? For me it's the Reference 250s, whose spaciousness, tonality, perspective, bandwidth and dynamic capabilities mesh with my preferences for musical reproduction (not to mention my chosen preamp). But it's also more than that. I would argue that as spectacular as the Ypsilon Aeliuses can sound, the Reference 250s have less of a sound because they possess more of the tonal diversity of live music. Put another way, while the Ypsilon amps may tell you more about the recordings, the Reference 250s will tell you more about the music on them, and ultimately that's what I prefer.

Summing up

've reviewed a series of Audio Research Reference products over the past decade, including CD players, preamps and amps. All of them have possessed a sonic family resemblance that is an aggregate of many of the qualities I've identified about the Reference 250s. However, only the Reference Anniversary displays them in the same proportions and concentrations as the Reference 250s -- evidence of the steady forward push of sonic performance, I suppose. This has roots in a number of things, including both products' sophisticated combination of tonality, spaciousness, density and dynamics, all of which make for a presentation that's both highly resolving and naturally rendered -- an appealing character to be sure.

But what about the assortment of amps I've reviewed since the launch of The Audio Beat three years ago? Weren't they appealing too? Without a doubt, though not all in the same ways. The fact is, I've had the good fortune to review a cross-section of the best amps extant. I began with the Ayre MX-R solid-state monoblocks, and progressed through OTL (Atma-Sphere MA-2 Mk 3.1s) and SET (Lamm ML2.2s) tubes as well as a hybrid design (the aforementioned Ypsilon amps). To describe these (and my reviews of them), a friend of mine coined the phrase "shades of wonderful." He meant it mockingly, but it actually reiterates two important points: these amps are sonically different and also sonically "wonderful." If you think, as my friend does, that in order for one of these amps to succeed the others must fail, you simply haven't heard enough equipment in this lofty performance strata -- or these amps in particular. Instead of mocking, "shades of wonderful" really does capture this group of amps.

Where does all of this leave the Reference 250s? Right where they started: as the sonic offspring of the Reference Anniversary preamp, the best Reference-series amps I've heard, and one of the very best amps I've heard, period. If you missed out on buying a Reference Anniversary during the year it was available and have always regretted it, Audio Research has soothed that ache with the Reference 250s.