I've heard a lot of great audio components over the years, but even in that steady stream of excellence, a few have stood out as something special. These are the products that, in their day, set a new standard for performance, and many of them are ones I wish I'd hung on to. Among these products are three preamps from Audio Research: the SP3A, the SP6B, and the SP10 (footnote 1). I know I'm not alone in viewing these models as classics.

Rumor has it that in the mid-1990s, when William Z. Johnson set out to design the original Reference 1 preamplifier, his jumping-off point was the SP10. What's not a rumor is that the Reference 1 carried on the tradition and lineage of those classic preamps. It was fully tube driven, it was the best preamp that ARC could build with the technology available at the time, and, like its predecessors, the Ref 1 redefined what was possible in a tube preamp—and, to some ears, what was possible in a preamp of any type. It thus became an instant classic. Time and technology marched on, and the Ref 1 was replaced by the References 2, 3, and 5, each one in turn carrying on the lineage and setting a new standard of performance (footnote 2). The latest in this series is the Reference 5 SE ($12,995), introduced in late 2011 to replace the Reference 5.

The Reference 5 SE
The Reference 5 SE benefits from several advancements that Audio Research developed for the cost-no-object, limited-edition Anniversary Reference preamplifier ($24,995), including two readily visible changes from the Ref 5. First, the 5 SE's front panel mirrors that of the Anniversary: the buttons are now tidy, round, metal, and set directly into the faceplate, rather than the Ref 5's square, recessed plastic buttons. Added on the inside, to the left, is a vertically mounted circuit board, stuffed to the gills with capacitors to nearly double the Ref 5's already massive energy storage. Other changes include new Teflon and hybrid coupling and bypass capacitors, and changes in the internal wiring.

Otherwise, the Reference 5 SE's basic circuitry resembles that of the Ref 5: it's mostly contained on a single-sided board that stretches from wall to wall. Additional, smaller boards handle I/O, switching, and display functions. As in the Ref 5, the Ref 5 SE's transformers—an R-core for the audio circuits, a toroidal for the display, relays, and microprocessor controls—are mounted on the sides of the chassis, above the board, to reduce vibration in the audio circuits. The audio circuitry's power supply is a hybrid-regulated design employing both FETs and a 6H30 dual-triode tube controlling a 6550 pentode tube. The audio circuit itself is "as simple and good as we could make it," according to ARC's David Gordon: a single gain stage with a cathode follower tube in a zero-feedback, fully balanced, pure class-A triode layout. There are six tubes: four 6H30s in the analog stage, and the 6550 and 6H30 in the power supply.

The Ref 5 SE's cosmetics and layout are handsome in Audio Research's traditional fashion, and it's the same goodly size and weight as the Ref 5: 19" wide by 7" high by 15.5" deep, and weighing 30 lbs. The chassis is of heavy aluminum, with a thick, elegantly machined aluminum front plate and two rack-mount handles. The review sample's top plate was of tinted polycarbonate, with slot vents. A cover of perforated aluminum is also available, but ARC recommends the polycarbonate, perhaps because of lower chassis resonances. The Ref 5 SE is available in natural aluminum or black.

Like the Reference 5, the 5 SE has at the center of its faceplate a large window that prominently displays the volume setting with both a bar graph and large numeric display. Also displayed, in far smaller characters, are the selected input, whether it's single-ended or balanced, and the status of several functions: mono/stereo, mute on/off, and polarity non-inverted/inverted. The display can also be used, via the remote control, to show the hours each tube has been used, handy for confirming what your ears will tell you: that you've reached the 600 hours of break-in time that ARC recommends. Via the remote, the display's level of illumination can be set to one of eight levels, from off through bright. But even when the display is fully dark, any change in status lights it up for 10 seconds to display the change.

Flanking the display are two return-to-center knobs, attached to a microprocessor, that turn through an arc of about 60°. The one on the left changes the volume across 103 steps; the one on the right selects among the six inputs. Below the display are six small buttons: Power, Mute, Mono, Balanced/SE (input selection), Processor, and Phase Invert. All front-panel controls are duplicated on the utilitarian remote control, which also includes buttons for adjusting balance and display brightness, and to display tube hours.

On the rear panel are six line-level inputs, a processor input that bypasses the volume control to allow the Ref 5 SE to be integrated into a home-theater system, a set of line-level output jacks for recording, and two sets of outputs. All inputs and outputs have both single-ended and balanced connections. For each input, the type of connection is selected with the Balanced/SE button on the front panel or remote.

Use, Handling, and . . .
The Reference 5 SE was a delight to use. All of its controls were intuitive and positive, and the single-ended and balanced connectors were all nicely solid. An interesting feature of the Ref 5 SE is that it's supplied with a 20A IEC power cord, not one of the standard 15A cords we're all used to. I would have liked to experiment with different power cords, but no dice: Although ARC's extremely heavy cord is removable, it takes a different type of IEC connector. I used the Ref 5 SE with both single-ended and balanced sources, and though I experimented with both types of outputs, I found that I marginally preferred the sound with the Ref 5 SE's balanced outputs driving my VTL amps. In any configuration and throughout my listening, the ARC was completely quiet.

Performance
The Audio Research Reference 5 SE's lineage and $12,995 price led me to expect a lot from it, and it didn't disappoint. I can sum up my impressions of it in one word: "Wow!"

The Ref 5 SE was no shrinking violet of a preamp. It was big, bold, brash, and brassy, in exactly the way live music is big, bold, brash, and brassy. While listening to Howard Hanson and the Eastman-Rochester Orchestra's Fiesta in Hi-Fi (CD, Mercury Living Presence 434 324-2), I was repeatedly taken aback at how the Ref 5 SE reproduced this recording's full dynamic spectrum and harmonic richness. Bass drums exploded and boomed with a wonderfully round, taut tone. Castanets cracked with perfect woodiness. Cymbals rang and shimmered, maracas hissed and clattered, violins sang, cellos swooned, basses groaned ominously. You name it, the Ref 5 SE nailed it.

Nor did the Ref 5 SE shine only with sonic spectaculars. It was equally impressive with small, intimate recordings, such as Doug MacLeod's Brand New Eyes (CD, Reference Fresh FR-703CD). Its incredible resolution and recovery of fine detail made it seem as if several layers of the recording chain had been wiped away and I was in the room with MacLeod.

Regardless of the genre or scale of the music, the Ref 5 SE brought about a fundamental, not-subtle jump in my system's performance. It was like hearing a good direct-to-disc record for the first time, or maybe an analog master tape. It wasn't just a question of being a little bit better, or better in one or two ways—it was a broad-brush, wholesale improvement, like going from two dimensions to three, or from black-and-white to color. It was almost the sort of fundamental change in the fabric of the music that (dare I say it) separates live music from recorded. This isn't to say that my system magically became the real thing with the Ref 5 SE installed, but it was definitely a step closer.

The ARC's reproduction of timbres was uncanny; it seemed to be painting the music with a tonal palette both bolder and more nuanced than I'd heard before. Tonal colors were richer and denser, harmonic structures more complex. With the Ref 5, instruments sounded more like themselves, more alive. Violas were more distinct from cellos and violins, and each woodwind retained more of its inherent characteristics, whether it was the woody, airy tone of a clarinet or the reedier buzz of an oboe. The rich way the Ref 5 SE portrayed instruments also made it easier to discern individual musicians in a chorus or orchestra, the differences in timbre and texture enhancing the image specificity to create a nearly holographic effect. With recording after recording, I noticed more complexity in background voices and passages—and not just across the midrange, but from the very bottom to the very top of the audioband, and across the full range of volume, from the faintest pppp shadings to thundering ffff climaxes.

---

Footnote 1: I bought a sample of the Audio Research SP-10 after I reviewed it for the English magazine Hi-Fi News in May 1984. It was both the most expensive and most enjoyable audio purchase I had made at that time and I still have it, occasionally taking it out for an satisfying canter around my music collection.— John Atkinson

Footnote 2: The Reference 1 was reviewed by Wes Phillips in the November 1998 Stereophile; the Reference 2 by Michael Fremer, in September 2000; and the Reference 3 by Paul Bolin, in December 2006.

Article Continues: Page 2 »

Company Info Audio Research Corporation 3900 Annapolis Lane N. Plymouth, MN 55447-5447 (763) 577-9700 www.audioresearch.com		Article Contents Page 1 Page 2 Specifications Associated Equipment Measurements

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Recently, I thought about all the audio shows I've attended over the last 27 years, looking for any pattern that all of them might have shared. I came up with a handful of audio manufacturers that have earned at shows a reputation for getting, year after year, consistently good sound—rooms in which I could reliably depend on being able to chill out and enjoy music in good, involving sound. Those companies include Audio Research, Music Hall (distributor of Creek and Epos), Vandersteen Audio—and Definitive Technology. Since their founding, in 1990, Maryland-based DefTech has been a major presence at shows, displaying an increasingly wide range of high-value speakers for two-channel and surround-sound systems. But I'd never reviewed one of their models. I thought it was about time.

Of the 25 two-channel loudspeakers Definitive Technology designs (in the US) and manufactures (in China), ranging in price from $290 to $3998 per pair, I chose the StudioMonitor 55 ($598/pair). (You can read Stephen Mejias's thoughts on the SM55's little brother, the StudioMonitor 45, in his "The Entry Level" in the August 2012 issue, with a "Follow-Up" from John Atkinson in October 2012.) This two-way, biwirable bookshelf model has two drivers and a passive bass radiator. Its 1" aluminum-dome tweeter has been heat-treated to relax the crystal structure of the metal, then coated with ceramic to extend its high-frequency response. The 6.5" mid/woofer has a cast aluminum basket and incorporates DefTech's patented Balance Double Surround System (BDSS), which supports the cone at its inner and outer edges for longer, more linear excursions.

Having reviewed several dozen reflex-loaded bookshelf speakers that achieve low-frequency extension via a front- or rear-firing port, I was intrigued by DefTech's unique solution: Instead of a port, the SM55 has a top-firing passive radiator measuring 6" by 10". As DefTech's Paul DiComo explained to me, the company prefers bass radiators over ports because: 1) they effectively block midrange frequencies, which can create midrange smearing; and 2) they produce less distortion and avoiding "chuffing" noises. The bass radiator has a basket and spider but no magnet or voice-coil; its cone is of MDF with a vinyl laminate. DefTech claims that using a passive radiator provides greater control, definition, and speed.

The SM55 has two grilles: one each for the front and top panels. I listened to the speakers with all grilles on, all grilles off, and with the front grilles off but top grilles on. In all three arrangements, the sound was identical—hats off to DefTech for designing a couple of transparent grilles. However, I preferred the SM55's looks with its front grille off and its top grille on. Removing the front grille reveals the sexy, gloss-black baffle while leaving all grilles on gives the black wood-grain SM55 a drab, boxy appearance. Removing only the top grille gives the speaker a ridiculous scalped-cowboy look. I biwired the SM55s and placed them on my Celestion Si stands, which are loaded with sand and lead shot. (DefTech doesn't make a dedicated stand for the SM55.)

Listening
To test a speaker's midrange integrity, I begin most listening sessions with well-recorded voices. With all recordings I sampled, the Definitive Technology StudioMonitor 55 passed with flying colors. In "You Are but a Dream," from the Frank Sinatra singles compilation This Is Sinatra Vol.2 (LP, Capitol W982), the voice of Ol' Blue Eyes was bathed in a reverberant and silkily mellifluous glow. Bob Dylan's voice in "Sad Eyed Lady of the Lowlands," from a pristine reissue of his Blonde on Blonde (LP, Columbia/Simply Vinyl 0162), was as natural as I've heard through any speakers, regardless of price. Higher in the audioband, Carole King's "You've Got a Friend," from her Tapestry LP (Ode SP 77009), floated in natural holographic splendor throughout her vocal range; and the harmonies in "Twelve-Thirty (Young Girls Are Coming to the Canyon)," from the Mamas and the Papas' Farewell to the First Golden Era (LP, Dunhill 50025), assumed angelic qualities. Finally, the SM55 enabled Louis Armstrong and Velma Middleton's voices in "Honeysuckle Rose," from Satch Plays Fats (LP, Columbia CL708), to seamlessly blend while retaining each artist's unique phrasing.

The SM55's midrange created an open and natural window for jazz recordings. Anat Fort's piano in her A Long Story (CD, ECM 1994) emerged as warm and inviting in its lower register, with perfectly linear low-level dynamics. And John Coltrane's tenor saxophone in Thelonious Monk with John Coltrane (LP, Jazzland 946), and Roland Kirk's tenor in "Oh, Lord, Don't Let Them Drop That Atomic Bomb on Me," from Charles Mingus's Oh, Yeah (LP, Atlantic 1377), generated identical responses for my notes: "forward, natural, and silky."

Jazz recordings also brought out the SM55's greatest strength: its midbass definition. Mingus's double bass in the title track of his Pithecanthropus Erectus (CD, Atlantic AMCY-1036), and in "Theme for Lester (Goodbye Pork Pie Hat)," from Mingus Mingus Mingus Mingus Mingus (LP, Impulse! AS-51), was rich, woody, open, and airy, with perfect definition.

Satch Plays Fats showcased the DefTech's ability to reproduce natural, extended highs. In his solo in "Ain't Misbehavin'," Armstrong's trumpet was biting and extended but without a trace of harshness. The SM55's high-frequency resolution also showcased its ability to render fast transients with abundant detail and no hint of smearing. It revealed all the subtle details of Monk's piano phrasing in "Epistrophy," from the aforementioned Coltrane-Monk record. Similarly, the SM55 revealed all the subtle nuances of pianist Jesse Stacken's phrasing in his lyrical, fleeting solo in the title track of trumpeter Liam Sillery's Phenomenology (CD, OA2 Records 22061). And I was able to individually follow each of the six percussionists on Tito Puente's Top Percussion (LP, RCA 1617).

However, all the jazz recordings mentioned above also revealed the SM55's single shortcoming. Although its high frequencies were natural, extended, and completely devoid of coloration or smearing, over them was layered a subtle powdery texture, and particularly over the drum kits. This created a slight lack of liquidity and continuousness in the highs that was not a problem in the midrange and bass. This was a minor problem, and I didn't notice it with most recordings.

I found myself unusually engaged and focused when listening to rock music on the SM55s—and my feet wouldn't keep still. Speaking of moving feet, the notes I took—when I could stop moving long enough to write—for "Let a Man Come In and Do the Popcorn," from James Brown's Soul Classics Vol.2 (LP, Polydor SVLP 127) read, "Churning! Coherent!" I then cued up a recent acquisition, The Complete Animals (LP, EMI EM1367), to discover that the compilers of this 40-track set had misled me. I looked for my favorite Animals tune, "Don't Bring Me Down," and found it missing (a royalties problem with songwriter Carole King, perhaps, or perhaps because it had not been produced by Mickie Most. (The collection's subtitle is The Complete Mickie Most Productions for EMI.) So I went to my second choice, "We Gotta Get Out of This Place," which moved me more than when I saw this Newcastle band perform the piece live at a reunion concert in the 1980s.

With other rock recordings, the SM55 favored electric guitars. Jack White's axe in the White Stripes' White Blood Cells (CD, V2 63881-27124-2) was buzzy, raspy, and forward—as it should be. And the interaction of the Ventures' two guitars in "Out of Limits," from The Very Best of the Ventures (LP, Liberty LN-10122), was clearly delineated; it was very easy to follow individual lines and phrasings.

I've saved the best news for last. The StudioMonitor 55 proved to be an awesome classical-music speaker. Louis Andriessen's orchestral blockbuster De Stijl is a combination of subtle delicate orchestral textures and a bombastic sense of drama. When I listened to a recording of this remarkable work by the Schoenberg and ASKO Ensembles conducted by Reinbert de Leeuw (CD, Nonesuch 79342 2), the DefTechs revealed its full dynamic range, from ppp to fff, on a wide, deep stage with gobs of ambience. And Marcel Dupré's solo organ performance of his own Prelude and Fugue in G Minor, from his Organ Recital (LP, Mercury Living Presence SR 90169), revealed delicate, airy highs along with a big, bloomy organ sound and a sense of a real instrument playing in a real space. And for a bookshelf speaker, the SM55's reproduction of the pedal notes in this recording were startlingly realistic.

Comparisons
I compared the Definitive Technology StudioMonitor 55 ($598) with the Epos ELS3 ($350 when last offered), the Epos M5i ($899), and the Wharfedale Diamond 10.1 ($350). (All prices per pair.)

The Epos ELS3 had a silkier midrange, with longer decays and more ambience, than the DefTech SM55. Its highs were also more sophisticated, detailed, and delicate. The ELS3's upper bass was cleaner and its midbass leaner than the SM55's, and the Epos was far inferior in high-level dynamics.

The Wharfedale Diamond 10.1 had a warmer midbass than the DefTech SM55, and more delicate and detailed highs, but its high-level dynamics were slightly inferior.

The Epos M5i had the richest, most detailed midrange of all four speakers, the best low-level dynamics, and the most delicate and sophisticated highs. The M5i's bass went as deep as the DefTech's, but more cleanly. The M5i's and SM55's high-level dynamics were equivalent.

Summing Up
Definitive Technology has produced a detailed and uncolored bookshelf speaker that provides superb value for the money. And in terms of its midbass definition and high-level dynamic drama with orchestral works, you'd probably have to spend more than $1000/pair to exceed its performance. I'm sorry I waited so long to review a Definitive Technology speaker. Next time, I won't wait.

Article Continues: Specifications »

Company Info Definitive Technology 11433 Cronridge Drive, Suite K Owings Mills, MD 21117-2294 (800) 228-7148 www.definitivetech.com		Article Contents Page 1 Specifications Associated Equipment Measurements

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Ever since encountering KingSound electrostatic loudspeakers at an audio show several years ago, I’ve looked forward to seeing how their line would develop. This time around, KingSound was showing its King III Full Range ESL ($12,000/pair). Driven by Bob Carver Cherry tube monoblock amplifiers ($7400/pair), Purity Audio Design Statement 2 preamplifier (approx. $12,500), and an AMR CD777 ($12,000), all hooked together by Kaplan Cables from John Atkinson’s adopted hometown of Brooklyn, the system was a joy to listen to on Chet Atkins’ recording of “Mr. Sandman.”

No one knew whose period-instrument rendition of Bach’s Brandenburg Concerto No. 2 was playing next, but I thought the sound warm, lovely, and delicious. Despite some weakness in the lower line of instruments, the color of the higher violins was spot on.

Apologies if names and prices are slightly off. Shall I tell you how many exhibitors at RMAF had neither equipment sheet handouts nor correct reference information for their products on hand?

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When I was seven years old, my father brought home an air-conditioner: a flimsy, hulking thing that sat in an open window just as a frame of X-ray film sits in an open mouth. It worked by consuming a steady diet of ice cubes and water, then blowing a fan across the slurry and into our grateful living room. It was loud and it stank and it wasn't very cold at all.

Air-conditioners have improved since 1962. So have automobiles and contact lenses and light bulbs and copiers and television sets. So have lots of other consumer goods.

It's reasonable for the average person to assume that hi-fi gear has also improved over the years, but that isn't entirely true. Although the audio industry has made strides in eliminating tonal distortions and in creating for the listener a more exciting spatial presentation of recorded sound, to me early playback equipment endures in sounding far more tactile, dramatic, and colorful. In those respects, contemporary gear has not only failed to hold its ground since 1962, it has markedly declined.

In other consumer industries, developmental gains appear to come at relatively little cost. In the quest for cars that are safer, faster, and more comfortable, fuel efficiency hasn't merely remained steady: It has improved, along with everything else. Yet audio engineers often seem no better at banishing distortions than I am at shooing flies out of the house: They can't get rid of one without letting in another. Amplifier designers sacrifice immediacy and speed for power, phonograph designers sacrifice musical dynamics for low tracking force, and loudspeaker designers swap efficiency for extended bass and treble.

Think of it: A loudspeaker's most fundamental task is to convert electricity into sound—a task at which the average new model fails more resoundingly with each passing year. What the hell kind of progress is that?

Funny you should ask
The up side of living inside an engineering swap meet: One can choose from a very wide range of commercial products, each with a different combination of strengths. As I noted in last issue's column, all in this hobby are free to prioritize for themselves those characteristics of live music to which the highest playback fidelity should be applied—and some of those aspects are indeed served well by combinations of powerful amplifiers and inefficient loudspeakers. In years past I have been not only impressed but delighted by such things as Stax F-81s driven by a Krell KSA-50, and Magneplanar MG-IIIs paired with a Conrad-Johnson Premier One. Those and other such systems can be considered every bit as good as the sort to which my present tastes have led me: just as faithful, but faithful to different aspects of the art of music.

As I've also pointed out, listeners who assign a greater importance to impact and color in music playback will find much to enjoy in certain vintage loudspeakers. Yet to find, buy, or install those speakers may not be as easy as one might wish. RCA LC1s don't turn up for sale very often. Western Electric 753s are even less likely to appear on the market—and when they do, their prices are horrifying. Altec Flamencos and Valencias weigh nearly 100 lbs each and tend not to fit in many compact cars—or compact rooms.

In this month's column, then, the question switches from Why not vintage? back to Why not new? In response to a growing interest in all things antique, not to mention an enduring shortage of full-range loudspeakers capable of being driven by low-power tube amplifiers, some contemporary manufacturers are bringing to market speakers whose vintage DNA is unmistakable. Five such models spring immediately to mind, two of which I've auditioned at length, another two of which—the DeVore Fidelity Orangutan O/96 and the current-day version of the Klipsch La Scala—are due to arrive here within the next month or two, and one of which remains elusive. The last is the classic Western Electric 753: not the extinct original, of course, but faithful reproductions presently offered by two different companies, with a third on its way. Shindo Laboratory and Déjà Vu Audio both offer custom-built speakers inspired by the original 753—in both instances, price varies with the exact driver complement desired—and Auditorium 23 plans to release one in the near future, as part of their Hommage series.

That brings us to two interesting and very different vintage-vibe speakers of my recent experience:

Oswalds Mill Audio Mini
Knowing that the other three loudspeaker models from Oswalds Mill Audio are built—and priced—on a grand scale, one might expect that the comparatively small and affordable OMA Mini ($18,000/pair) might also be comparatively disappointing: a compromise product for which people with smaller rooms and budgets are forced to settle. Nothing could be further from the truth, and after two visits to OMA's showroom in Brooklyn's Dumbo district—Down Under the Manhattan Bridge Overpass, for you non-New Yorkers—the Mini remains my favorite of their speakers in a number of ways.

OMA's loudspeakers are distinguished by their use of conical horns, cast from aluminum at a foundry near their workshop, in rural Pennsylvania. So it goes with the Mini, in which a proprietary horn-loaded compression driver handles the high and mid-frequencies. The two-way Mini also uses an 8" woofer in a ported enclosure made of the buyer's choice of birch ply or bamboo ply. The cabinet walls are thick, the quality of the joinery just as impressive; the front-firing port runs nearly its entire 14" width. The Mini's carbon-fiber woofer has an impregnated fabric surround and appears rather stiffly suspended; that plus the narrow port—the enclosure seems to split the difference between bass-reflex and aperiodic designs—combine to make what one assumes is an intentionally low-Q system in which smoothness of response and sensitivity (rated at 95dB) are emphasized over bass extension per se.

Listening bore that out. Aesthetically, the Mini's upper-frequency horn stands out from the rest of the speaker, but sonically the opposite was true: OMA's smallest loudspeaker is remarkably coherent. Its crossover, which I'm told is quite simple, is apparently also quite well done.

That quality is complemented by a well-chosen frequency range. The Mini's bass response, specified as 3dB down at 60Hz, is balanced by a treble range that leaves the speaker sounding neither dull nor lightweight. At OMA's Dumbo showroom, a pair of Minis gave fine weight to smaller-scale orchestral music—a 1961 recording of Igor Stravinsky conducting the Columbia Chamber Ensemble in his L'Histoire du Soldat Suite sounded amazingly lifelike—and modern pop alike. Far more important was the generous sense of touch the OMA speakers gave to that and virtually every other recording I tried over the course of two listening sessions there. Plucked strings—never in short supply in Stravinsky—leaped from the system as they should have. (And what are horns for if not for tactile musical thrills such as that?)

The styling of the OMA Mini, by the industrial designer and lighting specialist David D'Imperio, is distinctive: wood grain here, steel legs there, curves, angles, visual weight, and open spaces. One assumes a pair of them would go nicely with contemporary furnishings, yet I've seen proof of how their whimsical modernism jibes with the rich textures of antique furnishings and Oriental rugs. What I haven't seen is how they look in my listening room; as of this writing, review loaners aren't available. But hope springs eternal: I think this is a wonderful speaker that asks prospective owners to take a few chances, and richly rewards them with real-sounding music.

Line Magnetic 755 I
I wrote last month about the famed Western Electric 755a, a full-range drive-unit admired by most vintage-speaker experts of my acquaintance. Now a company called Line Magnetic Audio has begun to manufacture their own version of this classic. Their new 755 EX driver is a faithful reproduction of the 755a in every way but two: Its nominal impedance is 8 instead of 4 ohms, and, instead of a permanent magnet, the Line Magnetic 755 EX uses a field coil—an electromagnet—that's energized by DC from an outboard power supply.

This is no mere affectation, vintage for the sake of vintage: The magnetic flux density of a permanent magnet is known to change in response to vibrational energy and heat—of which moving-coil speakers are reliable producers—whereas a field coil with a very good DC supply can focus a far stiffer, stabler, sturdier energy field on the voice-coil gap, allowing dynamic contrasts in particular to be resolved with far greater efficiency.

This new-old 755, keenly anticipated for years, is available now in a loudspeaker called the Line Magnetic 755 I ($8995/pair), in which two field-coil 755 EX drivers are installed in floorstanding cabinets of original design and bundled with a pair of Line Magnetic PR-3 field-coil power supplies, all made in the People's Republic of China. System sensitivity is specified as 94dB.

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With the introduction of Audio Alchemy's Digital Transmission Interface (DTI) more than three years ago, the company created an entirely new category of hi-fi product: the jitter filter. The original DTI was a good start, but didn't always improve the sound of the better-quality digital front-ends.

The DTI•Pro, released 18 months later, bore little similarity to the rather simple DTI. The Pro added a more sophisticated dual Phase-Locked Loop (PLL) input receiver for much lower jitter, and also performed a new type of Digital Signal Processing (DSP) that Audio Alchemy called "Resolution Enhancement." This processing reportedly increased the resolution of the compact disc's 16-bit digital words, approximating 18- or even 20-bit resolution. It did this by looking at the 16-bit words over a time window, and calculating the additional bits that would have been present had the signal been originally encoded with higher resolution than 16 bits.

In my November 1994 review of the DTI•Pro (footnote 1), I called it a "breakthrough in digital audio reproduction" and "a musical revelation." When inserted in the playback chain between a transport and digital processor, the DTI•Pro significantly improved the sound of my system. Moreover, experimenting with the output word length suggested that the Pro's benefits were indeed rendered by Resolution Enhancement, not just jitter reduction.

Shortly after the DTI•Pro was launched, Star Semiconductor, the manufacturer of the Pro's DSP engine, temporarily went out of business. Although Audio Alchemy scoured the globe and hoarded all remaining Star DSP chips, it was clear that the DTI•Pro's days were numbered.

Audio Alchemy took the opportunity to redesign the Pro with a 32-bit DSP chip from Texas Instruments, hence the product's new name, the DTI•Pro 32. The new chip reportedly has 50% more computing horsepower to run the Resolution Enhancement algorithm. The Pro 32 also includes some operating improvements over the Pro. Fortunately, I still have an original DTI•Pro on hand for comparison with its successor.

Description
The DTI•Pro 32 is housed in Alchemy's standard 8.5" wide and 2" high chassis. From the front panel, the DTI•Pro 32 looks identical to the DTI•Pro. The front panel's 10 LEDs indicate the input selected, power to the 32's three main sections, when the unit is locked to a source, and whether the phase or polarity has been inverted.

The lock indicator actually requires two LEDs, marked "Primary" and "Secondary." The Primary lock shows that the Pro 32's first PLL stage has locked to the source, and the Secondary LED indicates the low-jitter "double-lock" condition. The Pro 32 should double-lock to most CD transports. More on this dual PLL later.

The tiny rear panel is consumed by input and output jacks. Input is via coaxial (BNC) jack, TosLink optical, or Alchemy's I²S bus. The addition of the I²S bus input is new on the Pro 32, and is included to accept I²S output from Alchemy's forthcoming CD transport. For an additional $179 you can swap the TosLink input for an ST-Type optical jack. The Pro 32's output appears on a BNC jack, AES/EBU connector, ST-Type glass optical jack, and I²S bus. A DC input connector, which accepts a plug from the Pro 32's small external power supply, finishes the rear panel.

An I²S connector looks like an S-Video jack, but has five pins rather than S-Video's four. The five pins carry left and right audio data on one line, bit clock, word clock, master clock, and the emphasis flag. By separating the clock and transmitting it independently of the audio data, the unit receiving an I²S-formatted signal need not "lock" to the incoming datastream and "recover" a clock. The result is nearly total immunity to interface-induced jitter. Indeed, the whole idea of extracting a clock signal from the audio data is a fundamentally flawed approach to transmitting digital audio. Note that when one of Alchemy's processors with I²S input is connected to the DTI•Pro 32's I²S output, the familiar and comforting "lock" LED doesn't illuminate on the processor's front panel. Don't worry; the processor will still decode the I²S data.

Seven user-selectable operating modes optimize the Pro 32's output for your digital/analog converter. These modes set the output word length (16, 18, 20, 22, or 24 bits) and other output conditions. One mode turns off the dither, but performs Resolution Enhancement. The output word length in this no-dither mode changes with the program source, with the algorithm deciding how many additional bits it can reliably interpolate. Turning off the DTI•Pro's dither can be a benefit with some digital processors that are already dithered internally.

The seventh mode turns off the Resolution Enhancement processing for HDCD$r playback. HDCD decoding requires that the control code buried in the least significant bit of the CD's 16-bit words arrive at the HDCD filter in your digital processor. Any change in the data—such as that introduced by Resolution Enhancement—corrupts this control code and prevents HDCD decoding. This is why the Pro 32 offers the HDCD bypass mode. Note that this HDCD bypass option wasn't offered on early versions of the Pro.

Choosing the correct output word length is crucial to getting the best performance from the DTI•Pro 32. Let's say you have a digital processor with 18-bit DACs, and you set the Pro 32's output word length to 20 or more bits. The DACs will simply truncate (cut off) any bits below 18, introducing noise and distortion. As JA described in his review of the Meridian 518 Mastering Processor in the January '96 Stereophile, truncation also hardens midrange textures and reduces the sense of space.

Conversely, setting the Pro 32's output word length to 18 bits if you have a true 20-bit processor prevents you from experiencing the full benefits of Resolution Enhancement.

Just because your digital processor has 20-bit DACs doesn't mean that it will pass 20-bit data from input to output. The NPC 5813 and 5813 digital filters, for example, truncate the incoming data to 18 bits. The older Yamaha YM3623 input receiver chip will pass only 16-bit data. For comparison, the Crystal Semiconductor CS8412 and UltraAnalog AES21 input receivers will pass up to 24-bit data, as will the Pacific Microsonics PMD100 HDCD decoder/filter.

There's yet another trap to be aware of. Digital processors with the Crystal or UltraAnalog input receivers, the PMD100 filter, and 20-bit DACs still may not pass 20-bit data. Some processors that have been retrofitted with the PMD100 weren't redesigned to pass the PMD100's full resolution to the DACs. Two such examples are the PS Audio UltraLink Two and Enlightened Audio Designs DSP7000 Series III. Each of these processors has only a 16-bit data path from the digital filter to the DACs. Consequently, they'll truncate 20-bit input data, even though they have a 24-bit input receiver, 24-bit digital filter, and 20-bit DACs. Any processor designed from the ground up around the PMD100 (as opposed to an existing design retrofitted with the PMD100) should pass at least 20-bit data. The only way of finding out which "20-bit" processors truly pass 20-bit data is by the measurements included in reviews. Reports from readers suggest that some processor manufacturers either aren't aware of their products' capabilities, or provide misleading information.

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Footnote 1: See my review of the DTI in Vol.16 No.5, DTI measurements in Vol.16 No.11, and DTI•Pro review in Vol.17 No.11.

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