Pass Labs XA160 monoblock power amplifier
Michael Fremer, November, 2003
Or are they? While the industry-feminizing tiny triode set has made a great deal of noise in the past few years (I can hear them hissing now), soft-walking, big-stickcarrying, mega-power amplifiers still circle the globe.
Softspoken audio veteran Nelson Pass has been building such behemoths for years. Between 1993 and 1998, his Pass Laboratories marketed the Aleph series: massive, powerful solid-state amplifiers designed to buck the industry trend toward ever-increasing circuit complexity. Aleph amplifiers were "single-ended, class-A designs biased by a constant DC, current source which negatively 'ghosts' the speaker load with simple but effective current modulation," according to patent No. 5,710,522.
If that sets your head spinning, consider that an Aleph amp contained but two gain stages—compared to nine, for example, in a Mark Levinson No.333. The simplicity of these class-A designs came at a price: inefficiency which meant lots of heat. And while the Aleph's sound was said to be incredibly pure and in many ways tube-like, its bottom end apparently lacked punch. In other words, the Aleph was the antithesis of what you might expect from a Krell amplifier, for example.
In 1998, Pass Labs introduced the X Super-Symmetric series, designed to attack the Krell-Levinson market with added bass and dynamic Viagra. Another patented design (No. 5,376,899) using a balanced circuit, the X amps used the same simple two-stage approach used in the Alephs. The X1000, Stereophile's "Amplifier of 1999," which put out 1000W into 4 ohms, was scaled down to produce the X600 and X350 monoblocks, as well as stereo versions of each.
Negative feedback was used in these dual-differential designs, not to reduce overall distortion but to make each complementary half of the circuit as identical as possible, to maximize the noise- and distortion-canceling benefits of the balanced design. According to Nelson Pass, the typical balanced design cancels out noise and distortion by a factor of 10. His approach, he claims, reduces overall distortion by as much as 99% without large amounts of feedback.
Enter the XA series
Nelson Pass says the X and Aleph designs are not mutually exclusive, so he set about combining them in order to produce a "warm, sweet" X amplifier or a "powerful, dynamic" Aleph. Or, as the white paper puts it, "Like Peanut Butter and Chocolate."The resulting XA design has two stages: a balanced pair of MOSFETs acting as an input stage, and an output stage consisting of a balanced pair of power MOSFETs biased into class-A operation. As in the X amps, feedback is used only to set the gain and bring the halves into symmetry. There are no adjustments, and no frequency-compensating capacitors.
According to Pass, the XA amps operate at approximately 33% efficiency, which means they idle at three times their rated output. In the case of the XA160, that means a bit over 500W—and twice that in stereo. That's a lot of heat! So when you're not listening, Pass suggests you leave the XA160 in standby mode, which keeps the caps charged and ready to go.
When powered up, the XA160 is rated at 160W RMS of class-A power into 8 ohms. Reducing the load impedance will not increase power delivery—nor, as the tonguein- cheek instructions say, will driving the amplifier into a short. According to the manual, "160W driving a 87dB/1W/1m efficient speaker will deliver approximately 110dB average acoustic signal in a 100 cubic meter room," with transient peaks somewhat louder.
The XA160's chassis is a refined version of the one used for the massive and powerful X600. The XA160 weighs 200 lbs, which means each monoblock requires two not-on-steroids people to schlep it. The amp's exterior is among the most dramatic-looking and aesthetically pleasing I've ever seen or stroked with quivering fingers. Your reaction may be different, but I doubt it. The industrial design (by former Krell designer Desmond Harrington, who, Nelson Pass says, moved west for the kayaking possibilities) speaks for itself, so I won't describe the faceplate's exquisitely milled multiple angles, which result in smooth, satiny surfaces; or the beefy, curved cutouts exposing, caressing, and cradling the massive, blue-backlit meter with its long, stiff needle, or the machined perfection of every cool-to-thetouch facet, or how ambient light slides seductively off of its muted façade.
Inside is a massive Piltron 3KVA toroidal transformer operating at a fraction of its rated power into high-speed, soft-recovery rectifiers coupled to six computer-grade 25,000µF, 75V capacitors, creating unregulated output-stage DC rails, which are then RF-filtered. Pre-packaged T0-3 metal-can power MOSFETs (HexFETs from International Rectifier and Harris), sourced from the same lot codes, are "hypermatched" with gate voltages matched to 0.05%. Each output device is rated at 125W/14 amps, but operated well within that rating. According to Pass Labs, the massive heatsinks run at about 25 degrees C above ambient temperature, meaning the output devices run about 70 degrees C below rated operating temperatures. Thus, Pass Labs conservatively estimates the XA160's lifespan at 50,000 listening hours. Given their cost of $18,000/pair, that will be just enough time for some of you to get them paid off.
Hoisting them into place
You shouldn't need a forklift to review any piece of audio gear, and the XA160s didn't require one—but they came pretty close. Placing them on any kind of platform—even a stand a few inches off the ground—is a two-person job. There are handles, but you might as well bolt a handgrip onto your aluminum siding and try separating your house from its foundation. A friend and I put the XA160s on a pair of Finite Elemente's Pagode stands. Two sets of manly (as opposed to Manley) binding posts, featuring finger-friendly sculpted plastic wings, make connecting cables a pleasure. Just don't overdo the torque.There are single-ended RCA and balanced XLR inputs. The rear panel offers 12V trigger turn-on, a standard 15 amp IEC AC jack (the XA160 draws about 5 amps), and a power switch and circuit breaker. Plug the amp in, flip the breaker, and a tiny blue LED at the top of the meter lights up. Push the large front-mounted button and the meter face glows cool blue as the needle rises to about a quarter past dead center. It never moved from that position during use. An hour's warmup is suggested for maximum sonic performance, but these amps sounded pretty deep from a cold start.
Warmed up, they sounded even deeper
As advertised, the XA160 sounded sweet, delicate, and rich. If Parasound's Halo JC 1 amp, which runs in class-A for the first 25W, smudged the dividing line between tubes and solid-state (see my review in the February 2003 Stereophile), the XA160 erased that line—even at ridiculously high volumes. I'm not suggesting it sounded precisely like a tube amp, just that most listeners' objections to solid-state will be erased.As I had for the Halo JC 1, I immediately fell for the XA160. "Who wouldn't go for this?" I heard myself say, so rich, coherent, relaxed, and inviting was the sound. Yet the amp delivered sufficient transient speed and detail to also be exciting, immediate, and involving with every type of music.
The XA160s' presentation was more about context than counting cymbal rivets. On good live recordings such as The Weavers' Reunion at Carnegie Hall, 1963 (45rpm LPs, Vanguard/Classic), it emphasized the size and the weight of the space in which an event occurred, rather than shining a spotlight on the players and letting me see them individually in stark relief. Rear-wall reverberant information was sufficiently well revealed to clue me in to the shape of the stage and how far forward on it the performers stood. From that Weavers LP I've heard more stagefront image presentations, more distinct rear-wall reflections, and longer trails with other amps, although those were somewhat to the detriment of the overall picture's coherence and to the sense of "weight" and body. The convincingly dense, solid images produced by the XA160s were about the most tangible I've heard in my system.
The XA160s effectively jelled the picture without coating it with goo, though some listeners will prefer a more open, airy sound. Others will say that open, airy sound lacks weight and solidity, and around and around we go...which is why you'll hear such a wide variety of presentations at hi-fi shows, all of which will be declared "lifelike."
As much as I admire Classic Records' vinyl edition of Norah Jones' Come Away with Me for its music and its sound, and used it extensively during the Parasound review, I've had enough of it for a while. I turned to the new Jacintha album, Jacintha is her name (Groovenote GRV101)—a tribute to Julie London and modeled on Julie is her name—on SACD and two 45rpm LPs. I had no context for the sound, but I listened to the album through four different amplifiers, and I certainly know the sound producer Joe Harley and engineer Michael C. Ross get at Oceanway Studios. The XA160s had a subtle, velvety character that floated images on soft cushions of air rather than suspending them starkly in space. The XA160's sound was particularly suited to the female voice, highlighting the lower register and displacing any tendency toward thinness and reediness, but without thickening or adding unnatural chestiness.
You don't want to hear Jacintha sing a jazzified version of the Doors' "Light My Fire" backed by flute, seven-string guitar, and congas? Maybe that's because it sounds worse on paper than it does in the ear, which is surprisingly good. Should you listen, you might like it (though right now I'm more in the mood for musical assaults from groups like MarsVa and Melt-Banana). But you'll definitely appreciate the butteryrich recording. Through the XA160s, the flute had a bit less air and a bit more roundness than through some of the other amps I tried, the congas had more wood and less hand slap, and the guitar had more body than string—but the overall presentation was so subtly turned in that direction that the result was more warmly seductive than smothering. A Merlot-like finish for the ears, for you winos—er, oenophiles.
Great audio gear is about balance, not "neutrality." In my experience, while neutrality would be the ideal, it doesn't exist. What do exist are some nearly ideal recordings, such as the new Jacintha album (or any of hers), which doesn't sound bright or etched, as many commercial recordings do. If a system leans too far toward warm and rich, a warm, rich recording will get smothered. While the XA160's overall flavor was lush and velvety, it wasn't so to the point of throwing a wet blanket over the sound. However, I'd be careful about mating the XA160s with speakers that sound overly warm. The Wilson Audio WATT/Puppy 7s and Audio Physic Avanti IIIs turned out to be good matches.
There was nothing soft about the XA160's bass performance. It effectively controlled the WATT/Puppy 7's woofers on "It Don't Mean a Thing if it Ain't Got that Swing" on the Modern Jazz Quartet's European Concert (2 LPs, Atlantic SD 2-603), which, because of what sounds like sympathetic stage vibrations, can sound sloppy and bloated. Through the XA160s, I heard each of Percy Heath's finger-slaps distinctly, even if leaner amps delivered them in a somewhat more nimble manner. If you want nonmechanical bass weight, heft, and texture, the XA160 should deliver. With them, I found the bass weight, speed, texture, and drive of many familiar LPs and CDs to be outstanding, if slightly on the "buttery" side of ideal.
As you might expect, amplifiers that don't blow a great deal of air around the room won't throw the most enormous soundstage, and the XA160s didn't—not when compared to the far more expensive Halcro dm68s, which I was fortunate enough to have here for a spell. The Halcros produced a noticeably wider, taller, deeper, airier stage, with greater image projection in front of the speakers, accompanied by a brighter, crisper, "faster" overall picture—almost like the difference between a classic warm Koetsu cartridge and one of A.J. van den Hul's detailed, razor-sharp Colibris. Advocates of either cartridge can be found, but not of both.
While the Halcros proved to be detail champs, separating out musical strands almost to the point of isolation, the XA160s did a better job of presenting the wholeness of the event and of imparting a sense of relaxation and flow. The XA160s' ability to subtly present "room sound" and a recording's spatial context was noteworthy, but they did a better job with big halls than with small clubs.
On Chesky's excellent-sounding 4 generations of Miles SACD (SACD243—yes, I know, it was originally a high-resolution PCM source), recorded live in a modestsize nightclub, the XA160s' warm presentation somewhat obscured the room's short reverb time, and thus the sense of the space in which the event occurred. With the Halcro dm68s, and later the Musical Fidelity KWs, the stage opened, instrumental images (especially Mike Stern's Chorused electric guitar) moved forward in the mix, and I could hear the delay on the guitar with far greater clarity. George Coleman's tenor sax was more reedy while losing some of its body, and Jimmy Cobb's drums, especially the snare, had more percussive crackle, but with less skin. Audience handclaps were somewhat more reserved via the XA160s, and while the folks sitting closer to the mikes protruded forward from the front of the speakers with the Halcro and MF amps, the Pass Labs placed them against the baffles.
Overall, the XA160s' picture had more weight, focus, and density; the other amps, more expansiveness and effervescence. I wouldn't say either perspective was right or wrong; they were very different but equally valid takes on the same event. One of the XA160's most appealing qualities was its nonelectronic presentation—its freedom from etch, edge, grain, and grit while not sounding rolled-off or dark. Switching to the Halcro revealed greater transient speed and detail and a lighter, more nimble overall touch, but also apparent were more of the recording artifacts found on most discs, black or silver. What was all the more impressive about the XA160s' presentation was that, despite its sounding darker and richer, I couldn't say it was any less transparent than the other amps with which I compared it, and it resolved important ultra-low-level musical information from familiar discs as well as any of them, if falling somewhat short spatially.
Like choosing tires for a high-performance sports car, choosing the correct associated gear is key to getting sonic satisfaction from any high-performance amplifier. I found the Shunyata Andromeda speaker cables to be a bit too smooth for the XA160s, even driving the WATT/Puppy 7s. The same proved true of the Wireworld Electra Series III AC cords. Switching to the Harmonic Technology Magic Woofer speaker cables and Fantasy AC10 power cords tipped the balance toward just the right amount of treble extension and detail, subtle though the difference was. The balance tipped the other way with the Halcro dm68s—toward the Shunyata and Wireworld cables.
The XA160 produced a silky-smooth acoustic in which small dynamic gradations flowed effortlessly up and down the scale. When large excursions were called for, as in orchestral crescendos, the amp didn't wimp out but seemed to open up and deliver almost unexpected jolts of energy without strain or shifts in overall tonal character. If the XA160 had a slightly subdued overall character, this was noticeable only in direct comparisons with the Halcro and Musical Fidelity amps. Cymbals rang, but perhaps not as brashly as through other amplifiers. Handclaps placed more emphasis on the hands than on the claps, and male and female voices gave more weight to the body than to the larynx or mouth. But these emphases were so subtle—the whole message was delivered intact.
Edgy music such as the Japanese speed-punk-noise band Melt-Banana's cellscape (CD, A-ZAP AZCD-005) passed through the XA160 with its leading edges and rhythmic vitality preserved, yet the Pass Labs also delivered the same kind of convincing weight and solidity as when reproducing audiophile-quality recordings. An enviable balancing act.
Conclusion
At $18,000/pair, the 160W, class-A Pass Labs XA160 offers build quality and industrial design befitting a top-shelf amplifier. Every visitor to my listening room was wowed, including competing manufacturers, one of whom exclaimed, "Do you have any idea how difficult it is to machine a front panel like that?" Whatever your reaction to the XA160's sound, it's likely you'll have trouble taking yours eyes off its sleek, masculine good looks. As befitting an unusual design, the sound it produced was unique in my experience, which should be surprising—it's the first solid-state, pure class-A design I've encountered. The XA160 delivered a seductive combination of richness, solidity, purity, and cohesiveness that I'd not heard before. It was possible to relax into every genre of music at all times and not feel slighted. I've experienced other, very different, mostly brighter and airier presentations that also satisfied, but none has offered the remarkable image weight and density offered by a pair XA160s.The amp that has come closest to the XA160's sonic temperament was, not surprisingly, Parasound's more powerful and one-third-the-price Halo JC 1, which runs in class-A for the first 25W, and delivers 400W into 8 ohms and 800W into 4 ohms. Everything positive I said about that amp I can say about the XA160, perhaps more so; I say "perhaps" only because I didn't have Wilson Audio's WATT/Puppy 7s here when I auditioned the Parasounds.
That brings up the law of diminishing returns. Is the XA160 worth three times the price of the Halo JC 1? While it delivers less overall power, it produces six times as much class-A power, and that class-A sound can be mighty seductive—especially when you push the Pass Labs amp to deliver big orchestral climaxes. If you like the richness and purity of the XA160's class-A sound and you need more than 25W to drive your speakers, then yes, the XA160 is worth it. Also, given the balanced design's circuit configuration, I think you'd be wasting some of your money if you didn't run the XA160 in balanced mode.
Based on my listening, I recommend the XA160s to owners of WATT/Puppys, especially the 7s. The amp's top-end performance was a perfect complement to the WATTs' inverted-dome Focal tweeters. Is there enough power in your room for your speakers in your system? Given the XA160's unusual circuit configuration, that's a question I can't answer. You also should keep in mind the unusual amount of heat a pair of XA160s throw off: the equivalent of two 600W hair-dryers!
Your $18,000 also pays for some dramatic industrial design, and while that's difficult to put a dollar value on, if presentation is important to you, you get an endlessly satisfying eyeful. The Pass Labs XA160 is an amplifier that's easy to love, easy to listen to, and just as easy to recommend—if your equipment and listening conditions qualify, and if you can afford it.