Pass Labs XA160 monoblock power amplifier

Before the advent of big-screen projection televisions, manhood was measured more conventionally: by the size of one's crate-sized, boat-anchor-heavy, brushed-aluminum-fronted power amplifiers. Those days are long gone.

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-stick-carrying, 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 tongue-in-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-the-touch 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 stage-front 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.

Pass Labs
P.O. Box 219
Foresthill, CA 95631
(530) 367-3690