as reviewed by Ryan Coleman
Everything in life is a series of compromises.
There is no perfect spouse, no perfect child. No perfect job, no perfect situation, no perfect life, and no perfect ending. Life's not perfect, and neither are we.
And neither are our stereo systems. Every single one of them has flaws (even live music does: musicians are involved, and they're not perfect). If you want the continuousness of a planar, you give up the slam of a monkey coffin; if you want the warm organic flow of analog, you give up the non-existent noise floor of digital… and you might not find your software available.
But if we were so bold as to dream of what would be perfect, I know what I consider to be perfect in an amplifier. The perfect amp would have the sledgehammer wall-flexing grunt found in a solid-state bruiser. The perfect amp would have the crystalline blue water treble clarity of an OTL. The perfect amp would have the Victoria's Secret sexilicious midrange of a SET. And the perfect amp would be reliable, affordable, attractive, and at home with any loudspeaker.
I know of no such amplifier. None is perfect.
But let me tell you a bit about the Pass XA 60.5 monoblock amplifiers…
Pass Labs should be well known to every audiophile out there; their reputation for performance, reliability and value is an industry standard. While Pass Labs has branched into a successful line of preamps, integrated amps and even speakers, it's their amplifiers that gather the most attention. Indeed, if there's an amplifier designer out there who is more respected, renowned, or holds more patents than Nelson Pass, I know not who it is. In a hobby where manufacturers hold tight to intellectual property like the government does with little green men in Area 51, Pass Labs stands as the exception; there is a host of literature on Pass' web site chronicling Nelson's ideas, circuit design and engineering solutions, and perhaps unique to the industry he also tells you how to do much of it yourself (via the website passdiy.com for those interested in building some of his pet amplifier designs). Suffice it to say: any manufacturer who's this willing to share information on what they're doing clearly is proud of his intellectual property, and in Pass' case, the numerous filings with the patent office ensure his designs are protected.
The XA.5 series from Pass Labs, the subject of this review, are the latest top-of-the-line series of class A amps. As the XA.5 evolved from the prior XA series, I would direct the reader to peruse the Pass literature on what has changed (and how these designs differ from the X.5 series of class AB amps). But to save you the trouble, the literature suggests the new series of amps have benefits derived from a beefier power supply, more output devices and lower distortion levels. Typically, when you improve the power supply and add more output devices, you get more current delivery and better dynamics. Of course, any time you reduce distortion, you've removed additive electronic artifacts that can rob the playback of music's organic qualities, leaving you're with a more natural reproduction of the recorded works. As I've not heard any prior works from Pass Labs, my evaluation will not be relative to prior models; I'll simply assume the XA.5s are better than what preceded them, and judge these amps as I believe live music is supposed to sound.
Physically, these are handsome, well built (but not ostentatious or overbuilt) monoblock amplifiers. The front plate houses a simple toggle switch for moving the unit from "standby" mode to "on" and a meter which measures bias; if the amps are unable to deliver the current demands of the speaker in class A bias, this meter will move to the right (if the amps are more than adequately powerful, it will NEVER move). In back, there are XLR or RCA inputs, two sets of binding posts (spades), a master power switch and a 15amp IEC. Thankfully, I found the amps largely agnostic to power cords and conditioning, which is one of many advantages of class A circuitry. They do run warm, but not so hot that you cannot rest your hand on the chassis for an extended period. Do plan on giving them some room for ventilation, and they do require a few hours warm up before sounding their best (though they sound exceptionally good within 20 minutes).
So how do they sound?
Well, to go back to the 'perfect amp analogy' from earlier. When you read or hear an audiophile talking about how solid-state amps suck, I've got dollars for donuts that say the audiophile is talking about a class AB bipolar transistor amp. Class AB amps are ones where the bias of the output transistor receives a non-steady state of current to power the speakers, but this current variability is frequently heard (if not measured) as a distortion, a ringing, in the treble and midrange. The ringing in this frequency is where our ears are most sensitive, and anything electronic artifacts in this frequency range will be found objectionable, unnatural and fatiguing.
So now you know why audiophiles frequently complain that solid-state amps don't sound natural—they're objecting to the distortion endemic to solid-state amps. The Pass XA.5s have a topology exclusive to them (thanks to patents) called "single-ended class A," which means they have a constant amount of bias on the output transistor. Whether the listening results are due to the topology or not, I cannot say. But I can categorically state this: they do not have this obnoxious ringing at all. As a result, its easy to go higher on the volume control, as when distortion is removed, you can listen at louder volumes. They are one of the few solid-state amps that I can listen to all day without any listening fatigue whatsoever, and that is because of the removal of this distortion. For tube lovers who hate how solid-state amps sound, the XA.5s are a must listen.
While the XA.5s have no upper midrange glare, they are not what I would categorize as a dark sounding amp. They're exceedingly even-handed in their tonal representations, without any dryness, glare, or whitish coloration, and without any thickness or chestiness in the midrange. In Yoda speak, "a fine line, these amps walk." Cymbals are splashy and brassy, not ringing or bleached. In listening to male vocals, I never noted a thickening in the tone, or a loss of pace.
Over and over again in my listening notes I kept writing the words "timbre and tone," as I found the XA.5s to be as good in tonal representation as any amp I've heard—there's no part of the frequency spectrum that draws the listener's attention (always a red flag if it does happen). Tonally, these things simply sound right. Now, I'm not an engineer and I am not going to make representations that the "single-ended class A" circuitry is similar to a SET amplifier; frankly, it sounds like a clever marketing slogan. I will say that I've heard SETs in my system and despite their numerous flaws (inadequate power, rolled highs, tubby one-note bass), they are uniquely beautiful and believable in the midrange in a way that I've never heard a solid-state or tube amp come close to. The XA.5s do not suffer the same flaws in the bass or treble as the SET amps have; but by the same token, the XA.5s "single-ended class A" topology does not result in a midrange that reaches the sheer believability of a SET either.
But, they come close—closer than any other solid-state amp I've come across. The midrange is where music lives, and I've not heard a more tonally natural sounding midrange through a solid-state amp than through the XA 60.5s. There is an organic, palpable quality when playing pianos, cymbals, bells, vocals and horns through the XA.5s, though not as liquid, extreme and seductive as when I've used SETs, they do remind me of it—in a dreamy, mesmerizing, lost in the music kinda way.
Ever hear the phrase "mosfet mist?" I used to own an older Conrad Johnson solid-state amp which was class AB with mosfets in the output stage. Despite the class AB bias, it was always pleasant to listen to, but suffered from three serious flaws: 1) limp wristed and poorly defined bass reproduction, 2) fuzziness in midrange and treble, and 3) poor soundstaging cues as performers were lost in a flattened, homogenized stage that was compromised by an elevated noise floor. These are traditional sins associated with mosfet output stages in solid-state amps, though designers frequently use them because of their significant strength: they sound sweet and musical. If I had to create some guidelines on the flaws of solid-state output stages, it'd be that mosfets have a haze, and bipolars have ringing; haze is like a fog limiting transparency into the presentation, whereas ringing is an obnoxious distortion added to the mid-treble presentation.
Again, nothing is perfect, but for long term listening ease, I'd always choose the mosfets' sins over those of bipolars'. But to return to the Pass amps, allow me to offer an analogy: if "mosfet mist" is the haze on the early morning landscape before the sun has yet to fully illuminate the world, with each object in the horizon having lost its full color to a white and silvery watery coating, the Pass XA.5s, which use mosfet output devices, will render the landscape just moments before the landscape has every last vestige of the morning dew burned off (while an OTL will have noon-day sun clarity in the treble and midrange but without bass control, which I'll touch on later). In this reviewer's opinion, for satisfying musical reproduction, treble and midrange clarity isn't everything; what matters is the ability to provide clarity and accuracy while being musical and natural at the same time. That is the balance that the Pass XA.5s achieve so well. They remind me of mosfets' musicality but with clarity far more characteristic of a tube amp.
But if the Pass amps give up anything, it's absolute clarity. For a mosfet amp, the XA.5s do better than any other iteration of a mosfet output stage in the removal of electronic haze that I've heard, though they don't get every last bit of haze out. But that's OK. They don't try to get noon-day sun clarity, they've made the design decision that the view of the landscape is detailed enough with almost every vestige of water burned off for the music to be musically communicative, tonally natural and fully developed. And with such a low level of mosfet mist, the XA.5s can render a fully developed and highly specific soundstage—meaning that they will definitely not be the weak link in delivering a soundstage. They do not throw a billowy, gargantuan and in some cases, an ill-defined stage like some tube amps, but they present appropriately sized and spaced images with well-defined depth. And the reason they can do this is because they have such a low noise floor, and so little of the electronic haze I mentioned earlier.
It's prudent to continue this running analogy with the lower frequencies; if I'm willing to talk up the benefits of crystalline clarity of an OTL or tube amp, its only fair to speak of their weaknesses, which are manifest as a lack of bass drive and depth (to say nothing of their higher maintenance requirements). This is their critical flaw. Valves simply don't have balls. Textural shadings and micro-detail may be there through the mid-bass with a tube amp, but they're never delivered with a proper degree of thunder, drive or depth. Staging cues rely on deep bass reproduction, as that is where hall information comes in—echoes outline the dimensions of the recording venue, which allows the listener to place performers in the room with proper depth and dimension.
And to restate the obvious, music is visceral; bass guitars, kick drums, pianos, et al will all hit you in the pit of your stomach if your speakers can deliver it. Tube amps simply don't get these things right—or if they do, they require a gazillion valves and a dedicated air conditioner. Class AB solid-state amps can reproduce lower frequencies in a more realistic manner better than anything I've come across (but again, refer to the prior paragraphs which cover their characteristic problems in their midrange and treble). The Pass XA 60.5s are squarely in between the two topologies insofar as dynamic drive, depth and impact are concerned, delivering far more depth and impact than a tube amp, but not as much as a class AB current monster. The XA.5s are not head banger amps; there's less pop and impact behind dynamic swings with the XA.5s in the mix. Drum kits and other percussive instruments like piano do not pop as they would with a current dumping solid-state amp, but the XA.5s do give a fair share of impact while also delivering far more nuance and more texture than a class AB monster, with bass details reminiscent of a tube amp. The Pass XA.5s seem to have married the traits of both class AB solid-state amps and tube amps in their bass reproduction. (Of course, a larger XA model may have changed my thinking in the above, but I'm simply calling it as I heard it with the XA 60.5s)
Lets sum up. These amps are not perfect. No amps are. But their flaws are so trivial in the grand scheme of things, and their strengths so considerable in all the ways that relate to how music sounds, that it is only prudent to consider these amps a stunning success. The XA.5s have an organic tone that is seldom found in solid-state amps, and one that I've never found in a class AB amp. Tube lovers will never find the kind of bass control that the XA.5s deliver in their favorite valves. If one can accommodate the price, the space and the heat output, selecting amongst the 30 watt stereo amp or the monoblocks rated at 60, 100, 160 and 200 watts may very well be the last amps many audiophiles will buy. And allow me to make this blanket recommendation: if you're thinking of buying the XA.5s, and you think a particular model is the right size for your room/speaker/preferences, get the next larger model.
The primary weaknesses I found in these amps were a function of needing more power given how low the distortion levels were. As the music was so pure, so unadorned with annoying digital artifacts, I routinely found myself cranking the volume to the point where the amps ran out of juice, as evidenced by the bias meters jumping around. Had I a larger model of the amps, this would not have been an issue (however, I doubt its in the DNA of the XA.5s to deliver sledgehammer bass or explosive dynamics, regardless of the model size—if that's your primary listening predilection, I suspect you need to check out the Pass X-series). While I observed a small level of haze (again, mosfets do have characteristic qualities), I never felt deprived of any musical information, and I suspect any absolute loss of clarity will likely never be noticed by the majority of audiophiles—and I'm confident in saying it will have no adverse impact on the musical message the amps are trying to deliver. Sonically, even in a perfect world, there is little lost, little given up, and little more one could hope for than what the XA.5 series from Pass Labs deliver. Ryan Coleman