Pass Laboratories X150.5 Stereo Power Amplifier 
Home Theater Power Amplifiers Stereo Amplifiers
Written by Christopher Zell, Ph.D.   
Saturday, 01 November 2003

I have always considered Nelson Pass one of the founding figures in high-end audio. He has a long history of innovative amplifier and preamplifier designs, dating back to the ‘70s with one of the first successful high-end corporations, Threshold. I am very familiar with Pass’ amplifiers, and have owned a variety of Threshold components through the years. I have also kept an eye on his very active DIY career, which includes informative design articles and numerous DIY projects. Not having any exposure yet to any of the products from his latest company, Pass Laboratories, I was very curious and enthusiastic about auditioning the subject of this review, the X150.5 stereo power amplifier, and indeed I had high expectations to meet. Although the X150.5 is the baby in the current X series of amplifiers, it is by no means small in any way, specified at 150 watts per channel into eight ohms, and priced at $4,500.

Nelson Pass is a strong advocate of the concept that simplicity is paramount in producing an amplifier with superior performance. Perhaps most notable among his novel amplifier designs, up until recently, were the patented Stasis amplifiers sold first by Threshold, and subsequently by Nakamichi. I myself was a very happy owner of a Threshold S500 Stasis amplifier for a number of years, a true powerhouse that sounded musical and nimble. Pass Laboratories’ previous Aleph series of amplifiers, which featured a unique single-ended topology, have been very well received by the press and consumers alike.

This leads us to the current X series of amplifiers. They are based on Pass’ most recent patented design innovation, Supersymmetry. With this concept, Pass is moving closer and closer to the elusive goal of a very simple circuit, with only minimal local feedback, and vanishing low distortion across the output terminals. The amplifier is essentially a two-stage device, thereby simplifying the signal path circuitry. Using a balanced topology, any noise common to both sides of the circuit is canceled at the output. Taking advantage of this inherent feature, rather than striving for a perfect match between the input signal and output of each circuit half, Pass focused on making the two halves of the balanced circuit behave identically with respect to distortion and noise. He claims that Supersymmetry reduces the noise of a single device by a factor of 100, 10 times lower than a conventional balanced circuit. While there is a lot more to the actual circuit design and concept, I will force myself not to elaborate, interesting as it may be to an electronics enthusiast such as myself. The point is that the X150.5 stereo power amplifier is a clean, simple design, with extremely low distortion and noise.

The X150.5 is rated at 150 watts per channel into eight ohms, and 300 watts into four ohms. Surprisingly, it is the lowest powered member in the X family of Pass Laboratories amplifiers. This is a revamped version of the X150, adding the signature front panel blue meter featured in the higher-powered X amplifiers. The amplifier consumes 200 watts from the wall at idle, so it does run warm, even if not driven hard. Cosmetically, the amp is quite beautiful to the eyes of just about everyone who came into my listening room. The blue front panel power meter glows subtly through the round opening created by the decorative, two-tiered, anodized instrument gray aluminum front panel, striking yet not distracting or gaudy in any way. Curiously, the actual readout is not the typical instantaneous power output, but rather shows the bias current in the output stage transistors. Regardless, this added a touch of class to my setup, so I placed the amp squarely out in the open between the speakers, rather than off to the side with the rest of the electronics. Below the meter is a small silver toggle switch for placing the amplifier in or out of standby mode. Wrapping around from the top panel and all the way down the sides of the chassis are functional and stylish heat sinks. I say “functional” because they became uncomfortably hot to the touch when the amplifier is used for an extended period.

Moving around to the back panel, we see two pairs of input receptacles, single ended RCA and balanced. Below the inputs are the output loudspeaker terminals, which on my sample are surprisingly small and seemingly not very stout. In addition to not enabling banana plug connections, they do not easily accommodate large-diameter loudspeaker wires, unless spade lugs or something similar are installed. An amplifier of this pedigree, power and price should contain higher quality, more robust speaker terminals. Off to the left side are a replaceable main fuse, an AC power cord receptacle, and the master power switch, which must be on for the front panel standby control to function. I left this on unless I was gone for a few days, switching the amplifier on and off via the standby switch. A pair of five-way connectors for remote turn-on completes the available connections and switches. The X150.5 weighs a substantial but not backbreaking 70 pounds in the shipping carton, and measures 19 inches wide by seven inches high by 20 inches deep.

How much can be said about the setup of the amplifier? I placed the X150.5 between the front speaker pair in my listening room, utilized short 10-foot loudspeaker cables, and positioned the amplifier out in plain sight to fully enjoy the elegant cosmetics. The gain of the amplifier was a bit lower than my reference amplifier, so I re-calibrated the reference levels to match the rest of my home theater system for multi-channel audio and DVD playback. This is not a fault of the Pass X150.5; it just did not match the amplifier I normally use. The gain is certainly sufficient for use in any home theater or music system. The amplifier runs hot, so it should be placed where ventilation is good and the cooling fins are unobstructed. Before any serious listening session, I allowed the X150.5 a minimum of 30 minutes to warm up fully. Finally, although the Pass Laboratories X amplifiers do include both balanced and single-ended input connections, I did not have a balanced preamp available at the time of this review, so I was not able to take full advantage of the matched architecture of the X150.5.

After casually playing the Pass X150.5 in my system for a couple of weeks, I started my serious listening with Lightbulb Sun (Snapper Music), a year 2000 release from one of my current favorite artists, Porcupine Tree. I have been a fan of theirs for years, dating back to their early ‘90s spacey, progressive days, which were dominated by songwriter-guitarist-vocalist Steve Wilson. In the last few years, they have slowly evolved into more of a concept group, and their more cohesive and song-oriented albums show it, with less of a classical progressive wandering feeling. The title track leads off the album with crisp acoustic guitar, breaking into Porcupine Tree’s trademark punchy and dynamic kick drum and bass-fueled electric guitar riff. As the song climaxes, the fluid lead guitar competes with the raucous synthesizer, bass and drums, and the X150.5 expertly pulls the melody out of the mayhem. The delicate, harmonic side of the Pass Laboratories amplifier is evident on the Beatlesque track, “How is Your Life Today?” The purposely muffled lead vocals that open the song give way to Wilson’s clear falsetto, finally leading to beautiful, layered harmonies. Through the Pass amplifier, the soundstage goes from very small and intimate to large and wide as the single voice expands to many, and the snaps and clangs of the percussion and keyboards expand the image even more, before it shrinks back into a solid, center-focused small image again. The X150.5 was very impressive during this cut that I know so well, bringing out added three-dimensionality that I had not noticed to this extent before. The low end throughout was never bloated or overemphasized, occasionally causing me to wonder if something was missing, until suddenly it would appear with surprising power and impact. Also noticeable on this album was the extremely low noise floor, the music seeming more dynamic, not by perceived louder peaks, but by relaying the low-level portions with increased relative dynamics. With relatively efficient loudspeakers, such as the Paradigm Reference 100 v.3s, you can crank it to your heart’s desire without running out of steam with the X150.5. “Russia On Ice,” a classic progressive opus, really came alive. As I cranked it up, it moved the music up and down the rollercoaster of this exciting cut, and me right along with it.

In the mood for a little blues on a lazy Saturday afternoon, I selected Sam McLain’s acclaimed sonic jewel, Give It Up For Love (AudioQuest Music). I had forgotten how great this music and recording is, but the X150.5 immediately reminded me. This is how a band should be recorded, with big and powerful bass you feel in your gut, live percussion that jumps and snaps with the striking of the sticks, and vocals prominent, gritty, and clearly placed out in front of the band. I don’t know how to say it any other way than that it was easy to picture McLain standing and singing between the speakers, not smooth and gentle, but full of pain and emotion. Forgetting about the review, I threw my note pad down and let “What You Want Me To Do” rip, the bass slamming, the guitar and drums brash but still melodious as live music is and should be. Despite the fact that I could do without the occasional cheesy, soap opera organ riffs, I had a blast listening to this album through the X150.5. The Pass amplifier appropriately shrank everything down on gentler, simpler tracks such as “Don’t Turn Back Now,” the crisp cymbals jumping out of the silent background, the vocals intimate but still powerful, up front and center. Why can’t more music be recorded like this? One piece of advice – don’t play Give It Up For Love at the beginning of a listening session. It is too hard an act to follow, and most things will sound compressed and artificial in comparison.

Like many folks, I have been playing a lot of Johnny Cash lately as I grieve his recent passing. I am not sure how he managed it, but somehow he stayed interesting and relevant throughout the last 40 years across a wide variety of music genres, as is clearly evident in his 2002 release, Unchained (Sony Music). This is a wonderful album, well recorded, featuring great and varying original pieces and covers, with Tom Petty and the Heartbreakers as a backup band to boot. Cash’s vocals are clear, yet gritty and edgy where appropriate. The X150.5 portrayed the bite in his voice and the closely recorded acoustic guitars, but did it in a way that was not fatiguing to me, detailed and mercilessly revealing, but not brittle. The bass was lean and tight, but not so lean as to be overly polite. The Heartbreakers provide excellent backup, tastefully staying within the songs and in the background, with a few exceptions. One of these is the surprising and rocking Soundgarden cover, “Rusty Cage.” Midway through the song, the Heartbreakers were unleashed, the X150.5 setting the room on fire. If I detected someone sitting still during that tune, I’m afraid I’d be forced to kick them out of my house. Another tasteful reincarnation is the cover of the Heartbreakers’ own “Southern Accents,” which showcased the Pass X150.5’s ability to cover the full range of the throaty and rumbling vocals that only Cash can do.

I spent a very interesting period with a bare bones system, consisting of an Underwood HiFi modified Shanling SCDT-200 SACD player directly connected to the Pass X150.5, which in turn drove a pair of Revel Salons. This was one of the most enjoyable listening experiences I have ever had. The Pass Laboratories X150.5 and the Shanling SCDT-200 presented a truly beautiful, artistic and synergistic pair both sonically and visually. The gentle blue lights and classy front panel of the Pass X150.5 matched perfectly with the retro space age industrial design and similar neon blue lighting of the SCDT-200. With the lights dimmed and the music flowing, I was in audio and visual heaven. The X150.5 clearly differentiated between the Shanling’s softer, rounded tube outputs and the subtly but distinctively different direct outputs, with their additional detail, tautness and clarity. Guitar legend Ronnie Montrose’s Music From Here (Fearless Urge), perhaps his most tasteful album to date, was fantastic through this system. The underlying gurgling, water-like synthesizer in the gentle but complex “Fear Not” was clear and detailed, yet appropriately placed in the background. The bass guitar had good weight, in balance and solid but not bloated. The driving beat and rounded guitar of “Road to Reason” contrasted spectacularly with the sparkling, utterly lifelike cymbals and percussion. The Pass X150.5 clearly was able to rise up to the task of driving the demanding load of the Salons. Not until I cranked the inefficient Salons to extremely high levels did the X150.5 begin to run out of steam and the sound to slightly harden.

The Downside
As previously discussed, while I have no major complaints with the impressive sonic performance of the Pass Laboratories X150.5, I do have an issue with the loudspeaker binding posts on the review sample. They seemed out of place in quality when compared with the rest of the X150.5. An amplifier in this price range ought to have much better posts that are more versatile and robust. Not only would they not accept any kind of banana plug, they would not easily handle any large-diameter loudspeaker cables. One other minor item is that that the amplifier does run quite hot, which the buyer should be aware of when thinking about amplifier placement, as well as room issues such as ventilation and cooling. This is more of a warning than a criticism, one inherent in any amplifier with high idling bias currents in the output stage.

Despite the fact that there are plenty of more expensive amplifiers out there, most folks, myself included, would consider $4,500 a healthy chunk of change for a stereo power amplifier. A product at this price point had better deliver on all fronts, and with the exception of the binding posts, the X150.5 certainly does to both my ears and eyes. The Pass Laboratories X150.5’s stunning and tasteful cosmetics, combined with powerful, dynamic yet non-fatiguing sonics, make it a real winner. I am always wary of characterizing the sonic attributes of an amplifier using terms such as “soft,” “etched,” etc. since the effect depends so much on surrounding components and source material. Comparisons without matched level testing in the same room and with the same system are also extremely dangerous, as normally the conclusions are wrong or at best exaggerated. Still, I’ll do my best here to characterize my impressions of the X150.5. I doubt anyone would ever call this amp forgiving, euphonic and lush, or the bottom end rich and fat. The X150.5 consistently sounded very live and dynamic, partially because of what I perceived as a very low noise floor. The highs were very clean and extended and sounds such as high hats in particular were very impressive through the Pass Laboratories amp. The lowest octaves seemed a tad lean at times, which is again neither good nor bad, and the bass was liquid and tight, not bloated, while still powerful when called for. The X150.5 was very revealing and detailed, so that bad recordings were thrown right back at me, but without any extra sizzle to render them unlistenable, just revealing the ugliness that was already there. I noticed that vocals seemed a bit further back or very slightly recessed when compared with a few other top amplifiers. This is not a criticism, or a statement that the X150.5 was not accurate, just something that struck me over time. The X150.5 is an excellent amplifier that I will be sorry to see leave my sound system. And remember, if you wonder in the back of your mind if your loudspeakers are too inefficient, or your room too big, don’t forget that this is the baby in the X line. I can confidently recommend simply moving up the Pass Laboratories X series line to one of the X150.5’s more powerful siblings.
Manufacturer Pass Laboratories
Model X150.5 Stereo Power Amplifier
Reviewer Christopher Zell, Ph.D.

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