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Pass Labs X600.5 Monoblock Amplifiers Review

Ed Momkus contemplates the X-Factor: Pass Labs X600.5 Monoblock Amplifiers

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Pass Labs X600.5 Monoblock Amplifiers

Experience Matters

Sometimes I envy full-time reviewers who can dedicate all their time to the wide world of high-end audio. They listen to such a large numbers of products that they can develop a perspective about the “best” products over time. On the other hand, reviewers like me may hear 3 or 4 excellent models of a class of component and start thinking that they are the best available in that category. I’ve made this mistake before and subsequently discovered a component that beat the others decisively. Fortunately, this can be remedied by age and experience (yes, once you’re 53 you can start claiming to have some experience – you guys who think you’re experienced at 40 still have a way to go). (For crying out loud…-Ed.)

High-power amps (300 Wpc and up) are something that I’ve definitely experienced – but there are still many major exceptions. I’m going to start with full disclosure of those exceptions. The biggest are the Ayre MX-R (I’ve heard the lower-power Ayre’s), the big Krell’s (I’ve heard the lower-power Krell’s), Edge, Halcro, Innersound (heard only briefly at a dealer), Musical Fidelity kw (heard only the lower-powered MF’s), Parasound Halo JC-1, Sim Audio (I’ve heard the lower-power Sim’s and heard the W-10s only briefly at a dealer), Tenor and Theta (heard only briefly at a dealer). On the other hand, I’ve owned, auditioned in my system or listened extensively to Bryston, Bel Canto, Boulder, Carver/Sunfire, Classe, Electrocompaniet, Manley, Marantz, Mark Levinson, McIntosh, Pass Labs, VTL and XLH.

What’s With The .5?

You should check out the Pass Labs website for a technical description of what it means to be an X-600.5 vs. an X-600, but I’ve heard both, and the X-600.5 is definitely better in all respects. Like the X-600, the X-600.5 is hugely powerful, but the .5 version incorporates technology used in Pass Labs’ XA line, which are of pure class A designs. (Listening to the X-600.5 makes me think I should listen to the XA line, though I’d likely have to bi-amp to get the power I need).

I’m not technically proficient enough to discuss any truly “technical” audio stuff, but I’ve learned enough to get the main point of “Class A” operation. Class A amp’s output stage transistors are always on, and are thus inefficient, losing much power to heat dissipation. However, by staying on, the output stage transistors handle both the positive and negative halves of the audio signal. As a result, the output signal closely matches the input signal, while the amp avoids crossover distortion and the transistors remain at a more constant temperature. The factors collectively result in a smoother, more natural sound. My ears have repeatedly preferred Class A operation in amps.

The Pass X-650.5 is not a pure Class A design. Strictly speaking, it is a Class A/B design. However, I understand that it runs in Class A up to 110 watts, which is very good since you get Class A operation even at high volume levels. (I don’t know if the meters are a good indicator, but the needles only moved at extremely high volumes.) High bias causes the X-600.5 mono’s to idle at about 550 watts, most of which dissipates as heat across the heatsinks, so be prepared for a warm room. (This can be a real benefit in winter!)

Of course, I can’t go further without mentioning the head-turning appearance of these Pass amps. For me, these are among the handful of audio products whose distinctive appearance gets burned into your memory. This is especially true for amps, which tend to be the most vanilla of components. People who come in “The Room” want to know – “Wow, what are those?”

Setup & Break-In

I should note four aspects of my system (explained in detail below) that have a significant influence on amps: (1) dedicated 20 amp circuits for each monoblock; (2) Silent Source Reference High Current Power Cords (in my opinion a very natural-sounding PC); (3) Walker Audio Reference Prologue Amp Stands (they make most amps sound more energetic); and (4) entirely separate speaker cables to attach to each binding post of the amp.

The X-600.5 mono’s are big and heavy, weighing in at 150 pounds each. I’ve wrestled with 2 monoblocks that were heavier, but 150 pounds is still a backbreaker, so get some help in installing these behemoths. I initially put them on the amp stands without any cones or discs.

The Pass Labs X-600.5 monoblocks had no difficulty mating with my MBL 5010 linestage in driving my B&W Nautilus 800D’s, nor could I foresee any difficulty with any other good solid-state amp. However, its 11kΩ single-ended input impedance (22kΩ balanced) might be a little low for tube preamps.

The Pass X-600.5’s sounded good right out of the box. I must say that I like that. Though I’ve always been willing to give a component a chance, I’ve frequently wondered when manufacturers tell me that their product takes 750 or more hours of constant playing time to break in. I wonder whether the component really keeps getting better or whether it broke in at 100 hours and I’ve been getting used to the component’s sound for the last 650 hours. (Lest you think I don’t believe in break-in time – I leave my system on 80% of the time because its sonic’s will be in top form when I’m ready to listen.)

The X-600.5’s sounded even better after a week of “on” all the time plus 26 hours of playing time; and better still after a second week of full “on” time and a total of 60 hours of playing time. I really did not detect any material change after 100 hours of playing time, although I did think the amp sounded better when left on full time.

I used several different cones and discs with some success, but concluded that the Pass X-600.5 mono’s were relatively impervious to such items. I did, however, like them better on stands than on the floor.

I only used 1 brand of power cords – Silent Source High Current PC’s, modified with 15 amp connectors. It would have been interesting to try some others, but as you’ll see below, I did not feel like I was missing anything.

Listening Routine & Review Discs

After initial break-in, I have a routine when listening to amps that I must credit to Robert Harley’s excellent book, The Complete Guide to High End Audio. First, I like to play something with a powerful rhythmic drive (usually involving kick drum and bass) at moderate levels. After determining the amp’s basic character, I like to crank the volume to check for any sense of strain, and determine if the sound quality suffers at high volumes. In particular: Does the soundstage lose any of its coherence? Once these basics are established, I look for the listening perspective (what row am I in?), listen for hardness or etch in piano, flute, violin and female voice, and the realism and articulation of bass notes.

Once break-in reached one week of full-time “on” and 26 hours of playing time, I started five weeks of listening with a variety of material, constituting over 20 CDs. Even cursory listening during the first of those five weeks established that these amps have excellent rhythmic drive, effortless tight bass and a very wide, rock-solid soundstage.

For purposes of this review I will use the following discs to illustrate the X-600.5’s characteristics:
James Newton Howard & Friends, Lasting Impression Music LIM-XR-004 (XRCD)
Super Analog Sound of Three Blind Mice, TBM Music, Inc. TBM-XR-9002 (XRCD)
XTC – Apple Venus Vol.1, TVT Records 3250-2
Coldplay – A Rush of Blood to the Head, Capitol, CDP 72435 40504 2 B
King Crimson – In the Wake of Poseidon (30th Anniversary Ed.), Discipline Global Mobile, DGM0502
Chicago Transit Authority-Chicago Transit Authority, Rhino R2 76171

Listening & Conclusions

For a subjective review to have any meaning, the reader needs to have some perspective on the writer’s idea of good sound. My target is live music in a good acoustic venue, because you can clearly hear the nuances of the tones of instruments in such a venue. The sound of such music is never sluggish, yet the bass is full and has a physical impact. Flutes and violins can hit piercingly high notes without grating on your ears. You can close your eyes and not have to think twice about whether the keyboard player is playing an acoustic or electric piano. Cymbals are not confused with white noise and a triangle is not just a piece of metal being whacked.

Replicating all of this in a big room requires speakers that can fill the room and amps that can drive those speakers both very loudly and very delicately. I have tried to accomplish this by assembling a high-powered solid state system that (1) started with great PRAT and excessively etched overwhelming detail, (2) added full-bodied deep bass without diminishing the PRAT, then (3) tamed the extremes of the upper register and (4) added warmth to the midrange. Visiting audiophiles comment that my system is very natural-sounding, and are surprised that it is a digital-source-only solid-state system.

With this standard in mind, the Pass Labs X-600.5 monoblocks are, for my taste, one of the two best high-power monoblocks that I’ve heard. Even allowing for personal taste, any listener would regard them among the best-sounding amps they’ve ever heard. Furthermore, they sound better to me than 95% of the lower powered amps, solid-state or tube, that I have heard.

They may be 600/1200 Wpc into 8/4 ohms, but the word “delicate” comes to mind – really.

In my system, the Pass Labs X-600.5 monoblocks have a neutral soundstage with a perspective that changes in accordance with the recording, but is generally between row 8 and row 20, which is exactly how I like my listening position. They also have a very wide soundstage – similar to my Electrocompaniet Nemo’s, which cover the entire 30-foot width of my listening room. Their depth is similar to the Nemo’s, but with a little more layering of the performers, and excellent separation and “air” between musicians.

The X-600.5’s produced detailed, tuneful bass, with less bass weight than Nemo’s, but with a bit more nimbleness. The 600.5’s have a damping factor of 250, but sound cleaner and tighter than that rating. The X-600.5’s don’t have as much bass punch as the XLH M-2000 (the XLH may be one of the best dynamic “bass amps”), but bass articulation is very good, and especially superior when delicacy is required. This contributes to a subjective sense of speed, which is reflected in the amps’ slew rate of 50 V/uS. (My impression is that good “Hifi” amps may go from .5 V/uS to 80 V/uS. )

Most importantly, like the best lower-powered amps, the Pass X-600.5’s have a natural, detailed presentation in each tonal range without creating unrealistic “hyper detail”, and they do it without glare or that “glassiness” that seems to show up in many amps’ upper midrange. In short, they present what to my mind is a near-perfect balance between dynamics, detail and smoothness.

Let’s illustrate some of these points, starting with dynamics. King Crimson’s In the Wake of Poseidon covers a wide gamut of sounds. “Pictures of a City” is a horn-loaded piece that ranges from room-shaking volumes to very quiet, delicate passages involving bass, drums and cymbals. The dynamic range of the X-600.5 reflected this perfectly. Most other amps don’t sound delicate in the quiet parts if you have the volume set to shake the room at the loud parts. Conversely, if you set the volume to accurately reflect the quiet parts, you never get the big sound of the horns during the loud passages. James Newton Howard’s James Newton Howard and Friends is another disc that, properly rendered, has tunes that involve seriously dynamic drums and synthesizers, as well as delicate passages with only triangles, cymbals and wood blocks. As with “Pictures of a City”, even very good amps can’t render both properly. The Pass Labs X-600.5 mono’s did it perfectly in my system.

The depth and width of the soundstage are very important to conveying a “live” experience, but the thing that, for me, makes a superior soundstage is the separation and air around performers. XTC’s Apple Venus Vol.1 illustrated the X-600.5’s strength in this department. “River of Orchids” builds up to a point where horns come in, but are meant to be far in the background. Soundstages from some amplifiers are too flat and place the horns right behind the vocalist. Others place the horns far in the back at the expense of making them indistinct. The Pass got it right, with the horns set at the back end of the stage, but very distinct without calling too much attention to themselves. Similarly, “Knights in Shining Karma” showed excellent separation and a soundstage placement with the up-front guitars.

The tunes on the Super Analog Sound of Three Blind Mice are definitely a tribute to the TBM studio sound, designed to let you hear all the details and nuances of each performer. This is the type of recording that many love to listen to via a lower-power amp that highlights these details and nuances. The bass player adds a little growl to his instrument in “Blonde on the Rocks” and the vocalist in “My Memories” puts some edge on the voice, while “I’m a Fool To Want You” would not be the same tune without the distinctly subtle brushes keeping rhythm. All were beautifully rendered by the X-600.5.

Coldplay’s A Rush of Blood to the Head is an average recording, with some good spots and bad spots. However, the good spots can test the ability of the system in reproducing piano, vocals and drums, and some amps just make this CD sound aggressive. However, this was not the case with the Pass. On “Politik”, the piano sounds natural and the drums emanate power the way they do in a live concert. Similarly, “God Put A Smile On My Face” renders a very natural vocal performance, powerful drums and ringing guitars with a you-are-there feeling. A similar feeling of “real” was produced when I played King Crimson’s “Cadence and Cascade” and “In The Wake of Poseidon”. Greg Lake’s voice was live-sounding, natural and full-bodied, the flute was piercing but without a trace of glare, the guitars highly detailed, and the drums physically powerful.

Finally, I made sure everyone was gone from the house and that the neighbors were out when I turned to really high-volume listening. I wanted to see what the X-600.5 sounded like when it slipped out of Class A operation. I played a couple CDs for this, including some Who rock n’ roll and Perez Prado mamba, but I focused on Chicago Transit Authority’s self-titled album (sometimes referred to as Chicago 1 after the group shortened it’s name to “Chicago”). I really cranked up “Introduction” and sat back to listen.

The soundstage and separation of performers remained rock solid, and there was never any sense of the amps straining at all. The differences between the X-600.5’s and some of the other amps I’ve auditioned became a bit more obvious. The separation and layering of the Pass was a bit better than the others. The bass articulation was among the best I’ve heard, although with slightly less weight than some other amps. Still, the needles on those meters did not move, so I cranked it some more. It was at this point that I couldn’t tell any longer whether the slight edge that was creeping into the piano meant that the amps were operating in class B or whether the volume was just so loud that my ears started to hurt. Either way, I concluded that it didn’t matter – unless I somehow acquired a 50’ x 75’ x 10’ listening room, I was never going to be playing the system this loud.

My B&W 800D speakers demand lots of power and sound best with high current amps. My room is fairly large and needs a lot of power to move all that air around. The X-650.5’s easily fit this bill. If you are building your system around speakers that need dynamic yet delicate high-power amps, you have got to have the Pass Labs X-600.5 monoblocks on your short list. This is especially true if you listen to a wide variety of music and need an amp that has the power, dynamics, delicacy and soundstage to cover all the musical bases of rock, jazz, classical, opera, synth, techno, etc. Highly recommended.

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