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July 2012
Superior Audio Equipment Review

Pass Laboratories X350.5 Solid-State Stereo Power Amplifier
Able to perform the magical suspension of disbelief.
Review By Tom Lyle


Pass Laboratories X350.5 Solid-State Stereo Power Amplifier Unit

  It is no secret that I became a huge fan of Pass Laboratories' products ever since I reviewed their XP-15 phono preamplifier. I now have the privilege of reviewing one of their power amplifiers. Pass Labs manufactures four different series of power amps and a line of integrated amplifiers: the dual chassis X power amps, the newest and quite extravagant Xs (pronounced excess) line, the pure Class A XA.5 power amps, the INT prefixed integrated amplifiers, and the power amplifier series that includes the subject of this review, the Class A/B X.5. The 350 Watt per channel X350.5 is the most powerful stereo amplifier in Pass Laboratories' X.5 series, beyond it lays Pass Lab' massive 600 and 1000 Watt monoblock amps. That's not to say that some wouldn't consider the X350.5 "massive" in the same way as these more powerful amps. This 132 pound monster is almost one foot high, two foot deep beast of an amp! It is rather imposing resting on the lower plate of the equipment rack in my listening room. Plus, its large blue meter glowing in the center of its thick, sculpted aluminum faceplate is enough for anyone, audiophile or not, to take notice.


I have to thank manager Kent English of the Pass Laboratories team (and just a bit of Pass' website) for the following few passages of background and technical information. Kent's passion for all that is Pass not only seems to be transmitted as some sort of positive virus from founder Nelson Pass, but it seems as if the entire team at Pass Labs has a great passion for their products and each of their contributions to the products they produce. The beneficiaries of that passion are those that have the privilege of owning one of their products, but anyone that sits in the listening chair in front of a system that contains any of their products. Those who visit Pass Labs' website can find links to the writings of Nelson Pass, who is unquestionably an electronic genius with a flair for translating his passion into the English Language.

Pass Labs claims that the X350.5 brings more "spatial development and detail" than with any of the smaller X.5 series product to the reproduction of music. They go on to say that the X350.5 is capable of exposing very subtle tonal shading in recordings, even though they consider it their "workhorse stereo Class A/B amplifier".

All the amps in Pass Labs' X.5 and XA.5 share a similar topology, that is, for the most part they have identical front ends, but what separates them is their different output stages which run from 30 to 1k Wpc. All Pass Labs amps are two stage devices, where all the voltage gain happens in the front end and the output stages are mostly current followers. By modifying the bias of the circuits, namely, single-ended and balanced, and manipulation of their rail voltages/current, any of the amps' output stages can be optimized for wattage, such as in their Class A/B amps, or finesse, such as in their Class A amplifiers. So, even if the amps have identical output stages they can suite a variety of applications. And therein lies the beauty of a Pass Labs' method of their multi-use platform, that there is a Pass Laboratories "house sound", so to speak, so no matter what type of application an audiophile needs, and by that I mean a system that requires anywhere from 30 to 1000 Watts per channel, there is a Pass Labs amp that can meet their needs -- without anyone being penalized for their specific power requirements. And even though I haven't had much experience with multi-channel systems, an added benefit of the Pass Labs family sound is that one could mix and match Pass Labs' amps throughout the system, across channels, bi-amp'ing, tri-amp'ing, all without concern for spectral balance or phase anomalies between the amplifiers.

As one goes up in the Pass Labs line there is, as one would expect, more wattage and enhanced ability to drive difficult loads. But the engineers at Pass Labs also are aware that as one spends more on an amplifier one should expect something beyond an increase in wattage and physical size. So, while they maintain the sonic signature as one goes up the line an effort is made to optimize operational parameters through the output stages such that with each step up in the Pass Labs line comes with advantages such as a more refined soundstage and improved dynamic contrasts. In other words, characteristics that matter in real world listening, and as one Pass Labs employee puts it, things more important than bragging rights of "mine's bigger". This should be obvious to anyone who has heard a more refined audio product, and especially for those that pay for the privilege of owning one of these products.

In regards to the X350.5, its 350 Watt per channel power rating is a near perfect match for my very power hungry and impedance challenged Sound Labs electrostatic hybrids. The fact that this amp also looks super-cool is an added bonus, and as I said earlier, it looks quite impressive to audiophiles as well as ordinary citizens. But it is quite large and heavy, and as such there were not too many choices as to where it was to be located. There were only two choices in my listening room: on the floor resting on an appropriately sized amp-stand, or where it ended up, the lowermost shelf of the Arcici Suspense equipment rack with a healthy eight inches between it and the next shelf. I would like to personally thank the engineers at Pass Labs for including handles on rear of its cabinet that made placement not easy, but certainly easier.


The X350.5 is hardly a "green" component – it draws nearly 600 Watts from the wall socket when idling. When playing music at background levels the amp gets warm, at a healthy volume when serious listening sessions the amp got fairly hot, the manual states that one should be able to "put your hands on the heat sinks without discomfort for 10 seconds or so." I connected the amp's IEC outlet to the dedicated 20 amp AC line with the excellent sounding, but sadly no longer manufactured Virtual Dynamics David power cord, which has a triple run of untethered cables between its terminated ends. There is a power switch on the rear of the amplifier which puts the amp into its standby mode, with its front panel blue LED indicating that the power is on, which is where it stayed whenever I wasn't in the listening room. Pushing the large circular front panel button activates the amplifier, along with its back-lit blue meter.

The analog front end of the system consists of a Lyra Kleos phono cartridge mounted on a Tri-Planar VI tonearm on a Basis Debut V turntable. The tonearm's hardwired cable terminates in RCAs and is connected to the Pass Labs XP-15 phono preamplifier, which is connected with balanced MIT cable to the Balanced Audio Technologies (BAT) VK-3iX preamplifier, and for a very short time a solid-state Edge G2 linestage. I mostly used the BAT because the Edge lacks balanced outputs, and I was most happy using a set of MIT Shotgun S3.3 interconnects to connect it to the Pass Labs amp. The digital front end is fed from a 3.20 GHz Dell Studio XPS PC with oodles of external disc storage filled with FLAC files. These files are read with either Foobar 2000, MediaMonkey v3, or J. Rivers Media and sent via ASIO'd USB to either a Benchmark DAC1Pre, CEntrance DACmini, or Wadia 121 digital-to-analog converter via DH Labs USB cable. I also used an Oppo BDP-83 Special Edition with its analog outputs connected to the preamp to play SACDs, and its S/PDIF coax output connected to one of the DAC's for less than occasional spinning of physical Red Book CDs or even much less than occasional DVD-As. All the front end equipment is connected to a PS Audio's Power Plant AC Regenerator, with a separate Power Plant for the turntable. The dedicated listening room is treated with Echobuster acoustic panels, with the walls lined with LP and CD shelves and the floor covered by industrial grade carpeting.

I've been listening to the X350.5 for a few hours a day ever since it arrived, yet I'm not sure it is fully broken in yet. What I am sure of is that the X350.5 takes a while to sound its best after switching from its standby to power-on mode during regular use.  Even though the X350.5 sounds mighty fine when first switched on, during the listening session the sound of the amp improves in all of its sonic parameters until it levels-off, which takes about 45 minutes or so.

I'm probably not too far off in assuming that the sound of the X350.5, and all the other X.5 amps, really, are due to years of research and development by the folks at Pass Labs. This research includes some patents in the field of high-end amplification, and this research not only led to the betterment of the Pass Labs amps' specifications, but the amp's overall sound.


The upgrade in sound quality between the sound of the Pass Laboratories X350.5 and what many would consider an "acceptable" level of performance from an high-end amplifier was fairly obvious as soon as the X350.5 was up and running. I hope you will excuse me for perpetuating the stereotype of the American male, but one of the first things I noticed in regards to the X350.5 was its incredible bass slam. Not only were the low frequencies tighter, that is, each note and group of notes' transient response was more noticeable, but in areas such as pitch specificity and lightning fast attack I've never heard better in my system. And all this without the system's Velodyne HGS-15b subwoofer contributing to the bass response of the system! With the X350.5 driving the main speakers the sub was an option, not mandatory.


Yes, I was one of the lucky ones who saw and heard Kraftwerk play one of their shows at the Museum Of Modern Art in New York City in June. I am well aware that the promoter’s method of distributing the 450 tickets that were available for each of the eight nights was more than unfair, it was also inept (every Kraftwerk fan in the world logs on to a website at a specified time, what could go wrong?). But perhaps it was Karma that day that helped secure our two tickets -- or more likely that we logged on to snag our tickets with four different IP addresses with about 25 open tabs. All of my senses continued to vibrate from that evening's show for months, so naturally I played the Japanese EMI pressing of the LP of their Computer World album released in 1981 once the X350.5 had been running in for a while. Even though isn't a whole lot of subterranean bass on this album, mostly due to Kraftwerk's über-control of their analog synthesizers, as there seems not only to be not one misplaced note but the tonality of their instruments seems just as methodically planned, the Pass Labs X350.5 does an outstanding job of reproducing their extremely important bass lines. These sometimes overlapping lines are more often than not the Teutonic-funk-groove machine that drives the songs, but they also often serve as the songs' melodies. The X350.5 communicated the synth-bass with adeptness, command, and the dexterity that was needed to immerse me in sound, and it wasn't until after the album was finished that I, as I often do, wondered how these four German guys come up with their trendsetting musical ideas (let alone this album's spot-on prediction of the preponderance of computers in our daily 21st Century lives). So I played the album again.

Pass Laboratories X350.5 Solid-State Stereo Power AmplifierThis slam that was evident in the lower frequencies also made itself evident throughout the rest of the sonic spectrum. The transient response of the X350.5 is stupendous without ever, ever being overly detailed. And I think that is one of the best traits of this amplifier, the way it manages to deliver its ultra-transparent sound while at the same time imparting the meaning, drama, and excitement that I can only compare to the experience of hearing live music. The reviewer side of me is made patently aware that it is doing this with every audiophile-approved trait in full sonic view. Traits such as its extraordinary soundstage, explosive dynamics, pinpoint imaging, ultra-extended but natural sounding frequency response, etc. etc. etc. are all there. The take-away is always that the music lover in me is the beneficiary of all that the Pass Labs X350.5 can accomplish. The X350.5 has a way of communicating the massage of the music while at the same time presents an unadulterated representation of the signal that it is fed, which also means that the nuances of real instruments being played in real spaces are reproduced with expertise.

Gyorgy's Legeti's Cello Concerto as played by Reinbart de Leeuw conducting the Schoenberg Ensemble with cellist Siegfried Palm from FLAC files burned from the fantastic Teldec CD The Ligeti Project III demonstrates this nuance I speak of. As the cello's single pitch arises from the black as pitch background floating in the space between the two speakers, the peace doesn't last long as Ligeti demonstrates his compositional prowess as he further explores minute changes in the ensemble's texture, utilizing what he terms "micro-polyphony". Rather than cast this work as a traditional concerto where the soloist plays in front of the orchestra, Ligeti does away with themes and replaces them with an interweaving of the cello and the orchestra while they both build the complicated, yet soothing atonality that is characteristic of Ligeti during this prolific period of the 1960s. I sat mesmerized throughout the piece, not only due to Ligeti's compositional skills, but in large part to the accurate reproduction via the Pass Labs X350.5 that made it possible to notice the shifting sonorities that occur during through the two movements of the composition as never before. The modern recording methods that utilize many microphones connected to many channels of a multi-channel digital mixer have seemed to have done away with the accurate placement of the sections of an orchestra, and especially solo instruments, but the accuracy of the tonality of the instruments is more often than not preserved, as it is to the nth degree in this recording. The Pass Labs X350.5 manages to not only reproduce this recording with spooky realism, but managed to somehow transport me to the original venue as the acoustics of the space were also reproduced with supernatural clarity. When a solo instrument or a group of instruments enters the soundscape the air around these instruments allows one to hear the space in which they were recorded, and by this I don't just mean the reverberant space of the hall, but the actual air that surrounds the instruments – in front, to the sides and behind them. This is one of my favorite short pieces by Ligeti, and to hear it through the X350.5 was indeed a treat. This amplifier laid out the cello and orchestral in front of me and around the speakers in an amazing display of transparency, the real instruments in the real space, despite the file's plain vanilla 44.1kHz/16-bit resolution.

You've probably noticed that in the paragraphs above it seems as if I've been talking as much if not more about the music than the sound of the Pass Labs X350.5. This makes perfect sense to me. This is because the X350.5 does many things well, but what it does best is draw one's attention to the music instead of drawing one's attention to the gear that is reproducing the music. When listening to great recordings of great music through a great system, the ultimate goal is to somehow be, at least in one's mind's ear, transported to the original event. When that original event is of real musicians playing in a real space, the X350.5 was able to perform the magical suspension of disbelief. Of course that didn't happen constantly, I guess that is the Holy Grail we are all searching for, but often enough that getting lost in the music, and the sound of the music, was a regular event. And when the music is "artificially" produced, as in the example of the Kraftwerk, I felt as if I was hearing exactly what the creators of this music were intending, in both the message of the music and what its producers were attempting to convey via their music. Reveling in the sound is not such a bad thing when the sound is so fantastic, and through the Pass Labs X350.5 it is indeed fantastic.

It is also a fact that I have not mentioned any negatives in regards to this amp's sound. This is only because the only negative I heard was that its transparency brought the shortcomings of my system to the fore, such as the slightly tipped up midrange of my speakers, and that the digital front end was perhaps not as accomplished as the analog front end, as well as a few other associated-gear related things. I think I could live with these negatives (at least for a while).


Every Audiophile
I guess that the Pass Labs X350.5 is not inexpensive can be viewed as a negative. Most every audiophile is aware that there are more expensive amplifiers available these days, as well as amplifiers that cost less than the X350.5. But it is clear to me that the X350.5 is worth every penny of its asking price, and that one is going to have to spend a lot more on a power amp to hear a significant improvement in sound. Of course there is the aspect of system matching, so the fact that it matches so well with the rest of my system might not mean that it is going to be a perfect match for yours. But what I am sure of is that the Pass Laboratories X350.5 is a fine product, and gets a first-class recommendation from me.



Type: Stereo solid-state amplifier
Power Output: 350 Watts @ 8 Ohms per channel, 700 Watts @ channel into 4 Ohms
Frequency Response: 1.5 Hz to 100 kHz
Gain: 26 dB 
Sensitivity @ 26 dB gain: 2.65 Volt
Distortion (1 kHz, full power): 1%
Maximum output (Volts): 76 
Maximum output (Amps): 28
Damping factor: 150 
Slew rate: 50 V/uS
Output noise: 200 uV
Input CMMR: -60 dB
DC offset: 0.05 V
Input Impedance: 30/20 kOhms 
Leaves Class A @ peak Watts: 40
Dimensions: 19" x 11.2" x 22.5" (W x H x D) 
Weight: 132 lbs.
Price: $11,550


Company Information
Pass Laboratories
13395 New Airport Road
Suite G
Auburn, CA 95602

Voice: (530) 878-5350
Fax: (530) 367-2193
E-mail: info@passlabs.com
Website: www.PassLabs.com













































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