I have a tremendous respect for Nelson Pass and his solid-state designs – specifically his amplifiers. I don't go all the way back to his Threshold years, but I have owned the Pass Labs Aleph 0, Aleph 2, and now the X350.5. The first two are mono-block designs and the X350.5 is a single chassis stereo amplifier.

The big attraction for me is the synergy provided when pairing these amps with a tube preamplifier. I am one of those strange guys who insist that for my ears and my taste, there must be tubes somewhere in the system. Solid-state and tubes should be able to live in harmony. Maybe it's the way those second order harmonics are handled by tubes. Female vocalists, saxophones, and hi-hat cymbals can be eerily real. Maybe it's the slam and detail proved by solid state bass. Acoustic double bass, electric bass (love that P-Bass Fender sound) and drums of all types (conga, tabla, double bass, etc.) have correct texture, speed, and weight.

For whatever the technical reasons, when you can combine the positive attributes of both design philosophies in a single system, the magic begins. When matched with the right tube preamplifier and speakers, Pass Labs solid-state amplifiers can do this as well or better than anything that I have experienced.

Design Evolution

The first Pass Labs models that caught my eyes and ears were the Aleph series, still considered classics by many. Despite their unique aesthetics, they continue to hold their value in the used market. The Alephs were some of the first solid-state amps that were proclaimed to be "tube-like" in sound. This can be attributed primarily to the specific MOSFETs utilized and the minimum number of gain stages that were implemented in the design. Some might argue that the Alephs marked the beginning of one our more popular audiophile debates—that the sound of tubes and solid state designs have merged to the point where today there is little difference in sound at all. I have not totally bought into this argument, but I agree with the trend.

Next up were the X and XA series. These amps evolved from the X to the X.5 and the XA to XA.5 models. The X/X.5 models are Class AB designs and the XA/XA.5 models are primarily Class A at their designated impedance and power ratings. All .5 models replace MOSFETs with JFETs for less noise, higher input impedance, and greater linearity. They also utilize a new bias circuit that provides for increased current, power, and control.

X.5 versus XA.5

I have never auditioned any of the XA.5 series amplifiers in my current system. I am however, very familiar with the sound of the XA160.5 with a variety of speakers and supporting components. Two of my audiophile friends own the XA160.5, so there have been plenty of opportunities for long listening sessions and some interesting discussions about sound and synergy.

I am often asked about the difference between the X.5 and XA.5 series when considering the best match for a specific speaker with specific dampening requirements. Both are designed with unique voltage, current, and power specifications that can help drive that decision. Though the XA.5 series seems to get the most press and accolades, the X.5 series can be just as alluring and satisfying with the right speakers and supporting components. Of course, there are plenty of different sized round holes and square pegs out there, but at least Pass Labs offers two superb solutions that you can audition and experiment with to find your unique and customized nirvana. I think that Kent English from Pass Labs states it the best in a simple and clear way:

Voltage, current and bias are massaged to accentuate a particular effect. Class A amps are optimized for finesse in handling signal and in general have lower rail voltage and greater current reserves than the identical sized A/B package. Class A/B amps are optimized for maximal wattage and in general have higher rail voltage and correspondingly lower current reserves than the identical sized Class A amplifier. Additional current reserves are beneficial in loads of low impedance and with woofers that require lots of control. Higher voltage is beneficial with speakers that are already well damped or those which have capacitive reactance as a big part of their impedance plot (many panel speakers).

If I were taking the over-simplified view of the world, speakers with high "Q" would first audition with class A amps and those of low "Q" would first audition with A/B amps. Speakers of low impedance or high sensitivity tend to get auditioned with the XA.5 amps. Speakers of more modest impedance and low sensitivity tend to get auditioned with the X.5 amps. In other words, there are a number of variables in the decision matrix. That said, these are very much "rule of thumb" and nowhere near "first-principle". The ultimate effect is very much room, material and listener specific. Performance metrics aside, this is a very subjective hobby.

X350.5

The X350.5 is a Class AB single chassis stereo design that weighs in at 150 pounds and can provide 350 watts of output at 8 ohms and at 1% distortion (1KHz). The X350.5 is fully balanced from end to end. Pass Labs calls their circuit topology "Supersymmetry" and has patented the design. The details and nuances of Supersymmetry can be found on their web site.

The X350.5 accepts balanced and single ended inputs. Binding posts for speaker terminations are of an industrial type design—very stable and strong. The AC power input to the amplifier utilizes a RF filter that removes high frequency noise coming into and going out of the ampli­fier power supply. This could be the reason that Pass Labs does not recommend replacing the included standard Belden type power cord with an after market power cord—they don't think that it will make a difference. Regardless, I have experienced improved bass and definition with both my TG Audio Silver and Transparent Audio Powerlink MM power cords.

A breaker switch is located on the rear of the chassis, and a standby power switch is located on the front panel under the voltage dial. This standby switch helps keep warm up time to a minimum, and is a very useful function for my listening habits and schedule (which are unpredictable). The X350.5 runs slightly warm to the touch even at idle.

In general, the X350.5 is built like a tank. From an aesthetic perspective, these amps have a very dramatic industrial design and look to them. In my humble opinion, the thick stainless steel casework and the large current dial on the front panel are as sexy as it gets.

Sound

The following is a short summary of my evaluation of the sound associated with the X350.5. It goes without saying that many of these statements can be system-dependent. These are just a few data points based on my personal experience, my components, and my ears.

I find it difficult to describe a product on its own without including some information about the rest of the system. Please see the "Meet The Writers" section of Positive Feedback (https://positive-feedback.com/staff.htm) to get a profile of my entire system, and what musical tastes and priorities might influence my analysis.

For this review, I have included comments about several combinations of speakers, preamplifiers, and amplifiers. This should also give you a better feel for the sonic qualities of the X350.5.

Speaker Comparisons

I have to admit that I have a love-hate relationship with the sound of Sonus Faber speakers. I have owned at one time or another all of the Homage line, including the Guarneri, Amati, Amati Anniversario, and the Stradavari.

The X350.5 is a wonderful match with the Amati Anniversario and the Stradavari. Both speakers are known for a slightly musical and romantic sound—though the Strads can really turn into chocolate with the wrong electronics.

When utilizing the Hovland HP200 preamp and balanced interconnects, the additional control of the X350.5 took a stranglehold on both speakers at all frequencies. Bass had superior speed, slam, and texture. Mids had improved air and transparency yet still exhibited a rich lush weight and timbre. Highs had an impressive new level of speed and detail without excessive glare and edge.

I also had the opportunity to hear the HP-200 and X350.5 with the Magico V3 speakers. The speed and delicacy of the V3 complimented the rich detail of the X350.5 in a very special way. This combination was pure magic—especially with chamber music, female vocals and small jazz quartet recordings. The sound stage reached out well beyond the speakers and in all directions. Images locked in as well as any other system that I have heard.

Preamp Comparisons

When I first purchased the Aleph 0 mono-blocks long ago, I was told that I if really wanted to hear them sing, I should consider pairing them up with the Joule Electra LA-150 tube preamp. This was a match made in heaven, sparked true love, and started me on my long journey for combinations of tube preamps and solid-state amps. That LA-150 also provided me with excellent service when combined with the solid-state Krell 600 and the solid-state Hovland Radia amplifiers.

For this review, I auditioned the X350.5 with three preamps: the Pass Labs XP-10 (solid state), the Joule Electra LA-150 (tubes), and the Hovland HP200 (tubes). The Sonus Faber Amati Annirversarios were used for all comparisons; balanced interconnects were used throughout.

From the first note, it was obvious that there was a definite synergy when mating the XP-10 and the X350.5. The sound was extremely smooth and natural. Tremble had excellent speed and good detail. There was enough warmth and weight to satisfy my midrange cravings. Bass was as expected—powerful and fast. I can only imagine what the bass performance might be like with the top of the line XP-20 and it's standalone power supply. I would love to try the XP-20 in a follow-up review. (Are you listening Pass Labs?) Nevertheless, if I ended my review at this point and had to live with this system and only this system going forward, I would still be one happy camper.

Next up was the Joule Electra LA-150. The LA-150 can only provide single ended outputs, so single ended interconnects were used throughout. It never occurred to me that I was missing anything with the XP-10, but once I had the LA-150 in place, it suddenly seemed as though the sound included rushes of different textures and flavors that I had not experienced before. The output was slightly darker, but for my ears this had a positive effect at most frequencies. Female vocalists were creamy and sensuous, without bloat and chestiness. Cymbals had excellent detail and shimmer, though I was hearing more of the metal sound of the cymbal than the trailing edge. Bass was powerful yet a little more rounded and dark. I preferred the tighter, more detailed bass of the XP-10, but the LA-150 got my juices flowing at all other frequencies.

The Hovland HP-200 provided a third palette of performance and options to consider. Balanced interconnects were used between the preamp and amp—not a unique option for a tube preamp these days, but I was beginning to think that this balanced thing might be making a significant difference in sound quality. It would be hard to prove this without trying additional preamps. The darkness was now gone at all frequencies. Bass was as tight and powerful as the XP-10, but I was now hearing much more information in the lower frequencies. Different brands of electric bass were easy to recognize. When struck, I was able to sense the leather and skin effect on bass drums and congas. Cymbals had greater shimmer and decay without edge. Mids were not quite as rich and tasty as the LA-150, but still extremely satisfying. Reed instruments had the proper weight and timbre. Female vocals were still luscious and seductive. For my listening tastes, the HP-200 and X350.5 combination hit the nail on the head.

Amplifier Comparisons

One of my favorite amplifiers of all time is the Joule Electra Rite of Passage. This is an all tube mono-block design of considerable size and power (220 Class A watts at 8 ohms). I compared the X350.5 with the Rites using both the LA-150 and HP200 preamps and the Sonus Faber Amati Anniversarios. As required, single ended interconnects were used throughout. As expected, the Rites exhibited much of the same sound as the LA-150, but they managed to step it up a couple of levels in terms of proper weight, timbre, and what I can only state as "you-are-there realism."

The Rites are slightly dark, yet still addictive and mesmerizing. They have probably the best mids of any tube or solid-state amp that I have personally experienced. The X350.5 held its own, but that "reach out and touch" factor was slightly less apparent.

On the other hand, I preferred the bass and treble of the X350.5 for all the reasons mentioned earlier. All those tubes create a tremendous amount of heat, and on a nice summer day and it can be overwhelming in a small-to-medium-sized room. As with any tube component, you never really know if the tubes are breaking in, at their peak performance, or burning out. Yes, I love tubes, but if I can minimize the number of tubes in a system and still get a similar, if not better quality of sound, I will save in long-term maintenance and anxiety. The Rites can also be extremely noisy. Hum and grounding issues come and go. When I consider all of these factors, I prefer the X350.5.

I did not directly compare any of the other amps that have found time in my listening room over the years, so please view the following comments with the appropriate amount of caution. Based on my memory and notes, the X350.5 has it all over the Krell 600 and Hovland Radia. Bass quality compares with, or exceeds, the Krell. In terms of bass, the Hovland is not even in the same league. Mids and treble can obviously depend on your taste and system synergy, but I could now never give up the detail, warmth, and timbre that the X350.5 can provide.

Music

I find that one way to effectively communicate what I have been experiencing with a component is to describe the sound that I hear on specific CDs, SACDs, or LPs.

Dead Can Dance – Into the Labyrinth, 4AD Records. SACD

Talk about "wow factor"! It's hard to pin this group down in terms of music genre, but the primary focus seems to be on rhythm and percussion. World music? I'm not sure, but these songs can range from Celtic and Middle Eastern folk songs to Georgian chants with a dash of classical music and Tangerine Dream synthesizers thrown in. My favorite is track number 1 - "Yulunga (Spirit Dancer)." After a slow rhythmic opening utilizing a variety of synthesizer and string instruments, the congas kick in with a bang. With the X.350.5, you can sense the tightness of the leather and skin on each drum. You can feel it in your chest and ass as the percussion moves across the soundstage. No overhang. No huffing and puffing. Speed and dynamics are as good as it gets. The X350.5 keeps up and executes without flaw.

Beethoven – Sonata In G Major, Op.96 for Piano and Violin, Wilson Audiophile Recordings. CD

This CD is out of print and can be difficult to find. Just create a permanent search on Ebay and be patient. It is definitely worth the effort. It can also be found in LP format—which might sound even better, if that's possible. David Wilson is much more than a speaker designer. Simply recorded. Only the best equipment and instruments utilized. A single pair of Schoeps microphones, a Guarnerius violin, and a Hamburg Steinway all recorded in a small college hall. I have mentioned that the X350.5 has an incredible ability to produce proper weight and timbre. Check out the Guarnerius violin. You can almost taste and smell the rosin as it pops off the bow and strings. Piano is one of the most difficult instruments to record and reproduce. The harmonics are spot on. The dynamic interplay between the two is amazing. Reach out and touch! You are there!

Richard Thompson – The Old Kit Bag, Cooking Vinyl Recordings. LP

I love this recording – especially on LP. Richard Thompson is a master and deserves much more respect and acclaim, though I doubt that he wants it. As with most of his releases, it's all about love songs and relationships. Check out the solos on cut 1 - Gethsemane. The X350.5 gets the bass kick drum and tambourines just right. Electric guitar is as nasty and dynamic as it gets. Not sure what guitar amp Richard is using, but the sound has a delicious growl and texture. This is what all Rock ‘n' Roll recordings should all sound like.

Madeleine Peyroux – Dreamland, Atlantic. CD

Yes, "Careless Love" gets all the press attention and an audiophile CD and LP treatment to boot, but I feel strongly that "Dreamland" is her masterpiece. This is her first recording out of the gate and it is a sonic wonder. Some complain that Madeleine sounds too much like Billy Holiday, but I never sense that she is anything but a true original. I love female vocalists, and these songs are an excellent test for midrange accuracy. In some systems, I have heard a faint but annoying strident element to her voice. Not with the X350.5. Listen to Cut 2, "Hey Sweet Man." Recorded with just Madeleine and acoustic guitar, you can get a real sense of her delicate voice. What you hear is a golden glow and sweetness without edge. Clarinets, saxophones, marimbas, and banjos can be found throughout the twelve tracks. They hover in space and pop out like the real thing. Madeleine supposedly started her career on the street corners of Paris with just a few friends playing simple acoustic instruments. You can easily imagine that the guitar case full of loose change and bills is right there lying on the floor—right in front of your eyes and ears.

Summary

The Pass Labs X350.5 amplifier is an extraordinary achievement for both sound and execution. When properly matched with speakers that have synergistic voltage, current and power requirements, it can compete well with any solid-state amplifier in the market. This includes Pass Labs own XA.5 series of amplifiers, which have received so much attention and rave reviews. The X350.5 can easily be paired up with a variety of tube preamplifiers to produce a sound that exemplifies the merits of both solid-state and tube design. Though additional study is required, it seems that a balanced interface to supporting preamps can bring the sound to an even higher level of excellence. I am looking forward to the next generation of Pass Labs products. The current benchmarks will make it quite a challenge for Pass Labs to beat their own products! Robert Youman

X350.5
Retail: $11,550

Pass Laboratories
PO Box 219
24449 Foresthill Rd.
Foresthill, CA 95631 USA
TEL: 530.367.3690
web address: http://www.passlabs.com/index.htm