Published On: January 29, 2018

Pass Labs XA25 Stereo Amplifier Reviewed

Published On: January 29, 2018

Pass Labs XA25 Stereo Amplifier Reviewed

In this week's featured review, Terry London auditions the XA25 amplifier from Pass Labs. This pure Class A stereo amplifier is rated at 25 watts RMS into eight ohms and 50 watts into four ohms.

My review of the new and quite extraordinary Pass Labs XA25 amplifier, which retails for $4,900, marks the fifth time I've had the pleasure to experience yet another creation by Nelson Pass and his team to drive my reference system to its highest levels of musical enjoyment. For the last four years, my solid-state reference has been a pair of the XA60.8 mono-block amplifiers that have offered nothing less than superlative performance, regardless of what speakers I added. I knew that Pass Labs had introduced the XA25 about six months ago and had put it on my list of future reviews to arrange. However, I got sidetracked with other reviews and my professional work responsibilities. Then, in a casual conversation I had with Andy Collen, owner of the high-end salon The Audio Surgeon located in Michigan--who is a long-time friend of Nelson Pass, a Pass Labs retailer, and someone who has great taste regarding anything in audio--he asked me if I had heard the XA25 yet. When I said no, he responded, "You are in for a real treat. In its own way, it might be the best of all Nelson's amplifiers in the last 20 years!" This certainly motivated me to get the XA-25 in-house to review as soon as possible.

The Pass Labs XA25 is a relatively small amplifier compared with the XA.8 Series models. It measures six inches high, 17 inches long, and 17.315 inches wide, and it weighs 55 pounds. It lacks the bias meter found on the front of all the other XA.8 Series amplifiers. The silver front plate simply has a push-button on/off switch and a single blue LED that lets you know that the amplifier is on. Engraved on the left and right side of face are "XA25" and "PASS," respectively, which flank two grooves that are carved out of the face plate. Around back is one set of RCA inputs, one pair of very high-quality speaker wire terminals, and the IEC input. Finally, a pair of robust handles makes moving the XA25 a relatively painless procedure. As you can expect from Pass Labs, the chassis and build quality rank with the best on the market, regardless of the price. I found the XA25's appearance to be quite handsome in an understated yet classy "less is more" way.

Pass-XA25-silver.jpg

The XA25 is a pure Class A design that only uses two transistors per channel, yet it delivers 25 watts RMS at eight ohms and 50 watts RMS at four ohms. However, it can deliver peak current of 10 amp output (200 watt A/B peak into two ohms), which means that it can drive most speakers without any difficulties. Three other measurements worth noting are its damping factor, output noise level, and slew rate. When compared with the Pass Labs XA30.8 pure Class A single-chassis amplifier, the XA25 has a much higher damping factor (500 versus the XA30.8's 150), a lower output noise level (uv:50 versus the XA30.8's uv:200), and a higher slew rate (100v/us versus the XA30.8's v/us 50). The Pass Labs XA.8 Series amplifiers are some of the quietest solid-state designs on the market, and this one has a dramatically lower noise floor. What these measurements equate in sonic parameters are that the XA25 is a faster and more dynamic amplifier, has unbelievable transparency/clarity, and has the ability to exhibit great control over the low frequencies because of its damping factor rating.

Since the XA25 uses qualitatively different transistors than the other Pass Labs amplifiers, I asked Nelson if he could write a brief explanation of what the differences are between the XA25 and XA.8 Series amplifiers. He answered, "These types of higher power devices have been around for a bit, but in the past I have found it more practical to match a quantity of 150-plus-watt devices and run them in parallel, which requires resistive ballast for thermal stability. For years, we have noted the differences that output stages exhibit with varying amounts of this degeneration, and it was always clear that the best examples showed the most 'square law' character and largest Class A envelope, which is the 'simple' power FET model. We finally decided to remove all degenerative resistance from the output stage for sonic reasons. The design that did the job required new bias circuits, and it was easier to make with large single-die Mosfets. There are a limited number of such products on the market, and we bought samples and tested them in prototypes by measurement and listening, and we picked our favorites, which you see in the XA25. It is not simply the big transistors that create this effect--the bias circuit being a key element--but they are the most convenient approach."

The Hookup
The XA25 arrived with the usual first-rate packaging in which Pass Labs ships all of its gear. I inserted the XA25 into my reference system, replacing the XA60.8 mono blocks, along with other amplifiers that I use (Linear tube Audio ZOTL-40, Triode Lab SET 2A3, Usher 1.5 reference, AricAudio SET KT-88),to drive the Tekton Design Ulfberht speakers. The rest of the system was comprised of the CEC-3 CD transport, Concert Fidelity-040 hybrid DAC, Linear Tube Audio Micro-ZOTL preamplifier, Running Springs Dmitri power conditioner, MG Cable reference silver and copper wiring, and Audio Archon power cords, all placed on the Tomo rack/footers by Krolo Design.

Performance
My first selection was the brand new album by the great jazz pianist Ahmad Jamal: Marseille (Jazzbook Records) captures the sound of Jamal's piano with great accuracy as he plays it in a studio venue. I was immediately struck by two attributes of the XA25's performance. First, its overall liquidity was striking in how the music seemed to ooze out of the Ulfberht speakers and just flow/float into my room. I am not just talking about being grainless compared with other great solid-state amplifiers; this one had a special quality that I have only found in the very best tube amplifiers that seem to get you closer to the music in a relaxed way. Secondly, I believe that the XA25 is the quietest amplifier I have ever had in my system. I could hear every micro-detail easily and clearly, with no sense of the music being analytical or exaggerated compared with listening to live music.

The next selection came from the great jazz tenor saxophonist Johnny Griffin. The Kerry Dancers and Other Swinging Folk (Riverside) is an album that I always use as a test recording for timbres/tonality/color. It also helps me get a take on how well a component handles individual imaging, how three-dimensional the players sound, and how much air/space is around each player. I was very fortunate to hear Johnny Griffin play many times in jazz clubs in Chicago, so I use this recording as a gauge to see how close a system can get to producing the illusion that I'm hearing him play live. With the XA25 driving my system, it got very close indeed. The timbres and color saturation equaled what I normally experience only with tube-based SET designs. The sound of his tenor saxophone had a sensational lifelike presence in its tonality and came closer to his real-life sound than I have ever heard in my system before. The XA25 also rendered one of the highest levels of what I call image density or palpability for each player. The size of each individual player was accurate, along with that person's placement in the soundstage. If I didn't know better, I would swear that the XA25 was a world-class tube amplifier, due to the overall liquidity and voluptuous timbres/tonality of the instruments and three-dimensional imaging. Yet it also delivered unbelievable transparency and extension/control of the bass frequencies.

One of my favorite pop vocalists of all times is Steve Winwood. His new album Greatest Hits Live (Thirty Tigers) contains extended lengthy jazz/blues versions of his greatest songs throughout his career. The sound of this live recording is one of the finest I've heard in the last 10 years. The XA25 created a wall-to-wall representation of where Winwood and his band were performing, with tremendous depth and height, and made the 21-driver, seven-foot-tall Ulfberht speakers totally and completely disappear into the soundstage. With this recording, the XA25 showed the type of control and macro-dynamics it has on tap. When the band was ripping at the highest volume levels, the XA25 never broke a sweat or showed any strain at all. According to my dB meter, the system was hitting peaks of over 110 dB. I quickly lowered the volume level to protect my hearing and let the foundation of my house settle back to normal.

My final selection was the album Dreams and Daggers (Mack Avenue) by the highly acclaimed jazz singer Cecile McLorin Salvant. The recording features both live and studio performances. To a certain degree, Salvant sounds like a young Billy Holiday and shines in her phrasing. She also has a pristine tonality that clearly conveys her emotions. This recording again showcased the extraordinary way that the XA25 produces a delicacy and sweetness of colors/tonality and, at the same time, reveals every little simmer and nuance of her voice, rendered in a very natural/organic fashion. When her bandmates kicked in, I could easily discern their placement around her on the bandstand.

The Downside
The XA25 amplifier is not a balanced design, so you can only use single-ended RCA connectors to drive it. The XA25 is so transparent that you'd better have a reference-level preamplifier to drive it; otherwise, you will not get the total magic that this amplifier has to offer. Finally, speakers with very low sensitivity and/or wicked impedance slopes might not be a good match for the XA25.

Comparison and Competition
In the $5,000 price bracket that this amplifier falls into, I have experience listening to two solid-state amplifiers that would be natural competitors. The first amplifier is the Sanders Sound Magtech, which retails for $5,500. The Sanders Sound Magtech has the ability to drive virtually any speaker. However, compared with the XA25, it lacks the overall liquidity and beautiful timbres/tonality, and it sounds somewhat grainy and dry in its presentation. Furthermore, it lacks the XA25's soundstaging ability in regards to image palpability and space between individual players.

The second amplifier, the Aesthetix Atlas, is a little more pricey at $8,000. The Aesthetix Atlas comes much closer to the XA25 in it produces timbres/tonality. It's an overall warmer/sweeter amplifier with a lot less sense of dryness. However, it still does not get the beautiful purity of tone/color that the XA25 nails down in spades. I also do not believe that the Atlas is quite as "cat-like" in its speed or overall macro-dynamics as the XA25.

Conclusion
There is an old platitude often used in high-end audio: the perfect amplifier would have the best of both worlds--the beautiful colors, lack of grain, and space/imaging of a great tube amplifier and the dead quiet nature, rock-bottom/tight bass, and transient speed of a great solid-state amplifier. In my 30 years of listening to great amplifiers from Threshold/Pass Labs, I believe that Nelson Pass and his team have come closest to this utopian ideal with the new XA25 amplifier than ever before in their long and illustrious history.

The XA25's performance in my system is still somewhat shocking to me. I thought I would never hear a solid-state amplifier that could create the gorgeous timbres/tonality/color of the finest tube-based amplifiers, yet that's what you get with the XA25. Another quality that distinguishes the XA25 from other solid-state amplifiers is its amazing liquidity. It's not just that this amp has a lot less grain and dryness. Rather, it's the XA25's total absence of grain and dryness. The XA25 also creates a soundstage with three-dimensional space and air between players, with realistic depth/width/height, and "meat on the bones" image palpability. Then add in the fact that the XA25 has virtually has no noise floor, leading to amazing transparency, and is also fast and dynamic, with a simmering sweet top end and full bass extension with control and grip on the bottom frequencies. The Pass Labs XA25 amplifier will not be leaving my system; it is definitely my new reference solid-state amplifier.

Additional Resources
• Visit the Pass Labs website for more product information.
• Check out our Amplifiers category page to read similar reviews.
Pass Labs HPA-1 Stereo Preamplifier/Headphone Amplifer Reviewed at HomeTheaterReview.com.

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