Pass Labs XA100.5


Well-Known Member
Dec 7, 2011
North Shore of Boston
I have been interested in high-end audio, and music in general, for about fifteen years. During this time I have experienced the steady evolution of Pass Labs class A amplifier technology. My first serious amplifier was the Pass Aleph 3. My system back then consisted of Symdex Gamma speakers and a portable Radio Shack Optimus CD player with variable output. It was a pretty basic system, but one which introduced me to the joys of music and good quality sound.

Aleph: Six years ago I decided to upgrade that simple system. I bought a turntable and a Pass Aleph P preamp and Aleph Ono phono amp. Soon afterwards I upgraded my speakers to the Egglestonworks Rosa which are 87db efficient and have a nominal impedance of 6 ohms and a minimum impedance of 4 ohms. Suddenly the 30wpc Aleph 3 no longer had enough power. I upgraded to the 60wpc Aleph 5 and then the 100wpc Aleph 2 mono blocks. These Aleph amplifiers sounded remarkably similar. They were each simple, pure Class A designs with only 2 gain stages. There was a naturalness, smoothness and ease to the sound. The midrange especially with vocals and horns just sounded right. It was immediate, transparent and incredibly palpable. The treble was sweet and airy. There was a sense of flow and lack of fatigue that made listening to music very involving and relaxing. But the lower frequencies were missing focus and control. Bass was slow and somewhat loose. Transients were blurred. Each upgrade brought increases in power and slightly improved dynamics, but the bottom end remained weak and poorly defined.

XA: Then the XA160s arrived. With the addition of super-symmetry, noise floor and distortion dropped precipitously. Grip of the lower end and overall control of the speakers improved. Macro and micro details emerged and the soundstage grew deeper. Music suddenly became much more involving. Instrument tone, timbre and texture all improved. I experienced the cliché of “hearing new things” on each of my LPs. The middle and upper frequencies reached new levels of clarity, transparency and detail, while retaining their incredible naturalness. Air and imaging also improved. But as is usually the case as things improve, expectations increase, and the system’s deficiencies become all the more apparent. The incredible current reserves in the Alephs were no longer available in the new XA. As the impedance of the Egglestons drops to 4 ohms in the lower frequencies, the XA160’s 160 watts remain constant. Lower frequencies became cleaner and more focused, but transients remained somewhat slow and dynamics seemed constrained.

XA.5: With an interest toward improving transient speed and bottom end control, I traded in the XA160s for Nelson Pass’s new XA100.5. On paper, the differences are increasing power as impedance loads decrease, much greater amounts of current and lower noise. The design is said to be about 20% more efficient than the XA, but being class A, the amps are still quite hot.

Listening to the XA100.5: I did not begin any critical listening until I had played music through the amps continuously for 200 hours. Changes were quite noticeable during the first one hundred hours or so, then there were some minor changes for the next 50 hours and then the amps settled in and behaved consistently.

When I first sit down to evaluate a new piece of equipment, I play whole sides of favorite albums to get a general sense of rightness. Is the music enjoyable? Is the experience relaxing? Is the sound pleasing? In essence, I ask myself if I am having fun. As I begin to listen more critically to determine if the new equipment improves the sound of the system, I ask if familiar music is more involving that it was before. Does it sound more real? Then I begin to analyze the sound with individual tracks in order to understand how instruments and vocals sound. Below are some specific listening impressions of the XA 100.5 amplifier describing the areas in which it truly excels:

1. Tonal Accuracy: John Coltrane and Johnny Hartman ( Impulse AS-40, LP)
“Lush Life” has never sounded so lush. Hartman’s voice has moved down into his chest. It is rounder with more flesh. Coltrane’s sax entering half way through the song used to be surprising. It is now startling in its impact and tonal purity. I found myself comparing the undertones of Hartman’s voice to those of the piano. Along with tone, timbre is incredibly accurate. The ultra low noise and lack of distortion accentuate the beauty of these songs. Cymbals sound like brass being struck and not like a bunch of white noise. I found I wanted to turn up the volume to more realistic levels in my small room. This is moody music.

2. Dynamics: Beethoven, Violin Concerto in D, Heifetz/Munch/BSO (RCA LSC-1992, LP)
Recorded in Boston in 1959, this piece has massive dynamic swings. I have heard the BSO perhaps eighteen times in the last ten years and this amplifier truly captures the space of the hall. The layering of the orchestra is much more accurately portrayed. The incredible swings from massed strings to solo violin are clearly delineated. I could “see” and feel the orchestra surrounding Heifetz’ enthusiastic violin playing. Individual instruments became distinct. The strings had real weight and the solo violin’s wooden timbre was much more evident. Near the end of side one, Heifetz’ aggression becomes almost sweet and delicate when it ascends into those last high notes as the hall becomes dead quiet. The silence between the music and the air around the violin was palpable and completely engrossing.

3. Transient Speed and Decay: Duke Ellington and Ray Brown, This One’s For Blanton (Pablo 2310-721, 45rpm LP) If you have never heard a well-recorded LP played back at 45 rpm, you can not know what this medium is capable of. This duet featuring two jazz giants exemplifies realistic acoustic bass and piano. Transients are explosive. Plucked strings and felt hammers striking strings sound lightning fast and accurate. Rhythm and pace is spot on. Decays are long, continuous in tone and utterly natural. You can hear the instruments vibrate the air creating sound. Though this music is ultra quick, it is very relaxing. This is toe-tapping at its best.

4. Imaging: Best of Chesky Jazz Vol. 2 (Chesky JD68, CD)
Tracks 14-33: “Percussion Imaging Tests.” As the drum set is moved away from the microphone in three-foot increments, and the studio goes from dry to live, the recording space is very clearly defined. Depth and image specificity is superb. The system can play much louder with less distortion. Drums never spanned the frequency spectrum so well. Track 47: “General Image and Resolution Test.” Four musicians play percussive instruments, walk up to and around the microphone three times. Previously, the individual instruments were overlapped and sounded unfocused. They moved together as one mass. Only one drummer sounded as though he jumped around my head. With the XA.5, the instruments are distinct and clearly defined. All four musicians now sound as though they are walking in succession out from the space between my speakers right toward me and three of the four actually seem to march around my head. The image resolution and instrument specificity is astonishing. I simply did not think this 3-D special effect was possible from a two-channel system.

5. Realism: Carla White, Mood Swings (Milestone, M-9159, LP)
“Love came on Stealthy Fingers”. Here is a song that sums it all up. This is one of my all-time favorite vocals from the late New York jazz singer. I’ve heard her live a dozen times. She sang at my wedding. This piece sounded good and fairly realistic with the earlier Pass amps in the system. With the XA.5, the tone of her voice became utterly natural. With the lights dimmed, late at night, the mood is recreated. Carla is here in the room centered in front of me singing every crystal clear note. The piano is a few feet behind and to her left. The bass is on her right. Intimate and perfect. This is how she would have wanted her voice to be remembered. Hearing this song so well reproduced in my room is what listening to music through a stereo system is all about.

Design: I have had five Pass amplifiers and four Pass preamps and phono amps. Nelson Pass is a legend in the industry. His reputation is well deserved. I have found each of his products extremely well designed and reliable. The amps are hot, heavy and big. I have not experimented with aftermarket power cords, amp stands or damping devices. My amps are directly connected to dedicated 20amp circuits. (See details in my virtual system page). Speaker terminals are big and easy to use. I could still hear some minor transformer hum when my ear was placed an inch above the top plate, but it is not nearly as noticeable at it was on the XA160s. The packaging is superb. Once powered up, the meters don’t seem to move in the XA.5s, but that aluminum faceplate is simply gorgeous. These amps are expensive and they look it.

Conclusions: Clearly, I am a big fan of Pass Labs products. The newest line is no exception. The XA100.5 is a highly resolving amplifier. It is the most neutral and transparent amp I’ve had in my system. Tonal accuracy, transient speed, dynamics and imaging are all significantly improved over the XA design. Noise floor is lower, making audible an astonishing amount of low-level detail. The soundstage is slightly wider and deeper. The XA.5’s 100 watts sound bigger and have greater grip on my speakers than did the 160 watts of my old XA160s. The amp has overcome the previous Aleph designs’ weaknesses while retaining their strengths. Could it be better? I suppose it could have even quicker, more explosive and effortless transient attacks, but that is probably more a function of my speakers and their efficiency. It could also have even lower distortion and less noise, but they are so low now that they may be approaching some theoretical limit. Good recordings sound better, while poor recordings sound worse. Unfortunately, with this amp’s increased transparency, system shortcomings have become more apparent and will someday need to be addressed.

In the end, is it mostly about enjoying the music? Of course it is, or it should be. But it is also in some small part about being able to depend on and, yes, even admire the equipment. The Pass XA100.5 amplifier is one of those rare components that enable its owner to enjoy this hobby that much more. Is it perfect? Who knows? But it certainly does make me want to improve the rest of my system in order to find out.

Musical tastes: jazz standards, small scale and big band, vocals, 80%, classical 15%, classic rock 5%. (vinyl 95% vs. digital 5%)
Sound preferences: neutrality, tonal accuracy, imaging
Room dimensions: 16 X 17.5 X 7.5, speakers on long wall
Previous amplifiers: Pass Labs Aleph 3, Aleph 5, Aleph 2, XA160

Associated equipment (more details on virtual system page)

Front end: SME Model 10 with SME V arm
Sumiko Celebration cartridge

Electronics: Pass Labs X1 preamp
Pass Labs XONO phono

Speakers: Egglestonworks Rosa, (87db, 6ohm nom./4ohm min.)

Cables: Hovland Music Groove II phono
Harmonic Technology Magic ICs
Harmonic Technology Fantasy AC
Harmonic Technology Pro 9+ speaker

My system has changed since I wrote this review. Here is a link to the current set-up:

Stereo, XP-20 003.jpg

Stereo, XP-20 019.jpg
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WBF Founding Member
Apr 21, 2010
Manila, Philippines
Very well written review Peter. Kudos to you. Just a couple of pictures and you would've gotten a perfect score. ;)


Well-Known Member
Dec 7, 2011
North Shore of Boston
Very well written review Peter. Kudos to you. Just a couple of pictures and you would've gotten a perfect score. ;)

You request, I provide. Thank you for your kind words.


Well-Known Member
Dec 7, 2011
North Shore of Boston
Nicely written review Peter. I enjoyed my XA-100.5's immensely.

Thanks Christian. This review did appear briefly on the Pass Labs website before they revamped it. Yes, the XA100.5's are very good. But I understand that your XS150's are considerably better. Progress marches on. I did find that the difficult load of the Mini 2 was too much for the 100.5, so I went with the considerably more powerful 160.5.

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