At the same time, as Nelson Pass makes it clear in the Pass literature on the Xs 300s, they are not an exercise in size for size’s sake or in specmanship. He stresses the harmonic structure of the Xs 300’s transfer curve and its ability to reproduce the quality and integrity of live music. He also responded to my request for background on how the changes in design affected sound quality by stating that “we had been working on the Xs amplifier design for several years, and the big hardware aspect of it was nicely covered, but the progress stalled when we found that a simply bigger version of what we had been doing wasn’t enough of a breakthrough in sound. Then a couple of years ago my batch of custom SIT transistors arrived, and we started to listen to the first prototypes of little 10 watt amplifiers using them. The transistors had a unique character, and while the SIT amplifier was not at all adequate to our needs, it still showed us what we were looking for. I remember remarking that ‘whatever the hell that has, we need to find a way to bottle it!’
“I began a comprehensive objective analysis of the SIT amp and began applying some new approaches to elicit some of those same qualities from the existing Xs amp circuit. Initially we used the SIT amps as a benchmark, but quickly began expanding the performance envelope for much greater power and control and much less distortion. While we retained most of the signature in the subtle relationships between the lower-order harmonics, it was not quite the same—it was better.
“While every part of the amplifier is important, playing with developmental tube and SIT designs showed that it is the character of the power output stage itself is most influential in shaping the sound of the amplifier. This is not a radically new idea—the output stage does the most work, generally has the most distortion, and is the interface to the complex variable and reactive load which is a loudspeaker. In the end, we found that adjusting the values for push-pull Class A biasing, and also the amount of single-ended bias in the output stage, gave us a major improvement. There are also two new and proprietary circuit techniques, one having to do with a form of local feedback around the middle stage (stage 2 of the 3), and the other fundamentally altering the use of the constant current sources that are used to contribute to the bias of Class A push-pull output stage and mold its sonic footprint.”
Nelson notes in the instruction book for the Xs 300 that his previous designs for Pass Labs led him to go from the single-ended/push pull Aleph Series, to single-end bias in balanced output stages in the early XA Series, to balanced push-pull/ single-ended Class A in the XA.5 Series, to finally making massive increases in the output stages of the Xs with a 10-fold increase in the bias current provided by constant current sources. This expanded the single-ended power range by a factor of 100, and brought the Xs Series to something far closer to pure single-ended Class A. The output stages eliminate even trim capacitors, have a 100kHz open-loop bandwidth with only limited feedback, and the circuit is DC-coupled.
He also notes that the Xs Series of power amps makes improvements in the drive stages and devices—using Toshiba MOSFETs and new forms of local feedback. Input impedance has been raised to 200k ohms in the balanced input, with minimal capacitance. Virtually any preamp will drive the Xs, and sensitivity to input cables is minimal.
Like the Xs preamp, the end result is a pair of amps I’d love to be able to afford and own—perhaps Pass will let me use one of my children as a hostage and loan me a pair. The Xs 300s are superbly musical with any kind of solo voice and instruments, depending on the recording. That added level of detail and dynamic realism is less forgiving of recording and front-end limits than the XA-160.5s—particularly any that are slightly bright or have an upper-midrange edge.
Going back to its ability to deliver immense levels of power and energy at real-world listening levels, the Xs 300 has enough power in the bass to drive my listening room—hell, my whole house—to vibration on those few recordings that really get down to the deepest bass. It can easily handle the electronic room compensation of my Wilson Alexias down below 25Hz (get out your copies of Rutter’s Requiem, the Fennell and Dallas Wind symphony recording of Pomp and Pipes, or the Jean Guillou organ recording of Moussorgsky’s Pictures at an Exhibition). Control, detail, and apparent speed are all improved for the lower bass into the bottom edge of the midrange.
It is the overall realism of the music in the lower to upper midrange, however, that is the area of clearest improvement. In a head-to-head comparison with the Pass XA-160.5s, and the excellent AVM SA8 power amp, the Xs 300s did as much as any amplifier I have yet heard to get the most musical detail and realism out of the full range of my recordings. Low-level musical and soundstage detail were particularly impressive. So was the purity of really good recordings of solo violin and piano, as well as soprano voice.
I was also struck by the fact the Xs 300s made two other improvements over the XA-160.5 and other Pass amps I’ve auditioned, which I had heard to a lesser degree in comparing the Xs preamp to the XP-30. The soundstage is not only more detailed, but also more open and wide, and apparent depth is more realistic when the recording permits. The mid-and upper-bass are also more detailed and lifelike, again when the recording permits.
The Xs 300 is the first Class A amplifier I have heard that does not even slightly soften upper-midrange and upper-octave dynamics. The end result is musically realistic and involving at every level. No aspect of music or sound quality stands out from another because none has to. This is an almost perfectly balanced set of incremental but very real improvements over the other amplifiers I’ve auditioned in my system, and it is an overall level of improvement my audiophile friends heard and commented upon without any prompting from me.
As a result, there is a natural synergy between the Xs preamp and the Xs 300 mono amplifiers. To go back to my earlier comments about the way manufacturers voice their electronics, the two are complementary in their improvements, and the sheer neutrality of their voicing ensures that improvements in the sound quality of one does not mask those in the other.
I should stress that the margin of superiority that the Xs preamp and Xs 300 amplifiers had over my reference XP-30 and XA-160.5 was limited, and was dependent on the quality of the recording, my front end, speakers, and cables. But hell, I’m an audiophile and that margin of superiority is really hard to resist. There are reasons why Nelson Pass warns in the manual for the Xs preamp that “I can only say that if you are on a restricted budget, you might be wise to avoid borrowing one of these.” That same warning applies to the Xs 300s, and even more to listening to both.
But, I’d temper Nelson’s warning just a bit. I’ve heard a lot of truly superb systems and gear over the years that were well beyond my budget. I’ve never regretted such experiences, for they have always taught me something about setup, sound, and music, and even when I’ve had to return gear on loan I’ve gone back to my reference system able to understand it and enjoy it more than before.
As has been the case with every bit of really good gear I’ve listened to—regardless of whether I could afford it—I have truly enjoyed my time with the Xs preamp and amplifiers, and done so without regret. Moreover, I know from personal experience you can get truly outstanding performance out of far less expensive Pass Labs preamps and amplifiers. I never come away without learning enough to make slight improvements in the way I place my speakers and listening position, in choosing interconnects and cartridges, in how I set up my tonearm and turntable, and in selecting front-end components. Oddly enough, I also find myself rethinking my ratings of the sound quality of both my older LPs and high-resolution recordings and downloads. Being an audiophile does not have to be a passive sport. Every time you learn from listening to other components, you can act on those lessons.
Finally, if you do ignore Nelson’s warning and somehow become addicted to the Xs preamp, the Xs 300, or both—and if find you desperately need a solution that goes beyond intelligent restraint—I have one that works every time: Just win the lottery. You deserve an Xs preamp and a pair of Xs 300s!
SPECS & PRICING
Pass Xs Preamp
Max output: 22V
Output impedance: 20 ohms main balanced per leg; 50 ohms aux balanced per leg; 120 ohms single-ended RCA, either
Noise floor: -122dB ref to 5V
Distortion: .001% 1kHz 5V
Frequency response: –1dB@1Hz and 100kHz
Channel separation: Greater than 110dB
Residual broadband noise: Less than 25mV
Max gain: 10dB
Volume control: .5dB steps
Power consumptions: 55W
Dimensions: 19" x 6.25" x 14"
Weight: 80 lbs.
Pass Xs 300 Mono Amplifier
Power output: 300W into 8 ohms, 600W into 4 ohms
Input Impedance: 200k ohms bal, 100k ohms single-ended
Power consumption: 1000W
Number of chassis: Two per amplifier channel
Dimensions: 19" x 11.5" x 27.5", each chassis
Weight: 168 lbs. (power supply), 130 lbs. (amplifier)
PASS LABORATORIES INC.
13395 New Airport Rd., Suite G
Auburn, CA 95602