TESTED: Pass XP20 preamplifier and XA160.5 monoblock power amplifier

Equipment report
Solid-state preamplifiers
Pass Laboratories XP20
TESTED: Pass XP20 preamplifier and XA160.5 monoblock power amplifier

Let me begin this review with the punch line. The new Pass XP20 is one of the best linestage preamps around, and the XA160.5 power amp one of the best Class A power amps I’ve ever heard. I am using them to update my reference system, and I would put them firmly in the top rank of audio components. I realize that this takes the suspense out of this review, but I want to make this clear from the outset, because I also want to caution you about the limits to the differences between today’s best preamplifiers and amplifiers.

The XP20 Preamp

I have to confess that I begin my review of each new effort to make the ultimate linestage preamplifier with an instinctive distrust. Somehow, each time, I have to relearn just how important a component a preamp really is by testing a new one. Unless I’m actually auditioning preamps, I tend to take them for granted. I keep thinking one should be able to largely ignore the differences between preamps until a product like the XP20 invariably convinces me I’m wrong.

I suppose that part of my prejudice is driven by the fossil in me. I grew up in a world where tube preamps were filled with features, phonostages, and as many complications as possible. I can’t look at a modern linestage preamp without thinking that it doesn’t have enough knobs and switches. Part of my distrust, however, comes from the impact of the post-modern reconstructionist school of existential high-end audio minimalism. I not only want complexity; I want simplicity. I can’t help feeling that a component with such basic functions should be easy to make and cheap: a few wires, an input switch, a volume control, a few RCA jacks, and a plastic box. Who could possibly need more?

Whenever I get a really good preamp to review, however, I can’t make any of these prejudices work in practice. Today’s better linestage preamps may be the least colored part of the audio chain, barring audio cables, but they really do make a difference. If you want a great system you do have to spring for a great linestage, and linestage preamps are still getting audibly better.

The Pass XP20 is an excellent case in point. It makes a number of sonic advances over its predecessor: the Pass XO.2. These differences are not striking, but they are audible. Given good source material and a good system, you can hear that the XP20 lives up to its design goals. It is less noisy than earlier Pass preamps (and the XO.2 still has some of the lowest noise around). Soundstaging detail is clearer and so is soundstage depth. Dynamics are better, especially in lower-level passages, and the mid and lower midrange have slightly more natural tone and even less upper midrange coloration.

Wayne Colburn, the lead designer of the XP20, explains the source of these improvements as follows: “Lower noise always seems to help a product out. Of course this improves the measured and audible dynamic range. Nelson Pass worked hard to do this on the new amps and it was really noticeable. I decided to pursue this in the new preamplifier.

“Starting with the power supplies, noise was lowered. More filtering was used on incoming AC lines, and a newer, larger, low-noise transformer installed. Rectification is done via new low-noise high-speed diodes. During pre-compliance testing for CE certification (we invested in a screen room, a spectrum analyzer, and associated test equipment), I was able to find new sources of noise and ways to deal with them.

“A new gain stage was developed for the XP series preamps and integrated amplifier. Very similar to our UGS4, but some newer tighter-tolerance components and a reconfigured feedback scheme gave us what we were looking for in better stability and linearity. If the component count can be reduced and the measured performance increased, this almost always turns out to be a good thing. Indeed, it did in this case in that it gives a more transparent sound then our previous efforts. The measured performance was improved almost an order of magnitude on both ends of our typical distortion curves at the extremes. These are critical areas in music reproduction: low-level information and high-level signals.

“The voltage-regulation section was changed to lower the noise and make the residual noise more symmetrical. Many IC solutions have unequal noise on plus and minus. I decided on discrete regulation with some references Nelson started using in the power amps. Some newer and ROHS-compliant semiconductors were included that turned out to be better performing and lead-free to boot.”

At the same time, the XP20 does not attempt to revolutionize any aspect of preamp design. You don’t have to be an electrical engineer to see that such changes are largely matters of detail and the result of empirical experimentation. The sonic improvements are not the result of dramatic innovations in circuit topology; they are the kind of differences that show up in listening sessions.

This emphasis on listening may also explain why the XP20 focuses on sound at the cost of some features available in previous designs. It has separate sets of XLR and RCA inputs, rather than offering both for each input position, and it now lacks a mono switching option. It does, however, have a far better and easier to use remote control and a notably more usable volume control with smaller differences in gain at each step. If you adjust level and balance for each recording, and particularly adjust balance to maximize depth and soundstage width at your listening position, the new remote and volume control make it easier to set gain at precisely the level that sounds best.

The XP20 also sounds better than its lower-priced new sibling the Pass XP10; the XP20 costs $8600 and the XP10 costs $5250. The basic difference between the two new Pass preamps is that the XP20 has a separate power supply and a tape loop. Wayne Colburn explained the rationale for the separate power supply: “The XP20 uses an external power supply, giving more space to work with in both chassis. The new power supply added more capacitance and then I followed that with a capacitance multiplier circuit that gives the on-board regulators a little less work to do. The XP10 doesn’t have this added level of power-supply sophistication. The added space gives more inputs and outputs and allows better channel separation compared to the XP10 or previous preamplifiers. The XP10 is very good, but the XP20 is even better.”

I am no preamp designer, but I find Wayne’s comments track with all of my prior reviewing experience. Virtually all of the preamps I have auditioned over the years that had power-supply upgrades also produced improved sound, and this gain was audible when I compared the XP20 and the XP10. The XP20 had slightly less noise, a bit more low-level detail, better high-level dynamic contrasts, and more soundstage detail. You hear these differences most easily when you go back from the XP20 to the XP10. It is easier to detect the degradation than the improvement. The level of difference you hear will depend, however, on the quality of your source material, the rest of your system, and your listening environment. Both preamps are so good that the investment cost of the XP20 may not be worth it relative to investing in upgrading a more colored speaker or front-end component.

The XP20 versus Other Top Linestage Preamps

As for how the XP20 compares to other top linestage preamps, there are so few remaining colorations in the XP20 and the best competing linestage preamps that it is difficult to make comparisons. The nuances in the XP20 will interact with the colorations in your other components, and these colorations will be notably greater in every other aspect of the audio chain. If the nuances of the XP20 complement the rest of your system, you will get a mild touch of synergy, but very mild. The XP20 simply is not colored enough to make a major difference. If your system needs a preamp with clear colorations to compensate for some other component, this definitely is not the preamp for you.

If I were to attempt to give you a reliable description of the differences between the XP20’s sound and that of two other great solid-state preamps—the Boulder 1010 and Krell Evolution Two—the differences would lie largely in timbre. The Boulder 1010 is slightly “warmer” with a touch more middle to lower midrange energy. The Krell has a warmth similar to the Boulder, but provides a bit more deep bass energy.

There also are differences in dynamic contrasts and textures, in upper-octave air, and soundstaging, but they are subtle and their sonic impact will depend heavily on associated electronics and especially on your speaker, setup, and listening position. They also depend on the music you are listening to. A given preamp may slightly favor or hurt the soundstage, imaging, or timbre of a given recording, although again one has to strain to say that most of these differences have great musical significance. With all due respect to some of my colleagues, the chances you are going to have some kind of musically valid emotional experience because of the sonic differences between the best linestage preamps are negligible. If you audition a preamp like the XP20, you have to listen to a lot of music, and make your decisions on the basis of the nuances you hear over time.

The story will be very different, of course, if you need given features. The XP is pretty basic in functions compared to the features in the Boulder 1010. If you are looking for a warmer and more forgiving sound, you can also find some tube preamps that offer this kind of coloration. I would, however, be careful about any generalizations about solid-state versus tubes, as I also had the opportunity to compare the XP20 to a prototype tube preamp that now seems too expensive to see the light of day in today’s economy. Although its noise floor was slightly higher, it too was truly neutral. The differences lay largely in low-level contrasts and dynamics, and they were no greater than the differences between the XP20 and other top solid-state preamps, at least in terms of musical realism.

Let me stress that none of these four preamps sounded exactly alike. Put each preamp in your system in your room with your listening material and you will hear enough differences in timbre, detail, and dynamics to prefer one sound over another. I would be happy to live with any of the four, and for all my prejudices, I again learned that a great linestage preamp is still a necessity in a great system. I also learned that the XP20 is a great preamp.

The XA160.5

I have found the sonic differences between the best amplifiers to be clearer and more audible than those between the best preamps. At the same time I have never been a believer in choosing one type of amplifier—tube or solid-state, or Class A versus Class AB. The best and most neutral amplifiers still don’t sound the same, but the way in which an individual design is executed now tends to be as important as category. I could live very easily with the best examples in each category. I also find that the sonic differences between tube or solid-state, and Class A and Class AB, are becoming smaller over time.

The best Class A amplifiers are still generally sweeter and offer a bit more harmonic detail and better low-to-mid-level dynamics. Much depends, however, on the amount of Class A power a given Class AB amplifier can deliver before it goes into Class B operation. Choosing a Class A amplifier also generally comes at the cost of slightly less coherence than you get in the best Class AB designs, and cost considerations invariably mean that Class A designs offer less power for high-level dynamics at a given price. I find myself moving back and forth between the two circuit topologies and many top manufacturers (like Pass) use both.

I was, however, as impressed with the new Pass XA160.5 as with the XP20. With the possible exception of the Krell Evolution One, it is the best example of solid-state Class A design I have yet had the opportunity to audition, It also is unusually dynamic for a Class A design. It offers Class A operation throughout the audio frequency range up to its full rated output of 160 watts, and it has a power supply and mix of output devices that allow it to produce peaks of up to 328 watts.

Once again, the change is evolutionary and not the product of major changes in circuit topology. The Pass XA160.5 continues to use the hybrid Class A circuit that Pass first developed for the Aleph .0 power amplifier in 1991. This circuit parallels a push-pull Class A output stage with a current source that biases it into single-ended Class A. The use of a single-ended Class A circuit delivers what Pass feels is the lowest and least damaging harmonic distortion, second harmonic, instead of the third harmonic of push-pull. The use of high bias levels also reduces that distortion to extremely low levels.

The XA160.5 does, however, have a new bias circuit with a bias generator that sharply reduces variation in the bias current in the output stage, and allows 20% greater power output for a given supply voltage. The output stage not only uses a heavy bias current for push-pull Class A operation to a large portion of the rated power, but also has a single-ended bias source for single-ended Class A at low wattage.

A cascode “JFET Symmetric” front end replaces the previous differential MOSFETs. This means the XA160.5 has less noise, a higher input impedance, and greater linearity. There also are more output devices, and matched complementary power MOSFETs are operated as followers to provide more current and power handling and improved linearity. The overall circuitry provides a very high damping factor (a nominal 200) and greater stability into demanding loads. The power supplies have been given more storage capacitance, larger and quieter power transformers, twice as many fast-recovery rectifiers, and improved harmonic filtering in both the primary and secondary supply circuits.

This combination of a better power supply and more output devices gives the XA160.5 the ability to deliver more than five times the current of the XA160, and more than 25 times the power into very low impedance loads, allowing it to drive virtually any speaker available. The XA160.5 is also stated to have about 10% of the noise of the XA160, and the XA160.5 also proved to be considerably less sensitive to AC line-quality and power cords than some competing amplifiers.

These differences produce more audible changes between the XA160.5 and the XA160 than I heard between the XP20 and X0.2—in part, I suspect, because power amps are sensitive to speaker loads and have more system interactions than preamps. The XA160.5 is a cleaner, sweeter, and more detailed amplifier—particularly in low-level detail and dynamic subtleties—and has better high-level dynamics into real-world speaker loads. The XA160.5s do an even better job of reproducing natural timbre, giving more musical life, and reproducing greater soundstage width and depth.

Comparisons with other amplifiers are harder to make. I did find that the XA.160.5, like most of the best solid-state designs, did outperform all but the most powerful tube amps in several areas. It produced tighter and better-defined bass, reproduced all of the deepest bass, and controlled speakers better. Amplifier design has advanced to the point, however, where warmth and sweetness are products of individual design execution—not solid-state vs. tube differences. I found the XA160.5 brought out the natural character of speakers in these areas, rather than imposing a sonic character of its own.

At the same time, I got excellent performance with a pair of Boulder 1050 Class AB monoblocks, as I did from a pair of Pass X600.5 Class AB amplifiers. Both also offered more power and dynamics than the XA160.5s, although I am not sure that this advantage would be audible at listening levels that did not damage your hearing.

SIDEBAR: Robert Harley on the Pass Labs XA100.5

The XA160.5 reviewed here is a scaled up version of the XA100.5 I reviewed in Issue 185. The XA160.5 employs the same low-level circuits and output-stage topology as the XA100.5, but adds more output devices and a larger power supply for 160W of output compared with the XA100.5’s 100W. 

The XA100.5 is, in my view, one of the world’s great amplifiers. It is distinguished, primarily, by a tube-like liquidity in the midrange. It’s not that the XA100.5 has tube-like distortions, but rather that it lacks the characteristic signature of other solid-state amplifiers I’ve heard. Specifically, the XA100.5 has richly saturated tone colors, a wonderful warmth and body of timbre, and a completely grain-free rendering of instrumental textures. Moreover, it achieves these qualities without being colored, overly ripe, or falsely romantic. Rather than being tailored to sound pleasant, the XA100.5 simply imposes less of itself on the music than other solid-state amplifiers I’ve heard.

In addition, the XA100.5 is a powerhouse, at least when driving speakers of appropriate sensitivity for its output rating. It has iron-fisted control over the bottom end, super-fast reproduction of transient leading edges without the slightest bit of etch, and a sense of ease on even the most demanding dynamic contrasts.

But what really sets the XA100.5 apart is its supremely seductive midrange—lush without sounding syrupy, warm without sounding bloated, and richly refined and detailed without sounding analytical. This is an immensely involving set of qualities, and ones that make the XA100.5 easy to enjoy for extended listening sessions. –Robert Harley

Summing Up

As I said at the start, I was impressed enough with the XP20 and XA160.5 to upgrade my reference system. I can’t see how you could go wrong with either of these components. At the same time, I have tried to emphasize throughout this review that whether you prefer the same nuances that I do is something I can only speculate about. I also cannot predict the extent to which the colorations in other components, amplifier/speaker interactions, and speaker/room interactions can largely mask many such differences.

This highlights the need for serious auditioning, particularly at such serious prices, and either getting a loaner or an exchangeable unit to try in your own system, if at all possible. If there is any one point I would consistently stress about creating a great high-end system, it is that an audiophile who relies on either reviewers or dealers to do their listening for him is little more than a puppet with money.


Pass P20 linestage preamplifier

Maximum output voltage: 7V single-ended, 15V balanced

Output impedance: 200 ohms single-ended, 1k Ohm/leg balanced

Input Impedance: 48k Ohm single-ended, 48k Ohm/leg balanced

Inputs: Two balanced, three single-ended

Chassis dimensions: (Two) 17” x 12” x 4”

Shipping weight: 55 lbs.

Price: $8600

Pass XA160.5 monoblock power amplifier

Sensitivity: 1.79V@ 26 dB gain

Low-frequency response: 1.5 Hz

High-frequency response: 100kHz

Power output: 160W (8 ohms), 320W (4 ohms),

Input impedance: 30k/20k Ohms

Damping factor: 200

Dimensions: 19” x 22” x 11.5”

Weight: 154 lbs.

Price: $22,000/pair

Pass Labs

24449 Foresthill Road

Foresthill, CA 95631

(530) 367-3690


Associated Equipment

Dynavector 20X, Sumiko Celebration, and Koetsu Onyx cartridges, VPI TNT HRX turntable and JMW 12.7 tonearm, Tact 2.2X digital preamp-room correction- equalizer-D/A converter, EMM Labs SACD/CD player, Pass Xono phono preamp, Pass XP.10 stereo preamp, Pass XA160.5, X600.5, PrimaLuna ProLogue Seven power amplifiers, Vandersteen 5A speaker, Audioquest Niagara and K2, Kimber Select, and Stealth interconnects, speaker and digital cables. PS Audio Premier AC power conditioner