Pass Xs Preamp and Xs 300 Mono Power Amplifiers

Major New Challenges to the State of the Art

Equipment report
Solid-state power amplifiers,
Solid-state preamplifiers
Pass Laboratories Xs,
Pass Laboratories Xs 300
Pass Xs Preamp and Xs 300 Mono Power Amplifiers

These criteria are why I now use Pass Labs preamps and amps in my reference system, why I use and have used equipment from other manufacturers, and why I praise the electronics I review when they deserve it. Overall balance and lack of coloration or voicing, and lack of equipment noise and interaction problems, are the reasons that I have used the Pass XP-10, XP-20, and XP-30 preamps as references in the past. Each has proved to be progressively better—as have the preamps I’ve used as references from other manufacturers.

I have wondered each time whether the new Pass preamp was really going to be all that much better than its predecessor. As with most of the top high-end manufacturers I’ve worked with in recent years, I’ve never heard any Pass equipment that did not provide excellent sound by most standards—regardless of price. I’ve also found the improvement to be progressively smaller and harder to describe. The fact is that most high-end preamps and amplifiers today are notable successes regardless of price, and the ones that have the least apparent voicing present more and more problems in describing the sonic differences.

The Xs preamp is, however, definitely better that the XP-30, and even more of a challenge to other manufacturers. Its price tag is an issue, but the Xs is better in every respect, and not just in sound quality. It has better ergonomics: The display is larger, and the volume and balance settings are more obvious; it has a polarity button that can make a real difference in sound quality; and the steps in the volume and balance controls seem more precise. It is a large two-chassis unit, rather than a stack of slimmer units—three in the case of the XP-30—which makes hookup easier. Although having an easy choice between XLR and RCA inputs is a strength of almost all Pass designs, the Xs interfaces perfectly with the Pass XP-25 phono preamp. It also retains the ability to independently adjust the outputs for bi-amping—a key feature with more complex systems.

As you might expect, the Xs also pushes the limits of its components and circuit design well beyond the capabilities of the Pass XP-30 preamp I use as a reference (along with the EMM Labs Pre-2 preamp). The noise floor is an incredibly low -122dBV. The Xs retains the features of the XP-30, but has more output and can swing enough voltage to drive a balanced follower output at 150 watts rather than 80 watts. There are three advanced power supplies, and they drive what are the equivalent of dual-mono, gold-plated ceramic circuit boards.

When I asked Nelson Pass to describe the design activity that led to the Xs Preamp’s sound quality and price, he replied as follows: “Wayne [Colburn] was given an unlimited budget to do whatever he wanted: ‘This is your chance to go all out, Wayne. We aren’t calling this Xs for nothing.’ And so the approach is more like obsessive attention to detail armed with lots of money.

“Gold Megtron and ceramic circuit boards, hyper-selected and matched NOS FETs, crazy regulation and isolation, fabulous relays and switches, bigger display, more expensive parts and more voltage, more bias current, and still more voltage and bias current. ‘I want to be able to drive Magicos directly with this, Wayne!’

“There are a couple tricks I can’t talk about, such as how he got this great performance out of the volume controls, or the bias regulation of the output circuits, but most of the technology is excessively straightforward.

“Starting with two prototypes that measured very well, the last year has been spent tweaking them in tandem with a totally subjective approach—probably more listening time than any product we’ve ever done. Back and forth. More time, attention, work, imagination, and money is what we put into these products.”

I also asked Wayne—the lead designer on the Xs Preamp—to summarize his view of the impact of the design improvements in the Xs and how making them improved sound quality, and he provided the following background:

“This preamp is the sum of a lot of small things, from the first one I worked on at Threshold (the Forte 44), to all the small circuit refinements over the XP-30 and the group of people involved in testing and listening. It was also fun to do, especially when finished. It is DC-coupled as well. The XP-30 used custom caps, but no cap at all is better. It provides better bass and extended resolution throughout the full range of music. I feel the servo-system I came up with works better than previous configurations I have seen or tried.

“The use of the optimized circuit-board materials has subtle effects. Sonically, it seems to lower the noise floor ever so slightly and give a bit of top-end extension. I also love the way solder works on the ceramic circuit boards and their gold-plating. I am not sure being rated for 280 degrees C does much, but it can’t hurt. The main board uses a new-to-us Panasonic material designed for high-speed computer routers, and the power supply has a high temperature board with heavy copper plating. There are thus a total of four different types of circuit board materials in the Xs.

“The new gain modules also use higher-power Toshiba devices for the cascode portion of the circuit and the pre-driver stage, allowing for more bias, which gave a bit more clarity and space in the sound. Two separate circuit boards for the power supply made for a marked improvement in instrument placement and bass control. The measured crosstalk is also better. The output stage runs twice the bias and sets itself automatically, so it is very stable and comes to its best operating point quickly. I think this makes for a better power amplifier interface.”