Since 1991, Nelson Pass and his company Pass Labs have been making some of the finest Class A and Class A/B amplifiers the world over. Unlike that of many other amplifier manufacturers, Pass Labs' signature sound has been forged out of a desire for simplicity - a simplicity that has resulted in numerous awards and more than a few patents for their amplifier designs. Based upon Nelson Pass' now infamous Supersymmetry circuit topology, the X Series amplifiers have been Pass Labs' gateway drug, if you will, in terms of getting consumers into the Pass Labs family. The X Series has a little something for everyone, with amplifiers ranging from 150 to 1,000 watts. For me, the entry point was the X150, Pass Labs' most affordable X Series amplifier at 150 watts per channel. While my first experience with Pass Labs was via their "baby" amp, for many, it was the X250. Many view the X250 as the workhorse of the X Series, for it strikes a near-perfect balance between price and absolute performance. As good as the original X Series was, in 2003, the X250 (as well as the rest of the X line) went under the knife for an overhaul. The result was an all-new, though still familiar, X.5 line of amplifiers.
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The improvements made to the entire X-line include replacing the input differential MOSFETs with Cascode JFETs, which are the same JFETs that Pass Labs uses in its phono stages. This change is said to lower noise and raise input impedance while improving linearity. The X.5 refresh also added more output devices, which improved current and power handling, as well as linearity. There is an all-new bias circuit, which allows for a 20 percent increase in power output at any given voltage. Speaking of bias, the X.5 models are also biased heavy for Class A operation at low wattage, which any audiophile will tell you is where the magic happens.
Focusing my attention on the X250.5 reveals that the X.5 update brought about several physical changes as well, specifically a new, more robust front fascia. The first-generation X250 sported Pass Labs' trademark blue voltage meter, though the thick aluminum surrounds sloped back as you approached the sides of the amp and its massive heat sinks. With the .5 update, the sides no longer slope; instead, they remain flush until close to the very edge, where they recess back at about a 45-degree angle. The X series now more closely resembles Pass Labs' XA Series of all Class-A amplifiers in terms of their outward appearance, which is a good thing. The X250.5 measures 19 inches wide by nine inches tall and 21.5 inches deep. Accompanying its larger footprint is the X250.5's weight, which rests at 110 pounds.
The X250.5's massive all-business demeanor doesn't fade when focusing your attention on its back panel. The first thing you notice is the X250.5's rear-mounted rack handles, which sit at the outer edges of the amp's wing-nut style binding posts. The X250.5's binding posts are nicely spaced and can accept all types of speaker cable, with the exception of those terminated with banana adaptors. Below the binding posts and centered along the bottom of the amp itself rest its detachable power cord and master on/off switch. Above the binding posts and centered along the top of the back panel are the X250.5's unbalanced and balanced audio inputs. There is a secondary pair of five-way binding posts that are present to facilitate remote start-up or turn-on.
Under the hood or behind the scenes, the X250.5 boasts a power output of 250 watts per channel into eight ohms and 500 watts into four. The X250.5's reported frequency response is 1.5Hz - 100kHz, with distortion (at 1kHz) rated at one percent. Input impedance (Kohms) is 30/20 and its damping factor is 150. Maximum output (volts) is rated at 64. While much of the world's products may be focused on efficiency, the X250.5 isn't among them, for its power consumption sits at 600 watts, which isn't insane, but it is constant, which is why the X250.5's heat sinks run more than a little warm to the touch.
The original X250 amplifier retailed for $6,000, which made it a true high-end value, for it not only competed with some of the best amplifiers in the business, it did so at a far more advantageous price. While the X250.5 retails for more at $8,250, it still competes with those costing much, much more. And of course, there's the fact that all Pass Labs' products are designed, manufactured and assembled by hand, right here in the USA.
Installing a solid-state amplifier is a simple and straightforward proposition, though with regard to the X250.5, I'd suggest getting a friend to help, for it's an absolute beast. I can speak to the X250.5's build quality and heft firsthand, for I didn't have a friend (or wife) to help me maneuver it into my rack. Thankfully, the X250.5 is easy to grip, for its heat sinks are so robust that they can double as rack handles, as can the X250.5's solid faceplate. Once in position on the bottom shelf of my Omni+ equipment cabinet, making the requisite connections was a breeze.
I began by connecting the X250.5 to my Integra DHC-80.2 AV preamp, which I used for home theater, as well as for some two-channel listening. For more critical two-channel sessions, I used the Wyred 4 Sound DAC-2's preamp functionality, since I've found it does little to alter or enhance the sound of amplifiers connected to it. Both preamps were connected to the X250.5 via unbalanced interconnects from Crystal Cable. The X250.5 was charged with powering my reference Bowers & Wilkins 800 Series Diamond loudspeakers, though I did substitute Focal's Electra 1038Be loudspeakers later in my evaluation period. Both pairs of speakers were connected via three-meter runs of Crystal Cable speaker cable. Source components included Cambridge Audio's Azur 751BD universal Blu-ray player, Wyred 4 Sound's DAC-2 digital to analog converter/preamp and my trusty AppleTV.
I ran both the 800 Series Diamonds and 1038Be full-range for a large portion of my evaluation, but also crossed them over at 80Hz so as to pair them with my dual JL Audio Fathom f110 subwoofers for home theater use. The subwoofers were EQ'd close to flat (in my room) using Room EQ Wizard and a Behringer Feedback Destroyer Pro. Minus some low bass EQ, the rest of my room was treated using a variety of room acoustic products from GIK Acoustics.
I typically don't believe in burn-in, at least not when it's followed by a figure in the hundreds of hours. That said, the X250.5 does sound best after being powered on for a good hour or so. Those of you who are fans of tubes will understand this phenomenon, for it takes time for everything to warm up to be at its peak operational temperature and performance. However, after maybe a few days of playback, I noticed no shift in the X250.5's sound after that initial 30-60 minutes of warm-up time.
I began my evaluation of the X250.5 with John Mayer's 1999 self-distributed debut album, Inside Wants Out (John Mayer LLC), and the track "Comfortable." The entire album has a near-live quality to it, as it was recorded in a single session, with all the musicians occupying the same space. The first thing that struck me about the X250.5's performance was the added inflection afforded to both instruments and vocals. Whereas many an amplifier will simply communicate the music, the X250.5 takes things a step further and conveys the subtlest of emotions behind every chord and/or lyric. Listening to the deeply personal "Comfortable," I got the sense that the X250.5 was in on the relationship that sparked the song, as if it had a deeper appreciation of the lyrics than other amps did. I know that sounds odd, for obviously an amplifier can't feel emotion, but if ever there was an amplifier that excelled at conveying it, it would be the X250.5.
Read more about the Pass Labs X250.5 stereo amp's performance on Page 2.