February 2002

Pass Laboratories Supersymmetry Balanced Single-Ended Class-A
X250 Stereo Amplifier
Review By A. Colin Flood
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  The Pass X250 is a silver plated monster amplifier that seems to do virtually everything well. It is a massive and powerful amplifier created by the "G-d of Monster Amplifiers", Nelson Pass. His simple yet brute power amplifiers have dominated the high power end of ultimate audio for decades. His Pass Labs Aleph 0, Aleph 3 and X1000 monoblock power amplifiers were Stereophile's Products of 1996, 1997 and 1999 for amplifiers. The X250 is the second lowest power model in his X series reference line. There is a 150 version. It is visually the same as the other, more powerful X series models.

Technicalities

The X250 is a single-ended "Class-A" topology with Pass's Supersymmetry™ Balanced approach to precision component matching. His Supersymmetry™ topology was granted a United States patent in 1994. Employing a balanced gain-stage, distortion and noise are made absolutely symmetrical at the two outputs. The signal is amplified, yet noise and distortion are canceled. The amplifier uses carefully matched components in a balanced "Class A" circuit. This simple circuit contains only two gain stages. A balanced single-ended voltage stage drives a bank of high power MOSFETs, which operate as followers for a minimal amount of feedback around the output stage. The massive reference level amplifier is fully DC coupled with no capacitors in the signal path. The result is a high-frequency smoothness belying its massive solid-state power.

Everything about Pass Laboratories amplifiers exudes awesome power and quality. The X250 has twenty computer grade capacitors at 10,000 uF ratyed at 50 volts each! These are used to create unregulated output stage rails, plus and minus 47 volts at 20 amps.

What we have here is a 20" block that looks and feels much larger than it is. The face of the amp is two carved aluminum plates that form a circle around a light blue dial and a smooth silver stand-by power button, the size of a quarter. In addition, the amplifier feels far heavier than its approximate 100 pounds. The thin black cooling fins are four inches long and guard the entire length of each side.

The X250 specs are impressive. Gain is a large 30 dB. Frequency response is beyond ideal, -0 dB at DC to -3dB at 100kHz. The power output is rated less than the way the amplifier appears to sound. Rated for 250 watts for two channels at 1% THD into 8 ohms, it would be easy to exaggerate this figure. Given its Olympian heft, it would be easy to tell guests that this was a 1,000-watt unit. Only the uninitiated would not suspect otherwise. The sound certainly would not give it away.

As any tweaking audiophile might suspect from such a super unit, the X250 pumps twice as much power, 500 watts, when pressed into 4 ohm range yet still provides for very good control of low impedance woofers. Under actual conditions the Pass Laboratories X250 it will produce approximately 1,800 watts for short duration!

The X250 manual is a wonderful resource and quite detailed with the philosophy behind it. Nelson Pass covers some very interesting design considerations. In it, he says "we are not aware of a speaker on the market that presents unusual difficulty with these amplifiers." I do not doubt it. Nelson Pass does warn that the DC line for electrostatic or ribbon speakers must be optimized to drive those kinds of loads. Power consumption when idling is 270 watts, up to 1,000 watts when pushing its output. Maximum current draw is about 20 amps, so this is the type of monster amp for which a dedicated power line would be a good idea. Dynamic range is all you would ever need, an impressive 148dB.

A little white power switch is located on the rear of the amplifier with the silver standby switch conveniently located on the front panel. Single-ended inputs are via RCA connectors and balanced inputs are via XLR connectors. Five-way binding posts are on the rear, topped by heavy-duty wing nuts to provide industrial strength speaker connections.

Surprisingly, the manual says that interconnects do not matter with this amplifier. "We have found that about 90 per cent of bad sounding cables are really bad connections, and we recommend that special attention be paid to cleanliness of contact surfaces and tight fit." Nelson Pass has several excellent articles at his web site. He makes convincing arguments for Class A operation of audio amplifiers, whether they are solid state or tube designs.

System Details

Coupling a giant amplifier with big old horns like my 1982 Klipsch Cornwall models may not immediately make much sense. Rated at an ultra-efficient 100 dB/w/m, the compression drivers with metal horns barely need the power the X250 puts out when idling. The amp also costs several times the price of the big old horns. In fact, it even weighs as much as one of the speakers (maybe this is the way it should be?). So putting them together is the very definition of amplifier over-kill. It is like dropping a heavy Vette engine into a VW Beetle. It does not make much sense, except as an novel experiment, but boy, it sure can go! And go it did.

With a politely British Rotel 951 CD player, the upgraded Series II version of the classic Dynaco Pas3 pre-amplifier and a pair of Klipsch sub-woofers, the Pass X250 powered the full-range of my old style horns. The only advantage to this ridiculous marriage is that the ultra-sensitivity of the big old horns reveals every sonic defect of the front-end chain. No mediocre product can hide in this system, for all flaws are exposed in ruthless 100-dB daylight.

Sounding Off

Yet, nothing was out of the Pass X250's league. It easily handled everything thrown at it. The current draw meter on the front never moved. For the record, this blue-eyed beast never blinked. It did get a tad warm, but that's all it did after an evening of musical fun. Nothing that we threw at it required any effort from the X250 at all!

Only the deepest bass scenes, not the loud action ones, on The Matrix movie made the current needles jump. And that was when we listened to a pair of B&W 803s. But on my big old Cornwalls, nothing budged the blue lit needles. Nothing, that is, at "normal" to "merely loud" listening volumes. No serious ear bleeding teenage tests were performed. Other than The Matrix, the only other movies watched were U-571 and The X-Men.

Pass says the X250 amplifier need about an hour to warm up. While I did notice the standby light was still on long after the power was disconnected, I did not notice any material difference in the sound after an hour or two of use. The X250 operates in "Class A" to roughly 75 watts. From there, it goes into Class A/B operation, which gives you momentary pulses of more juice should you need it. An inefficient loudspeaker generally needs lots of juice. For examplie, a cone loudspeaker rated at 85 dB/w/m in a average size living room when trying to reproduce momentary 118 dB peaks may required brief pulses of as much as 4,000 watts. The X250 is designed to give you the quality of "Class A" below 75 watts and even more power in A/B mode.

You can see this current draw with the thin blue-lit wavering line on the meter. The meter's needle stays at about 1/3 until the output stage begins drawing more than the 270 watts the X250 draws at idle. This didn't happen with music on the big old horns... ever. When very deep Matrix bass tracks on the B&W 803s required the extra juice, the added power flowed as smoothly and as effortlessly as if the borrowing power of the Federal Reserve stood behind it. No check was too large to cash. Not even depth charges on U-571. Loose electrical wires on The X-Men snapped and popped like lighting crackles.

Yet, the industrial strength construction and plain simplicity of design made for not just a very solid presentation, but also a very refined and detailed sound. Endless and effortless power seemed to flow directly from the power station's utility grid. This is very different from the soft clipping that "tubophiles" always expect. In my set-up there was no need for clipping; soft or hard. The Pass Laboratories X250 always seemed to have plenty of power. The X250 handle on the bass was authoritative, detailed, solid and effortless. The tremendous woofer control made for bass that was not taut or flabby, boomy or bouncy. It was not exaggeratedly fast or numbingly slow.

With oodles of power to spare, dynamics on the big old horns were superb. Everything played had range and extension. Multiple instruments played as if each had their own speaker and amplifiers. The X250 did not mash the chorus together. There was no lack of resources for musicians to draw upon. The closely-mic'ed dramatic vocals and instrumentation on Diana Krall's wonderful Stepping Out [JustinTime 2000] was as smooth as I have ever heard on much more expensive, and practical, amplifier and loudspeaker combinations. The crisp and clear flute tracks on the impressive DMP does DSD stereo SACD [Digital Music Products] were as smooth, liquid and refined as they are on the very best systems.

The closest midrange competitors that come to mind, where I have heard Krall and DMP recently, is the YBA and JM Labs system. By removing the bust-the-budget price from the equation, the Wife Acceptance Factor (WAF) was very high. "Little miss sensitive ears" though the X250 was the best she has heard on my sedentary system. (Unfortunately, the unit's price is a far-flung orbit from my down-to-earth budget!)

Unlike the warm and lush midrange that tubophiles have come to revere, the vocals with the Pass X250 on the big old horns were clear and neutral... and very easy to get accustom to. In this respect, as well as every thing else it did on these subjective tests, it is a reference grade amplifier in alomost every regard. It has all that power; just so it can stand aside and not be heard. Whatever the engineer wanted to put on his disc, you will hear it here. The X250 construction and design means that there is none of the wonderfully euphonic things that horns and tube amps add to the disc to make something akin to music. There is no added warmth or softness to this baby. Imaging is okay, though it is not the extended dimension (3D) that tube amps do so well. In fact, I double checked the phasing, just to be sure that something was not set wrong. Yet it is not cold or unemotional; it is merely majestically accurate.

The sound might be derided as dry, without any bloat or bloom. Musical horns, such as saxophones, although they sounded wonderfully natural, did not have some of the "edge" or "bite" that tubes on ultra-efficient horns give them. As might be expected with an amp, that can not only delve deep into the lower ohms, but also come up with oodles of A/B power when demanded, bass and drums were remarkable tight and well controlled. They knocked. They thumped.

It is hard for me to believe that any other amplifier could make a material dent on this concrete block's control of bass and drums. Only the $70K Martin-Logan electrostatic Statements, powered by Krell's biggest amps, with their mid-bass driver towers and deep-bass columns, had more tangible bass punch and drum kicks for me.

Summing Up

A super-symmetrical super-amplifier is "technophile" overkill for big old horns that need only six rude watts to pump out 110 dB in my living room. This is not one of the wonderful amp and speaker combinations that make audio history. Indeed, two low cost tube amps did a better job with these big old horns and the astonishing $275 Axiom Audio Millennia M3Ti loudspeakers (more on them next month). The Antique Sound Labs Wave 8 is an amazing 8-watt monoblock tube amp for only $99. These little charmers, and the refined Bottlehead 2A3 Paramours monoblocks at 3.5 watts per side, made embarrassingly good music when combined with the big old horns or the Axioms. Colored and inaccurate? Yes, but also very inexpensive, quick, lush, dynamic and detailed, with engaging euphonics, imaging and presentation too.

In fact the X250 sounded much better with the B&W 803s than with the big old horns. When the B&Ws thirsted for power, the Pass amplifier opened up a river of current. Now that is a classic combination. I think that any speaker with a wild, difficult or bumpy impedance curve would be easily tamed by this beast master. Created by the "God of Power Amplifiers" Nelson Pass, everything about the X250 exudes awesome audio power, control, definition and build quality. This amplifier always seemed to have plenty of muscle waiting in reserve. It is a massive and potent monster amplifier that, even when poorly mated to big old horns, still managed to do almost everything well.

Specifications

Gain: 30dB 
Frequency Response: -3dB at 100kHz 
Power Output: 250W @ 1% THD (1kHz 8 ohms) 
Storage Capacity: 200,000 µF 
Max Output Voltage: +/- 65 volts 
Max Output Current: +/- 20 amps
Input Impedance: 22k ohm balanced 
Damping Factor: 250 ref 8 ohms nominal 
Slew Rate: +/- 50 V/uS 
Output Noise: 300uV un-weighted 20-20kHz 
Random Noise Floor: approximately 2uV 
Dynamic Range: 148dB 
Balanced CMRR: -85 dB @ 1kHz 
DC Offset: < 100mV 
Power Consumption: 270 watts idle, 1000 w max 
Dimensions: 19" X 9.5" X 22" (WxHxD) 
Shipping Weight: 100 lbs. 
Price: $6,000

Company Information

Pass Laboratories
P.O. Box 219
24449 Foresthill Road
Foresthill, CA 95631

Voice: (530) 367-3690 
Fax: (530) 367-2193 
E-mail: Peter@passlabs.com
Website: www.passlabs.com