Pass Laboratories X350 Stereo Power Amplifier 
Home Theater Power Amplifiers Stereo Amplifiers
Written by Bryan Southard   
Tuesday, 01 August 2000

I received a call from my wife a few months back while I was traveling on business. She said that there was a box on the porch that she couldn’t possibly budge. I soon realized that it was the awaited Pass X350 that I was to receive for review. Knowing that it was heavy, I asked her to get a neighbor to help her drag it into the house. She later called and said that it was a huge job, even with a strong male helping. It wasn’t until I got home that I realized how massive and dense this box really was. From unpacking to positioning it onto my amp stand, this was a project. It weighs about 150 lbs., but that doesn’t tell the whole story. At 19 inches wide, 22 inches deep and 10.5 inches tall, the X350 is not a small amp, but it is not the largest by a long shot. However, it looks and feels as solid as a wrecking ball. Its front panel is constructed from a one-inch thick plate of aluminum with heavy machined front wings that dress a four-inch diameter power meter. The sides have been fitted with top to bottom heat sinks that at first appear to be overkill, but once this amp warms up, you see that the form fits function. The rear is simple yet fittingly sturdy, with large wingnuts for speaker connection. The price on the X350 is $9,000.

When powered up, the X350’s front power meter glows cool blue. It seemed to me like a window looking into a fish tank. I found it to be a very appealing feature. The funny thing is that the meter tells you nothing. I should perhaps rephrase this to say that it tells most of us nothing. What it reads is the amount of power being drawn from the wall into the amplifier’s power transformer. This means that when you turn the volume up, the meter needle moves but doesn’t provide you with any overly meaningful information. However, this said, I found the meter feature so aesthetically appealing that I would pay extra for it if necessary. Frankly, I find it to be visually astounding.

The Pass X350 is the largest stereo amplifier in the X line, their newer reference series, and a baby brother to the X600 and X1000 monoblocks. The X350 is rated at 350 watts at eight ohms. For those not familiar with the previous Pass products, for many years, Nelson Pass has been creating amplification that is considered by many to be among the very best that this industry has to offer. The Aleph series amplifiers of the past were run by some of the most discerning critics in the industry. Pass owns many patents for his amplifier designs, including multiple patents for the X series amplifiers.

The technology associated with the X350 amplifier is called Supersymmetry. The U.S. patent for this was issued to Pass back in 1994. The amplifier’s performance is achieved by using highly matched components in a simple balanced class A configuration. The X350 uses just two stages. These are inherently low distortion types of circuits, but their performance is improved when operated in balanced mode through cancellation. Distortion and noise identical to both halves of a balanced circuit will disappear at the output, and in a well-matched symmetric circuit, most of the distortion and noise is identical. Supersymmetry enhances this effect by providing a connection between the two halves of the balanced circuit that further perfects the match. Any distortion and noise not already identical to the two halves is made identical, and the result is improved cancellation at the output. You will find considerably more detail available with respect to design and engineering at the Pass Labs website listed at the end of this review.

After an approximate 200-hour break-in period, which is needed for nearly all amplifiers without question prior to any critical evaluation, it was time to hear what this powerhouse had to offer. One characteristic that became immediately apparent was the presence of detail and this amp’s superb dynamic range. Dynamic range refers to the range between the loudest and quietest pieces of information within the music. Dynamic range can be best evaluated in musical pieces containing detailed instruments of varying frequencies and volumes, such classical performances by an orchestra or a symphony. The Pass X350 did one of the best jobs I have heard in reproducing these details. The bass control of this amplifier was unquestionably superb, the best I have heard to date in my reference system. When listening to music with some dynamic impact, this amp had my Revel Studios hopping. The Revels are not particularly efficient - in fact, they are quite inefficient - but when driven with high power, they will knock you silly if you prefer to listen to music aggressively, as I do. The Pass provided bass control, along with the necessary finesse, without a hiccup.

The Music
When I review a product, I always start by just listening and just enjoying the music. It is counterproductive to try to assess a piece as soon as you receive it, because it not only takes time to break in a component, but it takes time to acclimate to the sound and determine what it is that is right and what is wrong.

Unless a component is bad, I will spend several weeks just enjoying the unit, saving critical judgment for later. One of the many albums that I listened to was Ellis and Branford Marsalis’ ‘Loved Ones’ (Columbia Records). The X350 recreated the instrumental timbre of the piano very well in this beautiful and open recording. The piano sat back nicely, yet had plenty of percussive dynamic impact. The X350 was decisively transparent and open.

When listening to Sister Hazel’s first album, ‘Somewhere More Familiar’ (Universal), the song "Starfish" is an excellent test for bass extension. At high volume, the kick drum got into my soul. It had enough bass slam to make my heart feel as if it were beating irregularly. During a drum solo, I love feeling as if someone is kicking me in the chest and the X350 provided that sensation. It has the power to drive every conceivable speaker to its fullest. Okay, you ask, if that’s the case, why do Pass and others sell amps that provide two and three times the power? Thanks for asking! Because, however impractical, until the gas shortages of the ‘70s, Chrysler manufactured a 426-Hemi engine that produced up to and exceeding 800 horsepower for the type of car body that my grandma owned. It’s the good ol’ American way. If one is good, two is twice as good! We Americans love excess! Otherwise, this amp is plenty capable of the toughest loads.

Back to the Music
And a little more rock and roll. Every time I use rock as reference, we get letters criticizing the music selection as poor quality for product evaluation. That might be the case with poorly recorded rock, but let’s face it, if you like rock, what Sonny Rollins sounds like on the given component maters little to you. I love all music including rock, so here it is.

Blind Melon from their self-titled first album (Capitol Records) benefited immensely from the power and the control of the X350. I was continually impressed with the X350’s ability to involve me in the event, and to provide very high volumes of this high-energy performance without fatigue. I noticed that this amp had me listening to levels far higher than I typically do. Power is addictive without question, and quality power is infectious.

I have owned tubes for quite some time now, not because I am a tube addict, but because I demand reality in my reproduction and have yet to find a solid-state amp that can reproduce the live musical experience the way that tubes do. I am aware that tubes have their drawbacks in both sonic and maintenance areas. They will not give you the authority and dynamic slam that the best solid-state amps will. Many tube amp manufacturers will claim that their amps have the benefits and naturalness of tubes, with the authority and sound of solid-state. Conversely, many transistor amplifier manufacturers will claim to have the control of solid-state and the sweetness of tubes. In general, I find this to be hype with little substance. Solid-state sounds like solid-state. The Pass not only confirms but quite possibly epitomizes this fact. I don’t think that Pass would ever imply that the X350 has the warmth of tubes - they would likely say that they spent considerable money in development in order to avoid the shortcomings of tubes. Many listeners, if not the majority, have a preference for the pluses of transistors and would be willing to overlook the benefits of tube electronics. Personally, I am simply looking for the most realistic reproduction achievable, and the most natural-sounding instruments. I found the Pass X350 to be a tad cool in nature, so it’s not among my favorite amplifiers in these categories. There were times when I fell in love with its dynamics, yet felt that I could not live without the timbre, wonderful texture and sweetness of the AR VT100 or my current reference, the Sonic Frontiers Power 2.

If you are looking to integrate a theater amplifier with the X350, Pass offers a good match with the X3, which supplies 150W x 3 to control your rear and center channels. As always, I recommend that you attempt to use same-manufacturer amplification for theater applications. Although I did not run this amplifier in a multi-channel application, its ability to provide the highest in multi-channel reproduction is unquestionable.

The Downside
There are few downsides to the Pass X350. Because of the class-A operation of this amplifier, it runs very hot. By "hot," I mean that you can heat your house with this amp and, depending on your local utility rates, not necessarily inexpensively. Although I previously mentioned that I love the front meter on the amplifier, I should emphasize that, although beautiful, it is as useless as . . . well, you can finish that for me. If it were to demonstrate some information for the music lovers rather than the engineers - not to imply that there aren’t music-loving engineers, - that would be a different story. However, let’s face it, a meter that reads the current demand to the transformers, measured in joules, is hardly presenting quintessentially crucial information.

When looking to buy an amp in this category, it is important to either audition the amplifier before you buy, or to consult your retailer and be sure that they understand your system and your personal likes and dislikes. An amplifier is a critical link in your audio chain. I consider the Pass X350 to be among the best solid-state amps that I have heard and, although pretty darn expensive, not ridiculously so when compared with similar quality products. Before purchasing in this class, I would consider the competition carefully. At a few coins below the 10K mark, there are other comparably-priced amps to chose from. A couple that come to mind are the Krell 250Mc monoblocks at $11,000 and the Mark Levinson No. 336 at $9,500. If you wanted to run out today and get an amp without the trouble of auditioning, the X350 would be a very safe choice. It is a fantastic amplifier.

I feel that the X350 is not for everyone but, for that matter, neither is any product right for all. The X350’s somewhat lean and ultra-transparent top end will offend some and energize others. If you are looking the warmth of tubes, you won’t find it here. If you are looking for a gentle, sweet, musical component, you will only questionably find it with the X350. If you are looking for dynamic slam and the many benefits from the best that solid-state has to offer, you’ll find that and more. Regardless of your taste in presentation, you are sure to find this amp an exciting one.
Manufacturer Pass Laboratories
Model X350 Stereo Power Amplifier
Reviewer Bryan Southard

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