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Pass Laboratories X150.5 Stereo Power Amplifier Print E-mail
Saturday, 01 November 2003
Article Index
Pass Laboratories X150.5 Stereo Power Amplifier
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ImageI have always considered Nelson Pass one of the founding figures in high-end audio. He has a long history of innovative amplifier and preamplifier designs, dating back to the ‘70s with one of the first successful high-end corporations, Threshold. I am very familiar with Pass’ amplifiers, and have owned a variety of Threshold components through the years. I have also kept an eye on his very active DIY career, which includes informative design articles and numerous DIY projects. Not having any exposure yet to any of the products from his latest company, Pass Laboratories, I was very curious and enthusiastic about auditioning the subject of this review, the X150.5 stereo power amplifier, and indeed I had high expectations to meet. Although the X150.5 is the baby in the current X series of amplifiers, it is by no means small in any way, specified at 150 watts per channel into eight ohms, and priced at $4,500.

Nelson Pass is a strong advocate of the concept that simplicity is paramount in producing an amplifier with superior performance. Perhaps most notable among his novel amplifier designs, up until recently, were the patented Stasis amplifiers sold first by Threshold, and subsequently by Nakamichi. I myself was a very happy owner of a Threshold S500 Stasis amplifier for a number of years, a true powerhouse that sounded musical and nimble. Pass Laboratories’ previous Aleph series of amplifiers, which featured a unique single-ended topology, have been very well received by the press and consumers alike.

This leads us to the current X series of amplifiers. They are based on Pass’ most recent patented design innovation, Supersymmetry. With this concept, Pass is moving closer and closer to the elusive goal of a very simple circuit, with only minimal local feedback, and vanishing low distortion across the output terminals. The amplifier is essentially a two-stage device, thereby simplifying the signal path circuitry. Using a balanced topology, any noise common to both sides of the circuit is canceled at the output. Taking advantage of this inherent feature, rather than striving for a perfect match between the input signal and output of each circuit half, Pass focused on making the two halves of the balanced circuit behave identically with respect to distortion and noise. He claims that Supersymmetry reduces the noise of a single device by a factor of 100, 10 times lower than a conventional balanced circuit. While there is a lot more to the actual circuit design and concept, I will force myself not to elaborate, interesting as it may be to an electronics enthusiast such as myself. The point is that the X150.5 stereo power amplifier is a clean, simple design, with extremely low distortion and noise.
The X150.5 is rated at 150 watts per channel into eight ohms, and 300 watts into four ohms. Surprisingly, it is the lowest powered member in the X family of Pass Laboratories amplifiers. This is a revamped version of the X150, adding the signature front panel blue meter featured in the higher-powered X amplifiers. The amplifier consumes 200 watts from the wall at idle, so it does run warm, even if not driven hard. Cosmetically, the amp is quite beautiful to the eyes of just about everyone who came into my listening room. The blue front panel power meter glows subtly through the round opening created by the decorative, two-tiered, anodized instrument gray aluminum front panel, striking yet not distracting or gaudy in any way. Curiously, the actual readout is not the typical instantaneous power output, but rather shows the bias current in the output stage transistors. Regardless, this added a touch of class to my setup, so I placed the amp squarely out in the open between the speakers, rather than off to the side with the rest of the electronics. Below the meter is a small silver toggle switch for placing the amplifier in or out of standby mode. Wrapping around from the top panel and all the way down the sides of the chassis are functional and stylish heat sinks. I say “functional” because they became uncomfortably hot to the touch when the amplifier is used for an extended period.

Moving around to the back panel, we see two pairs of input receptacles, single ended RCA and balanced. Below the inputs are the output loudspeaker terminals, which on my sample are surprisingly small and seemingly not very stout. In addition to not enabling banana plug connections, they do not easily accommodate large-diameter loudspeaker wires, unless spade lugs or something similar are installed. An amplifier of this pedigree, power and price should contain higher quality, more robust speaker terminals. Off to the left side are a replaceable main fuse, an AC power cord receptacle, and the master power switch, which must be on for the front panel standby control to function. I left this on unless I was gone for a few days, switching the amplifier on and off via the standby switch. A pair of five-way connectors for remote turn-on completes the available connections and switches. The X150.5 weighs a substantial but not backbreaking 70 pounds in the shipping carton, and measures 19 inches wide by seven inches high by 20 inches deep.

How much can be said about the setup of the amplifier? I placed the X150.5 between the front speaker pair in my listening room, utilized short 10-foot loudspeaker cables, and positioned the amplifier out in plain sight to fully enjoy the elegant cosmetics. The gain of the amplifier was a bit lower than my reference amplifier, so I re-calibrated the reference levels to match the rest of my home theater system for multi-channel audio and DVD playback. This is not a fault of the Pass X150.5; it just did not match the amplifier I normally use. The gain is certainly sufficient for use in any home theater or music system. The amplifier runs hot, so it should be placed where ventilation is good and the cooling fins are unobstructed. Before any serious listening session, I allowed the X150.5 a minimum of 30 minutes to warm up fully. Finally, although the Pass Laboratories X amplifiers do include both balanced and single-ended input connections, I did not have a balanced preamp available at the time of this review, so I was not able to take full advantage of the matched architecture of the X150.5.


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