One of the most venerable designers of solid-state amplifiers over the past 40 years is the legendary Nelson Pass. He has over seven patents that cover some of his most creative and groundbreaking circuit designs. From the very beginning, all of Pass' amplifier designs were based on Class A topographies, which offer the rich color and the overall liquidity of what tubes have to offer with the grunt, power, and control of the best solid-state amplification. Over the years, I have owned his Threshold Stasis 2 amplifier, Threshold SA-1 mono blocks, Pass Labs Aleph 1 mono blocks, Pass Labs X350.5 amplifier, Pass Labs XA100 mono blocks, and most recently Pass Labs XA60.5 mono blocks. Besides totally enjoying how each amplifier produced beautiful music, I also appreciated each amp's industrial build quality and rock-solid dependability, never needing repair.
Plus, I have great respect for two aspects of Mr. Nelson's design efforts. First, each new generation of his amplifiers offers significant improvement over the previous generation, which was already great in its own right. Secondly, unlike many other manufacturers who come out with new versions of their amplifiers during each buying cycle to maintain sales, Nelson Pass and his team only bring a new generation of amplifiers to the market when they believe they have learned how to make a more musically satisfying amplifier.
I have had a pair of Pass Labs XA60.5 mono blocks for over five years, and it took seven years for Pass Labs to come out with the new XA.8 series. The series includes a total of 10 new amplifiers in two-channel and mono-block varieties. I chose to review the XA60.8 mono blocks, which retail for $12,800 per pair, because of my familiarity with the XA.5 version. Numerous and major changes were made to the new XA.8 amplifiers, such as output stages that bias more deeply into Class A operating region, lower distortion rates, higher power MOSFETS, much larger power supplies, and much more extensive and larger heat-sinking on each mono block. During my audition of the XA60.8 mono blocks, I discovered that they run much cooler than my XA60.5s because of their more efficient heat-sinking.
Each of my XA60.5s weighs 62 pounds, while the XA60.8 weighs 88 pounds due to its heat-sinking and a much larger and heavier power supply. It measures 19 inches long by 21.25 inches deep by nine inches high. A large bias meter in the center of the matte-silver front plate glows blue when the amplifier is on; the needle within the meter only moves if the amplifier leaves Class A. The on/standby button is located below the bias meter. On the back is a master power switch, a pair of handles, a pair of single-ended (RCA) inputs, a pair of balanced (XLR) inputs, the IEC power input, and finally a pair of speaker-wire terminals. The speaker-wire terminals were the best I have ever encountered on an amplifier, composed of large wings that allow you to hand tighten your speaker wire spades until you hear a click, which indicates that the connection is secure. Unlike the upscale appearance of the XA60.5, the XA60.8 is rather austere and understated in its overall look. The casework and overall quality of the chassis are superlative, which is what you would expect from Pass Labs.
The XA60.8 mono blocks are rated at 60 watts pure Class A into eight ohms and 120 watts pure Class A into four ohms. The XA60.8 leaves pure Class A after 122 peak watts and moves into Class AB for tremendous headroom on very large-decibel peaks. Regardless of which speakers I connected or at what extreme sound pressure levels I listened, I could never get the XA60.8s out of their Class A biasing range.
To provide first-class packaging and to ensure safety and protection of the amplifiers during shipping, Pass Labs used extremely thick cardboard boxes for the exterior as well as the interior, along with component-fitted pieces made of high-density foam inserts to protect the amps from rough handling. Although the demo amplifiers I received had enough hours on them to be considered fully burnt-in, I still put another 50 hours on them before I began my serious auditioning process. My auditioning system was composed of the Aerial Acoustics 6T tower speakers and the Lawrence Audio Cello tower speakers, as well as the Backert Labs Rhythm 1.1 preamplifier (review forthcoming). The source was a reference MBL 1621 transport driving the Concert Fidelity -040 battery-powered hybrid DAC (review also forthcoming). All wiring was silver reference MG Cable ICs and MG Cable reference three copper ribbon speaker wire. The entire system was powered by Harmonix Studio master power cords and a Running Springs Audio Dmitri power conditioner.
Click over to Page Two for Performance, The Downside, Comparison & Competition, and Conclusion...