My review of the new and quite extraordinary Pass Labs XA25 amplifier, which retails for $4,900, marks the fifth time I've had the pleasure to experience yet another creation by Nelson Pass and his team to drive my reference system to its highest levels of musical enjoyment. For the last four years, my solid-state reference has been a pair of the XA60.8 mono-block amplifiers that have offered nothing less than superlative performance, regardless of what speakers I added. I knew that Pass Labs had introduced the XA25 about six months ago and had put it on my list of future reviews to arrange. However, I got sidetracked with other reviews and my professional work responsibilities. Then, in a casual conversation I had with Andy Collen, owner of the high-end salon The Audio Surgeon located in Michigan--who is a long-time friend of Nelson Pass, a Pass Labs retailer, and someone who has great taste regarding anything in audio--he asked me if I had heard the XA25 yet. When I said no, he responded, "You are in for a real treat. In its own way, it might be the best of all Nelson's amplifiers in the last 20 years!" This certainly motivated me to get the XA-25 in-house to review as soon as possible.
The Pass Labs XA25 is a relatively small amplifier compared with the XA.8 Series models. It measures six inches high, 17 inches long, and 17.315 inches wide, and it weighs 55 pounds. It lacks the bias meter found on the front of all the other XA.8 Series amplifiers. The silver front plate simply has a push-button on/off switch and a single blue LED that lets you know that the amplifier is on. Engraved on the left and right side of face are "XA25" and "PASS," respectively, which flank two grooves that are carved out of the face plate. Around back is one set of RCA inputs, one pair of very high-quality speaker wire terminals, and the IEC input. Finally, a pair of robust handles makes moving the XA25 a relatively painless procedure. As you can expect from Pass Labs, the chassis and build quality rank with the best on the market, regardless of the price. I found the XA25's appearance to be quite handsome in an understated yet classy "less is more" way.
The XA25 is a pure Class A design that only uses two transistors per channel, yet it delivers 25 watts RMS at eight ohms and 50 watts RMS at four ohms. However, it can deliver peak current of 10 amp output (200 watt A/B peak into two ohms), which means that it can drive most speakers without any difficulties. Three other measurements worth noting are its damping factor, output noise level, and slew rate. When compared with the Pass Labs XA30.8 pure Class A single-chassis amplifier, the XA25 has a much higher damping factor (500 versus the XA30.8's 150), a lower output noise level (uv:50 versus the XA30.8's uv:200), and a higher slew rate (100v/us versus the XA30.8's v/us 50). The Pass Labs XA.8 Series amplifiers are some of the quietest solid-state designs on the market, and this one has a dramatically lower noise floor. What these measurements equate in sonic parameters are that the XA25 is a faster and more dynamic amplifier, has unbelievable transparency/clarity, and has the ability to exhibit great control over the low frequencies because of its damping factor rating.
Since the XA25 uses qualitatively different transistors than the other Pass Labs amplifiers, I asked Nelson if he could write a brief explanation of what the differences are between the XA25 and XA.8 Series amplifiers. He answered, "These types of higher power devices have been around for a bit, but in the past I have found it more practical to match a quantity of 150-plus-watt devices and run them in parallel, which requires resistive ballast for thermal stability. For years, we have noted the differences that output stages exhibit with varying amounts of this degeneration, and it was always clear that the best examples showed the most 'square law' character and largest Class A envelope, which is the 'simple' power FET model. We finally decided to remove all degenerative resistance from the output stage for sonic reasons. The design that did the job required new bias circuits, and it was easier to make with large single-die Mosfets. There are a limited number of such products on the market, and we bought samples and tested them in prototypes by measurement and listening, and we picked our favorites, which you see in the XA25. It is not simply the big transistors that create this effect--the bias circuit being a key element--but they are the most convenient approach."
The XA25 arrived with the usual first-rate packaging in which Pass Labs ships all of its gear. I inserted the XA25 into my reference system, replacing the XA60.8 mono blocks, along with other amplifiers that I use (Linear tube Audio ZOTL-40, Triode Lab SET 2A3, Usher 1.5 reference, AricAudio SET KT-88),to drive the Tekton Design Ulfberht speakers. The rest of the system was comprised of the CEC-3 CD transport, Concert Fidelity-040 hybrid DAC, Linear Tube Audio Micro-ZOTL preamplifier, Running Springs Dmitri power conditioner, MG Cable reference silver and copper wiring, and Audio Archon power cords, all placed on the Tomo rack/footers by Krolo Design.
Click over to Page Two for Performance, The Downside, Comparison & Competition, and Conclusion...