Whilst our designer's book on the XA25 has long since closed, ours just opened. What would it say under 'listening impressions' for the smallest stereo amp from Pass Laboratories in their year 2017 catalogue? From the descriptions, it certainly seemed that being most affordable was far from synonymous with least sophisticated or interesting. For contrast, I had their XA30.8 and FirstWatt F7. And this, from HomeTheaterReview.com colleague Terry London: "Wanted to share that I just got through submitting my writeup on the Pass Labs XA25. I believe that this might be the best amplifier Nelson and his crew have yet created/built. I own a pair of XA-60.8, a FirstWatt SIT 2 and have had in my system an XA30.8. The XA25 does color, tonality, spatial qualities, bass control and overall macrodynamics different and better for my taste than these other wonderful amplifiers. I still cannot believe that I'm not listening to a great tube SET were it not for how quiet, quick and dynamic the XA25 presents the music."

One channel of a mid-power XA.8 amp output stage sporting 28 power Mosfets vs. the XA25's lone pair.

Strolling the hood. The last and only other amplifier I reviewed which ran such high-power transistors as solitary pairs was the €20K Reimyo KAP-777. It outputs 200/400wpc into 8/4Ω to instead bias class A/B and not advertise <1Ω stability. Others championing single transistor pairs with high power would be Gato's 250-watt TwinFet PWR-222 at £11'180; and Gamut's 220wpc D200i stereo amp at £9'300. Then there are the 55-watt LinnenberG Allegro monos at €5'000. There's probably more; but just as likely not many. And once we specify class A operation with a sub $5K tag, Slim Pickens isn't just the stage name of one Louis Burton Lindley Jr. Obviously FirstWatt has such options. From them we'd simply expect lesser current delivery with less low-impedance stability. For complex bass-heavy music into more reactive loads, far closer and heavier competition to the XA25 should come from the massively paralleled XA.8 models. The big question was how the different circuit approach and 700w/40A output devices would influence sonics. First, the obligatory peek...


... under the hood which...


... reveals this bird's eye view onto 18 power supply caps, the front-end board above those, a toroidal power transformer and, against the heatsinks, the output devices covered by floating circuit boards.


A side view shows more of an output board...


... and a close-up even more.


Here is the front-end PCB...


... followed by the final modules in the front corners.


As to the long view and what these big Mosfets might presage for Pass Labs, "the XA25 confirms our in-house view that removing degeneration delivers some nice effects. The primary rationale for the big Ixys parts however (as opposed to smaller parallel Fets) was that they are simpler to implement. Big Mosfets or not, more powerful product will have to take a different approach and that is under development as well as other refinements." So the primary attraction is the avoidance of ballast resistors. The big Ixys parts simply play enabler.


In a subsequent email, Nelson let slip that "the SIT-3 was recently revised as a result of marathon listening sessions. Another circuit version emerged the winner. I expect a pilot run to surface beginning of March followed shortly by production." This reiterates how final voicing decisions come about at Pass Labs: through an experienced listening panel, not a solitary decider.