XA25

Reviewers: Srajan & Dawid
Financial interests: click here
Main system: Sources: Retina 5K 27" iMac (4GHz quad-core with Turbo, 32GB RAM, 3TB FusionDrive, OSX Yosemite. iTunes 14.4), PureMusic 3.02, Audirvana 3, Qobuz, Tidal, Singxer SU-6 USB bridge, LHY Audio SW-8 network switch, Sonnet Pasithea DAC; Active filter: icOn Gradient Box 2; Power amplifiers: Kinki Studio EX-B7 monos, Enleum AMP-23R, Goldmund/Job 225; Headamp: Cen.Grand Silver Fox; Phones: HifiMan Susvara; Loudspeakers: sound|kaos Vox3awf + sound|kaos DSUB 15 on Carbide Audio footers, Audio Physic Codex, Qualio IQ; Cables: Complete loom of Allnic Audio ZL; Power delivery: Vibex Granada/Alhambra on all source components, Furutech RTP-6 on amps, Furutech DPS-4.1 between wall and conditioner; Equipment rack: Artesanía Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands; Sundry accessories: Acoustic System resonators, LessLoss Firewall for loudspeakers, Furutech NCF Signal Boosters; Room: 6 x 8m with open door behind listening seat
2nd system: Source: Soundaware D300Ref SD transport, Denafrips Terminator +; Preamp/filter: icOn 4Pro + 80Hz active filter; Amplifier: Crayon CFA-1.2; Loudspeakers: MonAcoustics SuperMon Mini, Dynaudio S18 sub; Power delivery: Furutech GTO 2D NCF; Equipment rack: Hifistay Mythology Transform X-Frame [on extended loan]; Sundry accessories: Audioquest Fog Lifters; Furutech NFC Clear Lines; Room: ~3.5 x 8m
Desktop system: Source: HP Z230 work station Win10/64; USB bridge: Singxer SU-2; Headamp/DAC: iFi iDSD Pro Signature;  Headphones: Final D-8000; Active speakers: DMAX SC5
Upstairs headfi/speaker system: Source: smsl Dp5 transport; DAC: Auralic Vega; Integrated amplifier: Schiit Jotunheim R; Phones: Raal-Requisite SR1a; Active DSP speakers: Fram Midi 120
2-channel video system: Source: Oppo BDP-105; All-in-One: Gold Note IS-1000 Deluxe; Loudspeakers: Zu Soul VI; Subwoofer: Zu Submission; Power delivery: Furutech eTP-8, Room: ~6x4m
Review component retail: $5'150

July 2017. "The XA25 has an RCA input and no balanced outputs. It's a very simple amp of surprising performance. However, it is the intellectual property of Pass Lab. So it remains under wraps. I will tell you this so that you can reverse engineer it: three stages; ~700 damping factor; high current; high slew; .00x% distortion; 40µV output noise; big Class A envelope; 25wpc into 8Ω, 100 into 2. And it sounds great." That was Nelson Pass. His amp today lacks a decimal point. It belongs not to the point.8 range and also lacks their power meter. Yet it's got more dissipation surface than the standardized FirstWatt enclosure. And unlike its occupants, it not only power doubles into 2Ω, it's stable down to—pop the cork!—0.5Ω Scintilla turf as long as you don't ask it to dispatch >10 amperes (200 watts peak into 2Ω) at which point protection would clamp down.

Meanwhile unlike other Pass amps, it gets by with just two output transistors per channel (one per half cycle), not an entire alley. That's a minimalist push/pull output stage. Here it involves these 800w/40A devices of the Ixys HiPerFet type. Those rock high power density and efficiency to not require beast-mode power supplies. Cosmetically and conceptually, the XA25 occupies middle ground between FirstWatt and Pass. Of course it remains pure Pass as 55lbs of 17×14.4×6" WxDxH hardware that runs ~25C° at the radiators, consumes 240 watts at idle and presents your preamp with 47kΩ. Voltage gain is 20dB, bandwidth DC-100kHz, noise sub 50µV, slew rate 100V/µs. "The signal path from input to output was simplified to fewer components, degeneration¹ as the other form of feedback eliminated. This increased efficiency and class A bias current for greater class A operation into low-impedance reactive loads. The push/pull class A output stage runs a new constant-current bias circuit to compensate temperature drift. That connects directly to the loudspeakers without ballast resistors for lowest possible distortion and highest damping factor. We still use the same NOS complementary small power Jfets and cascoded Mosfets in the classic current-feedback voltage gain circuit.

"The amplifier is still DC-coupled with no frequency compensation. The result is faster, with lower distortion, less noise, higher damping and larger Class A envelope all with a total of three pairs of push/pull gain transistors. In their ideal state, matched push/pull Fets give perfect square-law cancellation of distortion. That effect is somewhat spoiled by degenerative source resistance used to constrain the 'personality' of the devices. The removal of this form of feedback is an important element in the performance of the XA25. It is accomplished by new approaches to stabilizing gain and bias of push/pull Fets. This gives a lower output Ω, more gain and lower spectral content of distortion as evidenced by measurements. But the more important factor is the difference it makes to the music. We observed this in listening tests some years ago. Degeneration removed some of the organic quality to the sound. So it became the subject of further development work. The XA25 measures superbly but its subjective qualities are most special."

¹ Degeneration refers to the use of a small resistor between emitter and ground reference or power rail. This reduces the transistor's transconductance hence voltage which now depend mostly on the ratio of resistance/impedance rather than the transistor's intrinsic characteristics. Degeneration improves distortion and stability at the expense of gain. While those effects mirror negative feedback, they don't reduce output impedance or increase bandwidth as 'true' (or the other kind of) negative feedback does. This practice predates the invention of negative feedback. It's thus considered separate and usually goes unmentioned in the context of whether a circuit employs any negative feedback or not. So an amp can be advertised as having zero negative feedback yet still use degeneration. The secret sauce of the XA25 is how it controls bias and gain without it.

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

Now you'll want to know about the bolded date above. It's from first publishing this review in May 2018. When the company's Desmond Harrington contacted me January 31st 2023 to offer up another review sample, I sent him my old review link. When he replied that it was just a one-page preview, I looked. He was right. Yet I could have sworn that I'd heard the amp in Ireland then shipped it back to a Dublin dealer. It turned out that I had. The other pages had published, even ended in an award. For reasons unknown, the opening page simply didn't link to the subsequent pages. That essentially froze the review in endless preview limbo. As a 2018 issue it just slightly predates my change to adaptive HTML. I no longer have access to the static data base created in Adobe GoLive then DreamWeaver. It's been seamlessly integrated into WordPress but sits on a different part of my server. To fix the article's dead end required a reboot. It's what you read now. It's why this review relaunches five years later. What was old is new again. Wouldn't it be lovely if we could reboot ourselves in the same way?