Pass Labs INT-25 integrated amplifier

In my personal life I prefer the minimalist, one-box architecture of integrated amplifiers. Always have. Before I started writing for Stereophile, my only audio system consisted of a pair of 15 ohm Rogers LS3/5a loudspeakers (with factory wall mounts) and an ancient Creek integrated amplifier, connected to my computer via a Halide DAC HD, and to an Oppo CD player. That system had pitch-perfect tone and was satisfying with all types of music.


I would never purchase an integrated amplifier with a DAC or phono stage residing in the same box. To me, that’s like buying an active loudspeaker. Why would I pollute a fine audio component with a non-upgradeable and possibly third-rate ancillary For digital, I want a super-quality DAC like the HoloAudio Spring I used for this review. For my moving-coil cartridges I want a step-up transformer and the best tubed phono stage I can afford. Therefore, I need my integrated amp to be a simple, purist, high-bias class-A design, like the new Pass Labs INT-25 ($7250).


Description
If you look at the back panel of the INT-25, you’ll see only three pairs of gold-plated RCA line-level inputs and two pairs of Furutech loudspeaker connectors, plus a generic IEC socket.


If you look at the front, you’ll see a modestly thick brushed-aluminum faceplate with a small OLED display that tells me only where the volume level is set, plus a power-on button, three input-selector buttons, a mute button, and an IR window for the heavy, brushed-aluminum remote control. Very clean and minimal.


The INT-25 measures 17″ W × 6″ H × 17.9″ D and weighs more than 50lb, which makes it infinitely more manageable than the INT-60 I reviewed back in November 2016. The INT-60 measures 19″ W × 7.6″ H × 21.2″ D and weighs 93lb. I could not lift the INT-60 into or out of its box without help.


The INT-25 is simply a Pass Labs XA25 amplifier with a minimalist, single-ended version of the INT-60’s line-level preamp attached. I wrote Nelson Pass an email asking him to give me some new words to explain to my readers why the XA25 amp drives so many loudspeakers with so much dynamic authority and unmatched transparency. He responded:


“The XA25 has a very simple topology using some new parts and uniquely operating them in push-pull class-A without degeneration. Developing the .8 series, we clearly saw that degeneration—putting resistance in series with FET Source pins (or Emitter pins for Bipolar transistors) impacts the sonic performance, and not for the better.


“Of course, there is a reason why people use degeneration (the ‘other form of feedback’) in gain stages—it stabilizes the characteristics of the transistors so that you don’t have to do precise matching and compensation to keep circuits stable. “At the same time, it is a form of feedback, and carries some of the same baggage. Interestingly your ‘no feedback’ solid-state amplifiers routinely depend on degeneration in the gain stages to control the stability and distortion, and so do not achieve the same characteristics that help make SET tube amps so popular.


“The XA25 is our first example of undegenerated push-pull class-A with Vertical MOSFETs, enabled by new techniques. The result is more life to the character and improved dynamics. This is not a big surprise; degenerating resistors are usually inserted to ‘tame’ a transistor’s personality.


“So, with power FETs, it turns out that there are a couple more advantages that you don’t see with bipolar transistors: FET character is ‘square-law’: The current through the transistor is a square function of the Gate-to-Source voltage, and in this respect, the FET is like a tube.


“Push-pull class-A operation of purely square law devices results in a perfect cancellation of distortion (literally perfect), while degeneration adds measurable higher-order harmonic and intermodulation distortions to that arrangement.


“Also, square law devices in push-pull class-A have a naturally larger class-A envelope at a given fixed bias, along with a lower output impedance and higher efficiency.


“What’s not to like?”


Protocol
For the purpose of this report, I’m going to forgo my normal amp-testing procedure and not bother connecting the INT-25 to the whole pile of speakers lining my hall walls; if you read my XA25 amplifier review, you’ll know how the INT-25’s power amplifier section works with most of them. You’ll also learn how the INT-25’s amplifier section compares to the other power amplifiers stacked by my equipment rack.


Instead of all that heavy lifting, I decided to use only three transducers: the Harbeth M30.2 monitors; the DeVore Fidelity Orangutan O/93 floorstanders; and the RAAL SR1a ribbon headphones.


The Harbeths and the Rhinemaidens
The first music I experienced with the INT-25 driving the Harbeth 30.2s was Scene 1 of Wagner’s Das Rheingold with Kirsten Flagstad singing Fricka and Georg Solti conducting the Vienna Philharmonic (3 LPs, London OSA-1309). The moment the Rhinemaidens appeared, it was obvious the INT-25 was reaching extra-deep and extra-microscopically into this uniquely spacious recording. The voices of Alberich and Wellgunde and the huge force of the Vienna Philharmonic came from a more vast, more precisely described three-dimensional space than I had ever before noticed. Those qualities made this spellbinding recording even more vivid and dramatic.


I mean, who out there doesn’t enjoy “watching” a Wagner opera from microphones positioned just above the performers’ heads This slightly elevated view makes me feel like a detached spirit hovering above and among ancient gods. This landmark recording, produced by Decca’s John Culshaw and recorded by Kenneth Wilkinson, cast a top star in every role and had them perform on an enormous studio stage (with a grid painted on the floor). Each singer’s choreographed movements were mapped out to simulate a scaled-down version of an actual performance. Listening with the Harbeth 30.2s pointed straight at me in the extreme nearfield, I felt the weight of the orchestra and sensed the performers’ movements.


When I used this recording to compare the INT-25 integrated to the combination of Pass XA25 amp and PrimaLuna EVO 400 preamp, Decca’s studio space sounded distinctly more 3-D via the INT-25 than it did with the Pass-PrimaLuna combination. This newfound dimensionality appeared to be related to the INT-25’s greater depth of field: Performers’ voices seemed more in focus no matter where they were positioned on Decca’s studio stage. The walls of the Decca studio were more discernable.


More surprising was how the overall sound of this recording seemed finer-grained with the INT-25 than it did through my Rogue Audio RP-7 preamp and Stereo 100 amplifier combination—a combo that excels at grainlessness.


Driving DeVore Fidelity
The DeVore Fidelity Orangutan O/93 loudspeakers are, by design, tube-friendly. They couple well with single-ended directly heated triode amps such as the 845-tubed Line Magnetic LM-518 IA and the Elekit TU-8600R single-ended 300B integrated. They’re also friendly with low-power class-A solid-state designs by Nelson Pass. The O/93s are a chief reason I fell in love with Pass’s First Watt SIT-3 amplifier; now, with the INT-25, I’m discovering a new and different side of the Orangutan’s personality.

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