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).

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?"

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.

Pass Laboratories Inc.
13395 New Airport Road, Suite G
Auburn, CA 95602
(530) 878-5350

davip's picture

"... 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".

I've often wondered why magazines ostensibly dedicated to audiophilia justify reviewing pre-/integrated amplifiers without phono stages (let's forget about the IC that is a DAC), and this lamest of justifications above provides the context for asking. Why, firstly, would anyone have any faith in the sound-quality chops of such an amplifier when the highest-quality audio source (i.e., a turntable) was not used in its design? Does this reviewer kid himself (or, more importantly, his readers) that such designers use live, all-analogue FM broadcasts to design, test, and voice their amplifiers, or is he content for such amplifiers to be designed with digital audio signals as their source? Even if a turntable source was used (presumably using a non-'third-rate' phono stage), how could any end-user know how such an amplifier would sound with vinyl unless the same phono-stage used by the manufacturer in design or the reviewer in appraisal were also purchased alongside? It behooves amplifier designers -- people like Pass in particular -- to use the best quality source available in designing their kit and if there's no phono input then it wasn't used. If a turntable was used, then there is no justification whatsoever for not including a phono input.

The reason a phono stage belongs in an amplifier is because it equalises and amplifies the output of a phono cartridge, both of which are the only jobs of an amplifier (and, similarly, why a DAC does not belong in an amplifier, any more than a tweeter does). If Nelson Pass or indeed anyone else cannot design an integrated amp ('integrated' being the operative word here) worthy of being reviewed by Stereophile without putting a 'third-rate' phono stage in it, then he is worthy only of our opprobrium and certainly not of our patronage. Of course, he is perfectly capable of doing so, and it's silly reviewer statements like the one starting this review heading this comment that let him and many like him get away with not doing so.

Come-on Stereophile -- you aren't 'What Hi-Fi'. There's plenty of 'magazines' that pander to the computer-as-music-source crowd. Stick to the audiophilia that is implicit in your name...

JackWlwi's picture

The convenient fiction would be the assumption that a turntable is the most superior source which frankly, is just not true. The most pleasantly coloured perhaps, but the most superior? Sorry, nope.

DWMB's picture

I respect Herb Reichert very much. And as an audiophile, I perfectly understand the rationale behind separates. I'm also a minimalist and a purist. But is Herb suggesting that a flagship pure Class "A" integrated like the Sugden IA-4 is "polluted" or will have degradation in sound simply because it has a built-in phono stage? Although I'm not an expert in this field, I somehow doubt that to be the case given Sugden's pedigree with pure Class "A" architecture. I personally think that both the Sugden and the Pass Labs would be fine products sonically.

Ortofan's picture

... (or DAC) phobia, Accuphase makes a pair of FET output, class A biased integrated amps, with card slots for plug-in phono stage and DAC option boards.

Have no fear of a built-in phono preamp or DAC?
Then get the option boards.

Rather have an external phono preamp or DAC?
That's covered, too.

I'd want to audition the Accuphase amps before declaring something else to be reference grade.

Incidentally, an "arcane art form" such as opera should have been composed by Verdi and sung by Bergonzi.

Bogolu Haranath's picture

May be HR could review the new NAD M33 with Purifi 'Ultra-Quiet Amplification Technology' 200 WPC integrated amp, $5,000 ......... M33 has built-in phono-stage, DAC, Dirac-Live, tone controls, Wi-Fi access and headphone output :-) .......

Ortofan's picture

... give HR palpitations.

Bogolu Haranath's picture

We don't want that to happen to HR ....... We need HR for a long long time and keep reviewing audio equipment :-) ........

Bogolu Haranath's picture

Well ...... HR could use the M33 as a pre-amp and connect the pre-amp outputs to his favorite tube power amp :-) .........

dc_bruce's picture

I'll only go part way with Daveip's comment, to the extent that he implicitly flags the silliness of the review's expressed prejudice against integrated amps with phono stages (and, presumably, preamps with phono stages). Although I have a great fondness for vinyl (having grown up listening to it) and I own a separate all-tube phonostage (BAT) with a step up transformer, I would not claim vinyl as "state of the art." The best vinyl is certainly very good -- but so is the best digital. The limiting factors seem to be the quality of the recording and mastering, not so much the medium.

I do have an issue with reviewers --almost all of whom are unqualified to opine on the subject -- repeating with praise the manufacturer's description of the advantages of his unique circuit design and so on. And if we're going to talk about fetishes, why is this amplifier not balanced (which seems to be a fetish praised by many).

The proof, of course, is in the listening and in the measuring to some degree. Thankfully JA's measurements suggest that this is not the amp to use with 4 ohm speakers unless one like an additional flavoring in one's music reproduction.

JackWlwi's picture

Nailed it.

The power supply noise on the output is pretty substantial too.

tonykaz's picture

Exciting and thought provoking. What more could we hope for from Stereophile's Poet Laureate.

A reviewer or even a dedicated neurotic/psychotic Audiophile should own gear that encourages separates evaluation. ( I prefer Active Loudspeakers and/or Manufacturer-Designed complete systems as in : Meridian, Linn, PS Audio, Schiit ( in-spite of their horrible name and rear mounted power switches ) and even Diaveliet from France .

No phono, No problem.

The World's population today doesn't own Vinyl, doesn't plan on owning vinyl or doesn't even know what vinyl records are. Designing new Audio Gear for a 21st Century Marketplace won't include 33.3 ( why should it ? ) There are plenty of previously loved ARC Vinyl Preamps for any of us old geezers that are still holding large collections and want something vintage and interesting.

There are some very nice vinyls coming out but I can't play any of them on my mobile rig, so why bother? When I'm home, my family don't want the house filled with Russian Pianos, maybe if I'm a single guy but no way around wifey.

I will probably never own PASS gear but I do like reading HR's opinions. I'm waiting for Mr.HR's review of the 1,800 Watt Class D from PASS ( the size of an iPhone ), when it finally comes out next year or so. ( doubles as a remote control )

Tony in Venice

ps. I miss Enid Lundley and wire comment.

davip's picture

"...The World's population today doesn't own Vinyl, doesn't plan on owning vinyl or doesn't even know what vinyl records are. Designing new Audio Gear for a 21st Century Marketplace won't include 33.3 ( why should it ? )".

Straw-man argument. Do you really think that Nelson Pass' few-10s-of-watts multi-$1000 amplifiers are designed for "...the World's population" (when they can get a Bluetooth-enabled one with 10x the power for 1/50th of the price)?

Pass and his peers are not designing for 'the World' but a tiny subset of that world in the audiophile. Were the analogue element to disappear from Stereophile the magazine would go bust in fairly short order -- not everyone (at least not every adult) wants their musical enjoyment to be endlessly portable or regards their music collection as being on an SD-card as a good thing.

tonykaz's picture

what argument am I refuting? i.e. "straw-man"

Vinyl isn't an argument, is it?

Mr. Pass's concept of "First Watt" is certainly pertinent to the music reproduction by the vast citizen population. Who needs more than one Watt?

Of course his pricing and high manufacturing quality isn't aimed at "everyman", neither is vinyl now-a-days.

I do agree with you on your important point : Pass builds for a tiny segment of Audio gear Marketplace.

but disagree on Stereophile's not covering vinyl

Stereophile would do just fine because Stereophile exists and thrives on the journalistic integrity of JA1&2, the superb writing of some of it's staff and it's maintaining a strong connection to a thriving Industry. ( Automotive Audio itself exceeds $20 Billion )

Vinyl is a minute niche of hoarder/collector hobby that cannot itself support a proper print magazine. ( I myself own a rather large collection of 33.3 & Moving Coil phono Cartridges - kinda frozen in time ) Today's Vinyl greats are folks like Cassem, Chesky and a small few Europeans creating beautiful pressings, all of them combined would probably fit in a high-school gym. god bless em.

Vinyl's aggressive proponents tend to be Gas-lighters and have been since the mid 1980s . Some are acid spitters. I'm a vinyl guy that loves Redbook - a disruptive person from some Vinyl believer's point of view.

Tony in N.H.

mrkaic's picture

Look at the distortion at low powers — really bad stuff. Not cheap either. I will pass (pun intended).

jimtavegia's picture

Not what one would expect from Nelson. I suspect something is amiss. The HF at 8 ohms is also very odd. Should we not expect some component failure in this amp?

Surprised this could be considered a "reference" amp. It is true that I do not trust my ears anymore. These is so much at play to make an audio system likable, that we miss some things that aren't quite right. I know I do everyday.

jmsent's picture

..about the high frequencies at 8 ohms?

jimtavegia's picture

For an amp at this price range to not be extremely low at either 4 or 8 ohm and be consistent across the entire band at something under .0 something % I don't get. I am also not saying that low distortion is all of it, but it does raise the question brought by another of what % of distortion can we hear, does it matter in the overall presentation, and in the context of microphones and mic pre-amps does it matter. Obviously it was still like by HR.

I guess for some a straight wire with gain is out the window, but I think that as your speakers get more and more revealing and many that are power hungry, these things do matter.

tonykaz's picture

Can we actually hear low distortion?

The people that manufacture the Distortion measuring gear contend that humans can't quite detect distortion below 20%, ( strange as that might seem ). They prove their claim at every Audio Show they display at. It's quite surprising.

Tony in Venice

Archimago's picture

Great question Tony. Starting this weekend on the blog, we can test this out in the comfort of one's home :-).

tonykaz's picture

Maybe you can test it out and report.

I'd ask JA to make some sort of contribution as well as those Precision Gear Manufacturers.

It was the Japanese in the 1970s that got us all those zeros infront of a distortion number, wasn't it. Zero distortion doesn't sound good from our Absolute Sound perspective.

Personally, I'm fond of what ever distortion tubes yield.

So, In my opinion book, distortion is a good thing, which might explain my Alcholic life ( recovered in 1999 ) .

I love JA and his measurements, I just don't judge my gear based on the numbers. ( maybe I should )

Tony out in the dam cold winds of the Planes.

Archimago's picture

"Is high Harmonic Distortion in music audible?" Internet Blind Test is up now on the blog.

This is step one. To see if THD can be discerned and at what kind of level among audiophiles / music lovers who want to partake in the test.

In time, we can perhaps looks at things like high order vs. low order distortion. And of course odd vs. even order harmonics.


tonykaz's picture

I wonder if revealed facts will change our decision making decisions.

I'm related to a still suffering alcoholic that loves Beer so much that he has it with his breakfast cereal. ( he's divorced and living in his 85 year old mother's basement )

Your explorations will make you a legend. ( at least, in my mind )

Tony in Venice

Bogolu Haranath's picture

Compare the measurements of this Pass INT-25 integrated ($7,250) with twice as expensive BorderPatrol amp ($14,500) :-) .........

Les's picture

At the volumes Herb was listening, the distortion levels are quite reasonable. (And of course at lower volumes the distortion would be even less perceptible.) Also, the rising THD+n per frequency wouldn't come into play for opera or really any reasonable music. In what circumstances would we listen to such high frequency content at such levels?

This is not a defense of the amp but rather to point out that its "weaknesses" don't really come into play in real life situations. YMMV.

volvic's picture

But, for me who has quite a few components; tuners, cd players, computer audio, SACD and several turntables, only three inputs is a bit of a deal-breaker. It is not a knock on Mr. Pass and his gorgeously sounding equipment but on some of these manufacturers who make fewer and fewer inputs. Guess I'm old school.

Gerd Bethge's picture

I cannot understand why the difference in distortion at 100mW is so different between the two amps.
What went wrong with measurement.....


AaronGarrett's picture


Have you heard 75 Dollar Bill's album "I Was Real"? I think you will enjoy it!

Herb Reichert's picture

"I Was Real" right now. I am listening with the RAAL AR1a headphones and the new Schiit Jotunheim R dedicated amplifier.

You are right! I dig 75 Dollar Bill a lot.

Thanks for pointing it out.


AaronGarrett's picture

Glad you like it!

Bogolu Haranath's picture

As long as you don't print it, that should be ok ........ Just kidding :-) .......

helomech's picture

Herb has a new reference - most, most, most predictable.

unitygain's picture

Price-equivalent comparisons to the Pass INT-25 are just waiting to be made, namely:

- The venerable Sugden Masterclass IA-4 solid state class A integrated putting out 33wpc (particularly given the history of Devore / Sugden show combinations)
- The just-released Modwright KWH225i hybrid integrated biased 25 wpc Class A then A/B to 225 (with the added twist of 6922s in the preamp section and enough juice to push those lovely Studio Electric F2s)

Always a fan of the chimeric admixture of a very reasonable form factor (integrated amp) allied to a wasteful, Dionysian impulse (Class-A-or-be-damned!).

aRui's picture

I has Pass Labs XA25 and it is an excellent power amp, great measurement. I wonder Why the measurement of this INT-25 is much worse than the XA25 on the power side? Isn't the INT-25 is built based on the XA25?