Pass Labs INT-25 integrated amplifier Page 2

The Pass Labs integrated elicited a pristine, extremely well-sorted behavior from the O/93s. It played Wagner's Die Walküre, with Birgit Nilsson and Gré Brouwenstijn singing and Erich Leinsdorf conducting the London Symphony Orchestra (44.1/16 FLAC, Decca/Tidal), with an unprecedented amount of detail and spatial acuity. Better still, the INT-25 allowed the O/93s to play this utterly stunning Kenneth Wilkinson recording at highish volumes—with no noticeable clipping. No blurring. No clumping of detail. And no solid-state hardness.

Please forgive my focus on opera recordings. Probably only a few of you listen to this arcane art form. I use them in my reviews because I like how they sound, and opera recordings present difficult information-recovery challenges for any hi-fi system. I no longer have the Decca LPs (which were among the most thrilling-to-play, information-rich discs I've owned). But even from a Tidal stream, this Wilkinson-recorded Wagner is a musical supernova that instantly shows me how low-distortion clear (or smeared-and-blurred) my system actually is. The INT-25 let the O/93s make that Die Walküre into something so beautiful I just laid back and basked in it. Life was good—and poetic.

No question: With the O/93s, the INT-25 made a righteous, musically satisfying alternative to tubes.

Compared to Line Magnetic triodes
My Russian neighbor was totally zonked out, snoring on the couch, while I quietly exchanged the INT-25 for the Line Magnetic LM-518 IA tube integrated. When I turned the volume back up, he snapped bolt upright and began a disoriented barrage of Russian curses.


I was playing exquisitely recorded guitar music from the album Tassili (44.1/24 FLAC Anti-Epitaph/Qobuz) by Tinariwen, a Saharan Tuareg group that performs a type of Malian music known as Tishoumaren. My neighbor's response: "What the &%*# is this?" He couldn't tell I was laughing. "Be quiet and listen," I instructed.


This album has a unique, extremely vivid sound because it was recorded in a series of unedited takes performed live in the silence of the desert, near the town of Djanet on the Tassili n'Ajjer plateau in Algeria. The Line Magnetic's 845 tubes driving the DeVore O/93s amped up Tinariwen's vividness factor. Playing Tassili, the sonic force produced by the LM-518 made the INT-25 sound not threadbare, nor dry, but slightly skeletal.

I and the now-calm Russian sat in silent awe, listening carefully to every track as rendered by the Line Magnetic amp. Vlad broke the silence first, saying how intense and real the guitar on "Tameyawt" sounded. How every note was "%*#@ exploding."

It was true.

Tinariwen's music is rooted in moody, sometimes-droning electric guitars and relentless drum rhythms that shift unexpectedly. With the Line Magnetic LM-518 (driving the O/93s), goatskin drums pounded the floor and note decays appeared as touchable room-filling densities. With the INT-25, these densities were rendered more as resolved detail and less as energy, as through the 845 tubes.

Bass was tighter, and drumbeats pounded the floor more distinctly with the INT-25 powering the O/93s. Nevertheless, it was the crisp clarity with which the Pass labs projected vocals into the room that held my attention. The INT-25 made the O/93s image more precisely; but images were not quite as weighty or tangibly present as with the LM-518. The INT-25 seemed uncanny in how it showed fingers on guitar strings and tappings on the guitar body. In comparison, it was uncanny how physical and painterly the Line Magnetic LM-518 IA made the guitar amp's reverb sound.

Fancy-pants audiophile audio is all about where various components direct the listener's attention: The INT-25 focused more on the visions in my head, while the LM-518 IA addressed feelings in my body. Both honored the music on Tassili.

The INT-25's ability to excavate detail was even more apparent on Tinariwen's recording, from their album">Live in Paris (48/24 FLAC Anti-Epitaph/Qobuz), of "Tinde Final Tinariwen," featuring 75-year-old tinde master Lalla Badi performing at Théâtre des Bouffes du Nord. Lalla Badi's voice made me cry in 30 seconds. The powers of rhythm and tone forced my surrender. The word tinde in the title refers to the goatskin drum as well as the intimate celebratory nature of the chosen songs. Again, on this album, the LM-518 exceeded the INT-25 in its ability to deliver the dense sound bodies of the performers and their instruments. It also edged out the Pass Labs INT-25 at making goatskin "feel" like real stretched skin.

At an average level of only 84dB, the INT-25 made bass that physically shook the floor and the couch. (I don't know how Live in Paris was recorded, but damn! It sounded live in a way I think live actually sounds.) The reverb drone from the electric guitars made the air in my room feel like it does at concerts with enormous speaker stacks. Rhythm delivery kept my head nodding in time.


While comparing these two drastically different amplifiers, I fell back in love (again) with the DeVore Fidelity Orangutan O/93 speakers. I forgot how warm, vibrant, and satisfying their midrange could be. I had never realized how elegant and detailed their top octaves could be. Best of all, I discovered how fresh and transparent they could sound with some sparkling clean solid-state. With the DeVore O/93s, the INT-25 was a highly recommendable alternative to tubes.

Compared to XA25 + HPA-1
The most relevant comparison I can make is between the INT-25 integrated, which costs $7250, and the $4900 Pass Labs XA25 amplifier connected to the $3500 Pass Labs HPA-1 line stage/headphone amplifier. Together, these Pass Labs separates cost slightly more ($8400 plus interconnect) but include a world-class headphone amp designed by Jam Somasundram.

After extended listening, I could not describe the sound of either the INT-25 or the combination of HPA-1 and XA25 as vivid in a tube-like way, or one that presents musical tone in its most completely saturated form. Both Pass Labs setups sound fresh, alive, and clear, but a little Apollonian compared to the above-mentioned Line Magnetic SET amp. However! The INT-25 does generate a delightful amount of "FET sparkle" and a distinct but subtle "tube-like" glow. The HPA-1/XA25 combo does not. Neither does any other solid-state amplification I know. This is an important distinction. This subtle radiance enhances the INT-25's breezy, clear-sky transparency, making the INT-25 the most musically satisfying yet neutral (I hate that word) solid-state amp-preamp combination I've encountered.

With RAAL-Requisite SR1a headphones
Because its drivers are full-range ribbons, the RAAL-Requisite SR1a headphones have a DC resistance of only 0.018 ohms; this requires a power-absorbing resistive network (included with the headphones) to bring the ribbon's dead-short impedance up to a level capable of being driven by a loudspeaker amp like the Pass Labs INT-25. (The RAAL ribbon headphones were designed to be used with loudspeaker power amplifiers in the 50W to 150W range—not a traditional headphone amp.)

I played Act 1 of the previously mentioned Die Walküre and Scene 1 of the previously mentioned Das Rheingold and, as I often do while listening to opera or large orchestral recordings via headphones, I sat there shaking my head in amazement. The INT-25 put the uncanny resolving powers of this unique transducer on full display. Even more than my JPS Labs Abyss AB-1266 Phi headphones or the HiFiMan Susvaras, the RAALs resolve—by which I mean something conceptually and perceptually different than the dry, gray, unsupple, artificially etched sounds some audiophiles mistake for resolution. The SR1a shows the INT-25 about 5.8 ohms of series resistance, and while said resistance does consume amplifier power, the RAALs have no crossover or time-twisting reactances. When I use these full-range ribbons, I feel like they are exposing the kaleidoscopic inner workings of a recording better than any other transducer I've encountered.

On the Decca Rheingold, this exposure didn't just show me an avalanche of tiny, previously unseen details, nor did it show me just the Decca studio space and the singers moving about on the gridded floor under the microphones: It seemed to reach back into the guts of the INT-25 and show me, in sonic terms, the clock workings of its separate amp and preamp sections. What the RAAL headphones exposed was something that confirmed my developing suspicions: The INT-25's minimalist, Wayne Colburn–designed preamp and its Nelson Pass–designed amplifier (with its lack of degenerative feedback) are the "secret" Pass Labs ingredients that raise the INT-25's sonics to the best solid-state I've ever heard, along with the First Watt SIT-3 and J2 power amps.

Every day, the Pass Labs INT-25 sounded like a purer, more austere instrument than the combination of the Pass Labs XA25 amplifier with any of my in-house preamps. Every day, I noticed how the INT-25 offered a less obstructed and more transparent view of what my source components were excavating from my recordings. In addition, it made my hi-fi less hi-fi and the musicians more there—with a lower level of editorializing than I've gotten from any previous amp-preamp combination I've reviewed. My new solid-state reference.

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?