Pass Laboratories INT-25 Solid-State Integrated Amplifier
Lest one think that this review is simply not only more evidence that I have somehow formed an untoward relationship with Pass Laboratories, I'd build my defense around the fact that every time I reviewed one of their high-end audio components, I was "assigned" the task. Sure, the fact that I use not only the power and preamplifier but also the headphone amplifier I use as references, must not help my case. The next thing I know I'm asked to review their INT-25 integrated amplifier. It's tough work, but someone has to do it. Yes, I'm grateful for my problems.
Pass Labs INT-25 Integrated Amplifier
Class A is the least efficient of all the classes, as most of its energy is converted to heat and the least converted to a musical signal. But aah, the advantages of this seemingly feeble circuit lie in its sound quality, which in my experience it has proven to be the most amazing sounding solid-state circuit that can be had, as it is truly "tube" sounding, with zero of the disadvantages of tubes (other than the heat produced!).
The one and only Nelson Pass also prefers Class A operation and has gone on record saying that he prefers Class A to all other circuits. He says that "it allows the best, most linear performance from very simple circuits, and the best sound comes from simplicity and linearity." He adds that this simplicity also makes design, manufacture, and reliability a lot easier.
It is easy to hear the advantages of Class A circuit design, but because of the nature of the beast, high wattage Class A amplifiers are less common in many domestic situations. Because of their heat production, Class A amplifiers require heavy-duty hardware compared to most other approaches, but this also comes with the benefit of being able to drive speakers that would normally require higher wattage. Class A amplifiers conduct high current even at idle, and so they end up sending much of that current to the speakers. I've heard audiophiles attempt to be funny by stating something like "this amp isn't rated at xx, Watts, it's rated at xx Class-A Watts!".
One of the reasons Class A amplifiers sound so great is that they have very simple linear topologies. Fewer parts in the signal path equal greater transparency. All this, along with Pass Labs' reputation for very high-quality internal components and reliability, make this integrated amp attractive inside and out.
Pass Labs strategy includes replacing banks of smaller transistors with a single pair of FET devices that have a 700 Watt / 40 Ampere rating. With no "current-hogging" issues and a new constant-current bias circuit to compensate for temperature drift, they connect directly to the loudspeaker without ballast resistors for the lowest possible distortion and highest damping factor. No degeneration also increases the efficiency and Class A operating current, allowing greater Class A operation into low impedance and reactive loads. No degeneration also allows for a more pure "square law" operation, which as with tubes, improves dynamics and will also improve distortion cancellation.
I was lucky enough to be able to use the INT-25 with many different pairs of speakers, and to my surprise, or perhaps It shouldn't have been a surprise, that the INT-25 was not only able to drive each of them but sounded great on all of them.
Even though in their literature Sonus faber recommends that the amplifier rating should be at a minimum of 60 WPC, I suppose that this doesn't apply to the number of watts that are coming from a Class A Pass Laboratories amplifier. This comes back to the statement made to me by Nelson Pass, that Class A amplifiers have (because they require) bigger hardware compared to the other "low powered" amplifiers, and that they are already conducting such high current, even at idle, and this higher amount of current makes its wattage rating less important.
All this wouldn't matter much if it couldn't be proven in my listening room, but not only could the INT-25 drive the Sonus faber Olympica Nova V speakers, but the music coming out of these speakers sounded marvelous. I used the INT-25 when I reviewed the Sonus faber speakers within last month's issue (November 2020), and believe me, I also used the INT-25/Sonus faber combination when listening to music in my "free time" (there is no reason I need to explain why "free time" is in quotes, do I?).
Other speakers I used during the INT-25's review period included a pair of inexpensive, but rather large pair of "vintage" Magnapan SMGa flat panel speakers, which usually only sounded good with higher-powered amplifiers, and also a pair of my reference two-way floor-standing EgglestonWorks Isabel Signature, which also sounded marvelously connected to the INT-25. These speakers, along with the Magapans, needed a bit of bass reinforcement with the SVSound SB-2000 subwoofer I had on hand, an inexpensive but surprisingly good sub, especially when all that's needed is a bit of augmentation in the deepest bass.
The EgglestonWorks are very transparent full-range speakers, models that I've recommended to many audiophiles who have a small listening room but prefer floor standing speakers over a stand-mounted pair. They were able to demonstrate the magic that this Pass Labs integrated amplifier was able to inject into the music. It did this not by exaggeration of any of the qualities that were in the recording, but instead, by bringing out the best in the recording, qualities that one might not even know were present.
The sonic magic that the Pass Labs INT-25 was able to deliver was made very evident as I was playing just about every "Krautrock" fan's favorite album, Tago Mago, by the early 1970s German band Can. I played two copies of this album that I had on hand, the first an SACD that was released in the early ‘00s, played on an OPPO BDP-83 Special Edition universal disc player, and a Japanese pressing of a double LP played on a Pro-Ject X2, using an Allnic Rose phono cartridge (review forthcoming), connected to a Pass Labs XP-15 phono preamplifier.
The Pass Labs INT-25 was able to separate the instruments on this sometimes crowded, direct to a two-track recording, not only by isolating each instrument and voice within a huge soundstage, but separating each instrument and voice with dynamic distance, that is, unaffectedly separating them from each other, despite their appearance in the same area of the soundstage, and at the same time emphasizing very slight differences in their apparent volumes. The INT-25's level of dynamic distance is a type of trait that I normally hear with the best amplifiers and equipment powered by vacuum tubes.
On the leadoff "Paperhouse", Holgar Czukay lays down his Fender bass, energetically plucking its strings with his fingers, each string slapping against the instrument's fingerboard, the bass guitar's low-end thud mixing with the hypnotic motoric drumming of Jaki Liebezeit. This "you are there" recording still isn't what one would call a typical "audiophile" recording, yet its two-track origins have been praised by many an audiophile. The many notable sonic traits of this particular Can album are easily passed on by the Pass Labs INT-25 to any of the speakers that I had in my system during the review period.
Regardless of which speakers I had in the system when I played "Tago Mago" I was drawn into their world of rhythmic complexities mixed with psychedelic overtones. The INT-25 was able to let me "see" deeply into the recording, creating space between the sounds that enabled me to hear sounds around me as I passed deeper into the sonic labyrinth that the band created. On "Paperhouse", Michael Karoli's single-string fuzz-guitar floated atop the proceedings, as vocalist Damo Suzuki's Japanese accented English turned his lyrics into rhythmic sounds more than words, as it was intended, adding to the pulse that also highlighted the percussive aspect of leader Irmin Schmidt's electronic keyboard.
I suppose I'm choosing yet another non-audiophile-approved recording in which to demonstrate the Pass Labs INT-25's prowess in being able to take an "ordinary" recording and make it more than listenable due to the way it takes the music and expands its vista to the listener. Not only is it able to separate each instrument, sound, and voice -- but along with this, make each of these sounds more "visible" to the listener.
The signal of Carlos Alomar's guitar travels through the INT-25, and its circuit seems to be able to think, "Oh, that's Carlos Alomar's guitar!", and so it makes more of the nuances of his playing style more evident when the sound of his guitar comes out of the speakers into my listening room.
I hope this obtuse way of describing the sound of the Pass Labs INT-25 doesn't seem too odd. I also hope one understands that a piece of audio gear such as the INT-25 deserves and take advantage of the best and most appropriate speakers one can afford, and the best and most appropriate front-ends one can afford (at least eventually, as often assembling a system is a process).
Which gets me to the next important thing about the Pass INT-25: it is a relative bargain. I am aware that not everyone has $7,250 lying around. But, I'm convinced that there is no way one is going to be able to put together a preamplifier/power amplifier combination, let alone acquire an integrated amp, that will sound as good as this integrated amplifier anywhere near its asking price. One that has more wattage? Sure. Has more features? Probably. Sounds as good? No way.
The Pass Labs INT-25 isn't the first audio component I've heard over the years that seems to have some sort of magical circuit where it "knows" what type of instrument, voice, or sound is passing through it. But it is definitely one of the most affordable integrated amplifier or amplifier that can perform this feat at this level.
I can understand why one would add a speaker system and front-end that costs more than the INT-25. As I mentioned previously, I had excellent results using any of the speakers I had on hand during this integrated amplifier's review period. But it sounded a hell of a lot better when using the Sonus faber Olympica Nova V at $16,500 than it did with my vintage Magnapan SMGa, which if one can find a pair in working condition can be had somewhere between $250 and $400, depending on its cosmetics. The same was true with its front end, as the Pass Labs INT-25 could easily pass on its traits because of this integrated amplifier's astounding level of transparency.
But this integrated amp was reviewed in a high-end audio magazine by someone who considers themself a serious audiophile, so that's how I reviewed it. Its sound is first-rate, but the INT-25 is lacking in intangibles. What's missing? Pretty much everything. There are no XLR ins or outs, no subwoofer output(s), no tape loop, etc., etc. Not only would the INT-25 cost more if more features were added, but after considering this for only a short time, I remembered what Nelson Pass said -- that the best sound comes from simplicity and linearity. And so, the Pass Laboratories INT-25 lack of extra features, not to mention its Class A circuit, is an indication that its simple signal path from input to output is very likely responsible for a huge part of why it sounds so darn good.
Even though the Pass Laboratories INT-25 integrated amplifier might be missing just about all the mod cons, it is a component that I would think that many audiophiles would not only love to use in a second system, as I did, but I could easily imagine it being used as one's resident amplifier as the centerpiece of one's high-end audio system. It isn't perfect, as it is missing many of the conveniences that many modern listeners expect these days. But these things are easy to overlook and work around. This is because the Pass Laboratories INT-25 is a fantastic sounding component that should be considered by anyone who loves music.