This review first appeared in the magazine. You can also read this review HERE in its original version. We publish this article in a mutual syndication arrangement with publisher Srajan Ebaen. Everyone can write to him – if you have questions or wish to send feedback – you’ll find an e-mail address under his name. All images contained in this review are the property of - Ed.


Srajan Ebaen

My inbox showed a message from Subas Audio. Hans I Jonsson was quick and to the point: "Thank you again for your ongoing work in the audio community. I must say that I am a bit surprised to see your non-use of the Superior X.5 or XA.5 series from Pass Labs. If you like I can send you an integrated Pass Labs INT-150 for comparison with your F5 and expensive preamp setup." Asked whether he'd compared the Pass integrated and First Watt amp, he admitted no. "The coherent (?) reasoning builds on the assumption that the SuperSymmetry concept reserved for the industry products is the solution to seek. Much lower intermodulation distortion is indeed detected in large and complex musical passages. The F5 is a fine amplifier. Maybe the finest. But in the case of the INT- 150, you have his buffer volume control and the SuperSymmetry Endstufen. My idea was to interest you to investigate the much more expensive F5/ModWright against a simpler/cheaper and more versatile solution like the remote-controlled and balanced Pass Labs INT-150.

"I do love this amplifier and know that too many trophy audiophiles would miss the chance to instead dwell in endless comparisons over amplifier combinations intertwined with cable semantics. But maybe the sonic results of SuperSymmetry can enlighten the road to Damascus. Keeping the solution simple does include the overlooked integrated amplifier. In this case both with buffered and remote controlled volume and a fully balanced high-watt amplifier that performs much better in micro- and macro-watt levels than the single-ended 45, WE421A, 2A3 and 300B amplifiers I have heard and owned. Less number of almost linear amplifier stages and an optimum use of gain is the cure. No other product has this unique combination (buffer line stage/SuperSymmetry power amp) of intrinsic value. The outer looks and features put it however in a never-ending conga line of me-too products. If you want, I can send you one to test. I am still crazy enough to enjoy a lot of different amplifiers in my music listening praxis. But the INT-150 is much less fuzzy. People still talk about the Aleph 3, 5 or 1.2 today, blissfully ignoring the SuperSymmetry solution. I really do not understand the audiophile market."
With my arm twisted to breaking point, I needed Hans to ease off pressure. "Okay." Truth be told, I had been mighty curious about the XA.30.5 and compadres. But what with having worked my way through Nelson's entire First Watt line, I didn't want to seem greedy. Let others graze on his other meadow. Yet having a Swedish importer intercede surely was a sign. Like cheating without getting caught. It was time also to revisit less is more and more for less.

With the INT-150, that's of course relative. And on a number of fronts. 7K isn't small fry. Nor are 150/300wpc into 8/4 ohms. Nor 120.000 microfarad capacitance, 10/20 amperes RMS/peak current, 20 Mosfets per side and a hefty Plitron toroid. In fact, this chassis houses a complete, fully balanced X150.5 2-stage amplifier circuit with 32dB of gain, an input selector and volume control related to the new XP-10 linestage and a nice display with two intensity levels plus off. On the less-is-more front, enter DC coupling to avoid any and all capacitors in the signal path. Class A operation to 10 watts. A 64-step resistor-relay attenuator with discrete 1dB steps and balance offset via remote. Preamp outputs for biamping on RCA and XLR. And THD performance the German Hifi & Records measured as being superior even to the big X250.5, detecting only very low levels of 3rd and 5th-order.

As the quintessential American transistor amp designer, Nelson Pass had not authored an integrated before even though the big boy competition -- Krell, Plinius, simaudio, Rowland et all -- had. Why now the Pass? Should one suspect Nelson's work on the new Pass Labs preamps and the parallel First Watt B1 JFet buffer? "No, there is little relation to the B1 outside of the requirement for a buffer at the output of the volume control, since the intrinsic input impedance of the amp is 10 or 15 K per input leg. There is some similarity to the XP10 but not enough to talk about." On the input stage, "yes the details are scarce, and deliberately so".
So, the INT-150 is a 4-input, high-power, remote-controlled, dual-differential integrated with an input buffer. It's more than an amp + passive pot, less than an amp + active preamp. Its half-wave amplification circuits are 'super' matched -- that's the patented SuperSymmetry bit you can read up on here -- to cancel distortion across the speaker terminals rather than rely on steep feedback. Operation is class A/B, with the transition occurring around 10 watts. Hence low-level listening over reasonably efficient speaker will occur mostly or entirely in class A. Below 2 watts in fact, it'll occur in parallel single-ended mode. "In 1991 Pass Labs developed a hybrid class topology which paralleled a push-pull Class A output stage with a current source which biased it into single-ended Class A. The Aleph 0 amplifier operated as a single-ended Class A amplifier to its output rating of 75 watts into 8 ohms, and at currents beyond that it continued to deliver current as a push-pull Class A circuit.

"Subsequently in the X amplifier series we retained a small amount of single-ended Class A bias on our output stage as a means of controlling the amount and character of the distortion at the lowest power levels - that all-important first watt." Without too much license, one could call the INT-150 a convertible which morphs from class A single-ended micro-power mode with 20 paralleled output devices per side to high-power class A/B push/pull and back on full automatic. [For more information on "Leaving Class A", refer to Nelson's article.]

At first, I ran the INT-150 against the F5 off the ModWright DM 36.5 preamp. The 150's attenuator trimmed its higher gain by hovering around 47 on the display. Now the valve pre's volume control setting tracked both amps the same. I just swapped leads and compared. Naturally, the whole integrated notion is to decommission preamps as a breed. Hence later sessions removed the ModWright. That was the scenario our Swede wanted tested, that was the scenario most readers would want to read about. To set the stage, Critic's Corner on the Audio Asylum netted a telling bit by Morricab, former Positive Feedback reviewer: "...One name that Stereophile tends to drool over that leaves me far from impressed is Musical Fidelity. Another is Pass Labs (at least the X series amps). I have a lot of respect for Nelson's ideas and writing but the sonics of his current products IMO don't live up to the ideals. I haven't heard the First Watt stuff but I think it could be a lot better sounding..."

For 'approved' electronics, we get KR Audio, Lamm, Convergent, BAT, Vacuum Stage, Acoustic Plan, Silvaweld, Einstein (CDP excepted), Antique Labs' Hurricane, Edge NL Reference amps and battery preamp, Weiss digital and Monarchy Audio amps and DACs. Morricab, for amplifiers, thus favors full-on valve designs or as with Acoustic Plan and Monarchy, hybrids. He hasn't heard the Pass Labs XA amps but allows them chances. Translation, they're class A. He hasn't heard the FirstWatt amps but thinks they could be closer to his tastes. Again, most the Fs are As. Plus, FirstWatt is esoteric niche stuff, Pass Labs muscular mainstream. Serious audiophiles could thus exclude the INT-150 from consideration for being "mainstreamish, proletarian A/B and without valves". We're back to Hans' opening gambit.

Without at this stage knowing whether the ModWright preamplifier compromised or upgraded the INT-150's performance with redundant circuitry or commendable signal conditioning, there was a distinct sonic family resemblance to the F5. But the smaller, lighter and far cheaper amp booked a sonic lead. With hi-rez 24/96 DVD-A source material compliments of Aix Records and Laurence Juber's Guitar Noir played back via 20 paralleled 32-bit AKM converters per channel, this played out as
More tonal substance or inner warmth
More elucidation of the percussive background accompaniment
A more suave top end on metallic ring-outs
A soundstage that had moved farther behind the speakers by a few feet
A cooler, thinner, sharper and 'bluer' overall gestalt for the INT-150 in turn
Extricating the ModWright from the chain, the INT-150 lit up. Its innate bluishness didn't really change but it became more robust, substantial and immediate. It suggested quite conclusively that the signal detour through the preamp had been detrimental to prevent the 150 from fully opening up. Resolution improved. An interceding filter or block had been removed, stalled energy liberated. In musical terms, the focus now was on crispness, precision, separation and focus. This was a highly optimized, undiluted but civilized 3rd-order sound. That's how Nelson Pass references amplifiers whose remaining low THD is mostly 3rd and some 5th as opposed to SETs and other critters which are 2nd order (and up) machines. The sound now had one essential core quality from the SET camp: Unmitigated directness. Very un-SETish remained the absence of inner radiance. That's a triode effect some relate to as warmth; others as higher color temperature; other as saturation and others again as ripe tone or wet textures. It also telegraphs as spatial suchness. That isn't so much holographic from a sight/eye sense as it is 'deep space' from a feeling dimension. Whether such qualities are ultimately FX tricks, colorations or unpredictable distortion interactions remains irrelevant. Triode lovers recognize and fancy them. The 150 is texturally far leaner, drier and more 'matter of fact'. It doesn't cater to that big part of the triode gospel.

Neither does the F5. Fully. But it comes a bit closer. And then of course there is deliberate seasoning with a valve preamp. For perhaps the ultimate La Triode dose I've come across in that sector, think Thorens TEP 3800. Such preamp seasoning will certainly shift the balance. Another way to differentiate Inty's gestalt from premium SETs -- many lesser SETs are very sloppy -- is inner flexibility or buoyancy. Superior triode amps perform a tangibly breathing, wave-lapping action. The Pass is unwavering and solid. Factual. That's different. It celebrates what perhaps might be called a more cerebral sight-based clarity. There everything is assigned its proper place to be well-sorted, accurate, defined and precise. Unfazed. Crystalline nearly. With that, a cool temp reading. And great transparency. Morricab-type listeners will miss the minor sweetness and a certain inner redolence of enhanced tone compared to my DM-36.5/F5 combo. Or, they could snub their noses altogether and call anything less than my Yamamoto A-09S insufficient. Conversely, I also appreciate Hans I Jonsson tiring of lesser triode sound especially if he failed to arrive at an amp/speaker combo that truly meshed to compromise the stuff he wants to listen to. So I can appreciate why he likes the INT-150 but I favor what I already have more - on the Tango R.

The intermediate scoop.
One definite strong suit of the INT-150 is dynamic brio. Its reflexes are very well oiled. If you need close-miked hammer falls of a Cuban piano ace to thunder with appropriate percussiveness; rim shots to crack; popped power bass strings to go pow; you're home. In fact, think 'fast sound' in general. It's very decisive, direct and to the point in all regards. Those adoring harmonic padding will find this kind of directness a bit too stripped down, bare-boned or clean. (That their preference would then be dirtier by implication is just one of many hi-end peculiarities, syntax and otherwise.) But even those folks would have to admit that such non-abrasive, non-bracing clarity moves volumes needed for clarity downward. One can listen later into the night without stirring those already asleep. Inherent in all that clean clarity is a very grand soundstage of superior intelligibility. Image allocation is very specific but not chiseled. For that, the high frequencies are too benign. Bass is well damped and powerful. By virtue of the overall coolness, vocals, though very direct, lack that peculiar intimacy I get from the Emission Labs 300B XLS in my Yamamoto. It's another interesting bit of syntaxation how directness or immediacy need not equal intimacy. Immediacy yes, intimacy no. This was more of a captive voyeur's than immersed participant's perspective.

Obviously, this amplifier is in control. You notice that during crescendos macro and micro, from orchestral forces gathering for the final swell to the emphatic upward rush of a guitar arpeggio that peaks with a fat vibrato on the highest note. You notice it during complexity when diverse strands remain discrete and don't blot out. What you won't get is some of the 'connective tissue' that valve amps which are otherwise neutral enough insert between the notes. Whether that's an overlap of harmonic halos; low-level noise like subliminal tape hiss that seems to raise contrast ratios; or some other octave-doubling interactive side effect - the INT-150 banishes that stuff. Again, great clarity from perhaps superior noise performance and low colorations.

Because my ASI Tango R speakers run brilliantly on 8 watts of premium triode, any relevant test of an integrated muscle amp would of course run speakers thirsty for higher power. The arrival of the 85dB Davone Rithm from Denmark provided just the right opportunity.

To truly square the circle, ModWright's KWA-150 arrived as well. For convenient comparison between the two American amps, I defaulted to Esoteric's C-03 transistor preamp. It was set to zero gain to essentially become a passive preamp, albeit run off 38V rails. Final conclusions would again report on the INT-150 driven direct from the source but for the amplifier comparison, I wanted a simple cable swap scenario. Now the Pass amp clearly shifted into a far more optimal torque zone of its engine when asked to put out more and handle a grown-up load.

Clearly, now I had the right job for the tool at hand. The equivalently powered KWA-150 maintained context and a side excursion via my DeVore Fidelity Nines established more to gauge how these amps compared. We'll jump straight to the conclusion. The INT-150 is very capable in the bass, getting from a 7-inch SEAS coax driver in a small though very clever enclosure far more LF extension and heft than seems possible. It's perhaps appropriate to call this aspect heroic. This integrated combines reach, pressure and control and is of proverbial muscle amp caliber.

The INT-150 also gets fleshier and fuller as load requirements increase. Compared to the ModWright KWA-150, the Pass integrated puts more emphasis on the speed, impact and attack elements of the performance. This pertains to the entire bandwidth but particularly in the bass creates a wiry mien when called for. While the KWA-150 extends just as low, its overall character is more relaxed and includes a modicum of warmth. This emphasizes the weight aspect over impact.

The treble quality between these powerful amplifiers is different too. The Pass is black coffee, the ModWright adds some sugar and just a whiff of cream. In keeping with the caffeine image, the INT-150's top end is more energetic to key in on quick alertness while the KWA-150 tones down the illumination some for less highlighting of recorded reflections. In the eternal mix'n'match game, the ModWright + DeVore Nines created a very comparable set of strengths to the Pass Labs + Davone Rithm combination. That's because the 0.75" tweeter of the Americans is the more lit-up and finely nuanced operator over the 1" concentrically loaded Dane; and the enhanced mid-bass heft of the Rithm is better complimented by the thinner blood of the Pass than the inherently leaner and more linear Nines.

Adding it all up
The first integrated amplifier from Nelson Pass majors on dynamics and speed, a very informative treble and exacting soundstaging. Transient reflexes are very keen to create a subjectively cooler climate but some warmth comes into play as the load burdens increase to get the amp out of 1st gear. Apparent resolution is very high to translate into great visibility of and into the virtual stage. Bass is prodigious in the best sense of the word. The INT-150 sounds completely in command which minimizes the legato effect -- the ebb and flow -- one recognizes from low-powered tube amps. That is replaced by factual imperturbability. Presumably highly load-invariant into more challenging speakers, the same does not exactly apply in reverse. If my experiments translate more widely, sonics bleach out into non-reactive loads. This is not a 1st-to-last watt egalitarian then. This machine comes into its own as it begins to 'pull' some real speaker weight rather than 'rev empty'.
The deliberately color-shifted shot above reflects the somewhat distanced cooler 'Scandinavian' personality of the INT-150 into less copasetic low-watt speakers. This warms up with appropriate transducers but the underlying core virtues of incisive articulation, high resolution and great organizational power remain stable. Likely due to its very low distortion and formidable power reserves, the INT-150 is ideal for complex large-scale works that rely on unerring separation and stability; and a machine par excellence for bass freaks. You could remain ignorant of your smaller speaker's true LF virtues until it ends up in the grip of the Pass to then rewrite secret notions about needing bigger and more expensive boxes. This is also a highly dynamic amplifier which tracks voltage fluctuations very tightly to peel out of the surrounding thicket momentary spikes like percussion hits and bass accents in high relief. In fact, high relief is a perfectly suitable all-around abbreviation to summarize the INT-150's very linear sonics...

Quality of packing: Very stout.
Reusability of packing: Multiple times.
Ease of unpacking/repacking: Easy.
Condition of component received: Perfect.
Completeness of delivery: Power cord, owner's manual, remote control.
Quality of owner's manual: Very good.
Ease of assembly: None required.
Website comments: Recently updated. Very good.
Human interactions: Always professional and friendly.
Pricing: By containing a full-bore X amp and high-quality buffered preamp section, not as high as apparent at first glance.
Final comments & suggestions: Not its best on 'wimpy' speakers. As true muscle amp, wants to be put to work to fully come alive.

Source: APL HiFi NWO 3.0-GO
Preamp/Integrated: ModWright DM 36.5; Esoteric C-03
Amp: First Watt F5; ModWright KWA-150
Speakers: Acoustic System Tango R; Davone Rithm [on review]
Cables: Acoustic System Liveline interconnects and speaker cables; Crystal Cable Ultra loom; Crystal Cable Reference power cords
Stands: 4 x Ikea Molger with Ikea butcher block platforms and Acoustic System footers
Powerline conditioning: 2 x Walker Audio Velocitor S
Sundry accessories: Furutech RD-2 CD demagnetizer; Nanotech Nespa Pro; full-house installation of Acoustic System resonators, noise filters and phase inverters
Room size: Sound platform 3 x 4.5m with 2-story slanted ceiling; four steps below continues into 8m long open kitchen, dining room and office which widen to 5.2m with 2.8m ceiling; sound platform space is open to 2nd story landing and 3rd-floor studio; concrete floor, concrete and brick walls, converted barn with no parallel walls nor perfect right angles; short-wall setup with speaker backs facing the 8-meter expanse
Review Component Retail: $7,150




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