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 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".
"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
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.
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
Quality of packing: Very stout.
Source: APL HiFi NWO 3.0-GO
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