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Reviewer: Srajan Ebaen
Financial Interests: click here
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

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.

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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.

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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.]