June 2009

Pass Laboratories INT-150 Integrated Amplifier

by Uday Reddy

For more than 30 years, Nelson Pass has established a first-class engineering reputation, initially with Threshold Corporation and then with the company that bears his name, Pass Laboratories. Pass also produces low-power amps under the First Watt name and supports the DIY community with advice and project plans. To top this all off, he’s a genuinely nice guy, proving again that some people have all the luck.

Having first established his reputation with class-A amplification, Pass also produces class-A/B amps, and the company's current production includes its first integrated amplifier, the INT-150. Essentially a Pass Labs X-150.5 stereo amplifier mated to an XP preamp, the INT-150 incorporates technologies first developed for the Pass Labs X.5 series of amplifiers. These include Super-Symmetry, which produces high power and performance from simple circuits. The INT-150 is fully balanced, another Pass design point, in order to take advantage of such a circuit's ability to cancel out noise. Unlike the class-A, zero feedback amplifiers of the Pass XA.5 series, the INT-150 uses minimal feedback. The INT-150 will run in pure class A, up to 13Wpc, thereafter running in class A/B to its full output, stated as 150Wpc.

After spending the past 12 months reviewing costly products with an average retail price of almost $15,000 USD, it’s nice to get back to the real world -- sort of. The INT-150 is not inexpensive, but at $7150, it’s certainly more affordable than the other integrateds I've been covering, though no less interesting.

Features and specs

The INT-150 is a minimalist integrated, with controls for power on/off, mute, input selection and volume. That’s it -- no tone controls and no phono stage. Rather industrial-looking compared to other Pass Labs products, it’s still handsome, if a bit somber. The minimally sculpted front panel has a centrally located luminescent display with large characters that are easy to read at a distance and display the input selected and the volume level. Below the display, arrayed from left to right, are the power button, mute button, buttons for four inputs and the large volume control with a 63dB range in 1dB steps. All functions, including balance, can be controlled from the machined aluminum remote control.

The upper portion of the rear panel has, from left to right, RCA and XLR preamp output jacks, both RCA and XLR input jacks for inputs 1 and 2, but only RCA inputs for inputs 3 and 4. The lower portion of the rear panel has a centrally located power button, flanked by an IEC power-cord receptacle and a fuse holder. Rounding out the rear panel is a signal ground and horizontally arrayed left and right plastic-shrouded brass binding posts. These binding posts have to be amongst the worst I’ve ever experienced. Because they are placed so low on the rear panel, only bare wire is easy to use and banana plugs are not an option. Spade lugs, particularly on stiff speaker cables such as my Transparent MusicLink, are very difficult to connect and have to be inserted from above and tightened with a binding-post wrench.

The speaker binding posts led to a story. Two weeks into the review period, the INT-150 went dead silent, even though it remained powered up. Although I had been listening at reasonable levels, I suspected a blown fuse and replaced it with one supplied with the amp. I was able to power up the amp once again, but as soon it was fed a signal, the amp went silent within 5 seconds. The amp was shipped back to Pass Labs and a new one was shipped to me, which worked flawlessly. Evaluation of the initial unit showed nothing wrong with it after replacement of the fuse, and the suspicion at Pass Labs is that because the binding posts are so close together, the spade lugs must have come in contact with each other, resulting in a short circuit. As the first instance occurred the day after my housekeeper had cleaned in the listening room, I’m assuming that she knocked the cables while cleaning behind the amp.

After hearing about Pass Labs’ suspicions, I mentioned the poor design and layout of the binding posts and suggested a vertical array like those on my Jeff Rowland Concentra. The good news is that Pass Labs liked my suggestion so much that future production will have this improvement. Nelson, you know where to send the royalty check!

The INT-150 is beefy both in power output and in dimensions. Rated at 150Wpc into 8 ohms and increasing to 300Wpc into 4 ohms, the INT-150 should have no problem driving most speakers. Frequency response is rated at -3dB from 1.5Hz to 60kHz. Slightly wider than a standard rack width, the INT-150 measures 19"W x 7"H x 19"D and weighs 60 pounds, giving you a tangible sense of owning something for your hard-earned money.

You'll want to give the INT-150 plenty of space. Although its amplifier section runs in class A/B amp, it still gives off plenty of heat. Its sleek, widely spaced heat-sink fins and top plate never got too hot to touch, but the unit still ran hotter than any other amp I’ve had in my room, so much so that there was a noticeable difference in temperature whenever I walked into the room and, with prolonged listening, I had to crack a window open to let the warm air vent out.

The first INT-150 that I had received was very cool-sounding and analytical for the first 24 hours, then settled in with no appreciable change in character thereafter. The replacement was run-in at the factory for 48 hours prior to shipping, with the result that none of the initial coolness or analytical quality was apparent. Apart from the hiccup with the first amp, I had no further problems.

Sound

Like most well-designed integrated amps, the INT-150 is quiet, with sonic images that emerge from a black background. The soundstage was rendered deep and wide, with sound extending well beyond the outside perimeter of my speakers. Given the narrow width of my listening room, this was not only a pleasant surprise but also certainly better performance than that of any of the other amps that preceded it.

However, one thing that I noticed right away was that the soundstage was more diffuse, instead of the usual holographic, three-dimensional performance typical of most high-end equipment. While certainly not veiled or gauzy, this was atypical of my experience with other integrated amps, which have more of a laser-like focus. At first I found this a bit off-putting, but the more I listened, the more I realized that this is what most live music sounds like. Unless you’re sitting in the front row of a jazz club or next to the orchestra section of a concert hall, much of the sound is going to be reflected, which seemed exactly the kind of perspective the INT-150 provided. Once I got used to this, I enjoyed it and found myself listening more to the music alone, paying less attention to the placement of musicians and their instruments within the soundstage.

I’ve often noted when evaluating a component that once I get used to its sound, I’ll favor one type of music over another with it. Because I listen mostly to jazz these days, I was quite surprised to find myself listening to a lot of rock'n'roll and jazz fusion with the INT-150. Because rock recordings are usually overly compressed, I find them fatiguing after just and hour or so and end up switching over to jazz or classical music. With the INT-150, I could listen for longer periods and derived more enjoyment from the experience. I’ve been a fan of the Black Crowes since their debut release and have kept buying every album, hoping that they’ll equal or exceed the work on their sophomore release, The Southern Harmony and Musical Companion (American Recordings CK 69398). "My Morning Song" is a harmonically dense, grinding jam with layers of sound. The INT-150 handled this track well, without breaking a sweat and showing no congestion or loss of clarity, even at high volumes. And when lead guitarist Marc Ford tears into his ecstatic, sinuous solo, it was relayed distortion-free (except for whatever distortion Ford was adding to it, of course).

One night, while my wife and daughter were out of town, an audiophile buddy and I spent many hours drinking beer and listening to one Stevie Ray Vaughan album after another. Like his hero Jimi Hendrix, Vaughan had monster chops but a lousy singing voice, flawed with slurring and lack of enunciation. The INT-150 did nothing to improve this, but it did seem to de-emphasize these qualities so that they were less noticeable. More obvious was the reduction in the stridency of his voice, readily apparent on "The Sky Is Crying" from Blues at Sunrise (Epic/Legacy EK 63842). I have always loved this tune, but the singing was somewhat grating to me. Via the INT-150, this grating quality was softened ever so slightly, easing the rough edge. Lest you think this was due to "beer ears," rest assured this result was still present with repeated listening. This improvement in Vaughan’s vocals would seem to indicate some softening in the midrange, but I didn’t notice anything particularly wrong with the midrange, which was tonally neutral with no perceptible sonic character.

Dynamically, the INT-150 performed strongly, especially with large-scale orchestral recordings, in which the momentum of the piece was preserved, even in quiet passages. On the "Mars, the Bringer of War" section from Gustav Holst’s The Planets played by the Philharmonia Orchestra and conducted by John Eliot Gardiner (Deutsche Grammophon 445 860-2), the piece begins with a low-key but incessant march that builds to a cacophonous crescendo. This brooding sense of impending disaster was captured perfectly, the INT-150 never having to strain. However, the INT-150 was quite capable of being light on its feet when called upon. On "Jupiter, the Bringer of Jollity" from the same recording, the opening fanfare gives way to a flowing melody that feels joyous and hearty, then segueing into a dainty and delicate segment through which the INT-150 conveyed the work’s innate gaiety.

One of the reasons that I preferred rock and jazz fusion with the INT-150 was due to the integrated's forceful bass output. Like Brian Bromberg, Christian McBride is a monster bassist, with both acoustic double bass and electric bass guitar. On Chick Corea and John McLaughlin’s Five Peace Band Live (Concord Records CRE-31397-02), McBride’s playing is propulsive, and the INT-150 highlighted this quality, capturing his ability to anchor the bottom end and flesh out the rhythm section.

My only complaint with the INT-150 lay in one aspect of its treble reproduction. As I’ve mentioned before, I listen to a lot of jazz, especially piano-trio recordings. Through the INT-150, the upper registers of the piano sounded hard at times. I am at a loss to explain why this was the case only with piano when all other aspects of the INT-150's treble were reproduced cleanly and with fine detail. Also, why was this accentuated more via the INT-150 as compared to my Jeff Rowland Concentra and all the other integrated amps I’ve evaluated over the past year? Perhaps different cabling would have made a difference.

Comparisons

As I mentioned, I really enjoyed the bass reproduction of the INT-150. Although it didn’t have quite the bass control of the Luxman L-509u ($10,500), the best I’ve heard yet, it was at least as good as that of my Jeff Rowland Concentra ($5600 when available) in many respects and exceeded the Concentra when it came to the tunefulness and ability to follow the bass lines. While both the Concentra and the INT-150 are tonally neutral, the Concentra leans more toward the warmer side of the spectrum in the midrange, but I didn’t feel that the INT-150 suffered in comparison. I did prefer the treble of my Concentra, which was more liquid and tubey in its quality, but the INT-150’s ability to reproduce the upper frequencies is still comparable -- with the one caveat regarding the upper registers of the piano. Even if you listen to a lot of piano jazz or piano concertos, I don’t believe this character is a deal-breaker. Still, careful system matching and cable selection are important.

Conclusion

Despite my reservations with regard to piano reproduction, I thoroughly enjoyed my time with the INT-150. As for its presentation of the music, I really came to appreciate the INT-150’s more diffuse staging, and I suspect that others will enjoy this too. You just have to give it time so you can get used to the perspective, which, to my ears, replicated the live experience in an obvious way.

And here’s something else. Whenever I review a component, I consider the following situation: If what I own fails tomorrow, would I consider the piece under review as a potential replacement? With the INT-150, I’d have to say the answer is a wholehearted "Absolutely."

...Uday Reddy
uday@soundstage.com