Pass Labs INT-150 integrated amplifier

Ten thousand hours. According to Malcolm Gladwell, in his book Outliers: The Story of Success, it takes 10,000 hours of practice to become truly exceptional in your field. For any endeavor, Gladwell contends, what will get you to Carnegie Hall isn't inborn talent but practice, practice, practice.

I can relate. I've worked hard to put in my 10,000 hours as a critical listener, both as an audiophile and as a full-time, professional musician. In one area, however, I feel I'm still working up to that 10,000 hours: writing reviews for Stereophile. Few people can say they've spent 22 hours a week for 10 years actually writing about high-end audio. (Well, I suppose John Atkinson and a few others on the Stereophile staff can.) But as I come up on my first anniversary of writing reviews for this magazine, I know I'm still learning how to tell you, dear reader, what I'm hearing in my listening room in the most accurate and useful way possible.

By combining in equal measure factual reporting, descriptions of what I hear, comparisons with other, similar products, and my own opinions, I hope that each method of describing a product fills in one shade of the paint-by-numbers portrait that is an audio review. I know there's more to it than this, and that the devil is always in the details, but I hope with each review I write to become more in tune with the ins and outs of communicating what I've trained myself to hear, and to help the readers of this magazine on their own journeys.

There—I've just put an hour into writing this. I wonder what my approach will be in another 9999.

He heard, he listened
Nelson Pass founded Pass Laboratories in 1991 after leaving Threshold. At the time, he was intrigued with the idea of making class-A–biased, mainly single-ended solid-state power amplifiers. Unlike the Krells of its day, the Aleph series of amplifier, Pass Labs' first products, found a home with audiophiles who wanted to get tube sound from transistors. The Alephs did very well over their run of nearly 10 years.

But somewhere in that decade, Pass hit a critical mark in his own life as a listener. As he told me in an illuminating interview, "Over the course of the last 40 years, I finally got my magical 10,000 hours of listening in, and this has given me a more mature viewpoint on a lot of things. The ability to listen has become a larger part of the picture for me when designing and manufacturing products." Pass credits the development of his current X series of amplifiers, and their Super-Symmetry circuit, not only to his constant curiosity as a designer of electronics, but to his increased powers of listening.

The Super-Symmetry circuit, featured in all X-series models, works much like a balanced circuit. Essentially, the two halves of the signal are compared with one another in a balanced differential output, a small bit of feedback is applied to better match the signals, and the leftover distortions are naturally canceled out by the circuit. The circuit has been around for a while, but at the end of 2009 Pass Labs released its very first integrated amplifiers, the INT-30A and INT-150 ($7150), which feature this circuit. According to Nelson Pass, the INT-30A and INT-150 are, respectively, the XA30.5 and X150.5 power amplifiers, but with multiple inputs and a high-quality volume control. The INT-150 puts out 150Wpc in push-pull class-A/B, while the INT-30A produces 30Wpc in full class-A with a balanced single-ended output.

There is no preamplifier per se in the Pass integrateds, but instead, a slightly modified version of the gain stage found in each of the regular Pass amps. As Nelson Pass told me, "First there is a selector switch, which is just relays, and then there's a buffer that drives the volume control. The output of that goes to the amp, and then you're done. There's not a lot there."

How come no one ever wants to gap the bridge?
According to its owner's manual, "The INT-150 was designed specifically to bridge the gap between so called high-end audiophile product and the need for a simple 'no nonsense,' convenient, user-friendly, high satisfaction audio product." In these regards, the INT-150 is exactly what Pass Labs says it is. Setting up and using it were a breeze. I removed the gold jumpers from the XLR inputs, plugged in my balanced cables, attached my speaker cables, and turned the puppy on.

All the functions I needed were easily found and commanded by the remote control, which is as functional and beautiful as any I've ever used. I also found the looks of the Pass amp to bridge the gap between an elegant amp that would [ahem] integrate with the décor of a living room, yet still inspire grunts of approval from other males of my tribe who might wander into my dedicated man cave. The INT-150 has a blue, vacuum-fluorescent display that was easily readable from across the room. But if you like it really dark, you can attenuate the display's brightness or shut it off entirely.

Switching inputs with the remote or via the buttons on the INT-150's front panel resulted in a satisfying mechanical click each time the amp switched to the new input. The INT-150 has two inputs that can operate in balanced or single-ended mode, and three other SE-only inputs. In other words, you can connect up to five SE sources, or two balanced and three SE, or one balanced and four SE. I used the balanced inputs almost exclusively. The INT-150 has no built-in phono stage, and there's probably little chance of one ever being added.

Passing muster
I'm pretty familiar with Pass Labs gear. I've owned an Aleph 3 for a number of years, and I reviewed the XA30.5 for Stereophile in August 2009. I wanted to review the INT-150 for a number of reasons. First, I've been on a kick of reviewing integrated amplifiers of various designs and prices. Second, having lived with lower-powered class-A amps from Pass, I wanted to see if the higher-powered class-A/B designs would offer me better control of my speakers while retaining some of the sound of the Aleph 3 and XA30.5. My only disappointments in those two lower-powered amps were that they couldn't give me the woofer control or the limitless dynamic range I sometimes crave.

Pass Laboratories
PO Box 219, 24449 Foresthill Road
Foresthill, CA 95631
(530) 367-3690