Nelson Pass: Four Decades of Innovation

Solid-state power amplifiers,
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Nelson Pass: Four Decades of Innovation

I recently had the great privilege of spending a couple of days with Nelson Pass at his home on the Northern California coast. Unlike most meetings between an audio writer and a manufacturer, this one came with no agenda. It was simply a chance for me to spend some time with a legendary amplifier designer. I’d never met Nelson before—he is somewhat reclusive and no longer attends shows—but during my time with him I discovered a man who, despite his great success and stature, is utterly without pretense or ego (rare in the high-end audio industry), who possesses a deeply inquisitive mind, who is extremely generous with his time and knowledge, and whose creativity has only blossomed with the passing years.

It is impossible to overstate Pass’ contribution to the high end. His 41-year (and counting) career is distinguished by many brilliant and innovative amplifier designs, any one of which would have secured his place in the hi-fi pantheon. Consider that Pass’ first commercial product was the landmark Threshold 800A in 1975, an amplifier that did the seemingly impossible: significantly increase the efficiency of Class A operation by employing a novel “sliding bias” system. The 800A was also pioneering in its use of 48 output transistors in a triple-series/triple-parallel output stage. After that opening act, Pass went on to design the first amplifier with cascode operation from input to output (Threshold CAS-1 and CAS-2), and shortly thereafter the landmark Threshold Stasis 1. Pass’ creativity in exploring new amplifier topologies continued (in a new and unexpected direction) with the Pass Labs ALEPH Series of amplifiers beginning in 1991. The ALEPH amps were in production for 14 years. His Super-Symmetry circuit (1998) broke new ground in reducing noise and distortion. Today, the Pass Labs “.8” Series and Xs amplifiers continue to push the envelope in power-amplifier design. I can think of no other electronics designer with such a rich legacy.

Nelson Pass has structured Pass Laboratories so that he’s not involved in the business side of the company, giving him the freedom to focus exclusively on devising new circuit topologies. He’s set up a fully equipped design laboratory and a separate listening room in his home. Nelson is a student of the history of solid-state amplifier design, and this knowledge informs his work. His lab is stocked with rare transistors, many of them of historical importance. Despite the four decades between his groundbreaking Threshold 800A and Pass Laboratories’ Xs, he’s still passionately driven to discover new designs that advance the state of the art.

Since many of the amplifier circuits he’s currently interested in exploring aren’t appropriate for Pass Labs, Nelson started a new company called First Watt. The First Watt amplifiers are ultra-minimalist Class A, employ little or no feedback, and have relatively low power (5Wpc to 25Wpc). He took the name from TAS reviewer Dick Olsher’s now-famous dictum: “If the first watt doesn’t sound good, why would you want 199 more of them?”

Nelson and I spent a morning listening to different projects, with his describing the circuits and how the design choices affected the sound. It was absolutely fascinating. We also listened to several First Watt amplifiers and monitored their output power on an oscilloscope. At satisfying playback volumes, the amplifiers never exceeded 5W. On some music, a realistic level was achieved with the amplifiers producing less than 1W. Nelson’s speakers are fairly sensitive at 95dB, but not in the horn-sensitivity region of 100+dB. Frankly, I was shocked by how little power was required for adequate playback volume. By the way, Nelson hand-builds each First Watt amplifier himself; the company is literally a one-man operation. Given the success of Pass Labs, hand-making very small runs of minimalist amplifiers is clearly a labor of love.

To give you an idea of how creative this man is, consider “The Beast With a Thousand JFETs,” a one-off amplifier Pass just had to build. Nelson had been using a particularly great-sounding Toshiba small-signal JFET in amplifier input stages, and wished that output transistors had such low distortion. In a wild stroke of imagination he designed and built a power amplifier with 2352 of these JFETs operating in parallel for about 70W into 2 ohms. He named it “The Beast with a Thousand JFETs” as an homage to horror filmmaker Roger Corman, whom Pass is a fan of. (Corman made a film called The Beast with a Million Eyes.) The amplifier has zero commercial potential, but it just begged to be made. Incidentally, in the photo above, you’re seeing just half of two mono amps; the other side of each chassis is packed with an equal number of transistors. In his article describing the amplifier on the First Watt website, Nelson writes, “Besides the fun of the craziness of such an amplifier, what’s the point? The point is to build a simple amplifier with exquisitely good raw performance and see what it sounds like.”

The two photos that opened this story speak volumes about Nelson Pass. Nearly 40 years, much critical acclaim, seven granted patents, and great commercial success have transpired between these pictures, but Nelson today remains the same brilliant designer he was then, passionately pursuing ever more innovative ways of amplifying audio signals.