NELSON PASS: Circuit Topology and … the End of Science

Nelson Pass is a consummate engineer, but he got his start in physics, earning a bachelor's degree from UC Davis. As he worked on his degree, he was already an audio designer, focusing on loudspeakers—great training for a designer of audio amplifiers. Soon, in 1974, he cofounded Threshold Audio with René Besne, of audio and folk-dancing fame; their goal was to build electronics, partly because the field is less competitive—it's harder than building speakers.

As he told Thomas J. Norton in "Simple Sounds Better," an interview in the November 1991 issue of Stereophile, Pass created one of the first high-power class-A amplifiers: the Threshold 800A. He'd dreamed up its key technical approaches in the back of a bus, on his way to visit a cave. Also in 1991 he founded Pass Labs, which focuses on high-power transistor designs, and in 2004 he started a second company, First Watt, to produce and market some of his more interesting experiments, mostly in low-power amplification, using an unusually wide variety of solid-state devices.

I wanted to know more about Pass's approach to amplifier design, so we had a chat by e-mail.

Jim Austin: Do you own any tube amps?

Nelson Pass: I have a couple of commercial EL34 SET [single-ended-triode] and P-P pentode amplifiers—nothing special—and a pair of point-to-point pentodes that Wayne [Colburn] made for me (footnote 1). The SET is interesting because it sounds a lot like the [First Watt] SIT-1 and SIT-2 [power amplifiers], but has more noise and lacks the bandwidth. It's nice to take a listen to them now and then. I enjoy listening to differences.

Austin: You've chosen to work with semiconductor devices, not tubes. Yet you've expressed appreciation for tubes, and in some of your designs you seem to be working to mimic the sound of tubes. Please explain.

Pass: Fundamentally, what interests me most about amplifiers are .......