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Nelson Pass Interview 2023

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This past September I had the opportunity to attend the Nelson Pass Amp and Speaker Camp, which is held each year in Sebastopol, California. The camp has been going on for some years and has grown in popularity. This was my first time and I found it to be a fun day out. I don’t get the opportunity that often to spend the day with a group of like-minded audiophiles.

I spent the morning building a pair of Nelson Pass’s Open Baffle Speaker designs and the afternoon building a pair of his 8-watt Class A mono-block designs.

One of the highlights of the day was sitting across from Nelson at lunch talking about the early days of audio. I discovered we have a lot of common friends and associates.

We planned this interview at the show, and I hope my readers find it interesting.

Marc: You have a long and prolific history as an electronics designer. Can you tell Dagogo readers about your background and education?

Nelson: Born in Massachusetts in 1951, I grew up in Santa Rosa, California, and after high school I attended UC Davis, graduating in 1974 with a BS in Physics.

Marc: Will you elaborate on how you got into the audio business?

Nelson: As a teenager, I was interested in electronics and audio, and I built loudspeakers and modified amplifiers and things like that. When I got to college I fell in with guys who had similar interests and the projects got more ambitious, resulting in amplifiers built from scratch and loudspeakers as ambitious as “The Claw,” a horn loudspeaker with a 40-square foot mouth and 9-foot length. This sort of stuff landed me in a partnership intending to build loudspeakers commercially, which did not work out, but through that, I met Peter Werback, a superb amplifier designer who ended up working for the newly founded ESS, where he was designing an amplifier that became the ESS 500. I visited him there one day and management ended up offering me a job doing loudspeaker testing and design.

Nelson Pass 40 years ago.


ESS Amplifier

Marc: Did you know Oskar Heil?

Nelson: About a month after I started at ESS, Oskar Heil knocked on the door and introduced me to his Heil Transformer, and for ESS the rest is history. In 1972-73 my job consisted of the design and testing of speakers built around Heil drivers.

Heil Transformer

Marc: How long did you stay at ESS and was that when you started Threshold?

Nelson: At the End of ’73 I left ESS and joined with their marketing director, Rene Besne, to form Threshold Corp. with the aim of manufacturing the innovative Class A amplifier the 800A. Shortly after the company was launched, we were joined by Joe Sammut, who had been a stockholder at ESS, and later by Wayne Colburn.

This was followed by other amplifier designs until around 1979 when we released the Stasis series. We continued to build amplifiers and grow the company but were always strapped for cash, so it was difficult. In 1987 we were approached by Dynatech who made us a good offer, so we sold it to them and remained running it for four years.

First Threshold Amp 1993

Marc: Is that when you started Pass Labs?

Nelson: I left Threshold in 1991 to form Pass Labs on my property in Foresthill, hiring my neighbors, and launching the Aleph series of Class A amplifiers. The company was again joined by Joe Sammut and Wayne Colburn and later by Desmond Harrington and more recently by Anastasia Protopapis. After a few years, the company moved into a building in Foresthill, and later to the current industrial park at the Auburn airport.

Eventually, the First Watt enterprise outgrew my hobby business model, and I turned it over to Pass Labs, freeing me up to just do design and playing with the “DIY” efforts in the form of Passworks.

Marc: I know you are sort of semi-retired, and still involved with Pass Labs, but I understand you are also busy doing something else.

Nelson:  I mentioned the DIY thing, which dates back to the early 70s with the Class A amplifier that appeared on the cover of Audio magazine and numerous projects that appeared in Audio Amateur. After their demises, I turned to putting my efforts on the web, and at this point, I have authored about 100 articles or projects.

Marc: How did you get involved with the Amp Camp?

Nelson: In 2001 someone informed me that there was a DIY Audio website and they had a “Pass Labs” forum, so I went there, settled right in, and remain there today. I also became involved with the Burning Amp Festival held annually in the San Francisco Bay Area and the Amp Camps and Speaker Camps in nearby Sebastopol.

Marc: Can you elaborate on what prompted the amp design we built for the camp?

Nelson: All my DIY efforts revolve around the notion of simple and inexpensive but good-sounding projects. I try to also inject some novelty into the designs that adds to the educational opportunity and provides some bragging rights. However, the “camp” projects also need to be buildable in the course of an afternoon.

Amp Camp 2023

Marc: I understand this is the second generation of the design. Is there any significant difference in the amplifier design?

Nelson: The first Amp Camp amp was a single-stage single-ended Class A design very similar to the Zen amplifier from the 1994 issue of Audio Amateur. The most recent is a 2-stage push-pull design, the ACA-mini, which most resembles the F5 amplifier from First Watt. They are quite different amplifiers.

Marc: What can we expect at the upcoming Burning Amp get-together?

Nelson: At next month’s Burning Amp/Amp Camp, Mike Rothacher and I will present the “Zenductor,” which is a single-stage, single-ended Class A mono amp using a power MOSFET loaded by an inductor. If you like your single-ended Triodes, you’ll probably like this one…

Nelson Pass 2023

Marc: I know you have a love of tube amplifiers as well as solid-state Class A amps.

Nelson: I appreciate tube amplifiers, but love would not be the word… Actually, I pay close attention to what makes tube amps sonically pleasing, and I don’t mind borrowing some of the concepts for application to solid-state amplifiers. Do I own some tube amps and have I designed any myself?  Yes.

Marc: Do you think you may do a commercial tube amp design in the future?

Nelson: Not very likely. At my age, I’m happy staying in my lane.

Marc: Let’s talk about the speaker camp. I think all my readers will agree you are one of the finest electronics designers of our day, but not many know of your interest in speaker design. Can you elaborate on your history as it relates to speaker technology?

Nelson: As a teenager, I started out with speakers. I built them in college and they were the basis for my employment as ESS. We even produced some speakers commercially at Pass Labs but found that the effort made for too much disruption in our marketplace, so we decided to stay in our lane, which is solid-state amplifiers. Speakers remain a hobby for me, and I have a small warehouse of drivers and enclosures, waiting for the day when I have something like spare time.

Marc: What was your impetus to design the speakers we assembled at the camp, and can you describe the speaker design?

Nelson: In that warehouse space I had 100 full-range drivers purchased many years ago as well as 100 open baffle SLOBs (slot-loaded open baffles) designed around them that I had made by a commercial outfit. When speaker camp was to be repeated, I volunteered them for two separate events, and now I am out.

Marc: I understand you wouldn’t want to name a specific brand, but what is your personal preference when it comes to speaker type?

Nelson: I have a particular affection for full-range drivers — Lowther, Voxative, Fostex, Feastrex, Mark Audio, Lii, and so on. It takes art to make a good full-range driver, and when done well I find that they deliver the midrange articulation that I appreciate. Yes, they typically need some help in the form of a woofer, maybe even a tweeter, and some EQ is often needed, but then that’s just part of the fun. After that, I enjoy everything, even the not-so-good products…

Marc: Nelson, this was a pleasure. I look forward to reviewing both the amp and speakers for upcoming articles. I will let you know when the article is finished.

Ion Cloud Addendum

For those of you who are familiar with Nelson Pass and were surprised about his history with speaker design, you may not know about the Pass Labs Rushmore or SR-2 speakers. Even I was not aware of his Ion Cloud design from the early 1980s, I somehow missed this speaker’s demo at CES. From what I have discovered, this design reminds me of a cross between an electrostatic speaker and an Ionovac speaker.

If you are curious about the design there are articles and interviews about this design online that I urge you to seek out.


Copy editor: Dan Rubin

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