We Ask 10 Questions For High-End
During Enjoy the Music.com's very special 25th Anniversary we're asking various high-end audio manufacturers to answer the same ten questions. Their answers may surprise you! This month we're featuring Nelson Pass of Pass Labs. Pass Laboratories was founded in 1991 by Nelson Pass. Operating out of his shop at home, Pass developed prototypes of a single-ended Class A amplifier design. At the same time, he and Mike Burley were constructing an in-house machine shop with three home-built CNC milling machines with which to produce the amplifiers when the design was finished. The first product, the Aleph 0, shipped in 1992. In 1994 Pass Labs released a newer Aleph design which simplified the circuit from three gain stages down to only two stages and used a newly patented power current source to bias the single-ended output stage. Also in 1994 Nelson was joined by Wayne Colburn, who began working on improved version of the preamp and on a new phono stage. The company received two patents this year, one of them the design that became the SuperSymmetric circuit in use today.
Fast-forwarding a bit, in 2002 the first XA amplifiers were released, combining the X and Aleph topologies into balanced single-ended Class A amplifiers with many of the characteristics of both the X and Aleph series. The XA200 was rated at 200 Watts mono, the XA160 was 160 Watts. Eventually, In 2007, Pass Labs is applying the '.5' advances to the XA series of amplifiers. These revisions are said to dramatically lower distortion and noise and also dramatically improved the performance of the amplifiers into low impedance loads.
Q. What is your first memory of falling in love with music?
A. My mother had a decent record
collection and we had music in the house. When I was quite young I had an AM
radio tuned to KGO (SF) all night. Weeknights I fell asleep to Ira Blue (talk
radio) and woke up at 3am every night when their Jazz DJ played Jimmy Smith's Walk
on the Wild Side. By the arrival of the white album, I was hooked.
Q. How did you first get introduced to high-fidelity audio gear?
A. My father's business partner had a
Fairchild turntable and tube gear and one of his friends had gone with Heathkit
Electronics, one of which I helped assemble. My first taste of separates:
Garrard, Radio Shack amplifier, full-range 8" speakers 8" in homemade boxes,
later fitted with tweeters. The Radio Shack amplifier had Germanium output
transistors that overheated, so I had to modify it for better heat sinking.
In college, I fell in with the audio crowd -
Dynaco, JBL, Shure, and McIntosh. In my second year, I was building things from
scratch, and by my third year working for ESS.
Q. What is your favorite piece of vintage hi-fi, and why?
A. I admire many of the classics in
speaker and amplifier design from the 1940s and 1950s which achieved such great
quality of sound. They are the exemplars of simplicity that inspire my work. We
have several pair of Tannoy 15" coaxial drivers from the 1960s, that we still
use as references, not because they are particularly the best sounding
loudspeakers, but because they are very revealing of amplifier differences.
Q. When did you decide to start a high-end audio company?
A. In 1971, after constructing a giant JBL
driven horn speaker (The Claw), I formed a little speaker company with a
buddy at UC Davis and did a production run of three-way speakers that somehow
all got sold at a profit. In 1973 I took a job at ESS about a month before they
met Oskar Heil and participated in their transformation from a little garage
operation to the big time over the course of two years. It was physics classes
by day and ESS by night, and where I met my future wife Jill. In 1973 I
graduated and left ESS to start an amplifier company, Threshold with partner
Rene Besne who had also been at ESS.
Q. What, and when, was your company’s first product?
A. We shipped the 800A power amplifier in
1975. It was a 200 Watt Class A amplifier using a newly patented circuit that
improved efficiency by about 50%. It put Threshold on the map and was the source
of income for the first couple of years. At that time Dan D'Agostino was with
Dayton Wright loudspeakers, and he introduced many of their dealers to the 800A,
as it was one of the few amplifiers that would drive them. ITALICS Thanks
Q. What challenges did you face during those early years?
A. The usual — not nearly enough money —
the company was started on a shoestring.
Q. How have your products evolved over the years?
A. Well, I guess it's been about 50 years
When I first started out, I was looking for the
best measurable performance, and that gradually evolved into Bart Locanthi's
(JBL) philosophy of making circuits as linear as possible and then applying
modest feedback. Initially, that meant Class A operation (still does), and
techniques such as cascoding to minimize the distortion of the transistors
themselves without recourse to feedback. By 1978 this approach resulted in
designs without global feedback, the Stasis amplifiers, still manufactured
In 1991 I left Threshold and started Pass Labs, shifting my attention to simpler amplifiers operated single-ended Class A. These designs relied even more on the quality of the gain devices, and I abandoned Bipolar transistors for Field Effect Transistors (FETs) having a square-law character similar to Tubes.
In 1995 Pass Labs developed and patented the "Super-Symmetric"
circuit, of which the first was the X1000, a 1000 Watts monoblock, with only two
stages and local feedback. This circuit remains the mainstay of our amplifier
products, having gone through three evolutionary upgrades over the course of 25
years. Currently, we offer fourteen "X" power amplifiers and six preamplifiers
Starting in 1978 I published a series of simple little Class A amplifiers for DIYers, starting with the A20, followed by the A40 and A75 (with Norm Thagard) and the Zen amplifier series. This activity ultimately morphed into First Watt in 2000 with commercial production as well as additional DIY designs. The most powerful First Watt amplifier is 25 Watts, and the least powerful is about five Watts. Now numbering fourteen with each having a unique design and very limited production.
The F1 and F2 were groundbreaking current source
amplifiers using MOSFETs without feedback, the F3 used a newly minted power JFET
in a cascode single-ended amplifier and the F4 was a push-pull power follower
with no voltage gain. The F5 used only two JFETs and two power MOSFETs to
deliver high performance, the F6 and J2 used the new Silicon Carbide power JFETs
and NOS Toshiba JFETs, the M2 achieved voltage gain with an input transformer
and current gain with a power follower, and the F7 (pictured above) was a
lateral MOSFET design with positive current feedback and a fantastic damping
In 2005 I started working with more exotic and
unique parts, starting with the Toshiba small-signal JFETs and Lovoltech power
JFETs, and in 2008 with the SemiSouth Silicon Carbide power JFETs. Iin 2008
there was an opportunity to have some SIT (static induction transistors)
custom-made by the same SemiSouth, power JFETs with curves that resemble Triode
tubes and deliver a similar sonic character. They have some advantages over
tubes - no heater, long life, and operation at voltages and currents directly
compatible with loudspeakers, so no need for output transformers. The results
were very successful SIT-1, 2 (pictured below), and 3 amplifiers.
At present, I have acquired an inventory of new
and old transistors, and am focusing on the unique character that each of these
can bring to audio amplifiers.
Q. What is your company’s most popular product(s)?
A. Last year the product with the most
units shipped was the SIT-3, which is coming to an end as we use up the last of
the SemiSouth SIT transistors. The XA25 and the integrated amplifiers have
become popular, but overall no product seems to dominate our sales, we are how
we like it.
Q. What is your next planned product offering and its’ features?
A. That would be telling. We do have some
cool stuff coming, though, so stay tuned!
Q. What advancements do you speculate high-end audio will offer ten years from now?
A. I don't spend any time thinking about
it. I'm just an old fart having fun refining simple topologies and playing with
new parts. No doubt the industry's propeller-heads will continue to dazzle
consumers with digital legerdemain and ever more exotic faceplates.