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INT-150 Owners Manual
Nelson Pass has been designing audio electronics professionally since
about 1971, first with ESS (remember Heil Air Motion transformers?), and
then forming a new company, Threshold in 1975. Threshold pioneered
the design of high power Class A power amplifiers and later, high power
amplifiers using only local feedback (the Stasis series).
Pass sold Threshold and created Pass Laboratories in 1991, where he
concentrated first on elevating single-ended Class A power amplifiers to
new power levels and performance, the Aleph series.
Along the way he found the time to design highly successful lines of
amplifiers for such companies as Adcom and Nakamichi, and has
contributed approximately 60 designs (so far) to the public “Do-It-
Yourself” audio hobbyist community.
Over the years, Nelson Pass has made power, simplicity, and performance
his design signatures. The hardware tends to run heavy and hot, but elicits
high performance and reliability from simple circuits with little or no
negative feedback.
In 1998 Pass Labs released the X series of audio power amplifiers, based
on the trademarked “SuperSymmetric” topology (U.S. Patent #5,376,899),
which elicits high power and performance from simple circuits with
minimal feedback.
The first X amplifier, the X1000 was intended as the premier example of
the power of this principle, delivering 1000 watts rms into 8 ohms at low
distortion. By itself of course, this is no miracle, but you have to consider
that products with comparable performance have complicated circuits
with as many as nine consecutive gain stages and lots and lots of negative
feedback. The X1000 had only two stages and used only minimal local
feedback.
The difference was the unique balanced circuit topology in which circuit
errors are replicated at both output terminals so as to cancel and disappear
across the loudspeaker terminals. The high quality of the sound reflects
both the low distortion and simplicity of the gain path.
The SuperSymmetric circuit consists of two identical matched circuits
arranged like the wings of a butterfly, showing symmetry from left to right,
and operating balanced to the loudspeaker. The amplified signal appears
with opposing phase and equal potential across the loudspeaker. Most of
the distortion and noise appears in phase across the loudspeaker, and is not
seen.
We start with simple FET circuits already having low distortion and
noise, and arrange them in two symmetrical halves. The two halves of
the amplifier channel are closely matched, eliminating a large portion of
distortion and noise without feedback. A small amount of feedback is also
applied, not so much for the purpose of reducing distortion but to make
Introduction