Pass Labs XA100 - Manual (Page 13)

13 XA100 Owner’s Manual
they vanish at the output, typically by a factor of 10 or so for matched
single-ended Class A circuits.
Supersymmetry™ extends this concept by using feedback only to make
the distortion and noise more identical on each half of a balanced cir-
cuit, not to eliminate it as such. This gives as much as a 100:1 reduc-
tion in unwanted distortion and noise without requiring the equivalent
amount of negative feedback. It is simply much easier to tweak the two
halves of the circuit into identical symmetry than to eliminate all the
distortion in each half of the circuit.
The result has been a successful series of high power amplifiers offering
low distortion, and stunning dynamic range and control, with only two
gain stages and very minimal feedback.
Supersymmetry™ is ideally used to obtain high quality performance
from very simple gain circuits, and particularly favors single-ended Class
A topologies due to their smooth second harmonic characteristic, which
cancels easily.
Single-ended Class A has long been known as the “King of the Class
A’s”, the simplest, most pure and least efficient form of audio amplifi-
cation. Before the Aleph™ amplifiers, it was reserved for preamplifier
circuits, high quality amplifier front-ends, and other very low power
output stages.
US Patent # 5,710,522 describes the single-ended Class A power ampli-
fier biased by a constant DC current source which negatively “ghosts”
the speaker load with simple but effective current modulation. If you
have followed developments in audio for the last 10 years, you know this
design as the Pass Laboratories Aleph™. The Aleph™ is an incredibly
successful ground breaking amplifier design, hailed in Stereophile maga-
zine as “The Amplifier of the Decade” and winner of virtually every
international design award for audio that we know of.
The key virtue of the Aleph™ design lies in its simplicity and effective-
ness. With only two stages and no adjustments it delivers measured
performance rivaling the most complex designs, but retains the sonic
elegance and nuance of a truly minimalist gain path – only two gain
devices in series from input to output.
The two design concepts are not mutually exclusive, and we approached
the development of the XA series with an eye to creating a warm/sweet
X amplifier, or conversely, a powerfully dynamic Aleph™. The success-
ful result is a circuit which is described as balanced single-ended Class
A, consisting of two balanced Aleph™ amplifiers sharing a single dif-
ferential input pair of transistors.
Aleph Single-Ended Class A
The Supersymmetery
Aleph™