Pass Labs XA100 owner manual Aleph Single-Ended Class a

Aleph Single-Ended Class A

The Supersymmetery Aleph™

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.

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