Pass Labs 1XA160.5 User Manual

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®

April 25, 2007

XA160.5 Owner’s Manual

XA160.5 Owner’s Manual

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Nelson Pass has been designing audio electronics since about 1971,
first with ESS (remember Heil 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 successful lines of
amplifiers for such companies as Adcom and Nakamichi, and has
contributed approximately 60 designs (so far) to the public “Do-It­Yourself ” hobbyist community.

Over the years, Nelson Pass has made power, simplicity, and
performance his design signature. 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 “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 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 across the loudspeaker.
Most of the distortion and noise appears in phase across the
loudspeaker, and is not seen.

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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 the distortion as identical as possible on both
polarities of the balanced output.

It is easier to make distortions more identical than to remove them
with feedback, and this is the operating principle of X amplifiers.

Since the X1000, Pass Labs products have continuously evolved
toward higher performance. The Class AB “X” series was joined
by the Class A “XA” amplifiers. Subsequent improvements to the
Class AB “X” amplifiers resulted in the “X.5” products, and their
phenomenal success has led to the Class A “XA.5” series.

As always the goal has been the best musical and objective
performance possible with minimal parts in the signal path and
minimal feedback. This process is not completely understood, and
many of the improvements are the result of trial and error and
extensive listening.

Measurement of performance is important to us, and we feel that
well-designed product sounds good and measures good. Apparently
it is possible to make an amplifier which measures well but which
does not please the ears, and so we let our ears be the final judge.

X.5 and XA.5 amplifiers utilize the same basic circuit topology
and gain devices. They differ in the sizes of the power supplies,
the chassis and heat sinks, and the number of output devices. The
output stages of the X.5 series are operated in heavily biased Class
AB, with an idling dissipation similar to the rated output. The XA
amplifiers operate pure Class A with idling dissipation more than
twice the rated output.

Within any given chassis size, the X and XA amplifiers are designed
to dissipate about the same power, and it is not surprising that this
leads to two channel amplifiers with half the output power per
channel of comparable mono amplifiers, and Class A amplifiers with
half the output of Class AB amplifiers.

The X amplifiers deliver more output power per dollar, and the XA
amplifiers have a better subjective performance. All of the amplifiers
drive known loudspeaker loads without misbehaving, and all are
unconditionally stable into low impedance and reactive loads.

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All the usual performance metrics - power, distortion, noise, input
impedance, damping factor and bandwidth have all been improved
in this latest generation. The power supplies are larger, with fast
rectifiers gating to larger capacitor banks. The increases in AC line
noise worldwide have been addressed with heavier EMI filtering
and dramatically quieter power transformers. In addition, through
improved biasing regulation, the circuits give much more consistent
performance under varying AC line voltages and over a variety of
ambient temperatures.

All this is very nice, but only if it improves your listening experience.
We listen very carefully and critically to our amplifiers with a wide
variety of associated equipment and we take our time with it.
Musical performance is most important, and is the basis of our
success over the years.

It’s true that we also value the other things. The design and
manufacturing decisions have to balance subjective performance
against reliability, cost, specs, and ego (in approximately that order).
That is why we make more than one amplifier, but all are built with
the same commitment to quality.

If you have purchased this amplifier, we thank you for your
confidence, and hope that you enjoy it for many years. If you have
any questions or comments, or if we can help you with your audio
system, pelase don’t hesitate to contact us.

[e-mail]

www.passlabs.com

Tel [phone]

Fax [phone]

Pass Labs,
PO Box 219
24449 Foresthill Rd,
Foresthill CA 95631

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