PassLabs Aleph 2 Owners manual

Pass Laboratories

Aleph 2 Owner's Manual

Serial #

Dear Customer,
The Aleph 2 is the second of the newest generation of amplifiers from Pass Labs. This design

results from my commitment to create the best sounding product, a simple circuit having the
most natural characteristic. The Aleph 2 integrates power Mosfet devices and pure single
ended Class A operation in a simple two-gain-stage topology with the sole purpose of
recreating subjectively natural sound.

The Aleph 2 is unique in a number of ways:
Unlike the earlier Aleph amplifiers, it operates in pure single-ended mode over its entire

operating region, and does not incorporate a “pull” stage for additional current beyond its bias
point. When the single-ended output Mosfets (six in parallel) reach cutoff, the amplifier clips.

Most amplifiers on the market have between five and seven gain stages in series between the
input and the output. The Aleph 2 has but two, and enjoys a very direct path from input to
output, further enhancing the purity of the circuit and the resulting sound. Earlier Aleph
amplifiers have three gain stages.

The output stage of the Aleph 2 is a unique blend of traditional design and innovation
addressing the unique requirements of loudspeakers. Previous methods of loading the output
stage have used networks consisting of resistors, coils, transformers, and active current
sources, all of which offer an optimal load line based on a resistive load. The Aleph 2 has a
current source topology which optimizes performance for a wide range of impedance and
reactance in the load, improving all aspects of performance into real loudspeakers. Pass
Labs has a US patent pending on this output stage topology.

The Aleph 2 is unique in that there are no adjustments of any kind in the circuitry. There are
no potentiometers to adjust. The operating parameters of bias currents and DC offset and so
on are defined by physical constants, and will not go out of adjustment.

Most important, the Aleph 2 brings a new dimension to the recreation of subjective sonic
reality. The amplifier delivers detail and subjective space rarely found in semiconductor
circuits, coupled with the authority and clarity rarely found in tube amplifiers.

Thank you for purchasing this amplifier. I have personally performed the tests on it, and
guarantee that it meets our production standards. Accompanying is the distortion curve for
this particular amplifier.

This amplifier is different. I have considerable confidence that you will like it, but as the
performance is subjective, you be the judge.

_________________________________

Nelson Pass

Setup

The amplifier has three sets of connections and one switch: The first connection and switch is
the AC line power system. The amplifier's voltage and current rating are indicated on the
bottom. It will be either 240 volts, 120 volts, or 100 volts, all with a 3AG slow blow type fuse.
The frequency rating of the AC line source is 50 to 60 Hz.

Your amplifier is provided with a standard AC power cord which fits into the line receptacle
located just below the power switch. The amplifier is equipped for operation with an earth
ground provided by the AC outlet. Do not defeat this ground. The chassis of the amplifier is
connected directly to this earth ground, and the audio circuit ground is connected to the
chassis and earth through a power thermistor, which gives a ground safety connection but
helps avoid ground loops. While the amplifier is equipped with an AC inrush suppresser, the
turn-on AC draw will peak (half cycle) at roughly 50 amps on a 120 volt system.

The second connection is at the input, which occurs through a balanced XLR or standard
RCA unbalanced connector. The input impedance of the amplifier is 10 K ohms unbalanced,
and 25 Kohms balanced differential.

The third connection is the amplifier output connection. Connect the 5-way output connectors
to loudspeaker plus and ground, using the cable of your choice. The space is tight around the
output connectors, so use some care to insure that the hot output connection does not short to
the chassis or other connectors. We have provided two sets of output connections in parallel
to facilitate bi-wiring to the loudspeaker.

The amplifier draws 300 watts from the wall at all times, and during idle operation nearly all of
this energy will appear as heat on the heat sinks. Good ventilation is vital to the proper
operation of the amplifier. It has been adjusted for optimal performance at room temperature,
but will work well between 50 and 90 degrees Fahrenheit (10 to 33 Celsius). You should
leave at least six inches clearance on the sides and top. The amplifier should not be placed
in a closed cabinet which does not have forced air ventilation.

This amplifier runs hot. The heat sinks will warm up in about an hour to a temperature which
will not be comfortable to touch for more than a moment or two, which is 120 to 130 degrees
Fahrenheit (50 to 55 degrees Celsius). This is normal, and there is a thermal shut off system
which will shut down the amplifier at internal temperatures in excess of 160 deg. F. and 70
deg. C.

It takes at least an hour of warm up time to get the best performance out of the amplifier. It
will take that long to reach operating temperature and exhibit lowest distortion and noise. This
is not a subjective judgement, but based on actual distortion and noise measurements. You
may find somewhat greater residual noise coming through the loudspeaker when the amplifier
is first turned on, but it will decrease as the amplifier warms up.

The amplifier does not require any maintenance. While the design is conservative, this is a
hard running amplifier, as single ended Class A operation is the least efficient operating
mode. In fifteen years the electrolytic power supply capacitors will get old. Depending on
usage, you will begin to have semiconductor and other failures between 10 and 50 years after
date of manufacture. Later, the sun will cool to a white dwarf, and after that the universe will
experience heat death.

Product Philosophy and Design Theory

When I started designing amplifiers 25 years ago, solid state amplifiers had just achieved a
firm grasp on the market. Power and harmonic distortion numbers were king, and the largest
audio magazine said that amplifiers with the same specs sounded the same.

We have heard Triodes, Pentodes, Bipolar, VFET, Mosfet, TFET valves, IGBT, Hybrids, THD
distortion, IM distortion, TIM distortion, phase distortion, quantization, feedback, nested
feedback, no feedback, feed forward, Stasis, harmonic time alignment, high slew, Class AB,
Class A, Pure Class A, Class AA, Class A/AB, Class D, Class H, Constant bias, dynamic bias,
optical bias, Real Life Bias, Sustained Plateau Bias, big supplies, smart supplies, regulated
supplies, separate supplies, switching supplies, dynamic headroom, high current, balanced
inputs and balanced outputs.

Apart from digitally recorded source material, things have not changed very much in twenty
five years. Solid state amplifiers still dominate the market, the largest audio magazine still
doesn't hear the difference, and many audiophiles are still hanging on to their tubes. Leaving
aside the examples of marketing hype, we have a large number of attempts to improve the
sound of amplifiers, each attempting to address a hypothesized flaw in the performance.
Audiophiles have voted on the various designs with their pocketbooks, and products go down
in history as classics or are forgotten. The used market speaks eloquently: Marantz 9's
command a high price, while Dyna 120's are largely unwanted.

There has been a failure in the attempt to use specifications to characterize the subtleties of
sonic performance. Amplifiers with similar measurements are not equal, and products with
higher power, wider bandwidth, and lower distortion do not necessarily sound better.
Historically, that amplifier offering the most power, or the lowest IM distortion, or the lowest
THD, or the highest slew rate, or the lowest noise, has not become a classic or even been
more than a modest success.

For a long time there has been faith in the technical community that eventually some objective
analysis would reconcile critical listener's subjective experience with laboratory measurement.
Perhaps this will occur, but in the meantime, audiophiles largely reject bench specifications as
an indicator of audio quality. This is appropriate. Appreciation of audio is a completely
subjective human experience. We should no more let numbers define audio quality than we
would let chemical analysis be the arbiter of fine wines. Measurements can provide a
measure of insight, but are no substitute for human judgment.

As in art, classic audio components are the results of individual efforts and reflect a coherent
underlying philosophy. They make a subjective and an objective statement of quality which is
meant to be appreciated. It is essential that the circuitry of an audio component reflects a
philosophy which address the subjective nature of its performance first and foremost.

Lacking an ability to completely characterize performance in an objective manner, we should
take a step back from the resulting waveform and take into account the process by which it
has been achieved. The history of what has been done to the music is important and must be
considered a part of the result. Everything that has been done to the signal is embedded in it,
however subtly.