Virtually all the amplifiers on the market are based on a push-pull symmetry model. The pushpull symmetry topology has no particular basis in nature. Is it valid to use air's characteristic as a model for designing an amplifier? If you accept that all processing leaves its signature
on the music, the answer is yes.One of the most interesting characteristics of air is its single ended nature. Sound traveling
through air is the result of the gas equation
PV1.4 = 1.26 X 104
where P is pressure and V is volume, and whose curve is illustrated in fig. 1. The small
nonlinearity which is the result of air's characteristic is not generally judged to be significant at
normal sound levels, and is comparable to the distortion numbers of fine amplifiers. This
distortion generally only becomes a concern in the throats of horns, where the intense
pressure levels are many times those at the mouth, and where the harmonic component can
reach several per cent.
I authored the first patent on the dynamically biased Class A amplifier in 1974, however I have
not used the technique for the last 15 years. The reason is that I found the quality of sound
associated with an efficient Class A operating mode inferior in depth and less liquid at high
frequencies, simply because it operates at reduced bias at low levels. Given the plethora of
cool running “Class A” amplifiers on the market, you might say I opened a Pandoraʼs box.
A very important consideration in attempting to create an amplifier with a natural characteristic
is the selection of the gain devices. A single ended Class A topology is appropriate, and we
want a characteristic where the positive amplitude is very, very slightly greater than the
negative. For a current gain device, that would mean gain that smoothly increases with
current, and for a tube or field effect device a transconductance that smoothly increases with
current.
Triodes and Mosfets share a useful characteristic: their transconductance tends to increase
with current. Bipolar power devices have a slight gain increase until they hit about an amp or
so, and then they decline at higher currents. In general the use of bipolar in a single ended
Class A circuit is a poor fit.
Another performance advantage shared by Tubes and Fets is the high performance they
deliver in simple Class A circuits. Bipolar designs on the market have between five and seven
gain stages associated with the signal path, but with tubes and Mosfets good objective
specifications are achievable with only 2 or 3 gain devices in the signal path.
Readers of The Audio Amateur Magazine will be familiar with my “Zen” design, which uses a
Mosfet in a power amplifier which has only one gain stage, and only one gain transistor.
Research continues at Pass Labs in improving the performance of very simple gain circuits.
Yet a third advantage tubes and Mosfets have over bipolar devices is their greater reliability at
higher temperatures. As noted, single ended power amplifiers dissipate comparatively high
wattages and run hot. Bipolar devices are much more prone to failure at high temperatures.
In a decision between Triodes and Mosfets, the Mosfet's advantage is in naturally operating at
the voltages and currents we want to deliver to a loudspeaker. Efforts to create a direct
coupled single ended triode power amplifier have been severely limited by the high voltages
and low plate currents that are the province of tubes. The commercial offerings have not
exceeded 8 watts or so, in spite of hundreds of dissipated watts.
Transformer coupled single ended triode amplifiers are the alternative, using very large
gapped-core transformers to avoid core saturation from the high DC current, but they suffer
the characteristic of such a loosely coupled transformer as well.
The promise of the transconductance characteristic in power amplifiers in providing the most realistic amplified representation of music is best fulfilled in Mosfet single ended Class A circuitry where it can be used very simply and biased very high.
The Aleph 3 uses International Rectifier Hexfet Power Mosfets exclusively for all gain stages.
These Mosfets were chosen because they have the most ideal transfer curve for an
asymmetric Class A design. Made in the United States, they have the highest quality of power
Mosfets we have tested to date. We match the input devices to each other to within 0.2% and
the output devices to within 2%. The smallest of these, the input devices, are capable of peak
currents of 5 amps. The largest are capable of peaks of 25 amps each, and are run in parallel
pairs.
The power Mosfets in the Aleph 3 have chip temperatures ratings to 150 degrees Centigrade,
and we operate them at small fractions, typically 20% of their ratings. For extended life, we do
not allow chip temperatures to exceed 85 degrees C.
Regardless of the type of gain device, in systems where the utmost in natural reproduction is
the goal, simple single ended Class A circuits are the topologies of choice.
It is a very simple topology, which is a key part of the sound quality. Other solid state
amplifier designs have five to seven gain stages in the signal path in order to get enough gain
to use feedback to provide adequate performance. In this amplifier, we get greater linearity by
providing much more bias through two gain stages: a differential input stage, and the output
transistors.
Mosfets provide the widest bandwidth of solid state power devices, however they were not
chosen for this reason. The design of the Aleph 3 does not seek to maximize the amplifier
bandwidth as such. The capacitances of the Mosfets provide a natural rolloff in conjunction
with the resistive impedances found in the circuit, and the simplicity of the circuit allows for
what is largely a single pole rolloff characteristic.The slew rate of the amplifier is about 10 Volts/uS under load ,
which is about 3 times faster than the fastest signal you will ever see, and about 50 times faster than what you will be
listening to. In and of itself, the slew rate is an unimportant factor when evaluating tube and
simple Mosfet designs. It becomes more important with complex circuit topologies where
there is heavy dependence on feedback correction, but even then its importance has been
overstated.
The amplifier is powered by a toroidal transformer which charges .16 Farad capacitance. This
unregulated supply feeds the output transistors only with a full power ripple of about .2 volt.
The power draw of this system is constant regardless of the music playing through the
amplifier. As such, it does not depend on a high quality AC outlet or special power cords,
since the dynamic performance does not create a variation in AC line draw. If the AC line is
running low, the output stage will bias to a higher current level by way of compensation.
The amplifier is stable into any load impedance or reactance including a direct short, and will
deliver clean audio signal into as low as 2 ohms at 120 watt peaks.
The Aleph 3 is impervious to electrostatic shock at the input and dead shorts at the output.
You can safely plug and unplug inputs and outputs while the amplifier is running. (Do not try
this with other products).The Aleph 3 is protected from overheating by a 75 degree C.
thermostatic switch, and from internal failure by a slow-blow fuse.
The chassis of the Aleph 3 is made entirely of machined aluminum; no sheet metal is
employed. We mill the chassis components from aluminum stock on computer controlled
vertical milling machines. The pieces are grained and anodized at the finest plating house on
the West Coast, and engraved by laser engraving.
The Aleph 3 is warranted by Pass Laboratories to meet performance specifications for 3 years
from date of manufacture. During that time, Pass Laboratories will provide free labor and
parts at the manufacturing site. The warranty does not include damage due to misuse or
abuse or modified products and also does not include consequential damage
