| The Aleph 3 is the 30 watt per channel stereo member of the new generation
of Class A power amplifiers from Pass Labs which feature a two gain stage topology and
pure single-ended operation. Single-ended Class operation is often referred to as the
"king of Class A" because it has the smoothest transfer curve and operates at
the lowest efficiency of the family of Class A topologies. Properly designed, a
single-ended amplifier exhibits second and even order harmonic structure, similar to the
transfer characteristic of air, where the phase of the signal and harmonics reflects the
subtle difference between acoustic compression and rarefaction.
The Aleph 3 conforms to the Pass philosophy of having absolutely minimal parts in
series with the signal path. The input signal travels through a resistor to the input
Mosfet transistor, and from there through another resistor to the parallel output Mosfets,
and from there through a power resistor to the loudspeaker. There are only two sets of
gain devices and three resistors between the input and output.
This minimalist topology is supported outside of the signal path by regulated current
sources which separately bias the input and output stages.
The Aleph 3 is unlike the original Aleph 0s in two important ways. First, it has only
two gain stages instead of the original three. With one less gain stage, there is less
information lost within gain devices and in feedback. There is improved intimacy between
the input and output, resulting in greater immediacy, detail, warmth, and depth.
The amplifier also does not have the auxiliary pull circuitry which allowed the Aleph
0s to deliver very high currents into low impedance loads. This is a purer approach,
however it does not allow for greater power below 4 ohms. The amplifier will drive and is
stable into any load impedance or reactance including a direct short, and delivers clean
audio into very tough loads.
The current source which biases the output stage is a new design which maximizes the
performance of the output stage into reactive loads. It helps to optimize the load-line of
the single-ended output devices, lowering distortion. The design of this current source is
protected by US patent application.
The Aleph 3 uses power Mosfets for all gain stages. These Mosfets were chosen because
they have the most ideal transfer curve for an single-ended Class A design. Mosfets also
allow extremely high current outputs with low circuit complexity, delivering high
performance in a minimalist topology.
The input P channel and output N channel Mosfets are carefully selected from the same
manufacturing lot codes and matched to a tolerance of 2%. Precision metal film resistors
are use throughout.
The Aleph 3 has been designed for maximum reliability and years of trouble free
operation. The input gain devices are pulse rated 2 amps and are followed by output
devices with pulse ratings of 50 amps each. The amplifier employs 1.2 kilowatts of power
Mosfet output devices and massive heat sinking rated at .15 degree per watt. During active
operation the amplifier consumes 200 watts, so that each output device is operated at 16%
of its rated power.
The amplifier is powered by a toroidal transformer rated at 8 times the amplifier's
output rating which charges 160,000 uF of supply capacitance. This unregulated supply
feeds the output transistors with a full power ripple of about 1/2 volt.
The Aleph 3 is impervious to electrostatic shock at the input and dead shorts at the
output. The Aleph 3 is protected from overheating by a 75 degree C. thermal switch, and
from internal failure by a 4 amp slow-blow fuse.
The ultra-high bias current, the transconductance character of the Mosfets, and the
simple design topology all combine to deliver an amplifier having high power and strong
"bench" performance while still delivering the emotionally involving warmth and
depth associated with the finest tube amplifiers. The sound of the Aleph amplifiers has
been recognized world-wide as the most natural obtainable from solid state amplifiers. |
