| The Aleph 5 is the 60 watt per channel stereo version of the 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 5 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 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 5 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
deliver 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 5 uses power Mosfets for both gain stages. These Mosfets were chosen because
they have the most ideal transfer curve for an single-ended Class A design. Mosfets also
allow high current operation 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 used throughout.
The Aleph 5 has been designed for maximum reliability and years of trouble free
operation. The input gain devices are pulse rated 8 amps and are followed by output
devices with pulse ratings of 50 amps each. The amplifier employs 1.8 kilowatts of power
Mosfet output devices and massive heat sinking rated at .08 degree per watt.
During active operation the amplifier consumes 300 watts, so that each output device is
operated at 16% of its rated power. The Aleph 5 is protected from overheating by a 75
degree C. thermal switch, and from internal failure by a slow-blow fuse.
The amplifier is powered by a toroidal transformer rated at 5 times the amplifier's
output rating which charges 120,000 uF of supply capacitance. This unregulated supply
feeds the output transistors with a full power ripple of about 1/2 volt.
The amplifier is equipped with a balanced input featuring a common mode input noise
rejection of -60 dB, accomplished through a passive network tied directly into the input
stage of the amplifier, not with additional active input circuitry. Unbalanced input is
accommodated through RCA connectors.
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. |
