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 Pass Aleph 60 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 output devices to within 2%. The input devices
are matched in circuit for lowest noise and distortion. 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 Pass Aleph 60 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.
