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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 0 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 2 amps. The largest are capable of peaks of 25 amps each, and are run in parallel
banks of four.
The power Mosfets in the Aleph 0 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 80 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.
While it will not leave Class A on an unclipped positive signal, a single ended Class A design
would ordinarily clip at negative currents greater than the bias point, and for this occurrence
the Aleph 0 has proprietary pull circuitry in the output stage which allow push-pull operation at
negative currents beyond the bias point. This new topology has been designed to source
greater negative current than previous amplifiers with single ended bias, in the belief that
push-pull Class A operation is preferable to clipping.
It is a very simple topology, which is a key part of the sound quality. Other solid state
amplifier designs usually have five to six 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 three gain stages: a differential input stage, a
cascoded voltage gain stage, and the output transistors. The Aleph 0 uses Mosfet power
transistors throughout. The input stage uses power Mosfets and biases them at eight to ten
times the amount of current usually running through front end circuitry. The result is
significantly greater linearity and greater drive capability from simple circuitry.
Mosfets provide the widest bandwidth of solid state power devices, however they were not
chosen for this reason. The design of the Aleph 0 does not seek to maximize the amplifier
bandwidth as such. The capacitances of the Mosfets provide a natural rolloff in conjunction
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