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 four 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.
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.
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.
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.
The Aleph 1 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 1%. The smallest of these, the input devices, are capable of peak
currents of 8 amps. The largest are capable of pulses of 50 amps each, and are run in
parallel banks of twenty.
The power Mosfets in the Aleph 1 have chip temperature ratings to 150 degrees Centigrade,
and we operate them at small fractions, typically 10% of their ratings. For extended life, we do
not allow chip temperatures to exceed 80 degrees C.
A single ended Class A design would ordinarily clip at currents greater than the bias point,
and for this occurrence the Aleph 1 has a proprietary circuit which increases the value of the
constant current source so as to operate it in push-pull at power levels beyond the 150 watt
rating of the amplifier.
