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Designs that vary the bias against the musical signal will generally have bias currents at or
below the signal level. This is certainly an improvement from the viewpoint of energy
efficiency, but the sound reflects the lesser bias point.
I authored the first patent on the dynamically biased Class A amplifier in 1974, however I have
not used the technique for the last 15 years. The reason is that I found the quality of sound
associated with an efficient Class A operating mode inferior in depth and less liquid at high
frequencies, simply because it operates at reduced bias at low levels. Given the plethora of
cool running "Class A" amplifiers on the market, you might say I opened a Pandora's box.
A very important consideration in attempting to create an amplifier with a natural characteristic
is the selection of the gain devices. A single ended Class A topology is appropriate, and we
want a characteristic where the positive amplitude is very, very slightly greater than the
negative. For a current gain device, that would mean gain that smoothly increases with
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 five 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.
Readers of The Audio Amateur Magazine will be familiar with my "Zen" design, which uses a
Mosfet in a power amplifier which has only one gain stage, and only one gain transistor.
Research continues at Pass Labs in improving the performance of very simple gain circuits.
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. Bipolar devices are much more prone to failure at high temperatures.
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.
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