characteristic, but the scales have been altered by a factor of about 10, so that the voltages
are much lower and the current is much higher.
Audiophiles often go to great expense to achieve as little as 5 watts of power using Triodes
because of their specific sonic character. Unfortunately Triode performance is limited partly
by the need to transform the high voltage / low current operation of the triode down to the low
voltage / high current domain of loudspeakers. This means a transformer and all the
distortion comes with it.
Of course it would be nice if Triodes drove speakers without transformers.
It has been a goal of some designers to get transistors to sound like Triodes, with very limited
success. Fets can sound like Pentodes, but it takes a particular set of gyrations to make a
Fet do the Triode trick.
There are two things we want out of a solid state device for this purpose. First, we want a
“square law” input characteristic like that of tubes. Fortunately, Fets do that already – the
current through the Fet is a good square law function of the Gate to Source voltage.
Second, we want a low Drain resistance, equivalent to the Plate impedance of the triode.
This is where gain device can be regarded as a variable resistor instead of a variable current
source.
Why do we want this characteristic? Three reasons:
First, it allows a single gain stage with both voltage and current gain, and having a high input
impedance and low output impedance without a feedback loop or degeneration.
Second, this character allows “working the load-line”, the particular description of the path of
the gain device through the voltage/current region in the course of amplifying into the
loudspeaker. By choosing this line wisely, you can achieve intrinsically lower distortion.
Pentodes and Mosfets aren't very good at this.
Содержание SIT-2
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