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R11 120R
R13,14 MPC74 5W 0R56
C3 100µF
Schematics of the Original M2 Output Stage
MOSFET Choice
In our earlier MOSFET pairing comparison
[1]
. The best result by far is from a pair of curve-tracer
matched 2SK3497 / 2SJ618, both in H2 & H3. They also have the highest transconductance and best
pentode characteristics. Normally they are quite tricky to use in Class A, as they have a very large
positive tempco and can easily run away thermally. The M2 OPS, with its optocoupler auto-bias,
offers an ideal solution. This also explains why we are interested in a smaller-value C3, so that the
auto-biasing has a quicker response to counter bias drift.
Then IRFP240 / 9240 has essentially the same performance as the FQA10N80 / 9P25. These are
also readily available, though we did use the Harris IRFP9240 (also preferred by Nelson
[4]
) for the
measurements. And these are still available in quantities from Rochester. The single pair of
FQA28N15 / 36P15 that we measured turns out to be a bit of a disappointment. Maybe we just picked
a bad pair by chance without matching.
Power Supply
It should be obvious that changes in rail voltages will cause DC offsets. Thus, it is recommended to
use power supply with some form of voltage stabilisation. One may consider IC regulators such as