FirstWatt F5 Turbo V3, Assembly Instruction Manual, Page: 6 / 15

F5 Turbo Assembly Instruction Version 2 Page 6
d. Power up the amplifier, the output DC offset should be close to 0V. Voltage across R20 and
R23 should also be 0V. If not, power off the amplifier immediately and trouble shoot.
e. If there is no error, then adjust P1 slowly in clockwise direction until you see the output DC
voltage start to increase. Stop at about +50mV. Then adjust P2 in clockwise direction
slowly. The DC offset at output will then drop and then stop adjust P2 when at 0V (+/-1mV).
Note the voltage drop in R20 and R23. They should be have some readings but no more
zero.
f. Target bias of 1A (for the three pairs of mosfet in total or per channel), then the voltage
across R20 and R23 should be 0.167V. If target bias is 2A, then the voltage across the R20
and R23 should be 0.333V. Note that the voltage across R20 and R23 will not be the same
but should be close within 10% if the mosfet are well matched.
g. Then repeat the step “e” until the target bias current is reached. When the voltage across
the R20 is more than 0.1V, the output dc offset should only be adjusted to up 10 to 20mV
to ensure the bias is not increased too fast. In fact when you adjust, the heat sink is cold
and thus the bias should only be 80% of the target value. The reason is that when the heat
sink is hot, the bias will increase due to thermal drift. Let the heat sink temperature stable,
re-adjust down or up until the target bias is reached. This will take about 1 hrs to stable
temperature.
h. Check through the voltage drop across all mosfet to see how good the match is. The voltage
drop or bias should be within 10% across all the mosfets. This is to ensure that all the
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