Hafler TRM6.1, Installation & Operation Manual

Page: 29 / 36

CIRCUIT OPERATION
trans•ana Implementation
The transistor Q24 is configured to operate as a switch that controls the constant current source Q21 of the differ-
ential amplifier Q27 and Q28. Switch Q24 is under the control of the thermal and turn-on circuits, and provides a
Soft Start turn-on ramp according to the charging time of C70 through R13 and R156.
U8B is configured as a DC servo-integrator to null the output offset voltage. Class AB bias current is established by
Q29, and adjusted by R11.
The positive and negative input signals are conveyed to the pre-driver transistors Q25 and Q26 by the differential
amplifier Q27 and Q28, and by current mirror Q10 and Q19. Drivers Q22 and Q23 make the transition from the
low-voltage front-end circuit to the high-voltage output stage. Buffer transistors Q30 and Q32 enhance the front-end
drive current, and drive the positive output MOSFET Q20. The same is true for buffer transistors Q31 and Q33 and
negative output MOSFET Q4.
CALIBRATION
WARNING: Only a competent technician should attempt the following procedure.
Bias:
The bias control establishes the quiescent Class AB output current of the amplifier. The bias should not need read-
justment from the factory setting: however, if the amplifier is repaired and the output devices have been changed,
calibrating the bias is necessary.
Disconnect the power to the amplifier before removing the heatsink assembly from
the speaker cabinet. To adjust the bias, remove the jumper JW1 and connect an ammeter across the exposed pins.
Adjust trimpots R11 and R9 fully counter-clockwise before reapplying power to the unit. After the Soft Start turn-on
has settled, note the ammeter reading. Adjust R11 until an additional 50mA is created. Then adjust R9 until another
50mA is added to the ammeter reading. Replace jumper JW1.
– 20–
S
ERVICE
R
EFERENCE
Qualified Service Personnel Only