Hafler Trans-nova TRM8 Installation And Operation Manual
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CIRCUIT OPERATIONtrans•nova ImplementationThe transistor Q24 is configured to operate as a switch which controls the constant current source, Q21, of the inputdifferential amp, Q27 and Q28. When Q24 is off the emitter voltage is low, turning off Q21. Timing of the Soft Startfunction is controlled by the charging time of C70 through R168. The THERMAL Protection circuit uses Q24 to shutdown the channel when it activates.The positive and negative input signals are connected to the differential amp transistors Q27 and Q28. U8B is configuredas a DC servo-integrator to null the input offset currents.The output of the differential amp is fed to the driver stage by Q33 and Q34 which perform the DIAMONDtransconductance steering function. The cascode pairs Q29, Q30 and Q31, Q32 supply the signal voltage and currentneeded to drive the output stage Q20, Q25, and Q19, Q26.Class AB bias current is controlled by R117.Local feedback is supplied by the network R142 and C68, and global feedback by R145 and C69.CALIBRATIONWARNING: 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 needreadjustment from the factory setting; however, if the amplifier is repaired and output devices have been changed,calibrating the bias is necessary. Disconnect the power to the amplifier before removing the heatsink assembly. Toadjust the bias, disconnect all wires from the heatsink assembly and remove the jumpers JW2 (tweeter) and JW1 (woofer).Connect an amp meter across the exposed pins. Adjust R117 to get a current reading of 100mA at JW2, and R9 to geta current reading of 200mA at JW1. Before reconnecting the power cord to the heatsink assembly, connect the loosebrown wire from the power transformer to the remaining terminal of the IEC power connector.WARNING: Only a competent technician should attempt the following procedure.Calibrating Common Mode Rejection:The common mode null is adjusted by R8 (woofer) and R58 (tweeter). If common mode adjustment is needed, followthis procedure. Disconnect power to the amplifier before removing the heatsink assembly. Disconnect all harnessesfrom the assembly. Verify that the AC power cord has been disconnected from the amplifier, then connect the loosebrown wire from the power transformer to the remaining terminal of the IEC power connector. Reconnect power to theamplifier. Use a sinewave generator to provide a 100Hz common mode signal to the amplifier, making sure the DIPswitch is configured for balanced input. A common mode signal can be accomplished by connecting the generatorsignal to the tip and ring of a 1/4" plug (or pins 2 and 3 of an XLR) and ground to the sleeve (or pin 1 of an XLR). Connectan AC voltmeter to the output terminal of either amplifier, and adjust its common mode potentiometer for minimumoutput voltage.– 16 –S E R V I C E R E F E R E N C E! Qualified Service Personnel Only |
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