Lfa-150: A Fast Power Amplifier (Final Part)

42 EE December 19 LFA-150:AFASTPOWER AMPLIFIER (FINAL PART) Protection circuit. The protection circuit serves to: • delay the energizing of the output relay by a few seconds from power- on; • on switch-on, monitor the d.c. resist- ance of the loudspeaker: if this is lower than 2.2 ohms, the output relay is not energized; • deactuate the output relay if the direct voltage across the output ter- minals of the amplifier rises above 1 volt; • deactuate the output relay if the peak current flowing in the output tran- sistors rises above 10 A; from a basic idea by A. Schmeets • deactuate the output relay if one, or both, of the secondary a.c. voltages fails-this also ensures that the loudspeakers are disconnected from the output when the amplifier is switched off. The circuit diagram of the protection unit is shown in Fig. 9. Note, however, that the output relay and the peak- current detector are located on the current-amplifier board. The 24-V output relay is actuated by T4...