LA4287 sound circuit

Relevant key device information applied in this article: LA4287 TV audio signal is input from pin (1) of N601, AV audio signal is input from pin (3) of N601, and the TV/AV switching signal output by CPU (40) pin is passed through R601 Sent to the base of V601, V601 inverts the phase and controls the (4) pin of N601. After internal selection, the audio signal is output from the (9) pin of N601 to push the speaker to sound. V641 and V642 form a shutdown sound squelch circuit: when the machine is turned on normally, the 12V voltage is added to the base of V641 through R641, and then added to the emitter of V641 through VD642. Due to the presence of VD642, the base voltage of V641 is higher than the emitter voltage. When the voltage is 0.7V higher, V641 is cut off, and V642 is also cut off. This circuit has no effect on the volume control. At this time, C641 is charged with a voltage of nearly 12V. When the power is turned off, the 12V voltage disappears. At this time, because C641 is charged with 12V voltage, the base voltage of V641 is lower than the emitter voltage. V641 is turned on, V642 is also turned on, and the volume control voltage is short-circuited to ground, achieving shutdown silence. noise. LA4287 pin functions and reference voltage: Pin 1: 6.3V - Signal input 1 Pin 2: 0V - Ground Pin 3: 6.3V - Signal input 2 Pin 4: 2.1V - Signal switch Pin 5: 0.67V - Volume control voltage input pin 6: 9.8V - Filter pin 7: 9.8V - Feedback pin 8: 0V - Ground pin 9: 9.9V - Output pin 10: 21V - Power supply