Figure 3 shows the protection trigger and drive circuit of the odd-level AV-388D power amplifier. The DC detection circuit consists of a bridge rectifier circuit consisting of D4~D7, which is then amplified by Q15 and Q14 to drive the Schmitt trigger. Whether the positive or negative voltage appears on the left and right channels may cause Qi5 and Q14 to turn on and drive the relay to release the relay, so that the amplifier and the Speaker are protected. 
Tuqi sound AV-388D post-stage power amplifier circuit (click to see the original picture). The protection drive circuit is a Schmitt trigger with Q13 and Q12 as the core. Select the appropriate resistor values ​​of R28, R27, and R26 to ensure that the base state of Qi2 is high and Q12 is saturated. At this time, when the emitter current of Q12 flows through R26, a voltage is formed across R26, so that the Q13 emitter (ie, the input end of the flip-flop) has no high control voltage. Qi3 is in the cut-off state, achieving the first steady state. The relay is in the pull-in state and the amplifier is output normally. When the high level of the detection circuit or the power-on delay circuit output (this level must be higher than the trigger gate level of the flip-flop) is applied to the base of Ot3, Q13 is turned from off to on, and a positive feedback process occurs. : UQl3b↑→IQl3b↑→IQl3c↑→UQl3c↓→LIQl2b↓→IQl2e↓→IR26↓→UR26↓→IQl3b↑. Q13 is rapidly saturated and turned on. Its collector voltage is almost 0, which makes Q12 turn from saturation conduction to off. The output of the flip-flop is turned to the third steady state, and the relay is released and enters the protection state. When the protection voltage at the input of the trigger drops (eg, the end of the power-on delay protection or the release of the overload state), when the gate-off level is reached, Q13 exits saturation and triggers another positive feedback that is opposite to the first steady-state process. Q12 turns into saturation conduction again from the cutoff, the circuit returns to the first steady state, the relay pulls in, and the protection is canceled. In the circuit, R43 is a current limiting resistor, and D3 is a relay back electromotive force releasing diode to prevent the back electromotive force from damaging Q12. In addition. Since the relay requires a large pull-up starting current, the circuit has a capacitor C22 connected in parallel across the resistor R43. Before the relay is started, the capacitor is discharged by R43; when the saturation of Q12 is turned on, the voltage at the end of C22 cannot be abrupt, the starting current bypasses the obstruction of R43, and the C22 is turned on, so that the relay can quickly pick up. After the pull-in, C22 is also fully charged, and the sustain current of the relay is provided by the R43 attenuation. C8 is the delay capacitor and R3l is the current limiting resistor of C8. They form a delay circuit with R32, R30, Q13, and R26 to adjust the C8 and R31 values. The delay time can be changed. When the power is turned on, the power supply voltage is supplied to the trigger control terminals composed of Q13 and Q12 through C8 and R3l. The trigger is in the Q12 off state, the relay does not pull in, and the power output circuit is temporarily disconnected until C8 is charged to a certain charge.
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