A cost-effective active speaker 01

The power supply and speaker protection circuit is shown in Figure 9-65. The power supply is mainly composed of 1VD1~1VD4 and ic, i, 1C2, etc.
A large current bridge stack and a large-capacity filter capacitor are selected to ensure the supply of energy when the power amplifier is dynamic. 6 In order to make the sound playback system
work safely and reliably, a speaker protection circuit composed of VT2, 1VT3, 1VT4 and 1KO is designed. It has the following functions
:
    (1) The power-on delay is 5 seconds to turn on the speaker and turn it off immediately, eliminating the impact noise of the switch on the speaker. The time parameter
is determined by 1R10 and 1C13 {
    (z) When the power amplifier circuit is working abnormally, it passes the detection The circuit lowers the base potential of 1VT2, which can automatically cut off the output loop
. After the power amplifier returns to normal, the base potential of 1VT2 rises, and can automatically connect the eight- speaker loop, thereby protecting the higher-cost speaker
unit.
    The preamplifier circuit is shown in Figure 9-66. It completes impedance transformation and amplification of various signals to meet
the needs of driving the subsequent stage. In order to adapt to different sound sources, the amplification circuit is designed as an inverting amplification form composed of integrated operational amplifier 21C1 and its peripheral components
. TI can be used as 21C1. 0 82 or NE5532, the input end is equipped with three input jacks, which can be directly connected to various audio sources
.