Loudspeaker protection

loudspeaker protection 1105 This is an all-transistor design for incorporation in AF amplifiers that produce nasty clicks in the loud- speakers when turned on or off, jeopardizing the voice coils by pass- ing a large current surge. Assuming that AF amplifier and pro- tection circuit are off, Ci and C2 are empty of charge and Re is deac- tivated. At power-on, Di rapidly charges C1. Provided both the negative and the positive supply voltage are present and at the correct level, T2 and T3 conduct, while T1 is off, enabling C2 to be slowly charged via R4. If the voltage across C2 is suf- ficiently high for T4 to conduct, T5 will draw base current and energize Re, which connects the loudspeakers to the amplifier outputs. Zener diode D4 fixes the voltage across the coil of Re, so that differently rated relays may also be used in the circuit, provided D4 is changed accord- ingly. However the relay coil current should not exceed about 50 mA, while the changeover contacts should be rated i...