loudspeaker protection

elektor july/august 1985 There are many ways of protecting loudspeakers against the switch-on "plop": many of these rely on a clamp circuit across the power amplifier input to hold this at 0 V for a few seconds after switch-on. Others, like the one suggested here, depend on a relay to switch off the loudspeaker(s). Terminals A and B of the circuit in figure 2 are connected to one of the sensing circuits in figures la...1f, of which the pros and cons will be dis- cussed shortly. Whichever of these circuits is used, A is shorted to B immediately the power is switched on. This cuts off transistor T1 instantly, which causes capacitor Cl to charge. After a few seconds, the voltage across Cl causes zener D2 to break down. Transistor T2 and T3 then con- duct; the relay is energized, and the loudspeakers are connected in circuit. When the power is switched off, T1 conducts and this causes Cl to dis- charge very rapidly. The voltage across Cl quickly drops below the breakdown level of D2; ...