This protection circuit was built before I started the design of my power amplifiers and experimenting with them. My old-fashion acoustic systems went through three generations of these amplifiers and although my latest VK-5 model is an absolutely safe and reliable gear, each its turning on entails an intelligible click of the relay connecting the amplifier outputs to the loudspeakers.
     The relay is the most critical detail of any loudspeaker protection circuit, it should be of a non-sealed type and to have sufficiently large contacts to withstand high currents (at least 10A), these contacts should be accessible for visual checking and, if necessary, for cleaning. I use a 24V relay with gold-clad contacts to avoid any speculations about the relays that might deteriorate the reproduced sound.
     The second important characteristic is the delay time with which the relay isolates the loudspeakers from a faulty amplifier when a DC voltage of either polarity and more than, say, 2V suddenly appears at the amplifier output. Usually, the higher this voltage, the shorter is the circuit reaction to that and the more quickly the relay is deactivated. But there is also a time needed for mechanical disconnection of its contacts and this depends on the relay's property. Electronic part of the offered protection circuit is implemented on discrete components, its schematics is represented in Fig.1.

Fig.1. Loudspeaker protection.

     The stereo amplifier two output voltages are delivered to the circuit input via two simple low-pass filters (R3C1 and R4C2) to prevent the output AC signals from influence on the circuit operation. Normally, the analyzed amplifier DC outputs are less than ±0,5V, the transistor Q1 biasing is absent and it stays in the non-conducting state, providing the same state and for transistor Q3. The other transistors, Q2 and Q4, are at the same time turned on, thanks to resistor R11 sitting at +32V. The current flowing through Q4 energizes the relay K1 whose closed contacts connect the loudspeakers with the amplifier outputs.
     There is a delay in this connection when the amplifier starts working and the protection circuit begins simultaneously to be powered by supply voltage +32V. During the first seconds, the discharged capacitor C3 keeps transistor Q3 in the turn-on state, the Q4 transistor base and emitter are therefore shorted and its zero collector current can not activate the relay. Charging the capacitor C3 through resistor R10 leads to reducing the biasing potential on the base of transistor Q3, at the moment of its turning off transistor Q4 starts conducting and the relay is activated. Resistor R8 configures this transistor arrangement (Q3 and Q4) as a trigger to make the relay switching more fast and reliable.
     The delay duration can be approximately determined as T=2×C3×R10, in our case it is about 4sec. This time is sufficient for the amplifier settling process to be completed and for the disconnected loudspeakes to prevent the associated with this process thump noise from being heard.
     Appearance of the DC voltage of either polarity and more than 2V at the amplifier outputs immediately de-energizes the relay due to the non-conducted transistor Q4, but the circuit behavior differs whether this voltage is positive or negative.
     The positive amplifier output performs biasing of transistor Q1 through conducting diodes D4 or D5. The closed to ground Q1 collector quickly discharges capacitor C3 through a small resistance of R7, thus restoring the conditions for Q3 turning on and hence for deactivation of both transistor Q4 and the relay. The negative amplifier output, on the contrary, performs shutting the transistor Q2 off through conducting diodes D1 or D2, that leads to Q4 turning off and the relay deactivation too.
     Detailed consideration of the above process is carried out with the help of the circuit simulation in the Multisim 10 program, according to Fig.2.

Fig.2. Loudspeaker protection circuit – simulation.

     Screenshots of the virtual oscilloscope (Fig.3) show how does the relay deactivation (red color tracks) depend on the DC voltage applied to the protection circuit input (green color tracks). For VIN=3V the deactivation delay time is 0,8sec, for VIN=10V - 0,17sec and for VIN=25V - 0,07sec. After restoring the normal working conditions (VIN=0V), the relay becomes turned on in 3,8sec.

Fig.3. Protection circuit operation.

     The power amplifier turning off produces considerable voltage transients at its output, the circuit reacts to that almost instantly and the loudspeaker doesn’t suffer any stress and doesn’t emit unpleasant sounds. As the protection circuit is powered from the amplifier’s positive supply rail (within +25÷35V), it turns off as well after power is removed, leaving the relay deactivated and the loudspeakers disconnected.
     The circuit PCB has dimensions of 70×46mm, it is depicted in Fig.4. The picture clicking produces more detailed animated images of various color performance. The PCB layout may be slightly changed to fit the board to a specific relay. I use a 24V relay with the coil resistance of 1kOhm, the optimal for the circuit coil resistance value is between 800-Ohm and 1,5kOhm.

Fig.4. Loudspeaker protection PCB (scale 2:1).

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