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Follow these steps.
- First enable the most left switch, then enable the most right switch. You'll see the relay enable the speakers in a few seconds.
- To simulate transistor failing close the switch in parallel with one of the power transistors.
The idea behind it, is to delay the time for which the speakers connect to the amplifier, because at the first few seconds the amplifier goes through transient modes, before it settles in the position it was set to be in. Such transients may damage low power speakers, like a tweeter in a multichannel speaker system, and causes a loud pop in the speaker when the amp is first turned on. HI-FI systems always use such protection circuitry, sometimes a little different than this. The other type is to have something to sense when the amplifier has settled in the perfect position (such a device may be a simple comparator) which drives a thyristor, which drives a relay, that permanently enables the speaker, once the optimum mode is sensed, so that further crossing through the optimum point will not falsely retrigger the relay. The thyristor is the key moment here. The down side is that neither circuitry can sense and prevent catastrophic failure of some device in the circuit, and disable the speakers, so then a different circuitry is used. When an output transistor fails usually the speaker is subjected to the full Vcc (-Vcc) or close to it, so i guess a comparator is once again used to compare the maximum level to a closely set one, and disable the circuitry that drives the speaker protection relay. Since in a normal operation the amp never reaches a voltage value of above 0.707 the maximum swing, setting the comparator way up, close to the maximum Vcc will ensure that no false reading can be done. Basically Hi-Fi systems use many many ways to protect the amplifier and the speakers from the failure modes an amplifier can go through. I've combined both protection systems here. Same must be done for the PNP too. The maximum transistor current must also be limited i made such an example yesterday, i don't have the workspace to implement it here.
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