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This is the circuit of the audio amplifier used in the Speaking Clock.

Used alone, it might burn up due to too high a supply voltage and so a parallel zener was added to limit the voltage seen by the IC to 12V.

Another enhancement sought was automatic gain control, so that the volume control could be dispensed with, while the amp was being used as a tracer for faultfinding. A transistor and a couple of diodes provide that function. If the output goes above about 0.6V pk the BC557 conducts and sends a current through the two diodes. Their dynamic resistance drops and they shunt the signal being fed to the input of the amplifier. This keeps the output more or less constant.

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