When you turn off the power switch, it sometimes causes a shock-off noise, therefore it is necessary to break off the relay and then to keep the power amplifier apart from loud speaker at the moment that power switch is turned off. in other words, the protection circuit is required to have a function to detect that power-off time. However, in fact, it is difficult to detect that power-off time from actual DC supply voltage line. Because it cannot be turned 0 V instanta- neously due to a large capacitance inserted between the power supply line and GND. in case of ,uPC1237,it can detect this power-off time from AC power supply directly, that is, this is a function to detect AC level. The AC power supply level (usually 50 Hz or 60 Hz) can be transmitted to pin 4 through a half—wave rectification circuit as shown below. i And it works within a wide range of AC level by choosing a proper resistance as R4 (Refer to the characteristic curve shown as fig.5 for the choice of R4). If power switch is turned off while the relay is being made ON and the speaker is being connected to the power amplifier output, the relay will be broken OFF to disconnect the speaker after a time delay (AC OFF mute) according to the discharge time constant determined by the voltage on pin 4, the external capacitance C4, and the internal resistance of the IC.