Abstract: A direct current voltages detecting circuit is provided for monitoring output voltage of the power amplifier. A direct current signal is produced when a direct current voltage is detected. A cutoff circuit responds to the direct current signal and renders an output stage of the power amplifier a cutoff state. A failure determining circuit is provided to respond to the direct current signal and to produce an abnormality signal. In response to the abnormality signal, a power supply cutoff circuit operates to cutoff a power supply of the power amplifier.

Abstract: A protective apparatus for a power transistor which detects an abnormal temperature of a power transistor and cuts a load to the power transistor to prevent possible breakdown of the power transistor. The protective apparatus comprises detecting means which detects a junction temperature of the power transistor on the real time basis from a base-emitter voltage of the power transistor. When the junction temperature of the power transistor is excessively high, a protective circuit produces an output signal, and a muting circuit applies muting to an input signal to or a current flow through the power transistor.

Abstract: A karaoke device which automatically returns the level of a microphone volume to a presettled level at the beginning or end of individual music pieces, and which is capable of gradually reducing the gain of the microphone amplifier by a predetermined value when an output volume from the microphone amplifier becomes excessively loud during the playing of a music piece. The karaoke device includes an adjustor for adjusting the level of the microphone volume in accordance with a manual instruction or an instruction from a control circuit; a level detector for detecting when the volume level is greater than a predetermined level; a music piece detector for detecting a beginning or end of each music piece; and a control circuit that compares a standard volume level and the volume level of when a detected signal is outputted from the music piece detector, and controls the adjustor to equalize the output level to the previously stored standard level.