manufactured by oki electric co fig2 a to 2d show the circuit arrangement of the model msm 5203 gs fig3 shows the concrete circuit arrangement of the preamplifier 24 signal mixer 22 and power amplifier 18 the mute alarm signal eea is supplied to a volume controller 46 through a resistor r22 1 the slider of the volume controller 46 is connected to the noninverted input terminal of a first amplifier a18 through a resistor r18 1 and a capacitor c18 1 an output terminal of the first amplifier a18 is connected to a speaker 20 through a capacitor c18 2 and also to an inverted input terminal thereof through a resistor r18 2 the inverted input terminal of the amplifier a18 is grounded through a resistor r18 3 and a capacitor c18 3 the noninverted input terminal of the amplifier a18 is connected to a positive power source vcc18 through resistors r18 4 and r18 5 the junction p18 of the resistors r18 4 and r18 5 is grounded through the collector emitter path of an npn transistor q18 a resistor r18 6 and a capacitor c18 4 are connected in parallel to the collector emitter path of the transistor q18 whose base is supplied with.