The input circuit of the loudspeaker amplifier is composed of 4N36

The figure below is a circuit for the input end of a loudspeaker amplifier using a photocoupler. The left side of the figure is connected to the signal source of the loudspeaker equipment, and the right side is connected to the high-fidelity loudspeaker amplifier stage. The photocoupler isolates the signal source circuit (microphone, record player, record player output jack lead wire) to prevent the noise introduced by the potential difference between the ground wires from entering the loudspeaker and causing interference, so that the playback system has high stability and fidelity.

The electrical signal Ui from the signal source is first converted into an optical signal through the light emitting diode in the photoelectric coupler 4N36.

The phototransistor in 4N36 receives the optical signal and converts it into an amplified electrical signal. The DC is blocked by coupling capacitor C2, and the output audio AC signal is sent to the power amplifier for further amplification. R1 and C1 are the input current limiting resistor and DC blocking capacitor, R2 is the DC bias resistor of the light emitting diode (the working current of the light emitting diode is 5-10 mA), and R3 is the collector resistor of the phototransistor.