Typically, a headphone is connected to the loudspeaker output of the final amplifier stages through a voltage divider circuit. However, this simple design has two distinct disadvantages. Firstly, the headphone volume cannot be varied independently from the main speaker when the main speaker is switched on at the same time. Secondly, the voltage divider circuit causes attenuation at the same time affects the bass output negatively.
The solution to this problem is an independent amplifier for the headphones such as the circuit presented bellow. This headphone amplifier circuit is connected to the output of the final amplifier through the potentiometer P1. IF a stereo headphone is used, this potentiometer must be replaced with a stereo type. Furthermore, the entire circuit must be duplicated for the second channel.
The headphone amplifier delivers an output of around 1 watt. Use a power supply rated at 350 mA. The amplifier gain is dependent of the resistors R4 and R6. The values shown in the circuit gives a gain of 11. The voltage at the junction of R13 and R14 must be set at 50% of the power supply. This can be set through P2. The standby current through the final transistors is about 50 … 110 mA.