Converting a microphone preamplifier (RJ4558) into a headphone amplifier

As shown in the circuit diagram, the microphone signal is sent from the MIC1 and MIC2 sockets to the ③ and ⑤ pins of IC1 (JRC4558) through R1, C1, R8, and C5 respectively, and is output from the ① and ⑦ pins after amplification. It is added to the respective volume potentiometers W1 and W2 through R6, C4, R13, and C8 for volume adjustment. The two-way signal is added to IC2 through R7, R14, and C9 for mixed amplification and output from the ⑦ pin of IC2. The circuit is powered by a +12V power supply. This part of the circuit is designed as a small circuit board separately, which is installed on the inside of the panel together with the microphone socket, which provides convenience for modification. A slight modification of the components can be modified into a high-fidelity headphone amplifier. The specific steps are as follows:
1. Cut the copper foil at the "×" in the figure, and remove C1, C5, R4, R11, C4, and C8.
2. Replace C1 and C5 from 0.22μF capacitors with the original 10μF capacitors of C9 and C11, replace R4 and R11 from 1kΩ resistors with the original 5.6kΩ resistors of R7 and R14, replace C2 and C6 from 1μF capacitors to 100μF, replace C4 and C8 from 4.7μF capacitors to 220μF; short-circuit resistors R6 and R13.
3. Use the original microphone volume potentiometer to adjust the volume of the headphone amplifier. Take a section of high-quality double-core shielded wire from the AV board inside the machine to connect the R and L audio signals, connect them to the upper ends of W1 and W2 respectively, and connect the center connector of the potentiometer to R1 and R8 respectively as signal input. If the volume is too loud, use R1, R2 and R8, R9 to make an attenuation circuit by increasing the resistance of R1 and R8.
4. Use the microphone jack on the machine to make a headphone socket. Replace one of the microphone sockets with a standard ∮6.25mm stereo headphone socket, and connect points A and B to the R and L contacts of the socket with short wires. Next, listen to the sound, and the sound quality is good. This method does not require adding many components, and has no adverse effects on the original machine's functions and appearance. Interested enthusiasts may wish to give it a try.