Convert an idle VCD machine microphone preamplifier into a headphone amplifier

MP3 has taken over, and the glory of VCD players is gone, but idle VCDs cannot be wasted. Today we will use Shengke S-9001B VCD player preamplifier to convert it into a headphone amplifier .

As shown in the circuit diagram, the microphone signal is sent from the mic1 and mic2 jacks to the ③ and ⑤ pins of ic1 (jrc4558) through r1, c1 and r8, c5 respectively. After amplification, it is output from the ① and ⑦ pins and passes through r6 and c. 4 and r13, c8 plus Go to the respective volume potentiometers w1 and w2 to adjust the volume. The two signals are added to ic2 through r7, r14 and c9 for mixing and amplification and then output from the ⑦ pin of ic2. The circuit is powered by +12v power supply. This part of the circuit is designed as a small circuit board and installed on the inside of the panel together with the microphone socket , making it easy to modify. By slightly modifying the components, it can be converted into a high-fidelity headphone amplifier. The specific steps are as follows:

1. Cut off the copper foil at the “×” in the picture, 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 with 1kω resistors with the original 5.6kω resistors of r7 and r14, and replace c2 and c6 with 1μf capacitors. 0μf, c4 , c8 is changed from 4.7μf capacitor 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 two-core shielded wire to connect the r and l audio signals from the av board inside the machine, and connect them to the upper ends of w1 and w2 respectively. The center connector of the potentiometer is connected to r1 and r8 respectively for signal input. If the volume is too loud, use r1, r2, r8, and r9 to make an attenuation circuit by increasing the resistance of r1 and r8.

4. Use the microphone socket 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 socket r and l contacts respectively with short wires.

The next step is to listen to it, and I feel that the sound quality is good. Modification using this method does not require the addition of many components, and has no adverse impact on the function and appearance of the original machine. Interested enthusiasts may wish to give it a try.