1. Buy an SD card decoder board and a 7805 three-terminal voltage regulator.

2. Dismantle the transformer in the cabinet together with the AC input line, install a DC converter at the transformer position, connect two thicker and softer 48V input lines of red and blue to the input end of the DC converter, and weld the positive 12V, ground 12V and negative 12V of the DC converter at the corresponding positions of the rectifier output end.

3. Fix the SD card decoding board at an appropriate position on the edge of the speaker, and process an outward opening, so that the card slot faces outward, which is convenient for inserting the SD card. Decode board with 5 volts, connect the positive voltage of 12 volts in the machine to the input end of 7805, and connect the positive and negative poles of the output 5 volts to the decode board. The decoding board outputs two audio signals of about 1 volt, and the positive and negative polarities are distinguished and connected to the audio input of the power amplifier.

4. Turn on the 48V power supply and insert the SD card with MP3 or WMA songs to audition.

5. After normal, pack the box and hang it on both sides of the back frame of the car to enjoy music.