Homemade wireless audio repeater

First, let me introduce the first one which is fairly usable. In terms of grade, it is equivalent to the Alto, Xiali, Geely and so on among cars. It can be driven (it can indeed forward audio signals), but it is very uncomfortable to sit in (poor sound quality, mono), not to mention its safety (the oscillation frequency is unstable and easily interfered with).

Below is the circuit diagram of this Alto-class audio repeater.

The figure above uses a transistor 9018 to form an oscillator. The audio signal is input from the base of the transistor, and the frequency modulation is achieved by using the varactor characteristics of the BE junction. Although the circuit is simple, the sound quality is very poor, and the stability of the oscillation circuit is very poor. The change of voltage, the presence of human body or metal objects near the antenna will change the oscillation frequency and cause instability at the receiving end. Moreover, the circuit can only transmit mono audio, and users cannot listen to high-quality stereo music.

After two or three attempts, I finally made a transponder that I think is pretty good, at least equivalent to the grade of Camry and Accord. This time, the famous BA1404 stereo FM modulation chip was used. Because there is a pre-emphasis circuit in the chip, the sound quality has been greatly improved. However, the biggest improvement of this transponder comes from the leap from mono to stereo. This is equivalent to upgrading the car suspension system to independent suspension, which makes it much more comfortable to sit. The following is the circuit diagram of BA1404: