Homemade wireless audio repeater

First of all, I will introduce the first one which is relatively usable. It is of a similar quality to Alto, Xiali, Geely and so on in cars. It can be driven (it can indeed forward audio signals), but it is very uncomfortable to sit in (the sound quality is poor and the single-wheel drive is very uncomfortable). Channel), not to mention safety (the oscillation frequency is unstable and easily interfered).

Below is the circuit diagram of this Alto-level audio transponder.

The above figure uses a triode 9018 to form an oscillator. The audio signal is input from the base of the transistor, and frequency modulation is achieved by utilizing the varactor characteristics of the BE junction. Although the circuit is simple, the sound quality is poor, and the stability of the oscillator circuit is very poor. Changes in voltage, the presence of human bodies or metal objects near the antenna will change the oscillation frequency, causing instability at the receiving end. Moreover, this circuit can only transmit monophonic 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 on par with a Camry or Accord. This time, the famous BA1404 stereo FM modulation chip is used. Since there is a pre-emphasis circuit in the chip, the sound quality has been greatly improved. But the biggest improvement of this repeater comes from the leap from mono to stereo. This is equivalent to upgrading the car suspension system to independent suspension, which makes sitting much more comfortable. The following is the circuit diagram of BA1404: