The front-end amplification function is to complete the small signal voltage amplification task and improve the signal-to-noise ratio. I'm using a dual op amp NE5532. In practice, it is connected to an in-phase amplifier, the reverse phase of the operational amplifier is connected in series with an electrolytic capacitor of 47 F, so that the AC signal can be fully fed back, and the signal amplification factor is 1 + RFRO, where Rr is the feedback resistance, and a small capacitance is connected in parallel to it to suppress self-oscillation, ro is the resistance to ground for negative feedback. In order to enlarge the signal better, I set the amplification factor about 30dB, take rp 76kQ, ro 2.5K0, avf 1 + 76 / 2.531.4DB, because NE5532 needs power supply of 3V ~ 22V, so the earth 24v supplied by 7815 and 7915 is stabilized at three terminals. The power supply is decoupled by a capacitor of 10 F and 0.1 f.