Some knowledge about power amplifiers
The input signal of the power amplifier must be higher than a certain threshold. Therefore, instead of passing the original audio/RF signal directly to the power amplifier, it is pre-amplified with a current/voltage amplifier and sent to the power amplifier as input after making the necessary modifications. You can observe the block diagram of the audio amplifier and the usage of the power amplifier below.
In this case, the microphone is used as the input source. The signal strength from the microphone is not strong enough for the power amplifier. Therefore, it is pre-amplified first, and then its voltage and current are slightly increased. Then, the signal passes through the tone and volume control circuit to make aesthetic adjustments to the audio waveform. Finally, the signal passes through the power amplifier, and the output of the power amplifier is fed to the speaker.