555 is a single power supply, and the output is a positive voltage waveform, which means that the waveform is no more than zero. If you add a double amplifier behind you, the waveform is of course wrong.

Improvement: At the output end of 555, a bipolar electrolytic capacitor of 2.2uF or 4.7uF is connected in series after the center tap of R7 amplitude adjusting potentiometer (function: isolating DC, and passing AC signal, the waveform crosses zero), and then a resistor of 10K 1/4W is connected in series after the capacitor to connect the input end of dual-channel power amplifier, which can meet the requirement of 0-3V amplitude.