LM1875 non-inverting

[Erickuo]

I would like to ask, why when I input 40mVpp into LM1875, the voltage becomes strange, but when I input 1Vpp, the voltage is beautiful.
The following is my output waveform diagram and circuit diagram.

 

[jan.didden]

It looks like you have low-level oscillation or mains noise.
When you go to 1V the low level cannot be seen anymore because it is so small but chances are it is still there.
Try to play with your scope trigger to find the frequency of that stuff.
Is it still there when you turn the input down to zero?
Then you can trigger on it and see the frequency.
Turn your timebase to longer time periods to see if it is 50/60Hz mains.
Is it also on the input signal?

Jan