I am sorry to make all confusing. Maybe I ask the question more simply.
After testing the speaker, I find that current(AC) = 40mA, Vp-p = 7V can make the speaker become the highest volume, because the speaker is a kind of smartphone speaker.
But the max output signal of my devices is current(AC) before capcitor = 1-3mA, Vmax = 8V, Vmin = 5V, Vp-p = 3V, f = 15kHz Sine wave
The max output signal of my devices is current(AC) after 0.10 uF capcitor = 1-3mA, Vmax = +1.5V, Vmin = -1.5V, Vp-p = 3V, f = 15kHz Sine wave
These signal cannot drive the speaker in the highest volume, so I need to convert the signal to that.
Since the original has a very high DC coupling voltage, if i direct input to opamp like lm358, chopping must occurs. secondly, lm358 do not have much current ouput.
But if i series with a capacitor, the signal will have negative voltage, that I dont know how to process negative voltage with single 9V battery.
So I am confusing with the negative voltage (adding the DC component back to signal ?) and do the power amp LM386 can have my design output?
thanks