@minghierid At the opamp output 1.4 V is the maximum swing. With loud sounds of \$60\:{\text{dB}_{\small{SPL}}}\$ as the limiting factor this suggests a maximum voltage gain of about \$7000\$, or an opamp with at least \$150\:\text{MHz}\$ as the gain-bandwidth product if you want to reach \$20\:\text{kHz}\$. However, the OP484 spec says \$3.25 \:\text{MHz}\$ as the gain-bandwidth product. So, even ignoring the horrible power output stage there's already a problem. For voice, there's still a problem. You likely need a bootstrapped BJT stage prior to the opamp either way. Analog site isn't good.
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