Re: Small signal amplification.
I would bring the signal straight into the op-amp (use a BiFET type
for high input impedance) without a capacitor. Amplify it and then
use a capacitor on the output of the op-amp to isolate the AC signal.
Make sure the bias for op-amp is correct (again, a BiFET lets you
bias easily even with a single supply). Nice thing about op-amps is
that it is easy to design with them; controlling the amplification is
easy, just one resistor.