How to add NFB

Amplification of (100+4.7) / 4.7 = 22.3x (17dB).
Low turn over frequency is 1 / (2*π*4k7*0µ1) = 340Hz...
Exchange the 0.1µF for a 10µF for a nice 3.4Hz high pass.
(A 3µ3 non polarity cap results in a Fo of 10Hz!)
Phase check through V2A, V2B and transformer... ok.