This differential amplifier has an extremely high input impedance, and this version has a negative feedback already in place, giving it a gain of about 90.
Edit: I made a couple of changes: I changed the output section of the OP-amp part of the circuit to one that's based on the output of the 741 OP-amp, and I also added a 22pF capacitor to suppress the radio frequency oscillations that were occurring, although it does reduce the slew rate (I think that's the right term) enough that significant distortion occurs above 20kHz.
