First thing, C2 polarity is wrong, minus side should go to input (generator). Second thing, input impedance is quite low. To start with, increase R1, R2, R6 and R8 to at least ten times the value shown on the schematic. Lastly, install a resistor (something like 220 or 470 Ohms) between the +24V source and circuit. This will attenuate any hum on the +24V source.