Function of resistor in OpAmp circuit
Reducing R1 from 100K to 1K lowers gain from 39dB to 38dB but reduces rolloff breakpoint to 8kHz. R1 also lowers input impedance so source impedance affects rolloff and gain is affected by source impedance if >= R1.

ADDITION of R5 makes R1 redundant and forces Vin to be AC coupled which affects HPF point and causes pop on plugin.