Electronic – Function of resistor in OpAmp circuit

circuit analysisguitar-pedalinput-impedanceoperational-amplifierresistors

I came across this circuit as part of a larger circuit in a guitar effect pedal:

schematic

simulate this circuit – Schematic created using CircuitLab

I was wondering about the function of the resistor \$R_1\$. My calculations show that the same transfer function can be achieved with or without \$R_1\$ (leaving out \$R_1\$ just means that we need to change the other resistor values). The only difference that I can see is the change in input impedance of the circuit. Is this the only function of \$R_1\$ (i.e., reduce/define the input impedance), or is there anything else I'm missing?

Best Answer

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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.

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