Line input circuit

Question

I'm trying to design an unbalanced line input for an audio device using a single supply. I would like the input stage to provide variable gain between 0 and 2.

This is what I have come up with so far:

I have been running some basic simulations in LTspice that suggest the circuit is working as intended, so I hope I'm not too far off the mark.

However, I'm uncertain about a couple of things:

1. Am I correct in assuming that the input impedance of the circuit equals GAIN + R4 (i.e 14.7K)?
2. How do I calculate appropriate values for R1 and R2? 10K is just a guess...

Answer 1

Am I correct in assuming that the input impedance of the circuit equals GAIN + R4 (i.e 14.7K)?

For an AC input voltage, C2 can be assumed to dominate impedances and therefore your input impedance is purely the end-to-end value of your potentiometer (10 kohm).

How do I calculate appropriate values for R1 and R2? 10K is just a guess...

It’s a reasonable guess. You don’t want the values so low that your feedback network is taking several mA from the opamp output and, you don’t want them so high that parasitic capacitance ruins the top-end frequency response. 10 kohm is about right generally.

Use a simulation is my general advice for circuits like this.