Working with the circuit you already have, make these changes:
1. Place R3 and R5 in series between the output and inverting input.
2. Place the switch and C2 in parallel withR3 (three parts in parallel).
To start, make both R3 and R5 = 47K. With the switch closed (non-boost), the circuit gain is unchanged from before, unity and inverting (47K / 47K). With the switch open, the impedance of the R3-C2 combination changes with frequency. At low frequencies, C2 is essentially open and the circuit gain is 2, a 6 dB bass boost. At high frequencies C2 "shorts out" R3 and the circuit is back to unity gain.
The ratio of R2 to R5 sets the amount of bass boost, and The combination of R3 and C2 sets the corner frequency when boosting starts. Having the two resistors in series sets a limit on the amount of bass boost no matter how low the frequency. This is sometimes called a shelf equalizer, because the frequency response plot has a flat top.
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