Frequency selective operational amplifier circuit
If the pot is not linear (i.e. audio taper), k=0.5 will not be midpoint. However linear would be best.

However midpoint balances impedance ratio of gain so it is unity gain to nearly DC and well below audio range due to C2R4.  Otherwise Pot to fixed ratio looks like 16 to 20 db boost or cut treble amp. Which is flat in the 25kHz range.

Since C is huge impedance compared to R1 and R2 in the low f range, gain is unity regardless of pot position.