Here’s an idea you’re not likely to find in the op-amp “cookbooks,” doubtless due to a worldwide conspiracy to sell more op amps.
A single op-amp section can often “multitask” analog chores. This audio input stage combines low-pass, highpass, and variable-gain functions. Frequency response is within 0.25 dB of flat between 30 Hz and 15 kHz. The gain can be adjusted between unity and +15 dB.
The wiper of the gain control isn’t a very low impedance. Therefore, the lowpass and high-pass characteristics don’t quite live up to textbook second-order expectations. Nonetheless, the performance of this circuit is sufficient for suppressing subsonic rumbles and reducing RF pickup.
