# Volume Control Solutions

I am designing a small power amplifier similar to the circuit below, and I would like to have a volume control on the input. My understanding is that I can't just put a voltage divider on the input, since then the spectra of the filters on the input would change. I can't just have the pot change the gain of the amp continuously, since it has a minimum gain of 10. I figured I could just precede the input with a voltage divider and a unity-gain voltage follower, but I wasn't sure if there were some more elegant solutions or techniques available. Thanks!

The output will influence the node between $R_i$ and $C_i$ until the corner frequency at around $f = \frac{1}{2\pi(R_{f1}+R_i)C_i}\approx 16mHz$. This is far below the audible frequency range! I assume it is only used to bias the output and input at the same DC voltage. For all practical considerations, above this frequency, $C_i$ can be considered an AC short-circuit.
The combination $C_f, R_{f2}$ works at a frequency of $f \approx 160kHz$ which is well above the audible frequency range. Above this frequency, the capacitor $C_f$ can be considered an AC short-circuit as well, giving you a gain of 10.
Since the audible range lies between these frequencies, the useful gain is more or less $A \approx 1 + \frac{R_{f1}}{R_i} = 11$. If you don't mind having a minimum gain of 1, you can consider using $R_{f1}$ as a potentiometer. If $R_{f1}$ is turned all the way to $0\Omega$, you will get unity gain.
Alternatives are either using or cascading an inverting amplifier where you use a potentiometer in the feedback branch. An inverting amplifier has a gain of $A = -\frac{R_f}{R_{in}}$ so cranking the potentiometer all the way down will completely mute the output.