Im trying out how out how I could use an op amp as a band pass filter, and then use a potentiometer to vary the gain. the ultimate goal is to make an equalizer of sorts. but one step at a time.

I found these two links
http://www.ecelab.com/circuit-butterworth-hp.htm
http://www.ecelab.com/circuit-butterworth-lp.htm

If you look at figure 2 on each, which talks about "Circuit Diagram for a Second-Order Butterworth High-Pass/Low-Pass Filter". I see that the gain has a resistor which is independent from the cutoff frequency. Meaning, if I vary it, I can vary the gain without modifying the cutoff frequency. the only problem is, I dont know how I would do this for a band-pass filter (googling Butterworth bandpass filter left me short).

Any help anyone?

 

This is a really simple idea, but arranging the potentiometer as a simple volume control might work. Depends on the big picture, which means, a bigger drawing that shows the general layout of all the tuned frequency circuits. In fact, a drawing of a bandpass filter would be helpful!

Here's another idea:

circuit attached. It has the advantage that I have been using this for 33 years, and it has never needed repair.

 

Showing you the best 10 band equalizer that I know of kind of spoils the surprize, but it can save you so much work!