Bandpass filter w/ variable gain (That wont change bw, or Fc)

Discussion in 'The Projects Forum' started by Sutitan, Apr 12, 2010.

  1. Sutitan

    Thread Starter New Member

    Nov 12, 2009
    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

    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?

  2. Bychon


    Mar 12, 2010
    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.
  3. Bychon


    Mar 12, 2010
    Showing you the best 10 band equalizer that I know of kind of spoils the surprize, but it can save you so much work!