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
    4
    0
    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?


     
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    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

    Member

    Mar 12, 2010
    469
    41
    Showing you the best 10 band equalizer that I know of kind of spoils the surprize, but it can save you so much work!
     
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