# Audio equalizer circuit using multiple feedback bandpass filter for each frequency band

I have a project to design an audio equalizer circuit with the following specifications.

• Bass band, center frequency = 180 hz, quality factor =0.7, range of adjustable gain 0.4-8
• Midrange band, center frequency = 1 khz, quality factor =0.7, range of adjustable gain 0.2-4
• Treble band, center frequency = 5.5 khz, quality factor =0.7, range of adjustable gain 0.1-2

I did the calculations to satisfy the center frequency and quality factors, but I am not understanding the range of adjustable gain.

I only have 6- LM741 op amps (I understand the 741 is vintage, it is all I have, I'm just a college student. Trying to make it.) I don't understand where to fit my potentiometers.

Initially I thought I could place a variable gain inverting amplifier after each filter, but I then thought I would need a summing amplifier to combine the inverted outputs and that would be one two many op amps. Any help or advice would be greatly appreciated.

• You are probably wasting your time using the 1968 vintage 741. Read this Dec 7, 2020 at 16:58
• I don't think he is asking about component selection. Dec 7, 2020 at 17:05
• A single summing amplifier can have individually adjustable gains for each input, by making the input resistor variable. If the feedback resistor is 16 K, then the math for the input resistor ranges is pretty straightforward. Dec 7, 2020 at 17:07
• @AnalogKid the op said this: Any help or advice would be greatly appreciated. I also linked to a great article on Baxandall tone controls so reconsider your comment. Dec 7, 2020 at 17:15
• @AnalogKid So I can take the output of each filter and place a potentiometer then combine them into a summing amplifier? Dec 7, 2020 at 17:36