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

enter image description here

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.

  • 1
    \$\begingroup\$ You are probably wasting your time using the 1968 vintage 741. Read this \$\endgroup\$
    – Andy aka
    Dec 7, 2020 at 16:58
  • 1
    \$\begingroup\$ I don't think he is asking about component selection. \$\endgroup\$
    – AnalogKid
    Dec 7, 2020 at 17:05
  • 2
    \$\begingroup\$ 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. \$\endgroup\$
    – AnalogKid
    Dec 7, 2020 at 17:07
  • \$\begingroup\$ @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. \$\endgroup\$
    – Andy aka
    Dec 7, 2020 at 17:15
  • \$\begingroup\$ @AnalogKid So I can take the output of each filter and place a potentiometer then combine them into a summing amplifier? \$\endgroup\$
    – EE Padawan
    Dec 7, 2020 at 17:36

1 Answer 1

  1. Just a note: When designing these filters, one can set the center frequency, Q, and maximum gain at the center frequency. By choosing the capacitors to be equal, you have given up the ability to independently choose the gain at the center frequency.
  2. These filters have to be driven by a low impedance source for accuracy, so you can’t put a pot in FRONT of them, unless you want to add a buffer after each pot.
  3. It makes the most sense, then, to use a 4th op amp as an inverting summer after these stages to provide independent gain controls for the three channels. The 3 pots would be the input resistors for this summing stage, connected as variable resistors. Fixed resistors in series with the pots determine the maximum gain. Choose the pot values and feedback resistor value appropriately to meet your gain specs.
  • \$\begingroup\$ I noticed that the adjustable gain of the midrange is double the adjustable gain of the treble, and the adjustable gain of the bass is double the adjustable gain of the midrange, so if my Rf is a set value than will any combined resistance of the potentiometer and an input resistor simply need to be within the range of the respective gain values? \$\endgroup\$
    – EE Padawan
    Dec 8, 2020 at 1:05
  • \$\begingroup\$ I will do the first one for you: the gain of the bass channel you designed is -1. You need a max gain of 8. So, use 80k for Rf, the fixed input resistor is 10k, the adjustable resistor is 0 ohm to 190k. Now, you do the rest. ( this is likely homework?) \$\endgroup\$
    – user69795
    Dec 8, 2020 at 2:54
  • \$\begingroup\$ Your filters only boost a very wide bandpass, they do not cut. \$\endgroup\$
    – Audioguru
    Jul 5, 2021 at 16:01

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