I know I posted a question about this circuit before, but I'm running into a problem. I know this is a terrible equalizer design but I'm trying to learn to put in use what I learnt with Op Amps and transfer functions.

I did the following equalizer using 3 band-pass filters with each one made of a high pass filter connected with a low pass filter via a unity gain op amp for low frequencies, mid frequencies and high frequencies.

I used a inversing amplifier with \$V_{out}=\frac{R_2}{R_1}{\cdot}V_{in}\$ for the gain and then solved for: \$Gain=20\log_{10}{(|H(s|)}\$.

The problem is about the gain I'm trying to get in each band pass filter. I tested it with -10 dB gain on the low frequency band pass filter, 0 dB gain on mid frequencies and 10 dB gain on high frequencies based on the resolution of the above equation with \$R_1=1\$ \$k\Omega\$.

The schematics is the following: enter image description here

And the simulation yields the following for the amplifier at the 3 different band pass filters: enter image description here

Where the only working gain is the 10 db on the low frequency band pass filter.

For example for the -10 dB gain: \$20\log_{10}{(\frac{Z_2}{Z_1})}=-10\implies Z_2=Z_1{\cdot}10^{-10/20}\$

So I don't know what's wrong.

Thank you very much!!


1 Answer 1


You are using the gain equation for an inverting amplifier, but your circuit is non-inverting.

You either need to use inverting op-amps (and pay attention to the fact that the input impedance won't be zero) or you need to use attenuators where the gain is less than one.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thank you! Now I understood it clearly. I added a voltage buffer before entering the frequency band pass amplifier and it works perfectly. \$\endgroup\$ Jul 3, 2019 at 17:32

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