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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!!

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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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\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\$ – FelipeMedLev Jul 3 '19 at 17:32

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