I'm designing an simple equalizer for a first course in Electric Circuits and having some problems. Circuit diagram and AC Sweep Simulation (Multisim 14):

Circuit Diagram

Equalizer Circuit Diagram

AC Sweep Simulation AC Sweep Simulation

The Low-Pass filter was designed to have a cutoff frequency of ~900Hz. Before I put an amplifier in the circuit, the simulation shows a cutoff frequency as projected. But after, it gone to ~1272Hz (see AC Sweep Simulation) and gain at lower frequencies is no longer 0dB (went to -3.3dB). For the High-Pass circuit, gain remains unitary (0dB) at higher frequencies, but cutoff frequency was altered too. So:

  • Why is this happening? (Change in gain and frequency)
  • How to avoid it?
  • \$\begingroup\$ Maybe this is related? At least similar. \$\endgroup\$ Commented May 8, 2018 at 18:57

1 Answer 1


The low pass filter has two series resistors of value 1 kohm and when you add-on the impedance presented by the inverting amplifier (5 kohm) you will modify the cut-off frequency of that filter. The gain will also lower because the 5 kohm forms a potential divider with the two series 1 kohm resistors.

A way to avoid it is to make the 5 kohm resistor much higher in value. It connects to a virtual earth summing point so it is effectively acting also as a resistor to hard 0 volts.

  • \$\begingroup\$ I changed to 200k and fc gone to ~650Hz, if i put 100k it goes to ~1000Hz. So, how to calculate the precise value that makes frequency as close as possible of 900Hz? \$\endgroup\$ Commented May 8, 2018 at 19:36
  • \$\begingroup\$ You have a sim tool and I would use that to find a value that suits your needs. \$\endgroup\$
    – Andy aka
    Commented May 8, 2018 at 19:43
  • \$\begingroup\$ In this case it's ok, I can easily do it. But what if I have a hundred to find? It's impraticable! \$\endgroup\$ Commented May 8, 2018 at 21:04
  • \$\begingroup\$ If you had a hundred, you’d probably design the filter to be interfaced by a high impedance amplifier then post mix into an op amp summer. Or design the filter with the 5 kohm as part of the model. Or of you really need filter accuracy, do it digitally. \$\endgroup\$
    – Andy aka
    Commented May 8, 2018 at 21:14
  • \$\begingroup\$ Okay, I got it! \$\endgroup\$ Commented May 8, 2018 at 21:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.