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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?
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  • \$\begingroup\$ Maybe this is related? At least similar. \$\endgroup\$ Commented May 8, 2018 at 18:57

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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.

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  • \$\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

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