These are some of the basic concepts of active filters taken from Rod Elliot's website. For full articles and explanations, see the "Elliot Sound Products" link from the home page. This is just an overview for quick reference.

Simulated Inductor
Simulated_Inductor.bmp

Phase Shifter
Phase_Shifter.bmp

Simple Tone Control
Baxandall_Tone_Control.bmp

First Order Filter
First_Order_Filter.bmp

Second Order Filter
Second_Order_Filter.bmp

Subtracting Technique
Subtracting_Network.bmp

Resonance
Resonance.bmp

Common-Mode Rejection (Balanced Audio)
Balanced_I-O.bmp

Advanced EQ
Parametric_EQ.bmp

Multiple Feedback Bandpass Filter (Graphic EQ)
MFB_Filter.bmp
Notch_Filter_Bandpass.bmp

Fully Parametric EQ

      This schematic is not part of the Elliot Sound Products website, but I think it is the most important filter network ever invented. This is a fully parametric EQ with characteristics similar to those found on a high-end mixing desk. All three parameters can be adjusted virtually independently of eachother. At the extremes of the frequency sweep, Q adjustment has a slight effect on gain. Besides that, it is a very good parametric EQ.
Parametric.bmp

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