#### Audioguru again

- Joined Oct 21, 2019

- 696

Now look at the feedback from the 33k resistor and from the 68nF capacitor.

**It is positive feedback**to boost the response a little at the cutoff frequency to make a sharp Butterworth corner.

The datasheet for most opamps shows that the input impedance is Megohms or higher that have no effect on the 43k resistor.

A 52 years old 741 opamp has the lowest input impedance that I have ever seen. Its datasheet says "2M typical".

The input impedance of an OPA134 or TL071 audio opamp is 10 trillion Ohms.

The input of your circuit is a series 150nF capacitor that feeds the 100k bias resistor that has its AC grounded by the 22uF capacitor. It is calculated to have a highpass -3dB cutoff frequency of 10.7Hz so that it does not cut the 30Hz much from the Sallen-Key Butterworth highpass active filter.