Audioguru again
- Joined Oct 21, 2019
- 6,691
Look at the schematic. The opamps have 100% negative feedback (for a voltage gain of 1) since the output is connected directly to the - input.
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.
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.