Calculating Coupling and Decoupling Capacitors

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.

Thread Starter


Joined Nov 12, 2019
Okay thank you. I have measured an input impedance of 30kOhms at the Actual amplifier board. If i now use a 10uF coupling capacitor at the output of my schematic, Fc is 0,6Hz.
The higher the capacitor value is, the lower the cutoff is, since this is a high pass Filter, i can just use a 100uF capacitor and i should be completely fine? Or am i not able to do this because u always said something like:
"The input capacitor has to match the output capacitor otherwise you will hear a big POP sound"

What did you mean with that?
Please explain it, so i can finally order my parts :_D

Thank you in advance
if you use a 100uF output capacitor then it will take 6 seconds to partially charge or discharge and you will hear bad distortion during this time. 10uF takes only 0.6 of a second which sounds better.

Looking at your active highpass filter that feeds the amplifier input then its opamp +12V bias voltage rises slowly so there will be no POP sound.


The fairly old NE5532 audio opamp was used as an audio preamp for low impedance dynamic microphones so it did not need a high input resistance. I have never seen an NE5532 opamp.