# Calculating Coupling and Decoupling Capacitors

#### Audioguru again

Joined Oct 21, 2019
696
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.

#### Yeye

Joined Nov 12, 2019
47
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

#### Audioguru again

Joined Oct 21, 2019
696
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.

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#### muyustan

Joined Nov 29, 2019
10
A 52 years old 741 opamp has the lowest input impedance that I have ever seen. Its datasheet says "2M typical".
NE5532 30-300 k ohms.

#### Audioguru again

Joined Oct 21, 2019
696
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.

#### DickCappels

Joined Aug 21, 2008
6,096
How does an LM883 stack up?

#### Audioguru again

Joined Oct 21, 2019
696
I was also going to mention trying an LM833 dual audio opamp ( but my numbers don't have a typo).

#### DickCappels

Joined Aug 21, 2008
6,096
I was also going to mention trying an LM833 dual audio opamp ( but my numbers don't have a typo).
I am jealous your numbers are better than mine. The 833 seems to perform well noise generator up to 20 kHz. The specs are impressive and the price not bad.