Designed for the audio track, this circuit was found in a Popular Electronics from the 90's. Other operational can be used and R7 can be 470 k. The frequency is 1 kHz.
In this circuit of an old American documentation, the input impedance is the same as the output...
This circuit of old documentation is based on an operational amplifier that is no longer used...
We found this circuit in the March 1992 Radio Electronics magazine. The circuit provides 100 Hz...