This third-order Butterworth filter was found in American documentation from the 1980s. Formulas for component calculations are attached to the diagram.
We found this circuit in a 1976 documentation, but it can still be easily assembled using BC548...
This circuit of an old publication produces the insect-like noise that drives them away. The...
This circuit aims to increase the selectivity of receivers whose FI is 455 kHz. The circuit is...