This third-order filter was found in a 1975 publication. The circuit must be powered by a symmetrical source. The components can be changed for operation on other frequencies.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...