This circuit appears on this site from different sources. This is from documentation from the 1980s. The operating system used is uncommon and can be replaced by a more modern equivalent.
This second-order filter was found in a documentation from the 90's. It can be implemented with...
Charcoal microphones are rare today. In the figure we have a circuit that shows the use of one of...
This circuit was obtained from an English publication from 1977. The integrated circuit is a...