This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a Popular Electronics from the 90's. The operational amplifiers are the 741.
This circuit for old analog telephones is from period documentation. It controls a relay that can...
This circuit is from the 80's and can be implemented with modern operational amplifiers equivalent...
Found in Maxim's documentation from the 90's, this circuit uses a component from the time and has its...