This circuit was found in a 1967 EEE. The transistors are from the time and can be tested equivalent.
Found in a 1980s documentation, this amplifier uses an operational aplifier of the time with two...
This circuit was found in a Popular Electronics from the 90's. The frequencies depend on the...
This is a Philips commercial circuit from the 1980s. The circuit must be powered by 28 V and its...