This circuit from the 80's makes use of two transconductance operationals of the time. Modern equivalents can be used. The circuit does not need to be powered by a symmetrical source.
This circuit was obtained from a 1967 General Electric documentation. The circuit makes use of...
This configuration uses the 4 gates of a 7401 integrated circuit. The capacitors determine the...
This filter can be used to eliminate hum in sound equipment. Operational amplifiers can be 741 and...