Using 4 operational amplifiers this circuit generates 4 different tones. The circuit is from an American documentation from the 90's. Operational amplifiers are tTL084.
The circuit shown is from the Texas Instruments optoelectronics manual, applying to low power...
This circuit was found in a 1968 General Electric documentation. It has an input impedance of 2 M ohms...
This filter that rejects a frequency range determined by the double-T was found in the August 1993...