We found this circuit in a 1977 factory documentation. The operational amplifiers can be 741 and the symmetrical power supply. Capacitors can be changed for operation at other frequencies.
This circuit can use 20 to 100 mA relays with a general-purpose transistor such as the BC538. The...
The tone control in the figure below can be implemented with almost any operational amplifier. The power...
This circuit was found in a 1967 General Electric documentation. The components are from that...