This circuit was found in an old Popular Electronics. The integrated circuit is equivalent to a double 741. The source does not have to be symmetrical.
This circuit was found in a 1974 documentation. The bipolar transistor can be the BC548 and at the...
This circuit uses sunrise and sunset as a synchronization signal. The circuit is from Electronics Now...
This circuit was found in an Electronics World of 1966. It has a range of 5 to 25 000 Hz at 1 dB...