This circuit doubles the frequency of an audio signal, thus changing its timbre. The circuit was found in an Elektor from the 90s. The components are common today.
This is a commercial circuit for an AM radio with 4 transistors, published in a Japanese magazine in...
This circuit provides a few watts of output and can be used in the car as an auxiliary amplifier....
Other operators of the same family can be used in this circuit suggested by Motorola in its 1990...