for the reception of cw radiotelegraphy (morse code), the feedback is increased just to the point of oscillation. the tuned circuit is adjusted to provide typically 400 to 1000 hertz difference between the receiver oscillation frequency and the desired transmitting station's signal frequency. the two frequencies "beat" in the nonlinear amplifier, generating heterodyne or "beat" frequencies. the difference frequency, typically 400 to 1000 hertz, is in the audio range; so it is heard as a tone in the receiver's speaker whenever the station's signal is present.