A more extreme effect used with distorted electric guitars is amplifier feedback. In this case, the amplified guitar waveforms couple back to the strings with some gain and delay, as depicted schematically in Fig.9.4 .
The Amplifier Feedback Delay in the figure can be adjusted to emphasize certain partial overtones over others. If the loop gain, controllable via the Amplifier Feedback Gain, is greater than 1 at any frequency, a sustained ``feedback howl'' will be produced. Note that in commercial devices, the Pre-distortion gain and Post-distortion gain are frequency-dependent, i.e., they are implemented as pre- and post-equalizers (typically only a few bands, such as three). Another simple choice is an integrator for the pre-distortion gain, and a differentiator for the post-distortion gain.