Amplifier Feedback

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.4.28 [502].

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.

The distortion output signal is often further filtered by an amplifier cabinet filter, representing speaker cabinet, driver responses, etc. It is straightforward to measure the impulse response (or frequency response) of a speaker cabinet and fit it with a recursive digital filter design function such as invfreqz in Matlab or Octave. Individual speakers general have one major resonance--the ``cone resonance''--which is determined by the mass, compliance, and damping of the driver subassembly. The enclosing cabinet introduces many more resonances in the audio range, generally tuned to even out the overall response as much as possible.

For class A tube amplifier simulation, there should be level-dependent duty-cycle modulation as a function:^5.20

• 50% at low amplitude levels (no duty-cycle modulation)
• 55-65% duty cycle at high levels

A simple method for implementing duty-cycle modulation is to offset the input to nonlinearity Eq.(D.9) by a constant, such as input RMS level times some scale factor.

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.