1. An audio signal processor to perform an audio processing function upon an input audio signal, comprising: an audio processing device including a digital signal processor to modify an input audio signal in accordance with a signal processing instruction set, the audio processing device comprising a foot-actuated switch to allow a user to enable and disable digital signal processing functionality performed by the audio processing device; wherein the input audio signal is an input audio signal from an electric musical instrument; and a removable module for coupling and de-coupling from the audio processing device, the removable module having a signal processing instruction set and having a user interface including at least one control to set a control parameter, wherein, when the removable module is coupled to the audio processing device, the signal processing instruction, set is transferred to and implemented by the digital signal processor of the audio processing device to perform an audio processing function upon the input audio signal, wherein the user interface is particularly designed for the removable module and implemented as part of the removable module such that the control parameter from the removable module is transferred to the digital signal processor of the audio processing device to modify audio processing performed by the audio processing device upon the input audio signal.