Modeling Sallen-Key audio filters in the Wave Digital domain

Sallen-Key filters are widespread in audio circuits. Therefore, accurate and efficient digital models of such filters are highly desirable in audio Virtual Analog applications. In this paper, we will discuss a possible strategy, based on Wave Digital Filters (WDFs), for implementing all the analog filters described in the historical 1955 manuscript by Sallen and Key. In particular, we will group the eighteen filter models presented by Sallen and Key into nine classes, according to their topological properties. For each class we will describe the corresponding WDF structures. Finally, we will compare the output signals of WDFs to the output signals of the same models implemented in LTSpice.