CHAPTER 9 Bandpass & Notch Filters This chapter deals with bandpass and notch filters. Bandpass filters as such are rarely used in performing the basic band-splitting functions of a crossover (the filler-driver concept being the notable exception), but they can be useful for equalisation purposes, and are essential for putting together high-order allpass filters for time correction. Notch filters can also be useful for equalisation, but their most important use is in the construction of notch crossovers, whether based on elliptical filters or other sorts of filtering. 9.1 Multiple-Feedback Bandpass Filters When a bandpass filter of modest Q is required, the Multiple FeedBack (MFB) or Rauch type shown in Figure 9.1 has many advantages. The capacitors are equal and so can be made any preferred value. The opamp is working with shunt feedback and so has no common-mode voltage on the inputs, which avoids one source of distortion. It does, however, phase-invert, which can be inconvenient. Phase inversions are no problem in