ABSTRACT
Figure 9.1 shows the normal configuration of an MFB or Rauch filter. The minimum Q that can be normally achieved is 0.707 (1/√2), which requires R2 to be set to infinity, that is, not fitted at all. If you want a lower Q than that, you still omit R2, but the vital point is that a different set of design equations are used, which give different values for the other components and allow lower Q’s to be realised. These lower Q’s are required for time-delay compensation allpass filters, and for filler-driver crossovers. The design process for allpass filters is fully described in Chapter 10 on time-domain filtering, where low Q’s are especially necessary. This variation is on the standard MFB filter is sometimes called a Deliyannis filter.
