ABSTRACT
A practical version of an MFB lowpass filter is shown in Figure 8.35, designed for a Butterworth characteristic, (Q = 0.7071) and a cutoff frequency of 1 kHz; the passband gain is unity. Note that if C1 is a preferred value, C2 will in general not be; the value of 104 nF shown here would in practice be approximated by 100 nF in parallel with 4n7. As usual the resistor values do not work out conveniently, but very close approximations to these values can be cheaply made up using pairs of preferred values. MFB bandpass, highpass, and lowpass filters all inherently give a phase-inversion. This is no problem if they are used in pairs in a fourth-order Linkwitz-Riley crossover, but otherwise could be inconvenient, in the worst case requiring another stage that does nothing but invert the signal to get it in-phase again.
