240 Chapter 8 C2 R3 R2 3000 R - 220 nF R1 680 R 220 nF + A1 NE5532P R1 Out In 1400 R 1400 R C1 470 nF R2 - A1 NE5532P + 1400 R C1 104 nF Out 220 nF C1 In C2 Butterworth highpass 510 Hz (a) Butterworth lowpass 510 Hz (b) Figure 8.37: Second-order Butterworth highpass and lowpass MFB filters for distortion and noise tests. Cutoff frequency is 510 Hz for both. Figure 8.38: THD plot from the second order 510 Hz MFB highpass filter; input level 10 Vrms. The upper trace shows distortion from polyester capacitors; the lower trace, with polypropylene capacitors shows only noise. was still eliminated. Obviously, we need to see if the same applies to the MFB filter. I was not optimistic, because of the different ways in which the two kinds of filters work, but it does more or less work. In the highpass case only C1 and C3 need to be polypropylene, both capacitors contributing distortion. C2, however, can be a cheaper and smaller polyester type.