above and below fo is a small percentage such as 5 or 6 percent of fo 3 an interstage high pass type passive filter section 53 that is also excited by the a c signals passing through the initial high pass filter 51 and 4 a second bandpass type active filter 54 that is excited by the a c signals passing through the high pass filter 53 and is essentially a duplicate of the filter 52 except for being tuned to a center frequency substantially equal to the fundamental frequency of the 12 u00d7 ripple component of vdc persons skilled in the art will understand that a half power frequency is any excitation frequency at which the output voltage of a filter is 70 7 percent of its peak magnitude which in turn is obtained in the case of a bandpass filter at the center frequency in logarithmic units the magnitude at this frequency is down 3 decibels 3db from the peak magnitude the ripple sensor 48 also includes a pair of precision rectifier circuits 55 and 56 connected to the output terminals of the respective bandpass filters 52 and 54 these circuits rectify any signals passing through the bandpass filters the rectified signals are respectively supplied to a pair of smoothing circuits 57 and 58 comprising conventional low pass filters for deriving on the lines 360 and 720 corresponding output values that vary with the average magnitudes of the signals supplied by the rectifier circuits 55.