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Blockage of the amplifier previously ‘mentioned may be avoided by Zener diodes 155 and 156 connected to ' the output plates 152 and 153, or, alternatively, a neon tube could replace diodes 155’ and 156. Further, the gain of the amplifier is such that the smallest defect sig- ' nal is ampli?ed to say a 50 volt level. The voltage break down of the Zener diodes or the neon tube is so chosen that any output signal above the 50 volt level can not raise the potential of plates 152 and 153 due to the dis charge in such diodes or neons. As a result the A.C. negative feed-back pass does not receive signals above the minimum defect level and can not induce blockage. Additionally, the steady state DI). levels of the output plates 152'. and 153 are substantially de?ned by the pre cision resistors 165, 167, 145 and 166, 168 and 146, when the supply voltages +HT and —HT are regulated and have the long-tern stability of plus or minus one volt, the output plate potentials are de?ned with an accuracy of approximately plus or minus a volt and are equal, assuming, of course, that the resistance of resistor ‘165 equals that of element I456 and 167 equals that of ele ment 168 while element 145 equals element 145. When a black defect in the sheet material reduces the light collected by the photoelectric cells 98 or 99, a posi tive sign-a1 indicated by numeral 17% is produced at the output plate 1:32. Simultaneously, the cathode coupled pairs shown in H6. 10 produce an identical negative output signal indicated by numeral 169 at the output plate 153, such signal is negative only relative to the plate mean potential. It will be seen therefore that sig nals due to any defect result in two output signals which are equal and are not in-phase. When a defect viewed by any of the photoelectric cells 98 through 3911 is brighter or darker than the mean intensity of the sheet material, one of the output plates 152 or 1531's always positive while the other is negative. Signals indicated by the numerals 171 and 172 represent such push-pull anti-phase signals; the negative section of the output signals 169 and the positive section 176‘ are due to the ‘defect and sections 173 and 174» are due to the basic noise of the sheet material. Each of the output signals just mentioned are fed by two channels to the output pulse generator which may be a S-chrnitt trigger generator which is well known in the art and which consists of double t-riodes 175 and 176. Channel one of the invention is the amplitude differ entiating channel and its operation is described in what follows. The potential of the cathodes of the triodes 175’ and 176 is defined by potential pider chain 177, 173 and 179. Adjust-able resistor 179 sets this potential to say 250 volts; triodo 175 is normally cut off so that the con ducting triode 176 acts as a cathode follower. Potential pider 189 de?nes the potential of the cathode of the output cathode follower 131 of the amplifying system and is, for example, set at 225 volts. The 25 volt difference between the cathode potential of triodes 175 and 176 and the cathode potential of cathode follower 131 is suf?cient to cut off triode 175. > The potential of the control grid of triode 181 is de ?ned by the potential pider 189* through the grid-leak resistor 182 and is close to the potential of the cathode of triode 131 for the reason that triode 131 is a cathode follower. For purposes of description, the grid potential is assumed to be 223 volts. This potential of 223 volts is also applied through resistor 183 to the junction of the output plate coupling diodes 157 and 158. The anodes of the diodes 157 and 158 are connected to the output plates 152 and l53lwhile the cathodes are connected to the grid of the output cathode follower 181 through re sistor 183. ‘Therefore the cathodes of the said diodes are biased 23 volts positively relative to the steady state out put plate potentials which are set at 200 volts by the pre viously described feed-back network. The peak values of the defect signals 169 and 170 are limited to plus or minus 50 volts relative to the mean output plate poten tials .by the Zener diodes 155 and 156. Therefore, the peak level of the output signal 169 is +150 volts and that of the output signal 170‘ is +250 volts. Peak values of the basic noise signals 173 ‘and 174 are less than plus or minus 10 volts relative to the mean output plate poten tial so that the noise signals do not pass through the biased coupling diodes 157 and 153 while a portion of the defect signal 170 will pass through coupling diode 15S and will 10 appear at the cathode of the cathode follower 181 and ' at the grid of the trigger generator triode 175.