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When the number of the equal interleaved aperture sections is n, the maximum number of ampli?ers which can be connected according to the stated rule is 1/zn, and the minimum number of ampli?ers is 2 when n/ 2 inter leaved sections are connected to one ampli?er and the other half to the second ampli?er; n must be an even ninnber in order to preserve the symmetry of the sys tem and should preferably be equal to 4m, Where m can be any number. As an example, when m equals 1, 2, 3, 4, or 5 n equals 4, 8, 12, 16 or 20. Adjacent aperture sections are connected to di?erent ampli?ers in order to eliminate blind spots in the view ing aperture 7 shown in FIGS. 72 and 3. To serve the same purpose, the photoelectric cells associated with one section also view bordering parts of both‘adjacent sec tions. If adjacent sections, as for example It and section vn—1, were connected to the two control grids of the same ampli?er, and if a defect then passes under the piding line of the said sections, the ampli?er would re ceive two identical inphase signals and such signals would not be ampli?ed. An' amplifying system such as block '87 or 90 seen 10 15 20 25 35 40 45 50 55 60 65 70 75 12 in FIG. 8 or one of blocks 91 to 97 seen in FIG. 9, is shown in the circuit diagram of FIG. 10. This ampli?er system performs all the basic functions which are speci?ed in the amplifying system shown in FIG. 4 and is immune to in-phase interference signals, ampli?es and detects de fect-signals brighter or darker than the mean intensity of the sheet material under inspection. It is not sensitive to supply and heater voltage variations. While it is also an AC. ampli?er it is substantially free from blocking effects, i.e., it reliably detects small defects which follow large and intense defects, its integrator channels do not respond to the basic paper noise and are sensitive to uni directional weak defect signals which extend in the di— rection of travel of the sheet material. Additionally, the output cathode follower stages of several such amplifying systems are paralleled and connected to the output pulse producing stage common to all amplifying systems such as block 39 seen in FIG. 4, in such a manner that undesirable interaction between the amplifying systems, does not oc cur. Photoelectric cells .98 and 99 represent the cells which view one aperture section, e.g. section n——2 of FIG. 9. Photoelectric cells 100 and 101 view another aperture sec tion, eg 11 seen in FIG. 9. For purposes of description only, two pairs of photoelectric cells shown in FIG. 10 ‘view equal interleaved aperture sections. The number of photoelectric cells which view each such aperture may be one or more. The cathodes 102 and 103 of cells 98 and 99 are con nected to load resistor ‘106 as well as the control grid 108 of the cathode follower 110. The photocathodes 104 and 105 of photoelectric cells 100 and 101 are connected to the load resistor 1107 as well as control grid 109 of a second cathode follower 1111. The resistance of resistor 106 is equal to that of resistor 107. ' The DC. voltages developed across the load resistors 112 and 113 of the cathode followers 110 and 111 are proportional to the diffused re?ected light from the sheet material viewed by photoelectric cells 98, 99 and 100, 101. The output of the photoelectric cells 98, 99 and 100, 101 are matched to 'be equal by adjusting the vari able arms 1'14 and 115 of cathode follower load resistors 112 and 113. Thus, the in-phase output signals of the inspection head are matched and are applied "to both in~ put control grids 118 and 119 of the ampli?er. The ampli?er proper consists of long-tailed coupled triode stages, three such stages being shown in FIG. 10.
'Triodes 120 and 121 represent the ?rst long-tailed pair, triodes 122 ‘and 123 the second pair and triodes 124 and 125 the output stage. The number of stages used will vary according to the gain ‘and stability requirements of the ampli?er. The plates of the triodes are connected to the positive supply line +-HT Fby resistors 126, .127, 128, 129, 130 and 131. Such load resistors belonging to the ?rst, second and output stage have equal resistances, e. g. the resistance of member 126 equals the resistance of member 127. The ‘cathodes of the paired triode stages are connected by resistors 132, 133 and 134 to each other ‘and are con nected ‘by the “long-tail” resistors 135 to 140 to the nega tive supply line -HT. The cathode resistors have equal resistance values for each paired staged. Thus the re sistance of resistor 135 is equal to the resistance of ele ment 136, the resistance ofelement 137 is equal to the resistance of element 138 and the resistance of element 139 equals the resistance of element 140. The gain of the ?rst stage is mainly de?ned by the ~ resistance ratio of resistors 126/132 or 127/ 132 giving an identical number, the second stage \gain by 128/133. The third stage gain is by 130/134. Therefore, the gain of the whole ampli?er is de?ned by the value of these resistances and is substantially independent of the triode characteristics as well as aging loss of electron emission from the cathodes and heater voltage changes in these tubes. It is an essential feature of my invention that when a particular defect is sensed by one or more photo electric cells 93 to 1011 and is ampli?ed to some well de ?ned level by the “constant gain” amplifying system shown in FIG. 10, all other amplifying systems associated with the rest of the photoelectric cell row 6, shown in FIGS. 2 and 3, must have the same gain and characteristics as the amplifying system shown in FIG. 10, raising the de feet signal to the same well de?ned lev'el. According to the present invention and in order to make all the ampli ?er systems identical with each other, precision resistors are employed and such resistors de?ne the “constant gain” of the ampli?ers. All ampli?ers therefore are made to provide the same gain so that the differences in tube char acteristics will have only a second order effect upon such gains and the gain frequency characteristics of all ampli ?ers will ‘be identical. Discrepancies of the nominally identical precision resistors are compensated for by a slight adjustment of the variable resistor 132 which equal izes the vaht of all the ampli?ers. The amplifying stages herein are direct-coupled to each other by resistors 141 through 144. The resistance of element 141 equals that of element 142 and the resistance of element 143 equals that of element 144. The control grids of the amplifying stages are con nected to the negative supply line —HT by resistors 145 through 150. The resistance of element 145 equals that of element 146 and- the resistance of element 147 equals that of element 143 while the resistance of element 149 equals that of element 159. The Values of resistors 141 through 15:’) are so selected that the voltage levels of the control grids 118 and 119 and the control grids of tubes 122 through 125 are close to ground potential. By equalizing the values of all the resistors attached to triodes 111, 121, 123 and 125 with the values of the corresponding resistors of triodes 110, 126, 122 and 124, the ampli?er is made insensitive to variations of the sup ply potentials +111‘ and —HT. Supply voltage varia tions are reduced to in-phase signals at the control grids, the cathodes and the plates of the triode-pairs, hence such signals are not ampli?ed. The frequency response of the amplifying system should be constant from 1 cycle per second to 9 kilocycles. The low frequency limiting response of 1 cycle per second is de?ned by the requirement of the system to be able to integrate information on the sheet material when it travels slowly. When the slowest velocity of the sheet material is 20 inches per second, information present in the sec tion of sheet material 20 inches in length can be inte grated. The limit of high frequency response is de?ned by the requirement of amplifying without attenuation of the smallest defect of lip inch by 1X52 inch at the fastest sheet velocity. When the velocity of the sheet is 200 inches per second the high frequency limit must be 3 kilocycles per second and at a sheet velocity of 601') inches per second the high frequency must be 9 kilocycles per second. The ampli?er must also respond without blockage to small signals which follow high amplitude signals, for example, when 21 1&2 inches diameter full-black spot fol lows a defect of a much larger area. In order to satisfy the low frequency requirements, the arnpli?er shown in FIG. 10 is DC. coupled from the photoelectric cell cathodes to the output plates of the tried-e pair indicated by numerals 124- and 125. It is a further essential requirement of the output triode pair 124 and 125 that the steady state potentials of such output plates 152 and 153 shall be equal. These potentials are made exactly equal by balancing potential pider 154. The plate potentials must, of course, be equal in order that the coupling diode network, which consists of diodes 155 through 169, will function satisfactorily. As is well known in the art in DC. coupled high gain ampli?ers, it is difficult to achieve the stated balance of the output potentials for long periods of time without re balancing such systems. Therefore, according to my in 10 25 35 45 55 65 70 75 vention, the DC. ampli?er shown in FIG. 10 is con verted into ‘an AC. ampli?er by a feed-‘back network consisting of resistors 165, 166, 167, 168 and capacitor 169. Substantially all the DC. output at plates 152 and 153 is fed back to the input grids 118 and 119 by this network while the useful signal frequencies from one cycle per second to 9 kilocycles per second are decoupled by capacitor 169 ‘and consequently are not fed back from the output to the input. As a result the system ampli?es without the feed-‘back attenuation of such frequencies.