International photographer (Jan-Dec 1935)

. Ipril, 1935 T he INTERNATIONAL PHOT O GRAPH E R EL ven in a row in a one-inch wide bakelite strip 19 inches long, the bakelite strip holding 12 of the jacks. A small strip of paper slipped into a holder fastened over each jack gives the designation and circuit position of that jack. Several of these jack strips are installed in each bay ; and the input and output circuits of all amplifiers and other apparatus in that bay are connected to them. The jacks are "normalled" in such a manner that the equipment is connected together in the regular manner when there are no plugs in the jacks. An elaborate layout of jack strips is mounted in the central patch bay of large recording installations. This bay carries nothing but jack strips ; sometimes there are as many as twenty rows of them. These jacks are connected to the main input and output circuits of each channel ; and they are provided for the purpose of "patching over" entire channels. Patching within the individual channels is done by means of the jacks mounted in the bavs of those channels. **< AMPLIFIER u 3rr^ ^r?~* MPLiriER Y AMPLIFIER u D* rQO 3^ BOOJlfB MAIN RECORO I KlO> AMU AMXfR PA^ER /BRIDOINb (fi AMPLIFIERS RECORDERS FIC. 2. How Jacks are Employed Amplifiers. in the Connection of The special two-hole jacks can be represented by regular radio jacks as shown at A in Figure 2. It will be evident from an examination of this diagram that the output of amplifier U is normally connected to the input of amplifier V (U is "normalled" to Y), but that if a plug is inserted in either jack, the circuit between the amplifiers will be broken. When the plug is in jack V , it picks up the output of amplifier U ; when it is in jack W , it picks up the input of amplifier Y. How the output of amplifier U would be connected to two bridging amplifiers, Y and Z, through this arrangement of jacks is shown at B in Figure 2. Any of the three amplifier circuits may be picked up by inserting a plug in the corresponding jack. In block schematic diagrams, such as that of C in Figure 2, this arrangement of jacks is usually represented in the manner shown. Here the jacks are indicated by small circles, and the pairs of wires that connect amplifiers are represented by straight lines between the rectangles that symbolize the amplifiers. The arrows above the circles indicate the directions the jacks "face." Three of the amplifiers and three of the jacks are marked with By Charles Felstead, Associate Edito\ letters to correspond with those in B of Figure 2. Either of the two microphones, any one of the loud speakers, and the input or output circuit of any of the amplifiers or the mixer may be "picked up" and connected to another circuit by means of a plug and cord. This arrangement of jacks permits circuits to be changed with ease and surety; and in a case where severe trouble occurs in an amplifier and there is not time to disconnect it at the connection blocks, another amplifier from some other channel may be substituted for the defective one by means of the patch cords and jacks. Repairs to the amplifier that is not functioning properly can then be effected at leisure. Patch Cords For patching in other amplifiers in place of those normally in circuit, a special type of connector known as a "patch cord" is used. This is an 18-inch length of flexible two-wire conductor terminated on both ends by plugs that fit the jacks in the bays. In the absence of regular patch cords, two plugs of the proper type connected by a length of two-conductor No. 18 Tirex cord serves very well as a substitute, since the patch cords are so short it is not vitally necessary that they be shielded. Patch cords are usually made in two lengths, so that by selection of the cord of proper length for each connection unsightly lengths of cord will not be draped loosely about the equipment. A couple of patch cords of more than normal length are often found useful for elaborate patch-overs between portions of channels that are widely separated. A pair of high-resistance headphones equipped with a regular two-pole plug is necessary for plugging into the output-circuit jacks of amplifiers to check on the operation of the individual amplifiers. Special Purpose Jacks Besides serving in the input and output circuits of equipment in the bays, jacks are also provided in the channels for special purposes, such as to introduce into the circuit apparatus not normally there, and to facilitate connection between patch cords. A 500-ohm resistance shunted across a jack in the manner shown at A in Figure 3 furnishes a resistance termination that may be picked up by a patch cord and connected to any circuit that may need such a load. Another jack is connected to the ground bus, as at B. to provide a convenient ground connection in the jack bav. Other jacks — usually four or five of them — wired in multiple in the manner shown at C serve as "strap" jacks. B) means of this handy arrangement, the outputs or inputs of several different pieces of equipment may be patched together (that is, in multiple). The Multiple Jacks In elaborate installations where there are a number of channels, a row or two of "multiple" jacks in the cen(Turn to Page 26) Flease mention The International Photographer when corresponding with advertisers.