Projection engineering (Jan 1932-Mar 1933)

Page 24 PROJECTION ENGINEERING Fader attenuation circuits for public address and recording By W. S. Parsons* AFIELD with unlimited opportunities and growing with the passing of each day is to be found in sound projection. Large outdoor public-address systems are no longer a novelty, but the application of this type of service can be turned to other innumerable uses. Churches, schools, hospitals, funeral parlors, public ballrooms, lodge halls, factories, apartment hotels, and many other similar places, are all prospective customers for good sound equipment. This field offers a profitable business for both dealer and serviceman. There are several manufacturers offering good amplifiers to meet the various needs of this market, but the control of the input system and selection of controls and speaker arrangement of the output system are left to the ingenuity of the person installing the equipment. In the following paragraphs no attempt will be made to discuss the various amplification systems. That, after all, would be a duplication of the recommendations of any manufacturer of such equipment. Therefore, we will confine our remarks to a non-technical discussion of the use of attenuation units as controls in both input and output circuits. The requirements of sound projection are divided into two classes : First, a system of perfect fidelity, where both voice and music must be one hundred per cent. Installations of this type are necessary in schools, ^Engineer, Central Radio Laboratories, Milwaukee, Wis. I~1AD hotels, ballrooms, etc., where quality is primary and extreme volume secondary. Second, a class of equipment where volume is essential and fidelity only a necessity over the range of voice frequencies. This type of equipment is for announcing and call systems or outdoor public address where the major requirement is volume. Input Control Let us consider a typical input system for a requirement of the first class such as a centralized school installation. A microphone for announcements, pickup for recorded entertainment or educa traranrar > > VVWNA, — Vvwjv O -WwIa, vwyy^ — — ^vwvw CP VWWA Fig. 1. Fig. 2. tional subjects, and a radio set for broadcast features of interest to pupils. These three instruments must be properly mixed and controlled before input to the power amplifier to assure complete fidelity of the reproduced sound. See Fig. 1 illustrating connections. Also see Fig. 2 for connections for a 4 microphone mixer. A proper control at A, B and C. Fig. 1, is "T" pad No. 93-003, 200 ohms ; Fig. 2, A, B, C and D is "T" pad No. 93-005, 50 ohms. In the same type of mixer system as we have just described, the requirements may call for a double turntable installation for continuous recorded transmission. It is then necessary to provide a fader from one pickup to the other. See Fig. 3 for method of connections. A proper control is fader No. Fig. 3. 83-004 200 ohms. The pickups should be of 200 ohms impedance to match the 200 ohm fader. The secondary of the transformer in Fig. 3 should be 200 ohms for a 3-circuit mixer system and 50 ohms for a four-circuit mixer. A representative system of the second class would be a call system, of the type used in depots and factories, or a public address system devoted to announcements. The use of less expensive equipment will prove satisfactory in this type of installation as the prime requisite is ample volume, with fidelity necessary only over a relatively small part of the audio spectrum. Fig. 4 illustrates a four-microphone mixer panel providing a control for each microphone by means of an "L" pad. Four 50 ohm transformers in series feeding into a 200 ohm input transformer. A proper control for each microphone is "L" pad No. 96-005, 50 ohms. If a three-circuit control is desired, then the transformers should each be 200 ohms and feed into a 600 ohm input transformer, and the individual control "L" pad No. 96-006. 200 ohms. To summarize input mixer circuits, it must be remembered that the individual generator units such as microphone, pickup, radio set or other device, must be connected through transformers or direct in the case of pickups where they are of the same impedance value as the individual sections of the mixer, to individual controls. These controls are connected in series to match the standard line impedance values of 100 ohms. 200 ohms, and 500 or 603 ohms. The