Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, July, 1930 Page 9 Public-Address and Centralized Radio Equipment \OUfoQ.fiion\Jy <^> By E. W. UArcy* flat Won* tyifjbX* VoXAA/mS Q-onlnox c?f aren't ^d VI. Volume Controls THE author has received numerous inquiries requesting more information on volume controls and in response to this widespread interest has changed the schedule somewhat and for this article will deal with the subject of volume controls. It is obvious that the proper volume control to use in any given instance is dependent largely on the nature of the generator and load with which it is being used. We shall therefore divide this subject into separate discussions on several typical cases. Inductive Generators The most common case that comes up for consideration under this heading is that of an electric phonograph pickup. The nature of its impedance is purely inductive and therefore its impedance varies proportionately with freqency. The impedance curve of the common type of low impedance pickup is shown in Fig. 37. It can be quite readily seen that a volume control to operate between this pickup and its load, if we consider it from the standpoint of a power device, may be one of many compromises. If, on the other hand, consideration of the nature of this pickup can be altered to consider it as a source of potential and not power, a pickup volume control can be evolved that is both distortionless and efficient. Chief Engineer, D'Arcy Laboratories. IMPEDANCE CURVE OF 4-A REPRODUCER INDUCTANCE .C 74 3 HENF Y RESISTANCE 8 8 C HMS o o f t/> a. o 2 ' o FREQUENCY IN CYCLES Figure 37. A potential divider of the correct value fits the requirements exactly. A primary requirement for satisfactory operation is that of a sufficiently high resistance to insure that a minimum amount of current is drawn by the potentiometer. The effect of a potentiometer, of too low a resistance, is clearly illustrated in Fig. 39. Possibly a good rule to follow in this respect is to design the potentiometer to have a resistance at least three times the highest impedance of the pickup at its determined upper cut-off frequency. The transformer's input impedance should also be sufficiently high to insure a minimum of current drain. A suggested input impedance would be in proportion to the values given on the graph. By examination of the shunt resistance method of volume variation we can easily determine why the apparent lack of clarity and definition of several types of phonograph combinations manufactured. Function of Frequency As has been determined the effect upon the ear of loudness is carried largely by the lower frequencies, while musical definition and speech clarity are dependent upon the higher frequencies. It is fairly obvious therefore that when the shunt has reduced the volume to any noticeable extent the "highs" are lost. A curve of a balanced network operating between an inductive generator such as a reproducer and a potential load as furnished by an input transformer is given in Fig. 40. This particular illustration is used to demonstrate the effect of a constant output impedance device working into a potential load such as furnished by the input transformer. A variation takes place in this particular instance which does not occur when the generator is a carbon microphone or tube. As a matter of fact when networks of this kind are operated from a "resistive" generator the distortion caused by the network is not noticeable and for this reason they are used extensively in telephone practise. Where reproducers are used in connection with other equipment such as feeding through mixer circuits, the volume controls must be of such nature as to furnish satisfactory volume variation for several different types of generators, as carbon microphones, condenser microphones, telephone lines, and in some cases radio. As a rule any one of these sources of energy may be fed into the same position as the reproducer by means of jacks or switches. Volume control requirements in this case are influenced largely by the most important source of supply, which in the case of broadcasting stations, motion picture recording and public address systems are identical. Where several circuits are fed into the same amplifier and combined, as in mixer circuits, individual variation of the volume from any separate line should be possible without affecting the output of any of the other circuits. Some type of balanced network is necessary in order to obtain satisfactory results. In motion-picture projection where osound-on-film as well as magnetic pickups are used, it is desired to have 1 SHUNT INPUT RESISTANCE TRANSF. J ( F 1 50 ; pickup f*— 1 § || M 3ENERATOR INDUCTANCE .03t8 HENRY PRIMARY " 2 " REQUENCY Z1 Z2 DO CYCLES 20 OHMS 1256 OHMS 00 " 200 " 12.560 " 00 " 1000 " 62.800 " 0 -10 -20 (/) i/) 3-30 Ul £ "40 u Id Q -50 1 1 urn ji SHt JN / / 7.5 OHI A' SH 2 OHM SHU MT'^ c c 3 go 2 o FREQUENCY IN CYCLES " Figure 38.