Radio Broadcast (May 1928-Apr 1929)

Methods of Arranging VOLUME CONTROL SYSTEMS THE modern radio receiver has three controls on the panel — the tuning dial, the on-and-off switch, and the volume control. The electrical position of the first two controls is fixed — the on-and-off switch always is connected in the power circuit and the tuning dial always controls to the tuning condensers. The volume control, however, may be located at many different points in the circuit. From the standpoint of volume control a radio receiver might be divided into two main sections. In one section we place all the apparatus between the antenna and the input to the detector and the other section includes the circuits from the output of the detector to the loud speaker. Let us consider first the former section and determine at what points a volume control might be located. The First Section THE first section mentioned above, consisting of that apparatus between the antenna and the input to the detector, is actually the r.f. amplifier and so discussion now centers around where the volume control might be located in such an amplifier. In Fig. 1 we show seven diagrams of different parts of an r.f. amplifier system and each drawing indicates a different location for the volume control. Diagram a shows the volumecontrol resistor connected between antenna and ground. Sketch b shows the volume control connected across the primary of one of the r.f. transformers, in c the volume control is connected in series with the ground lead, and in d it is across the secondary of an r.f. transformer. In e the volume control is across the secondary of the r.f. transformer feeding the detector tube, and in f the volume control is a rheostat in the filament circuit. Diagram G shows a variable resistor, R, in series with the plate circuit of an r.f. tube and this provides another method of controlling volume. The characteristics of these various arrangements are briefly given below. Arrangement A: This control is used in many receivers and is considered quite satisfactory. Its one disadvantage is that when the control is adjusted to a point where its resistance is quite small (to obtain a low output from the loud speaker) the shunting effect of this resistor may lower the selectivity of the first r.f. transformer. Since, however, the volume is cut down when listening to powerful local stations, selectivity is not especially important and this is not a serious drawback. This volume control arrangement may be considered satisfactory. Arrangement B: This arrangement is practically the same as A except that the resistor is connected across the primary of one of the interstage r.f. transformers. This control may also be considered satisfactory. Arrangement C: With the volume-control resistor connected in series with the antennaground circuit, as in this arrangement, minimum volume is obtained when the volume control has a maximum value of resistance. This control will not tend to decrease the selectivity, but in many cases it has the disadvantage of making it impossible to bring the volume to absolute zero. Arrangement D: Connecting a resistor across the secondary of a tuned circuit is Table I i: il li. ii im ii ■ urn in" HI Resistance Arrangement Required Manufacturers 5000 ohms 10,000 ohms 100,000 ohms 100,000 ohms 15 ohms Carter type TP-5M, Frost type 1897 Carter type TP-10M, Frost type 1898 Bradleyohm type E, Carter type 11, Centralab type 100M, Frost type 1891, Clarostat Universal type, Electrad Tonatrol Bradleyohm type E, Carter type 11, Centralab type 100M, Frost type 1891, Clarostat Universal type, Electrad Tonatrol Carter type IR-15, Frost type 1815, Yaxley type 515, Clarostat Universal type, Electrad Tonatrol essentially similar to connecting a smaller low-value resistor across the primary, as was done in arrangement a, and both controls have essentially the same characteristics. Arrangement E: The input circuit to a leak-condenser-type detector tube is generally of much lower resistance than that of a tube used as an r.f. amplifier; for this reason the selectivity of the tuned detector grid circuit is lower than the r.f. stages. Therefore, a volume control may be connected across this tuned circuit without materially impairing the selectivity of the receiver. Arrangement F: A rheostat in the filament circuits of the r.f. tubes has long been a standard type of volume control in batteryoperated sets. However, it cannot be used with a.c. receivers, since it is not practical to control the volume by varying the filament currents of a.c. tubes. With the 226-type tubes varying the filament current would tend to increase the hum and in the case of the 227-type tube the electron emission from the cathode does not follow instantaneously the variations in current through the heater. Arrangement G: This type of volume control, consisting of a resistor in series with the B supply to the r.f. tubes, has been used very satisfactorily in battery-operated sets but cannot be considered a good control for receivers operated from a B-power unit. As the resistance is increased to reduce the volume the current drain of the r.f. tubes also decreases and, as a result, the voltage on the other tubes in the receiver is increased. In summary we would classify arrangements a, b, d, and e as satisfactory volume controls for any receiver, arrangements a and e being, in general, preferable. F is a satisfactory control for a battery-operated set. C and g are unsatisfactory. The Second Section ^SrOLUME controls in any part of the cir* cuit following the detector are generally unsatisfactory, for they do not prevent overloading of the detector tube when receiving strong local signals and detector overloading can produce serious distortion. A safe rule is always to locate the volume control at some point in the r.f. amplifier. In Table 1 we have listed resistors made by various manufacturers which can be used satisfactorily as volume controls. Fig. 1. — The volume of a radio receiver may be controlled in many different ways. The above diagrams illustrate seven different systems which are used frequently february, 1929 page 259