Radio doings (Dec 1930-Jun1932)

Page Thirty-two RADIO DOINGS December, 1930 THE EXPERIMENTER'S The fan who is experimentally inclined can readily improve the performance of the average radio receiver by employing one or more of the simple additions here suggested. Methods of Tone Control A suitable tone control device can be constructed and applied quite readily. In principle, it involves the suppression of certain audio frequencies, which can be selected at the will of the listener. The fact is utilized that a fixed condenser offers an easier path to high frequency currents than to low frequency ones, whereas the metallized resistor tends to obstruct the higher frequencies. In the diagram a portion of the current may be shunted through any one of six paths. With the switch at the left, the .05 mfd. condenser is in the circuit. The high frequencies will find a ready path through this condenser and hence will be by-passed to ground. As a FIG. \ A SIMPLE METHOD OF TONE CONTROL result, background noises will disappear, the effects of static will be lessened and a deep mellow response will be attained. As the switch is turned toward the right, condenser capacities are reduced, with a consequent increase in reactive impedance. Hence, less of the high frequencies are shorted out, until finally the switch reaches the open point and all frequencies will be reproduced. Parts Required — One .05 mfd. Condenser (1); one .01 mfd Condenser (2); one .00025 mfd. Condenser (3); one .5 meg. Resistor (4); one .2 meg. Resistor (5); one .1 meg. Resistor (6) ; one seven-Point Switch. If it is desired to eliminate a greater proportion of the low frequencies, a tone control such as that shown in Figure 2 should be assembled. With the 100,000 ohm resistance in the circuit, the high frequencies will pass through the .1 mfd. condenser, but the low frequencies will be blocked to a great extent. With more and more of the resistance cut out of the circuit, all frequencies will avoid the fixed condenser and will pass through the shunted portion, so that with the resistance entirely out, the fixed condenser OETECTOf* ODET. F I G. 2 TONE CON TROL " PERMITTING ELIMINATION o/ElTHER LOW OR HIGH FREQUENCIES will have no action whatsoever. The .01 mfd. condenser may be switched into the circuit, if it is desired to eliminate a portion of the high frequencies. Parts Required — One .1 mfd By-Pass Condenser (1); one .01 mfd. Midget Condenser (3); one Electrad Switch (4); one Electrad Royalty Variable Resistance, type "B" (2). A Band Pass Antenna Selector The Band Pass Selector shown constitutes an extremely useful addition to the radio receiver which tunes broadly. The selector will give greatly increased selectivity. Two identical Silver-Marshall r.f. coils are used, tuned by .00035 ^T7 LROTOR 7|[]| I U ROTOR T ,(° /"«• " p ""7 -iTOANTtN ) j-r-l 9Aof SET 8 Li?) TO GROU _ 1 T OF SET FIG.7 A BAND PASS ANTENNA SELECTOR. mfd. variable condensers. Both of these condensers are turned by the same drum dial. A small trimming condenser is included in the circuit to bring the two tuned circuits into exact resonance. Varying the positions of the coil rotors results in tight or loose coupling. Loose coupling gives great selectivity and it is possible to obtain 10 Kc. separation or any other conditions desired, by proper setting of the rotors. Parts Required — Two Silver-Marshall Plug-in type R.F Coils, No. 111A with coil sockets No. 515 (3,6); one Silver-Marshall Midget Condenser, No. 340 (5); four Binding Posts (1,2,9, 10); two .00035 mfd. Silver-Marshall Variable Condensers, type 320-R (4,8); one Silver-Marshall Drum Dial, 810 R (7). An Efficient Scratch Filter Those who have radio and phonograph combinations will be interested in the construction of a combination scratch filter and tone control. The circuit shown can be connected readily in the audio frequency amplifier and will be ex FIG.8. 6 o— wvyvw AN EFFICIENT SCRATCH FILTER. AND TONE CONTROL FOR RADIO AND PHONOGRAPH COMBINATIONS. tremely useful in reducing surface noises from the records and also in eliminating other undesirable higher audio frequencies. In the radio receiver, the filter tends to modify the effects of static and to give a more mellow tone to the music reproduction. This device removes practically all scratch and static noises above 4900 cycles. It consists essentially of a Sliver-Marshall r.f. scratch filter choke, an Electrad Royalty variable resistance and small Flechtheim fixed condensers. List of Parts Required — One Electrad Royalty Variable Resistance, type "H" (6) ; one .005 mfd. Flechtheim Midget Condenser, type M-H (1); one .002 mfd. Flechtheim Midget Condenser, type M-F (2) ; one .0005 mfd. Flechtheim Midget Condenser, type M-D (3); one Silver-Marshall R. F. Choke, type 278 (4); one Electrad Switch; one Roll Corwico Braidite Hook-up Wire. (Continued on Page 41)