Movie Makers (Jan-Dec 1953)

291 all these connections are carefully shielded and grounded to a common ground connection, as is usual in all audio systems. Use single shielded wire. SIMPLE MIXING SYSTEMS Mixing the sound signals from records and microphone is not really a difficult problem. It can be done simply either by using two volume controls and two resistors or by means of a mixing tube. The resistor method is shown within the dotted lines in Figs. 4 and 5; the mixing-tube circuitry is shown in Fig. 6. These mixing methods are included in the audio control center circuits which will be described in this article. Note in the diagrams that there is only one output connection for plugging into the sound recording medium. This plug-in will be either to the microphone or phonograph input, depending on the amplification of the system. If both are provided, it would be well to try each in turn to see which works better. In general, the best results will be obtained when the recording unit — tape recorder or magnetic projector — has its volume control set at about one half of full volume. Once the volume control of the recorder is set, this should not be adjusted again. The control center has all of the facilities necessary to make and to mix the complete recording. RIGHT HAND-LEFT HAND The turntable circuitry is so arranged that by use of the turntable selector switch either the right or the left hand pickup arm may be connected into the mixer circuit at will. (The switch should be marked 1 and 2, or R.H. and L.H.) By having two turntables in operation it is possible to bring in either one of two musical selections at the appropriate time. And to enable the operator to cut in either turntable at the proper time, there is provided the monitor headset and its selector switch. By this means the operator can listen to either record at will, although only the one cued in by the turntable selector switch will be playing through to the recorder. This is a great convenience when striving for exact transitions between one record and the other. AMBIDEXTROUS ARMS There is another important feature in this turntable design, and that is the ability of both pickup arms to play on any one record. This allows the same record, or portions of it, to be repeated at will by the instantaneous turn of the control switch. While the original or No. 1 pickup arm is playing, the second arm may be used to explore other parts of the record through the earphones. When the desired new portion of the record is reached, pickup No. 2 may be cut in via the selector switch. To assure this bi-pickup playing, the layout measurements given here should be followed closely. These measurements are correct for the generally used T^/o inch radius playback arms. [Continued on page 305] WIRING DIAGRAM -CRYSTAL PICK-UPS io&,m""""mu^^ V*vS. OHMS MICROPHONE '"""""""? k EARPHONES microphone: ? j t I f | \ VOLUME t ' \ 200000 OHMS TURN TABLE VOLUME 25000 OHMS 25000 OHMS EARPHONE SELECTOR '^ 'iiT— £ A < 2 TURN TABLE SELECTOR FIG. 4: Wiring schematic for crystal pickups shows use of tone control (top) but no pre-amps. WIRING DIAGRAM -MAGNETIC PICKUPS niiiuirinmuiiniiTTTT MICROPHONE I MICROPHONE VOLUME EARPHONES EARPHONE SELECTOR ,17 J TURN TABLE SELECTOR FIG. 5: With magnetic arms tone control is dropped, pre-amplification stage added at right. BOTTOM VIEW OF TUBE MICROPHONE / TURN TABLE SELECTOR d0^™_ OHMS 6.3 VOLTS AC (H CONNECTIONS TOR HEATERS) TO OUTPUT RECORDING SYSTEM 50,000 OHMS 12,000 OHMS 8 MFD MIXER-AMPLIFIER-CONNECTIONS 150 TO 18 VOLTS FIG. 6: Mixer here uses 12AX7 tube, while Figs. 4, 5 (see broken lines) use resistor units.