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162 TEMPLIN August
MIXER
The RA-1283 mixer is shown schematically in Fig. 2. Two microphone inputs are provided, working into separate preamplifiers. These amplifiers are arranged to operate from microphones having a 30-ohm impedance such as the Western Electric RA-1142 (cardioid) type. The preamplifiers have a gain of 33 decibels, an overload level of 0 dbm* for 1 per cent total harmonic distortion, and a background noise level of —93 dbm at the amplifier output.
Separate variable dialog equalizers are provided in each preamplifier. This feature is desirable not only to provide greater flexibility but because of special requirements in a low-cost system that it be possible to use the original film for release negative rather than undergo a costly re-recording process. In applications of this type, background music might be recorded simultaneously with the picture and the lowfrequency equalization for the music would generally be considerably less than for the dialog. As another example, in a two-microphone setup where acoustics differ markedly in the two positions, the equalizers may be adjusted so that they are balanced in the original recording, thus eliminating the necessity of frequency-response readjustment during a subsequent re-recording process.
The desired equalization characteristic is obtained by series and shunt elements in the plate circuits of the preamplifiers. The relatively high impedance of the plate circuit allows a smaller space and weight for the components than would be possible in the 600-ohm output circuit. The dialog-equalization characteristics are described in detail in a later portion of this paper.
Separate mixer potentiometers follow the preamplifiers. These are standard 600-ohm bridged-T type having l1/^ decibel per step (3/4 decibel per step when bridging two contacts) and tapered to full cutoff at the maximum counterclockwise position. The mixer potentiometers terminate in a combining network, the output of which is carried to the mixer output terminals.
Either phototube or direct monitoring may be selected by the mixer operator providing the optional RA-1278 phototube monitor assembly is installed in the recorder.
A volume-indicator, direct-monitor line is bridged from the main amplifier output. The volume-indicator circuit includes a highspeed meter and a control switch providing a range from +2 to +24
* Decibels with respect to 0.001 watt.