Movie Makers (Jan-Dec 1952)

44 Mixing sound for your movies Here's how to balance voice and phono signals for magnetic recording ALBERT B. DAVENPORT, ACL IN MAY, 1950, under the title A Simple Sound Control, Al Morton, FACL, discussed and diagrammed a mixing box for feeding voice and disc sound into the single input of his optical sound projector. The signals thus fed to the projector were then amplified by it and reproduced through its loudspeaker in direct accompaniment of a silent film screening. My problem was slightly different. But as I studied the Morton setup, it seemed to be — with proper application — just what I had been looking for. As far as I know, my application of the Morton mixing box has not been published. And it may not be theoretically correct. But it has worked out quite successfully for me. My aim was to use the box to mix and control the combined outputs from a microphone and a phonograph, applying them to the input of my Webster Chicago wire recorder (Model #180). Like most other recorders, this one has only a single input connection, which has a dual function depending upon the type of connection used with the input cords. As a result, when the microphone is in use there is no way of mixing background music with voice — other than picking it up through the mike itself, which is not so satisfactory as electronic recording. Likewise, when music is being recorded electronically, the mike cannot be used at the same time. The Morton mixing box, however, with only slight variations in hookup, has proven a very satisfactory answer to this dual problem — and without any internal changes in the recorder itself. Before beginning assembly, examination of the recorder hookup showed that, by the use of a threecontact "Jones" input socket, the microphone was plugged in directly for voice recording. But when the phono cord was to be used, by a variation in the connections of the input plug attached to it, a 1 megohm resistor is cut into the circuit to act as a voltage divider to take care of the increased voltage being applied. Since I intended using a single shielded conductor cord from the proposed box to the recorder input, it was evident that the extra resistor would have to be incorporated in the new circuit in some manner. The solution which was SYMBOL QUAN. "A" 1 R-l 2 R-2 2 R-6 1 C-l 1 C-2 1 C-4 2 C-5 1 6' 1 2 THE MATERIALS NEEDED PARTS AND DESCRIPTION Metal utility cabinet, 4" x 4" x 2", Type 3810 Mallory potentiometers, Midgetrols, Type U-53 1 meg., taper #1, audio Omite "Little Devil" resistors, 1 meg., V2 watt Omite "Little Devil" resistor, 1 meg., V2 watt Amphenol microphone connector, 75-PC1-M, male Amphenol microphone connector, 75-MC1-F, female Jones sockets. Type S-303-AB Jones plug. Type P-303-CCT Belden cable, =8412 Dial plate, 1-10 scale, "Microphone" Dial plate, 1-10 scale, "Phonograph" Bar knobs, red, IV4", setscrew type Shielded wire, nuts, bolts, etc. arrived at, as I say, may not be theoretically correct. But it seems to work successfully for me. Taking the hookup of the Morton box as published (see page 185, May, 1950 — Ed.), I inserted an extra 1 megohm resistor in the phono side of the circuit within the box, between the phono input and the R-l (potentiometer). This was to eliminate the use of the resistor of the same value inside the recorder. The input cord from box to recorder has a Jones three-contact plug attached, wired to points 1 and 2 of the plug, 1 being the ground and 2 the hot wire. The net result is that the box is now connected to the recorder just as a microphone would be, so that the resistor in the recorder is not used. This arrangement allows complete control of the phono input volume just as though the regular phono cord had been used. The only other major change that has been made is the type of input connectors used on the box. Since my setup uses Jones plugs for both the phono and mike cable connectors, Jones panel mounting sockets (Type S-303AB) were substituted for input connections, while an Amphenol-type connector is used for the output connection (75-PC1-M and MC1-F types being used). Admittedly the Jones sockets are harder to install, as it is necessary to cut out holes for them; but if mounted properly they make a neat appearance. The Amphenols originally recommended are self-ground [Continued on page 62] PLUG TO RECORDER INPUT MIKE CORD WIRING DIAGRAMS for the author's voice and phonograph mixing panel are outlined here.