International projectionist (Jan-Dec 1935)

14 INTERNATIONAL PROJECTIONIST May 1935 Questions and Answers on Sound Projection Aaron Nadell Note: This department is launched in response to numerous requests by readers for such a service. The worth of this section, which will be a monthly feature, depends wholly upon the cooperation given by readers: the more numerous the inquiries, the more varied the department and the greater its value. Correspondents are requested to be specific and as brief as possible. Only initials will be appended to questions.— EDITOR. Q. In your article you mention an "audio oscillator" for testing and comparing amplifiers with which I agree, but where can a satisfactory oscillator circuit be obtained with which to build such an audio oscillator? Too much depression for me to buy such as . . . and I must be content to construct one. I have searched several amplifier texts for a circuit and given up in despair. E. W. McG., Utah A. Sorry; but LP. believes that not only will you find an oscillator cheaper to buy than to build, but the whole practice of the construction of sub-standard and doubtful apparatus by projectionists or others not equipped to test their results is poor policy. Suppose you had the circuit and had built the oscillator, how would you know it was accurate enough for use without sending it for calibration to the Bureau of Standards or an electrical testing laboratory, either of which would charge you a substantial fee? And if they found it working improperly, you would have to rebuild and pay another test fee. You can probably rent a calibrated oscillator, if you need one, from some radio repair shop in your city. For all ordinary work test loops, test film and test records should prove sufficient. Q. I am constructing a microphone amplifier suitable for a two-button mike to be used with the installation in this theatre. Would appreciate it if you would send me a schematic diagram of a good two-stage amplifier suitable for this type of equipment. R. A. P., Georgia. A. A double-button carbon mike working into the system amplifiers you mention will not need a pre-amplifier. All you need is a microphone transformer to switch to your amplifier input, which in your case is probably 250 ohms. Q. My amplifier is a Radiart, last stage using two 47's in push-pull. I note a better sound quality when I can increase the volume control on account of increased audience. Could I use two 59's and adapters without other changes, instead of 47's, and so use a little more of the current from the photo-cell and lower stage tubes? W. H., Illinois. A. These tubes do not have the same characteristics. The last stage of your amplifier would have to be re-wired. I. P. feels it is not good policy to make such changes without full equipment for testing the results. Are you sure the fault lies in your amplifier? There seems to be a possibility that the auditorium is too reverberant, and that the addition of an audience provides sound absorption that improves the quality. Q. Any information as to where an analyzer for testing sound equipment can be purchased and the approximate cost of same ivill be appreciated. Also interested in the same information about a good output meter. R. M., Penna. A. Your name will be forwarded to the manufacturers and supply companies concerned. Q. In the March issue you recom and Screen Size", D. Lyman, Eastman Kodak. "The Photoelectric Cell and Its Use in Sound Motion Pictures", M. F. Jameson, Bell Telephone Laboratories. "The Radiant Energy Delivered on Motion Picture Sets from Carbon Arc Studio Light Sources", F. T. Bowditch and A. C. Downes, National Carbon Co. "The Photographic Effectiveness of Carbon Arc Studio Light Sources", F. T. Bowditch and A. C. Downes, National Carbon Co. "Lighting for Technicolor Motion Pictures", C. W. Handley, National Carbon Co. "A New Wide-Range Spot Lamp", E. C. Richardson, Mole-Richardson, Inc. "Sources of Direct Current for Non-Rotating High-Intensity Reflector Arc Lamps", C. C. Dash, Hertner Electric Co. Interim Reports of Academy Committees on the Release Print and Screen Brightness, G. S. Mitchell, Manager, Research Council, Academy of Motion Picture Arts and Sciences. "Wide-Range Reproduction in Theaters", J. P. Maxfield and C. Flannagan, Erpi. "The Projection Background Process", F. Jackman, Warners. "Technical Considerations of the High-Fidelity Reproducer", E. D. Cook, RCA. "Development and Design of the High-Fidelity Reproducer", F. J. Loomis and E. W. Reynolds, RCA "Technical Aspects of the Motion Picture", A. N. Goldsmith. "The Contribution of Dr. Lee deForest to the Electronic and Motion Picture Arts", G. A. Chambers, Eastman Kodak. "The History of the Talking Picture", W. E. Theisen. "Recording Music for Motion Pictures", M. C. Batsel, RCA. "Analysis of the Distortion Resulting from Sprocket-Hole Modulation", E. W. Kellogg, RCA. "A Comparison of Variable-Density and Variable-Width Sound Records", E. W. Kellogg, RCA. mend an analyzer, output meter, sound and visual test reel, film loops. As I am servicing nine theatres . . . please refer this to the makers of the above mentioned equipment. Also would like to have the names of some manufacturers of parts and supplies for W '. E. equipment. J. B., Oklahoma. A. Your name will be forwarded to the manufacturers and supply companies concerned. The sound and visual test reel can be obtained from the Society of Motion Picture Engineers, Hotel Pennsylvania, New York. Q. I desire information as to where or of whom I may be able to purchase an analyzer. . . . I am under the impression there are five meters of different types, also space for testing tubes, all on one panel. W. W. J., Texas. A. Analyzers differ in construction. The more modern types usually mount only one or two meters, which can be used for many different purposes, such as reading voltage, current or resistance, a.c. or d.c, by changing switch settings on the panel. The panel may contain a number of sockets for testing different types of tubes, or only a single socket, in which all tubes are tested by means of adapters. Your name will be forwarded to the manufacturers concerned. Wide Range Analysis Q. I would appreciate it if in future issues you can publish an analysis of the network used for wide range in Erpi systems. Why is the 250-ohm output of the amplifier used instead of 500-ohm output? Also an efficient method to test for continuity without upsetting the magnetic characteristics in audio transformers and voice coils. A. D., Penna. A. An analysis of the wide range speaker system is scheduled for an early issue. You can test continuity coils with good headphones, of approximately 1,000 ohms per receiver, and a B or C battery. A good quality, high-resistance voltmeter or ohmmeter can also be used. None of these instruments will pass enough current to do damage. Q. May both types of photo-cells [vacuum type and gas-filled type) be used in sound-on-film equipment? Do the two types have different operating characteristics ? H. W. W., Ohio. A. Both types have been used. Op erating characteristics differ slightly chiefly in that the plate current of trie gas-filled type is higher and the volume drawn from it therefore greater. Plate voltages are likely to be identical, although on this point you must consult the manufacturers of the particular cells you have in mind. With similar plate voltages little or no modification is necessary to substitute one for the other. You do not mention the type of equipment in which you wish to use these cells.