International photographer (Jan-Dec 1935)

Ten The INTERNATIONAL PHOTOGRAPHER November. 1935 The Practical Design of an Amplifier — Motion Picture Sound Recording CHAPTER XXIII May Be Used As Main Recording tr'%J\!i*l FTER twenty-two chapters of the theory of motion picIw/VVn ture sound recording, we come now to three chapters jfy 4Avj on *^e Prac''ca^ design of an amplifier that may be \ ~\<wj$sj used as the main recording amplifier in a film or wax recording system or as the main amplifier in a public address system. Since some of the readers may be MicHftol L_n_: PHONES] I If \^ CONTRot 3 -STAGE BASIC IounB ECODDIMCI PEVICC, S RECO FOR MOTION PICTURE SOUND RE Radio telephone TRANSMIT J ten — | FOR RADIO 6ROA0CASTINC H l OH POWER /MODULATORS OUTPUT r-ai ■a. FOR CENTRALIZED LOUO RADIO I N > SPEAKERS STALtAT.ONS l-du Figure 1. Block schematic of amplifier arrangement for various services. HlfrH POWER AnpiiriERS ^1 03 n-cn FOR PUBLIC ADORES* SYS loud TtMS SPEAKERS interested in this latter application, the following chapters will describe an additional high-power amplifier that may be added to this main speech amplifier to provide a public address system that will furnish sound coverage for as many as 5000 people. Design for Basic Amplifier In public address systems, centralized radio installations such as found in hotels and apartment houses, radio broadcast stations, and motion picture sound recording systems there is a basic amplifier that is practically the same for all these varied services. This amplifier is usually a three-stage affair. In the motion picture studio, it is known as the main recording amplifier; in the broadcast station, or broadcast station remote control room, it is called a speech amplifier. The input of this amplifier is fed from the mixer control panel, or main volume control, which in turn is supplied 'with sound energy by the microphones. This is clearly illustrated in the block schematic diagram of Figure 1. The arrangement of the circuit up to the output of this basic amplifier is the same for all the mentioned services; but the output connection is dependent upon the use to which the amplifier is put. In sound recording, the output connects to a recording device— a flickering lamp, a vibrating mirror, a light wave, or a wax cutting head — in radio broadcasting, it connects to the modulators, either directly or through another higher power stage of amplification; and in the centralized radio installation, or in the public address system, it connects, either directly or through another stage of amplification, to the loud speaking horns. The purpose of this basic amplifier is primarily to raise the electrical level of the speech current to a value that will be high enough to operate the recording device, modulators, or loud speakers at normal power. Design of the Amplifier We will start with two assumptions in the design of our amplifier: That it will operate at the input from a double-button carbon microphone or the equivalent, and that its power output will be about ten or fifteen watts. This requirement of a power output in the order of fifteen -watts means that we must use tubes of the intermediate power class, such as type 45 or type 2A3, and that they must be used in push-pull, particularly since we require a high degree of fidelity in an amplifier that is to be used for sound recording. For the input requirement, it is assumed that a high-quality double-button microphone of the Western Electric Type 600-A, or the older Type 387, or the equivalent is used. A microphone By Charles Felstead, Associate Editor of this type has an output of approximately 0.02 volts across 200 ohms. Since the output of this microphone is fed to the grid of the first amplifier tube through a microphone transformer that has a ratio which is usually 1 to 27, the peak voltage available across the potentiometer (marked Pot in Figure 2) is 0.02x27, or 0.54 volt. The potential we require for the grids of our type 2A3 tubes in push-pull reaches a maximum of 124 volts, since at a plate potential of 300 volts the grid bias voltage required for each tube is 62 volts and there are two tubes with their grids connected push-pull. This is represented in Figure 2. So we see that we must increase the input voltage on the grid of the first amplifier tube (0.54 volt) to 124 volts on the grids of the final amplifier tubes. This represents a voltage step-up of 124/0.54, or almost exactly 230, which is the voltage gain that must be provided by the amplifier. A little figuring, using the constants of the different tubes it would be possible to employ for our purpose, produces the result shown in Figure 2. The Developed Circuit Here we see that the first tube in the amplifier is a type 56, which has a voltage gain of approximately 10. Since the grid input voltage to this tube is 0.54 volt, the voltage that appears in its output circuit is 0.54x10, or 5.4 volts. Selecting a single-stage to push-pull transformer that has the proper input and output impedances for the tubes we wish to use and that has the normal ratio of 1 to 3, we secure a voltage for the grids of the tubes in our second stage of amplification that is equal to 5.4x3, or 16.2. The type 56 tube when operated at a plate voltage of 250 DOUBLE BUTTON CARBON mitKBoPHiiHI 56 2A3 Figure 2. Skeleton diagram of amplifier showing speech voltages. volts requires a grid bias voltage of 13.5; so, there being two tubes in push-pull, the tubes will stand a total peak grid swing of 2x13.5, or 27 volts. So our calculated grid swing of 16.2 volts is well within the safe limits, and distortion from overloading the grid circuits of these tubes in the second stage is not likely to occur. The voltage gain in these tubes being 10, a speech voltage is built up across their plate circuits that is equal to 16.2x10, or 162 volts. Now selecting a push-pull-plates to push-pull-grids trans(Turn to Page 25) M P G Re '.</■ 5. Patent Off. THE MOST TALKED OF FINE GRAIN DEVELOPER ON THE MARKET. IDEAL FOR MINIATURE CAMERA FILM. NO MIXING. ONE QUART SOLUTION GOOD FOR MORE THAN A DOZEN FILMS. FULL INSTRUCTIONS WITH EACH BOTTLE. AT YOUR DEALERS AT $1.25 A QUART. OR AT R. J. FITZSIMONS CORP. 75 FIFTH AVE. (Dept. B) NEW YORK CITY Please mention The International Photographer when corresponding with advertisers.