International projectionist (Jan 1963-June 1965)

Record Details:

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Voltage gain Power tubes Ton* Volume P^d T~p <i-s Stereo pickup T hUh /■wi il1 Balance r FIG 3 — A simple stereophonic phonograph amplifier circuit by 120v. AC Telefunken, Germany. Note the common "floating ground" (heavy wire) shared by the two sound channels. All monaural records can be played with a stereophonic pickup, but stereophonic records CANNOT be played with a monaural pickup! Attempts to play stereo records with ordinary monaural reproducers will result in severe needle chattering, distorted sound, and damage to the record. Connecting Stereo Cartridges The prevalence of stereo records should be reason enough for replacing all regular 33%-RPM pickup cartridges with stereo cartridges. (Yes, these play 45's, too.) Just make sure that the correct type and style of replacement cartridge is installed. There are no problems with crystal-type cartridges. If the theatre amplifier system is single channel, only the two outside leads from the crystals are used, and these are connected to the high-impedance phono input of the amplifier in the same way as a monaural pickup. The "common-ground" wire which connects the crystals of a flip-over stereo cartridge is not connected to the amplifier for single-channel reproduction. The two back-to-back units of a flip-over cartridge having a stereo unit on one side for all 33's and 45's, and a monaurel unit on the other side for 78's, are sometimes interconnected internally, but a commonground terminal or wire is brought out. If there are four terminals, two are connected to each other (the socalled common ground), and the remaining two go to the amplifier input. If a stereo sound system is available, the common ground (left connected) is tapped and connected to the "neutral" or "floatingground" terminal, while the right — and left — channel leads are connected to the proper input terminals. Most commercial stereo phonographs for the home have a double preamplifier sharing a common floating ground. Indeed, amplification in the voltage-gain stage is usually accomplished by a single tube having a double cathode-grid-plate construction, such as the 12AX7. This functions as two separate tubes. In the power-gain stage, however, two separate power tubes are the rule. Because a stereo record player has only two channels, right and left, it is necessary to obtain a mixed signal for the center channel of a 3-channel theatre sound system. This is absolutely mandatory when there is only one set of "woofers" for the non-directional bass tones. If the sound system has internal connections Tube filaments Rectifier -WAN M 2 International Projectionist January, 1963 for reproducing the low frequencies in all three CinemaScope channels through the single woofer, no problem arises. Otherwise the bass tones of the records would be totally lost. It is best to consult the theatre's sound service engineer about this matter. Compensation for the frequency-response characteristics of a record reproducer must be provided unless the stereophonic amplifiers have phono input terminals. A CinemaScope magnetic soundtrack amplifier has a rather strong treble boost; and the low-impedance inputs of the film preamplifiers are unsuitable for crystal pickups. The following amplifier stage may or may not provide sufficient amplification for a frequency-compensated crystal-cartridge signal. If not, the output of the preamplifier ol a commercial phonograph may be used. All of these difficulties may be avoided, of course, by using the monaural optical-sound system, which is exactly what is done in many theatres. The directional effect of stereo records is lost, but the quality of the sound is entirely satisfactory if the low-frequency reproduction is sufficiently strong to balance the highfrequency reproduction. A high-frequency response which is too strong makes the sound "thin" and harsh. High frequencies may be attenuated if a separate phono preamplifier is used (highly recommended) by shunting a capacitor of 0.01 to 0.1 mf. across the preamplifier output. With the technical details of record players and their connections out of the way, attention should be devoted to building up a good record library containing as many different types of music as possible — jazzy, moody, gay, atmospheric, etc. It will thus be possible to match the overture and intermission music to any conceivable type of feature film. And if "locale" music is at hand — French. Latin American, western, hillbilly, oriental, etc. — the matching of music and mood will be all the closer. Next month's article, however, will concentrate on theatre-organ music in response to numerous requests from projectionists anent suitable organ records. We'll review and recommend quite a few such discs; and more than that, we'll delve into the mechanics and audio magic of the "Mighty Wurlitzer"! (To Be Continued) 13 '."--;