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478 XIII. PEOJECTIOX AND PROJECTORS
some 10 db or more of feedback around the output tubes, such machines are still being sold in large numbers despite the lack of this simple yet effective design requirement. If sufficient inverse feedback is judiciously used with the customary tetrode amplifier, using output tubes such as the conventional 6V6 in push-pull, or, better, if a triode output push-pull stage such as a 6AS7G driven by a suitable cathode follower or other lowdistortion driver is used, the over-all performance of good film with a good loudspeaker would surpass even the optimistic hopes of the average 16-mm film user. It is a sad commentary that many present-day 16-mm audio systems have excessive intermodulation when compared with the 16-mm triode-output amplifiers used over 15 3*ears ago in the very first commercial 16-mm machines.
Most machines have both a volume and a tone control. In some machines, the volume control is an interstage control in the amplifier ; in a few others, it is a rheostat in the exciter lamp circuit. A disadvantage of the rheostat type is that the signal-to-noise ratio becomes poor when the lamp current is cut down to reduce volume ; however, the exciter lamps last longer. In a few cases the tone control is merely a variable resistor-condenser combination that reduces high frequencies in the manner of similar tone controls in radio sets. In most cases, the tone control is a variety of inverse feedback control that alters the low frequency end of the spectrum when turned in one direction, and the high frequency end when turned in the other direction.
The Loudspeaker.* The almost universal use of the direct-radiator type of loudspeaker — known often as the dynamic cone loudspeaker — is due to the simplicity of its construction and to its low cost. Because of the influence of price-competitive home radio set design, even the loudspeaker supplied today is not as satisfactory from the standpoint of distortion as the heavy loudspeakers of 15 years ago or more. The efficiency of a loudspeaker, like any other form of electric motor, depends upon the flux density in the air gap, the length of the conductor in the gap, and the current through it. To reduce cost in electrodynamic loudspeakers, the amount of copper used in the field structure was reduced from several pounds to a fraction of a pound; when permanent magnets were substituted for electromagnets, there was no attempt to provide the large field flux of the earlier heavy electromagnets. To offset the loss in loudspeaker efficiency due to the reduction in magnetic flux density in the air gap, the power output of the amplifier had to be increased. Unfortunately, high power output was obtained only with
* See pages 300-303.