Loudspeaker (Jan-Aug 1931)

PORTABLE SOUND (Continued From Page 8) throughout, strong AC line noises and other detrimental effects leaked in when the P. E. C. amplifier was incorporated in the same case with the main voltage amplifier. The engineers finally segregated the P. E. C. emplifier from the main voltage amthat they can be isolated during operation but put into one case for handling. They can be instantly hooked together as shown in the photograph, by means of the flexible detachable plugs and cords. The P. E. C. amplifier case contains the input stage of 224 and its associate stage 230-test jacks and meter, etc., the plate of the 230 connects through the connecting cord to the main voltage amplifier. The main voltage amplifier consists of two stages of push-pull amplification and also the full wave rectifier, filter system, volume control, test meter and jacks. The reason for using two stages of push pull is the exceptionally low AC hum and the large amount of undistorted output derived from its use. The reader will notice that we use the ux210 or 310 tube in the output amplifier. The reason for this is that our experience with 250 and 345 tubes for this type of work has not been satisfactory. In talking picture properly is approximately 27 per cent more than for a crater area of 90 square millimeters with two sources of the same intrinsic brilliancy. After correcting for the difference in intrinsic brilliancy, it is found that only 7 per cent more light can be expected from the 16 millimeter carbons at 145 amperes than from the 13.6 millimeter carbons at 120 amperes. Such calculations as these, which are made possible in part by the data given above, should be of some assistance in the more efficient use of this very fine source of light for projection of all kinds. reproduction any barrel distorted reproduction is met by the average listener with disfavor. Accentuation of the bass is detrimental. What is needed most are frequencies in the voice range and accentuation of the higher range. This has been done very satisfactorily. In locations where 110 volts AC is not obtainable, but 220 or 440 is handy to attach to, a step down transformer can be secured for reducing the voltage to its proper value. In case DC is the only supply available the equipment can readily be used in conjunction with a small DC converter. This makes it possible to use this system readily on trains, boats, etc., where no other type of electrical energy is available. There will no doubt be in the near future considerable use of the various types of portable sound apparatus. They are particularly suitable for use in churches, schools, cafes, dance halls, private homes, etc., where temporary installations are desirable. It is therefore extremely fitting that every projectionist should have a working knowledge of this as well as the other systems on the market. 1 1 i STRIP GEARS ON 707-A DRIVES If you should have a case where fibre gear on 707-A Drive is stripped and a disc show is being run, the 707-A Drive may be removed and film threaded through film compartment of 1-A Sound Unit. When removing this 707-A Drive, the 700-A Aligning Tool should be insei’ted in place of 707-A Drive in order to hold castings of universal base in place. However, if you do have a film show and not over three teeth of this gear have been stripped, the 707-A Drive may be left in providing the bevel gear on 708-A Drive is set just beyond where these teeth are torn out. 707-A Drive will then obtain enough momentum in one revolution to ride over these broken teeth of gear. Twenty seven