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270 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
next, particularly if they do not have both the specialized magnetic circuit design such as is found in modern Western Electric, Langevin, and Altec-Lansing units that also operate at low flux densities.
Hum induction in heater-cathode circuits of amplifiers is best controlled by eliminating the source of the disturbance ; unidirectional heater supply is the only complete answer — as has been pointed out.
Special mention should be made of the resistors used in low-level circuits such as in preamplifiers. There is a wide range of variation of noise among the presumably similar products of different manufacturers; the high price of a composition (" firecracker ") resistor is not an assurance of satisfactory performance. Usually, composition resistors* used in plate circuits and in grid circuits which are of the 1/2 and 1-watt rating are quite consistent in performance in the Allen-Bradley make. For consistent performance in low-level circuits over long periods of time, there is no substitute for wire-wound resistors such as the IRC WW-4 type.
Coupling condensers in preamplifiers and other low-level amplifiers should always be of the best moulded mica types** ; the performance demanded of a preamplifier cannot possibly justify the use of poorly sealed conventional tubular paper condensers for such operation. Bypass paper condensers are best and most costly in the hermetically sealed typesf, as used in military equipment; just how far a designer goes in this regard is controlled by the market price of the finished equipment and by the service expected from it. For permanently installed racktype equipment of long life expectancy, they are quite necessary ; for portable equipment where performance must be sacrificed somewhat to achieve light weight, lighter types may be used. Ordinarily, equipment designed for long life will be appreciably heavier than equipment designed for shorter life due to the extra weight of the hermetic seals and the special cans or cases used, in addition to the more generous design. Generally speaking equipment of reputable manufacture is well designed and can be relied upon to have good component parts. Under ordinary operating conditions, a thorough check of all equipment (condensers in particular) several times each year is sufficient to detect defective components before interruption in service or actual breakdown occurs. In general, electron tubes are not as consistent with regard to noise as are other
* American War Standards C75.7. and C75.17.
** American War Standard C75.3.
t American War Standard C75.16.