Silicon v. Nickel for Audio Transformer Cores

Radio Record, Volume II, Issue 10, 21 September 1928, Page 30

Silicon v. Nickel for Audio Transformer Cores

Improvements in Design.

AN interesting article bearing upon the improvement of audio transformers appears in the latest issue of the General Radio Experimenter, as follows :- EVELOPMENTS in loudspeakers made in recent months have re sulted in instruments which have extended the reproducable ‘range of fre. quency by some seventy-five to one hundred cycles downward. At the same time there has been a downward extension of the frequency range transmitted by broadcast stations. These factors have combined to revise the requirements for satisfactory performance of audio transformers. A year ago, there was little justification for audio transformers reproducing frequencies much below one hundred eycles, since none of the speakers then available were capable of producing an audible sound at ‘such frequencies, even though it was present in the broadcast transmission, which it was not. S a result of these developments, the low frequency cut-off of audio transformers has been moved steadily until transformers are demanded which will amplify sixty, or even thirty cycles. " Why High Frequencies are Lost. The design of such trunsfurmers has not involved any new principles, but rather the overcoming of practical difiiculties. involved in the adaptation of well-known principles. The prob- lem of raising of the lower end transformer characteristic is prinarily one of increasing the input inductance of the transformer, although the lowering of the plate impedance of tubes has had the effect of imPy

proving the characteristics of transformers of earlier designs. The inductance of the transformer de pends upon three factors, the number of turns of wire on the coil, the size of the core, and the permeability of the core material. The gain in inductance which may be had by ‘adding primary turns is limited by the fact that the secondary turns must ‘also be increased unless the turns ratio is lowered. The result is the loss of high frequencies as a result of coil capacity. Nickel Cores. HH high permeability nickel alloys are being used to an increasing extent for audio transformers. These alloys of nickel and iron have the property of high permeability at low flux densities, the conditions encountered in audio transformer primaries. These ‘alloys have. however, some disadvantages. The high permeability is maintained over a rather limited range of flux density, and falls off rapidly at higher or lower values. Advantages of Silicon. mIMPLY stated, such cores saturate easily. his difficulty is becoming more important as the plate currents of vacuum tubes are increased. A more serious objection yet is that the transformer is permanently damaged by an increase in field strength such as might result from accidental connection in a circuit without a °C" battery, or where a "C" battery was run down, or where the plate current was abnormally large for any other reason. Such temporary increase in flux through the core permanently changes the characteristics of the material. Silicon steel. on the other hand, is aad —

not permanently affected by increases in flux. The frequency characteristics of the transformer is of course affected by core saturation while it exists, but the éfféct "is not lasting. These considerations render the nickel alloy transformers particularly valuable for special laboratory work, or in commercial installations where care is taken to ensure proper operating conditions... The ruggedness of the silicon core -type of transformer, bowever, recommend it for general experimental use where conditions are frequently hard upon delicate apparatus. All the electrical advantages of the nickel alloys may be obtained with silicon steel by adjustment of other factors in the design. — Importance of Coil Design. T was found that when the lower end of the characteristic had been extended as desired, by changes in the coil and core, there was a tendency to resonance at high frequencies as well as a falling off of amplification. These ditiiculties were overcome by changes UE EEE ETE

SO in coil design. he resonance effects at high frequency are due to leakage reactance, ie, flux not linking botb primary and secondary coils, and by coil capacity. The loss of amplification at high frequencies is due to internal coil capacity, principally in the secondary. It was found possible to reduce both these effects by a form of coil construction which sandwiches the primary between two sections of the secondary. This type of winding not only: reduces leakage reactance by increasing .the coupling between primary and.. secondary,. but also reduces the internal capacity of the secondary by breaking it up into two sections. In the Type 585 General Radio transformers silicon steel has been used ‘as a’ core material. The coils’ are of the sandwich type described above. The result of this construction is a transformer possessing a practic: ally flat. frequency. characteristic from 80 to 6000 ‘eycles.. °° 2. cot:

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