Why to use an output transformer wrapped in air?
From always the output transformers they are realized with to the inside a package of thin sheet-steel to silicon, the requirement to use a such nucleus draws origin from some considerations notes from always to the technicians: the iron of the nucleus of a transformer introduces a magnetic permeability thousands of times advanced to that one of the air, that is the flow magnetic finds a distance extremely easy, is enough little coils of copper in order to obtain a efficient device, indeed how much the more the nucleus is large many little coils (and amount) of copper is demanded.
It would seem that all it spun smooth..... and in part spins, since the transformers are perhaps the machines electrical workers who have the highest energetic rendering, that is consume to their inside little energy, but what is the effect of the energy that it consumes a transformer?
Little badly if the problem were limited to the loss of some percent of useful power, but in truth the things go various: the losses are essentially due to five causes
1- the losses due to the resistance of the hanks of copper, to the passage of the current dissipate energy for Joule effect, are constant to all the frequencies, depend on the current,
2- the losses due to dispersed flows, not all the flow generated from the primary winding links with the secondary one, transferring all the energy,
3- a sure capacitivo connection between contiguous coils exists, and between these and the nucleus, gets worse with increasing of the frequency and the potential difference between near coils,
4- the alternated passage of the magnetic flow through the nucleus provokes to a current circulation called current parasite, is for that they are used lamierini thin sheet steel, in how much between the other things depends on the quality of the material and its physical dimensions,
5- perhaps more subdolous one of the problems it derives from the characteristic of the ferromagnetic materials with which the sheet steel of the transformers are realized, is it's characteristic to answer with a sure delay to the magnetization, and to maintain a sure memory of the received magnetization, (tasks to magnets) therefore that the magnetizing current must follow a particular curve dictates hysteresis curve, every shifting aside from this curve is come transferred on the f.e.m. induced on secondary, in the practical one the current of magnetization although of absolute value per cent low regarding that one of full load demands a remarkable member of third harmonica, forcing the final stage to work to exactly triple frequency of that one in execution, in order to diminish this effect are used to oriented grains etc.
in the end, the problem of the vibrations of the lamierini of the nucleus, also this produces audible degradation in the sonorous reproduction of quality.
The problems listed to the points 4 and 5 disappears with the use of transformers wrapped in air, then why do not come from always used? The answer to the question can be given analyzing which is the problems that rebel from the elimination of the nucleus of a transformer: because of the lowest relative permeability of the air is not possible to obtain a reactance of high the primary winding as we would want, in how much it would be necessary the highest coil number and/or a diameter of the enormous hank, with consequent increase of the problems seen to points 1, 2, 3, without to count to problems of practical type. Moreover it is necessary an amount of enormously advanced branch, (for an amplifier of medium power some kg) and the copper is much expensive one.
What involves one the not highest primary reattanza? In the first place one current of magnetization elevated to low frequencies, in fact the reactance is function of the frequency, in practical a such transformer to industrial frequency would absorb one elevated current to empty also without load, but in the audio employment that is what it interests to us it translate in a sure one and not steep lessening of the curve of answer to the low end, beginning from approximately 80 Hertz, wide compensated from the enrichment in harmonic content that contributes to supply an impression of I reinforce due to the so-called lacking effect of the fundamental one.
It is not simple to find the point of equilibrium between the parameters that enter in game in the dimensioning of a transformer wrapped in air, in fact everyone is legacy to the other and often they have opposite courses, like approval increasing the coils and the diameter increases the reactance, that it is positive, but increases also the resistance and the capacity coupling that is negative, during the first tests I have used various tipologie and realizzative techniques, until arriving to that current one, after to have wrapped some tens of Kg of enameled wire of copper, it appeals to to remember that I constructed a lathe purposely to me in order to carry out the first tests.
The definitive version previews the realization on a support in wood for the insertion in the piece of furniture and for constructive reasons, on which the 4 hanks of the 5 head physician and of secondary which are wrapped first and last with half coils regarding the connected others so as to obtain the resistance symmetrical.
Who wanted to have an idea of the dimensioning can try to make some account with the formulas of Kordorfer, varying the parameters in order to see the course of the reactance of the considered hank, holding account that gives alone an indication of the order of magnitude
by Alessandro Coppi