The supply transformer for audio applications must, above all, be able to transfer full power to the application (for example an amplifier) without putting limits on the application that would necessarily mean a reduction in the application’s performance in terms of power supplied.
TECA’s supply transformers for audio use transfer all the application’s power, dividing it into the voltages required by the different applications without limits, even after hours of continuous use.
It is thanks to a careful analysis of the transformer’s application at the design stage, so as to exploit all the company’s experience, technical knowledge and laboratory tests, that a product can be manufactured which fully satisfies the customer’s requirements.
Of fundamental importance is the attention given to the galvanic insulation which separates as much as possible the network supply from the application used. In fact the network supply can have quite significant falls in power and the supply transformer is the instrument which stops these dangerous spikes from causing damage to the application.
This insulation can be further increased by inserting an electrostatic shield between the primary and the secondary circuits (which, when connected to the earth, uncouples the entry and the exit), so that it is possible to effect a first filter for the application, allowing the network voltage to be stabilised.
The passage of high flow values in the magnetic field is registered with audio supply transformers, and as a result the imperfect canalisation of the magnetic field can mean an emission of electromagnetic waves which disturb the signal, introducing distortions which vanify all the precision design work in the whole system.
To drastically reduce magnetic disturbances, TECA acts at several levels in the design phase: on the size of the core, windings and position of the coils, so as to limit the quantity of possible emissions; in the production phase by positioning the coils so as to canalise the magnetic field inside the core. Finally it is possible to apply particular shieldings to the ring on the circumference of the transformer and to the disk in the upper part of the transformer itself.
The latter intervention in particular allows the electromagnetic emission of residual disturbance to be made uniform so as not to have points in the transformer where the magnetic field flow has higher values, transmitting directional disturbance to the rest of the application.
Another rather annoying source of disturbance in hi-fi systems is the noise made by the transformer: buzzing. The main sources of noise in a transformer are the following: electromechanical noise at a microscopic level due to the orientation of the Weiss domains, and mechanical noise at a macroscopic level due to vibration of the plates which form the core.
The core, made of ferrous magnetic material, is subdivided into regions called Weiss domains, bounded by Bloch walls, the volume of which is around (10-3-10)mm2. Inside these regions a molecular magnetic field exists, which causes the permanent alignment of all the molecular magnetic dipoles in a given direction and a spontaneous magnetisation of all the material which is not disturbed by thermic action (at least below the Curie temperature equal to around 750°C for the material in question). Since each of these regions is magnetised in a random direction, in the absence of an external magnetic field the material does not show a macroscopic magnetisation of its own. If an external magnetic field is applied, the Weiss domains tend to rotate so that the relative magnetic dipole lines up in the direction of the external field.
In the case of supply transformers, if a variable external field determined by alternating current (50Hz) is applied, this causes a continual realignment of the Weiss domains. This causes friction along the Bloch walls, which in turn leads to a background rustling when the transformer is working. The effect of the rotation of the Weiss domains is also known as bursts of magnetisation (each burst corresponds to the alignment of a single domain) or Barkhausen effect; this type of noise obviously cannot be eliminated because it is caused by the microscopic properties of the material.
TECA itself produces the cores on which its transformers are wound, it carefully selects the magnetic materials using materials with high permeability with orientated grains, lasar-worked or with special alloys which allow the noise to be reduced considerably.
At a macroscopic level, the core is generally made of a small plate 0.3mm thick, wound on itself until it reaches the size required. Of course the winding of the core leaves levels of air between successive layers of plates. Thus while the transformer is working, due to thermic dilation and the passage of the current, the geometric and mechanical form of the core undergoes modifications and the different levels of plate can vibrate, thus giving rise to further rustling.
Our technologically advanced winding machines for coils, together with our experience and advanced levels of technical know-how, permit the production of cores with cramming levels of over 98%, drastically reducing both the intensity and the frequency of this noise; in addition impregnation and resin solutions for the core completely block the different levels of plate, almost completely cancelling any vibration.
Another important characteristic of TECA’s audio supply transformers is their complete adaptability to the customer’s needs, In fact the design and production of the transformers is full-custom both as regards the choice of voltages and currents for the primary and secondary and as regards the choice of cabling (length and colour of wires, contacts, connections etc).
The quality of TECA’s transformers is checked at various levels: Each phase of the production is tested: voltages, sections and the number of coils are monitored at the end of each winding. Samples of the finished transformers are laboratory-tested to check that electromagnetic emissions are contained within the limits required by the customer, the average induction lost inside the transformer is measured as is the induction point by point to establish the presence of possible maximum points.
Once the production is completed, the transformers are tested (100% of the production) by automatic machines which again monitor the resistances, voltages and currents, carry out safety tests with a check on the presence of coils in hidden short circuit, and a test for the dielectric capacity of the layers of insulation; there is also an estimate of the lost induction of the transformer at different frequencies (100Hz, 1kHz, 10kHz, 100kHz).
In this way for every transformer produced it is possible to have a clear picture of its electromagnetic performance and the levels of safety guaranteed.