DESIGN OF AUDIO TRANSFORMERS
Most important elements in the design of an audio transformer are the selection of core type, size and material, turn ratio, windings and casing.
a) Core Size & Material
Choice of core size and its material depends upon frequency response, power levels, and harmonic distortion. Audio transformers should preferably use both high-permeability cores and the largest number of coil turns to create high primary inductance.
The most commonly used audio transformer core materials are M6 steel (a steel alloy containing 6% silicon) and 49% nickel or 84% nickel (alloys containing 49% or 84% nickel plus iron and molybdenum). Nickel alloys are substantially more expensive than steel but being high permeability materials require lesser number of turns.
Size of the core is determined based on the power requirements. Standard lamination tables needs to be referred. Once core is chosen, turns ratio to provide required inductance for a given size and magnetic material is obtained using the lamination catalogs.
b) Turns Ratio
Voltage induced into the secondary winding depends on the turns ratio of the transformer. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding.
If the turns ratio and the input voltage are known, the output voltage can be determined as follows
Where, E1 & E2 are primary & secondary voltages, N1 & N2 are the number of turns in primary & secondary winding.
Windings, primary as well as secondary, are the coils of conducting wires as a coil of conductors create a higher magnetic flux compared to the flux created by a single conductor.
The material used for the windings is application specific. Insulated solid copper wire is used for small power and signal transformers.
d) Hum Reduction
When signal levels are low, it is often essential to keep external magnetic fields to the minimum. For this purpose, transformers are enclosed in a case of high permeability materials.