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.
c) Windings
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.
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