Power transformers in audio equipment: the essentials

When designing audio equipment, it’s tempting to think that any power transformer with the required current and voltage ratings will do the job. Sadly, as experienced audio engineers will know, this is not true. We’ll look briefly at how transformers work and how they fit into audio equipment.

Power transformers in audio equipment

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How do transformers work?

These components essentially work by converting an incoming AC source into a variable magnetic field and then back again into AC. This is based on the phenomenon whereby a coiled wire, or ‘winding’, with a current passing through it, produces a magnetic field. Introducing a secondary winding into the magnetic field induces a current which is then used to drive the load.

The primary winding is powered with AC mains voltage, whereas the voltage for the remainder of the system is generated by the secondary winding. Windings are coiled around a core, usually made from iron alloy due to its high magnetic conductance, creating an efficient transformer.

Core types

Cores come in a variety of designs, including a doughnut-shaped toroidal transformer core and a C-shaped core. Some cores are more suitable for use in audio equipment than others.

EI-core transformers are the most common kind of AC transformer. Named after the appearance of its core, the windings are wrapped around the middle leg of the ‘E’ shaped piece, with the ‘I’ then joined to the ‘E’ to form a closed circuit. EI transformers are economical, but they are the least efficient for flux leakage, due to gaps between the windings at the E-I joints.

A toroidal transformer is commonly seen under the cover of high-end audio equipment. The core is doughnut-shaped with the windings, made from a single piece of wire, spaced out evenly around its diameter. The lack of discontinuities in the core assembly makes it a highly efficient core shape, reducing flux leakage by around 90 percent that of an EI transformer. However, toroidal transformers do exhibit some magnetic field leakage due to wider spacing between the windings on the outer rim of the core, compared to the inner rim.

Toroidal cores tend to be more expensive than EI cores, particularly at power levels below 500 watts. However, for high-end audio gear, the additional cost is often worth it due to the flux leakage noise that often occurs with EI-core transformers.