Other Losses

Author: N.H. Crowhurst

Primary inductance and core losses, together with distortion, are the principal things that complicate the behavior of an audio transformer at low frequencies. At high frequencies the behavior of the transformer is complicated by winding capacitance and leakage inductance. Winding capacitance allows minute audio currents to pass between points of audio voltage where there is no direct connection, in the way that stray capacitance allows audio current to pass at extremely high frequencies.

The magnetic field causing primary inductance follows a path through the magnetic core that embraces both windings. The current in the windings themselves, however, induces a magnetic field around each winding (apart from the bigger one in the core) that tends to "leak down" between the two windings. This means that the current in one winding will not produce a field that counter-balances the current in the other winding. Consequently, these uncounterbalanced fields will produce different voltages in their respective windings. Thus leakage inductance is like an inductance in series with the winding, because it allows an additional voltage to be developed between the terminals that is not accounted for by the counterbalancing effect of current in the two windings.

A well-designed audio transformer has to take into account all of these factors to make sure that its performance as an impedance-matching device is consistent over the frequency range for which it is intended.

Ordinary (left) and leakage inductance (right)