Output transformers must have very low leakage inductance in order to maintain high-frequency bandwidth with capacitive loads.Because of this, they rarely use Faraday shields and are often multi-filar wound. For low insertion loss, they use relatively few turns of large wire to decrease winding resistances.
Since they use fewer turns and operate at relatively high signal levels, output transformers seldom use magnetic shielding. On the other hand, input transformers directly drive the usually high-resistance, lowcapacitance input of amplifier circuitry. Many input transformers operate at relatively low signal levels, frequently have a Faraday shield, and are usually enclosed in at least one magnetic shield.
Audio transformers are used because they have two very useful properties. First, they can benefit circuit performance by transforming circuit impedances, to optimize amplifier noise performance for example. Second, because there is no direct electrical connection between its primary and secondary windings, a transformer provides electrical or galvanicisolation between two circuits.
Isolation in signal circuits is a powerful technique to prevent or cure noise problems caused by normal ground voltage differences in audio systems. To be truly useful, a transformer should take full advantage of one or both of these properties but not compromise audio performance in terms of bandwidth, distortion, or noise.
