A basic transformer is made up of two coils, a primary and a secondary and a core. When an electrical current flows into the primary coil, it generates a magnetic field. The field is concentrated via the core which in turn excites the secondary coil and regenerates electrical current at the output.  This creates a magnetic bridge between the input and output without any physical (electrical) connection. This allows alternating current such as an audio signal to pass, while blocking DC.

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