Transformer design is based on some simple physical principles like Faraday's and Ampere's laws, which are integral formulations of some Maxwell's equations. Intuitively, a transformer is a device in which two windigns are mutually coupled by a common magnetic flux, the currents adjusting themselves in order to mimize it. We will show in the following that for a perfect transformer with turns on the input and turns on the output, the ration between input and ouput voltages is , while the currents ration is . From those relations, it can be deduced that an input impedance Z appears as on the ouput. This ability to transform impedances will be crucial in audio applications.