Can a transformer alone be used for amplifying audio signals ? If yes how ?
Technically No. The transformer will not amplify a signal, however it can be used to make sure you use any signal you actually do have to your best advantage. A transformer is generally used to achieve an impedance match between the signal source and load. For instance if you were to connect an 8 ohm speaker directly to a 1 kilohm signal source, most of the available signal would be dissipated in the source resistance and less than 1% of the signal would produce movement in the speaker. If however a 1k to 8 ohm transformer is used to match the source to the load,up to just under 50% of the signal will make it into useful energy in the speaker. This is a substantial gain in performance of the overall system but is not actually amplification.
Transformers are AC (alternate current) devices and if 60 Hz is supplied to the primary windings 60 Hz will be produced at the secondary terminal. The transformers transform voltage and current at the primary to voltage and current at the secondary by magnetizing a mass of iron in their cores. The magnetic cores may not respond efficiently to very high frequencies (transmission frequencies) but might respond to audio frequencies (loud speakers employ magnets and coils of wire). If a suitable audio signal was applied to the primary coil a transformer might be used to step up or step down the voltage of the supplied signal while reversing its phase, however power (P EI) would remain nearly constant.
