Audio Frequency Transformer: Construction

Figure (a) shows center taped secondary winding to provide two anti-phase signals.
The dot near the winding indicates relative polarity of the signal on different windings.
The dot at point P shows secondary winding signal is in phase with primary winding signal whereas the dot at point Q indicates secondary winding signal is antiphase with primary winding signal.
Figure (b) shows two audio amplifier is used to couple the power output stage of an audio amplifier to the loud speaker.

Function of Audio Frequency Transformer

They allow only AC audio signal to the loud speaker whereas preventing DC from the amplifier.
They provide isolated external connection for the loud speakers.
They provide impedance matching between low input impedance of loud speaker to higher output impedance of an amplifier. It will allow maximum power transfer from the amplifier to speaker.

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The audio frequency transformer works in the frequency range of 20 Hz to 20 kHz. They are used in the audio amplifiers.
They are essential in the impedance matching. i.e., matching high impedance output of amplifier to the low impedance of loud speaker.