AUDIO TRANSFORMERS- FUNCTIONS

Like any other transformer, audio transformer comprises of a primary winding, secondary winding and the core and the primary function of an audio transformer is to couple the energy from the primary windings to the secondary without introducing any distortion. Audio transformers are designed for audio range of frequencies, i.e., for frequencies ranging from 20 Hz to 20 KHz and are used primarily in audio circuits.

The important functions of an audio transformer are:

1.      Impedance Matching

The impedance of the output stages of the amplifier circuits is often different from the impedance of the audio speakers. For maximum power transfer, impedance of the output amplifier should be same as that of impedance of the audio speakers. For lossless power transfer, impedance matching is done with the help of reactive elements, i.e., inductors and capacitors.

Since transformers can change the impedance (source sees the load with higher input impedance, while sink sees the source with lower output impedance), they are used for matching the impedance of the amplifiers and that of speakers.

The relationship between the impedances (primary and secondary) and turns ratio is as given below

Where,       

            Zp = Primary Impedance

            Zs = Secondary Impedance

            Np = No of turns in a primary

            Ns= no of turns in a secondary

Thus, if output amplifier stage has an input impedance of 1800 W and loudspeaker impedance is 8W, a transformer is needed having turns ratio of

With the transformer in between the amplifier and the speaker, amplifier will see an output impedance of 1800W, though actual load impedance is still the same, i.e., 8W. Similarly, loudspeaker will see an input impedance of 8 W though actual impedance is 1800W.

In a similar fashion (as explained above), audio transformers are used to change the impedance between the microphones and input amplifiers so that signal power is not reduced.

2.      DC Blocking

Typically the output amplifier’s AC signal is very low and is superimposed on the high voltage DC signal. It is often imperative to remove the DC signal from the composite signal before it enters the loudspeakers.

Since conventional transformers do not pass DC, audio transformers are used to prevent any DC signal from the amplifier entering the loudspeaker affecting the audio output.