How an Audio Transformer Works

by Ezmeralda Lee


Audio transformers are specially-designed for audio circuits. They block radio-frequency interference. They also block the DC components of an audio signal. They also split or combine audio components. Audio transformers are also used for impedance matching. Impedance is resistance. An example of impedance matching between high and low impedance audio components is the impedance matching between a high impedance amplifier and a low impedance loudspeaker. Telephone systems and professional audio systems use audio transformers.


The audio transformer works by aiding an AC input signal to produce another AC output signal. To achieve this, the audio transformer has two windings--the primary winding and the secondary winding. Windings are two or more coils of insulated wire wound around a magnetic metal core. The primary winding is the input and the secondary winding provides the output. When the primary winding gives a signal, it creates a magnetic field. The magnetic field starts fluctuating with the audio signal. The fluctuating magnetic field produces current. The current reaches the output or secondary winding and the signal is sent out of the transformer.


One purpose of an audio transformer is to increase and decrease voltage in an audio circuit. The other purpose is to convert the circuit from an unbalanced to a balanced circuit. Audio transformers are also used to block DC current and to isolate one audio device from another.


There are two types of transformers. The step up and down transformer's job is to increase and decrease voltage. The output has more windings than the input. This difference in the number of windings helps the signal to change as it moves through the transformer. There are two types of step up and down transformers--the single level compatibility transformer and the impedance compatibility or matching transformer. The Unity 1:1 transformer, unlike the step up and down transformer, has the same number of windings in both the input and the output. When the impedance of both the input and the output is the same, the signal does not change. Since the audio signal passes without modification, the Unity1:1 transformer can block DC and radio frequency waves. The benefit is that the signal can pass through many secondary windings and the sound in each can be isolated. Unity 1:1 transformers are of three types. One transformer is used for DC audio current blocking. The second type is radio frequency interference blocking (RFI) and the third type is a ground lift device used to isolate audio devices.


Audio transformers have limitations on the amount of sound decibels they can handle. They have limited frequency responses. They can block signals only below the range of 20-20,000 hertz. There is a maximum input level and this level cannot be exceeded without damaging the transformer and this point is called its saturation point. Audio transformers cannot set up a signal of more than 25 decibels of sound.


A good audio transformer should be capable of matching impedances. It should be able to increase or decrease signal level for up to 25 decibels. It should effectively isolate problem components in a group of audio devices. It should have a flat and broad frequency response. A hotter signal should go through the transformer without saturating it. A good audio transformer will have a good shield. Shields are necessary so that the audio device will not pick up any hum or sound from other devices. Sounds that are not required are kept out by the shield and the necessary audio signals are kept in by the shield.