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How Do Audio Transformers Work?

A transformer is a fundamentally important electronic component that couples two alternating current circuits. Electric utilities use large transformers to increase or decrease the voltage of electricity moving through power lines. Sound equipment, such as amplifiers and mixing consoles, uses small transformers specially designed for audio frequencies. They remove harmful direct currents the sound signal and act as a "bridge" between electrically incompatible equipment.

Transformer Action
- All transformers work on alternating current, or AC signals, whose current direction reverses rapidly. The transformer itself has a pair of fine copper wire coils fixed to a common metal frame. Alternating current in one coil produces an alternating magnetic field. The metal frame shapes the field around the second coil, which has an AC current induced in it. The transformer passes AC current from one circuit to another; in addition, differences between the two coils can increase or decrease the AC voltage or perform other useful changes to the current.
Line Balancing
- A long microphone cable acts as a radio antenna, picking up electrical noise from fluorescent lights and other sources. An amplifier or mixing board boosts the noise along with the signal, resulting in loud hums and buzzes in the sound. A balanced cable cancels the noise through the use of a pair of audio transformers, one at each end. The transformer at the sending end presents a lower impedance, or AC resistance, to the audio signal than the one at the receiving end.
Impedance Matching
- Impedance is an electrical quality of an electronic device, a measurement similar to resistance, but more complex. Devices such as guitar pickups, microphones and speakers have impedance, as do amplifiers and other circuits. In some instances, impedance values must match as closely as possible or sound power and quality suffers. For example, an amplifier with an output impedance of 500 ohms drives an 8-ohm speaker very poorly. If you place a transformer between the two, with an 8:1 ratio of wire turns between its two coils, the transformer compensates for the impedance difference between the two devices.
Isolation
- Because the transformer's two coils are connected magnetically, but not electrically, it passes AC but blocks DC. This property is called "isolation" and is important for vacuum-tube amplifiers, which have hundreds of volts of DC present in some designs. The transformer blocks the DC, preventing it from causing shocks or burning out equipment you connect to the amplifier. DC present on input signals can cause distortion and "thumping" when you make connections; the sudden appearance of DC voltage can damage speakers. Transformers on amplifier inputs remove DC and allow only AC sound signals into the equipment.

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