Audio Transformers are electromagnetic devices that transmit and modify input electromagnetic signals into output signals via inductive coupling. They isolate an input circuit from an output circuit and filter signals; operating on the audible band of the frequency spectrum (20Hz to 20kHz). As such they can have applications in the input stage (microphones), output stage (loudspeakers), as well as coupling and impedance matching of amplifiers. In all cases, the frequency response, primary and secondary impedances and power capabilities all need to be considered.
Materials and Structure
A transformer is an electrical device which allows an input signal (such as an audio signal or voltage) to produce an output signal or voltage without the input side and output side being physically connected to each other. This coupling is achieved by having two (or more) wire coils (called windings) of insulated copper wire wound around a soft magnetic iron core. Audio transformers are typically composed of copper wire windings around a steel or nickel-iron alloy core. Each core material transmits electromagnetic signals differently. Steel has a higher degree of hysteresis (magnetic signal lag), making it better for lower frequency transfer. The higher permeability of nickel makes it ideal for transmitting higher frequencies. The windings around the core determine the impedance level, which increases, decreases, or maintains the signal level as it passes through the transformer.
When the signal enters the transformer via the input (primary winding), it then gets transferred to the output secondary winding due to the inductive coupling of the soft iron core. The ratio between the number of coil turns on the primary winding (NP) to the number of coil turns on the secondary winding (NS) is called the “turns ratio”. The turns ratio between the input and output wire coils provides either an increase or a decrease of the applied signal as it passes through the transformer. More windings around the core correspond with a higher impedance, so if the primary winding has more than the secondary, the signal will decrease (step down). Conversely, if the secondary winding impedance is greater than the primary, the signal will increase (step up).
The number of turns on each winding determines whether the transformer provides a gain or loss of the signal:
- If there are more turns on the input winding, the signal will decrease or step down.
- More turns on the output winding will result in a step up.
Audio transformers are produced for a range of specific audio functions; many are similar in construction to power transformers and they often perform several functions at once. They can be considered as either a step-up or step-down type, but rather than being wound to produce a specific voltage output, audio transformers are mainly designed for impedance matching, isolation, and a variety of applications (see Data/Voice Coupling Transformers).
Transformers can step impedance up or down in the same way they do with voltage and current. Whereas they change voltage by the turns ratio and current by the inverse of the turns ratio, audio transformers change impedance by the square of the turns ratio. The same amount of voltage is induced within each single coil turn of both windings. The primary to secondary voltage ratio (VP/VS) will therefore be the same value as the turns ratio (NP/NS). Impedance matching audio transformers always give their impedance ratio value from one winding to another by the square of their turns ratio. That is, their impedance ratio is equal to its turns ratio squared and also its primary to secondary voltage ratio squared:
Impedance is determined by the efficiency of the conversion from voltage into magnetic flux. Audio transformers are ideal for balancing amplifiers and loads together that have different input/output impedances in order to achieve optimal power transfer, as in the case of a transformer at the amplifier input to match the impedance between microphones, connecting cables and the amplifier input. The input and output impedance levels are matched to create efficient power transfer without distortion or signal overload. Impedance matching transformers are similar in design to low frequency voltage and power transformers, but they operate over a much wider range of frequencies (for example, 20Hz to 20kHz voice range).
Isolation or Unity Transformer
Transformers have another very useful property, isolation. Since there is no direct electrical connection between their primary and secondary windings, transformers provide complete electrical isolation between their input and output circuits and this isolation property can also be used between amplifiers and speakers. A transformer with a turns ratio of 1:1 does not change the voltage or current levels but instead isolates the primary circuit from the secondary side. This type of transformer is commonly known as an isolation transformer.
As the impedance is identical for the primary and secondary, the signal level does not change. The transformer allows an audio signal to pass unmodified from the primary to the secondary while blocking DC voltage and radio frequency interference (RFI). Since the primary and secondary circuits are insulated from each other, the transformer will electrically isolate different pieces of equipment. This can solve hum problems by isolating or "lifting" the grounds of different devices. Other unity transformer applications include providing multiple outputs from a single mic input by using multiple secondary windings, and changing balanced signals to unbalanced signals or vice-versa.
Audio transformers are designed to operate over the audio frequency range, or much higher for radio-frequency (RF) transformers. Due to this wide frequency band, one of the main disadvantages of audio transformers is that they can be somewhat bulky and expensive. This is because a transformer's core size increases as the supply frequency decreases. Smaller designs can be achieved by using special core materials. Audio transformers have played an important role since the birth of audio electronics. When compared to modern miniaturized electronics, transformers seem large and heavy but they continue to be the most effective solution in many audio applications. The usefulness of a transformer lies in the fact that electrical energy can be transferred from one circuit to another without direct connection, and in the process the energy can be readily changed from one voltage level to another.