The term "Audio transformer" is used to describe a form of signal transformer used in audio applications. With few exceptions, these transformers consist of two “coils” of insulated wire wrapped around a common ferrous (steel) core. The coils are electrically isolated by the insulation, and can only transfer the signal from the primary (input) coil to the secondary (output) coil by electromagnetic coupling. This quality and the inductance of the coils allow the transformer to provide electrical isolation between the primary and the secondary for non-audio signals such as DC and RF.
Before audio electronics were developed that operated on bipolar power supplies; some method was needed to isolate the DC voltages required for audio circuits operating on single-ended DC power from different stages of the circuitry and from devices connected to the input and output. In many cases; "coupling capacitors" were used to allow the AC (audio signal) to pass while blocking the DC. In some cases, transformers could also serve this function; and provided other useful functions such as impedance and level matching. Special audio transformers made Balanced audio transmission possible. Please see balanced for more details.
Audio transformers are low-power signal devices optimized for transmission of wide-frequency AC signals, typically with very low distortion. Exceptions include transformers for voice communication with the associated reduced requirements for frequency bandwidth and quality.
This differs from more common power transformers where the main concern is efficiency.
Another quality important to audio is related to efficiency; insertion loss. Even with a 1:1 turns ratio there may be some loss of signal level due to insertion loss.
The number of times the coil of wire is wrapped around the ferrous core is referred to as the “turns.” Depending on the application, the number of turns in the primary and secondary may be the same or different. This is typically referred to as the “turns ratio.” In this discussion, it is assumed that the term voltage refers to an AC voltage because transformers do not allow DC voltages to pass between input and output.
For pure isolation, it is not unusual for the ratio of the number of turns in the primary and secondary to be 1:1. This means the signal voltage will be the same on the output as the input. In other cases, it can be desirable to use a different turns ration for level matching or impedance matching. For example, if a transformer has a turns ratio of 2:1, the voltage of the output will be one-half the input voltage. A typical application for this would be converting a balance audio signal to an unbalanced audio signal.
Analog Audio Transformer
In analog circuitry, the transformer transmits the audio signal from primary to secondary, so signal quality is directly related to the transformer quality. This results in high quality audio transformers being relatively expensive, and in the case of output transformers, relatively large.
Digital Signal Transformer
Transformers used in digital audio transmission are, strictly speaking, not audio transformers. They are “impulse” or signal transformers with limited bandwidth optimized for very high frequencies. By dramatically limiting the low frequency response of this type of transformer, excellent AC power line frequency rejection can be accomplished at the same time as DC isolation. Limiting response at frequencies higher than the desired signal frequencies decreases very high frequency interference which can increase jitter in transmission between digital audio devices.
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