Transformers and autoformers
Transformers are a ubiquitous circuit element, found in many electrical circuits in a wide variety of applications. Audio systems often benefit from transformers in several different ways. They can electrically isolate one device from another to prevent hum noise due to grounding problems, and to prevent shock hazards. They can also be used to create balanced audio lines, which help to reduce noise caused by electromagnetic interference.
Transformer basics
As diagrammed in Figure 1, a conventional transformer consists of two coils of wire wound around an iron core. The two coils are known as the “primary” (input) and “secondary” (output). Electrical current passing through the primary coil creates a magnetic field that induces an electrical current in the secondary coil. This allows two circuits to interact with each other without being physically coupled to one another. Additionally, depending on the size and configuration of the primary and secondary coils, the voltage and current induced in the secondary coil can be different than the primary coil, allowing the voltage and current to be "transformed" to different values.
In audio, we use transformers in microphone preamplifiers and other audio electronics and to distribute amplifier power to the loudspeakers in 70-volt and 100-volt distributed systems.
Transformers in 70V and 100V systems
In 70-volt and 100-volt distributed systems, each loudspeaker has its own transformer. This transformer, as diagrammed in Figure 2, can be “tapped” to apply a precise amount of power to each loudspeaker. A 70-volt or 100-volt distributed system allows a large number of loudspeakers to be connected in parallel to a single amplifier, simplifying installation. It also reduces power loss in the loudspeaker cable.
Figure 2: A Conventional 70V Transformer
Autoformers
An autoformer (also known as an autotransformer), as diagrammed in Figure 3, performs some of the same functions as a conventional transformer, but there are several differences in design and performance. In particular, an autoformer has only one coil which performs the functions of both the primary and secondary windings.
Figure 3: An Autotransformer
Since autoformers don't have two electrically isolated coils, they don't provide electrical isolation like typical transformers do. Instead, autoformers allow for the same voltage/current transformations that typical transformers are used for. Since 70V / 100V distributed loudspeaker systems don't require electrical isolation, autoformers are a good candidate for use in these systems.
The prefix "auto-" in autotransformer doesn't imply that anything "automatic" is happening. Instead, it comes from the Greek root for "auto", meaning "self", referring to the fact that an autoformer is a single coil that works with itself rather than a pair of coils working together.
Advantages
Because an autoformer requires fewer windings and has a smaller core, it is typically lighter and less costly than a conventional transformer with the same ratings. Autoformers have performance advantages too. They have increased power handling capability, flatter frequency response, lower insertion loss and lower distortion than conventional transformers of similar size and cost.
Because of these advantages, Biamp uses autoformers in many of its 70V / 100V speaker models. For example, the 200-watt autoformer used in Community R-Series and other models has a wide, flat frequency response and only a 1 dB core loss. This kind of performance is a significant improvement over a conventional 70V / 100V transformer of similar size and cost.