No, not “Robots in Disguise,” but rather the electronic kind.
Often misunderstood and mysterious, their basic function is to do as their name says: transform. Their primary purpose is to change something – a voltage or impedance. They can be very useful in a variety of ways, and are present in nearly every piece of electronic gear.
The most basic of transformers consists of two coils of wire wrapped around a core, usually of metal. The two wires are not connected to each other electrically. They interact due to the magnetic field they create and their close proxomity to each other. One other, but equally as useful result of their construction is that they will not pass any DC at all. Any DC applied to the “primary coil” is not induced into the “secondary coil.”
They change voltage in a ratio that is determined by the number of turns each of these coils has. For instance, if the primary has 10 turns, and the secondary has 100 turns, then 1VAC applied to the primary results in 10VAC signal on the secondary. This would be a 1:10 winding ratio transformer, and could be used as a “step up transformer.”
At first glance, it would seem that this would be an amplifier, but there is a very important law that comes into play. That is the law which states that “energy is never created or lost, but only converted into something else.” Therefore, while the voltage may be “stepped up,” the current available at the secondary is reduced by the same ratio. If the transformer were 100% efficient, these figures would be exact, but nothing is 100% efficient. A little energy is “lost” (mostly converted to heat) through the resistance of the wires used to construct the transformer, and various magnetic losses – including stray magnetic fields that ‘escape’ from the transformer and can affect nearby circuitry). A fairly standard efficiency rating for a transformer would be in the 85%-90% range. So, any time you use a transformer, there is actually some loss involved, but the benefits usually outweight the losses. A transformer is what is called a “passive device.” It draws no external power, and can generate no power gain, and in fact, always results in a slight power loss.
What, you may ask, are transformers used for in the audio realm?
Probably the most common use of a transformer (besides the internals of your equipment) is the lowly direct box (usually called a “DI“). It interfaces the high impedance (10K-12K ohms or so) of a guitar or bass to the low impedance (600 ohms to 1K ohm) of a mic preamp. In this function, it is called an “impedance matching transformer.” Without this, if you plugged a guitar or bass directly into a mic preamp, the relatively low impedance of the mic preamp would “load” the guitar’s pickups too much and would adversely affect frequency response.
Another use of transformers is in the splitter systems used to split the signal from a microphone and send it to two consoles (FOH and monitor). In this case, the transformer has one primary winding and TWO secondaries, and each winding is isolated from every other winding. In this design, the transformer is a 1:1 ratio – there is no voltage step up or down, and there is no impedance matching function. This not only splits the signal, it also isolates the two consoles from each other so there is no interaction as can otherwise happen if the consoles have a particular preamp design. If you simply used a “Y Cable” to split between the two consoles, the mic preamps of the two consoles would be in parallel, which would reduce the “load impedance” seen by the mic to one half, which may be low enough that it would begin to affect the response of the mic.
One thing to be aware of when using transformers is that they not only create magnetic fields, but they also will interact with magnetic fields that may be nearby. For instance, if you set a DI on top of a bass amp, the magnetic field created by the bass amp’s power transformer may be ‘picked up’ by the DI’s transformer and cause a 60Hz hum in the low impedance (mic) output of the DI. Simply move the DI. I would suggest putting it on the floor.
The coils of which a transformer is made are not purely resistive. They present what is called an “inductive load.”
The design of a good quality transformer for a specific purpose is a skilled art that requires extensive knowledge of electronics, magnetics, metallurgy. And, just like everything else, there are good quality transformers, and there are crappy transformers. It can be the difference in design or materials. Also, using a transformer for something other than it was designed for will result in poor results.