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Transformers (NOT the Movie)

by Steve LaCerra • in
  • May 2018
  • Theory and Practice
• Created: May 15, 2018

We hear a lot about audio transformers. “Transformer-balanced DI.” “Transformer-isolated split.” What, exactly, is a transformer, and what can it do for you? Let’s have a look.

A transformer is a passive electronic device that allows an AC input (like an audio signal) to be transferred to the output, without physical contact. As we’ll see, that last bit is very important, as is the fact that transformers do not transfer DC.

How does a transformer work? Insulated wire is coiled around a piece of magnetic metal or “core.” The coiled wire is known as a winding and, by definition, a transformer must have at least two windings (although some transformers have more than two windings). One winding is designated as the input, or primary. When an audio signal is passed through the primary, a corresponding audio signal is generated on the secondary winding or output. This is called inductive coupling.

Notice how I sneaked in the word “corresponding,” so I could be intentionally mysterious. The relationship between the input and output signals depends upon the number of turns in each winding. Let’s suppose the primary is wound around the core 100 times and the secondary wire is also wound around the core 100 times. This is called a “1:1 turns ratio.” In this case the output level and impedance will be equal to the input level and impedance.

Radial Engineering’s single-channel JDI passive direct box uses a Jensen 12:1 transformer to convert an unbalanced line/instrument level input to a balanced mic level output.

‡‡         Turn, Turn, Turn

If we wound the secondary around the core 200 times we’d have a 1:2 turns ratio. The output level would be higher than the input level. This is what a step-up transformer does. Free gain, kind of. Yes, the level steps up but so does the noise floor. A step-down transformer would have a turns ratio such as 2:1 — the input level would be higher than the output level. These differences in the number of windings between the primary and secondary result in different impedances. That’s how impedance-matching transformers are created, enabling you to (for example) connect the unbalanced, high-impedance output of a consumer CD player to a balanced, low-impedance line input while achieving the correct frequency response and level.

The transformer that you find in a passive DI (such as the Radial JDI pictured on this page) is a step-down transformer. The turns ratio is set so that an instrument-level signal is stepped down to microphone level. The transformer in that DI also balances the unbalanced instrument signal and changes the impedance from high to low, all of which make it possible to send the direct signal from that electric bass down 100 feet of copper without completely wrecking the way it sounds. [Note: Not all DI’s are transformer-based.]

Transformers can actually pass audio either way, so if you were to put a signal into the secondary, it would become the primary, the primary would become the secondary and you’d get signal out of the input. Got all that gobbledygook? Bottom line: Audio transformers can be used backwards without harm.

Fig. 1: Schematic for a simple 1:1 transformer isolation circuit.

‡‡         Isolation

A transformer with a 1:1 turns ratio is often referred to as an isolation transformer or a “unity” transformer: it transfers signal without changing the level. And, you ask, “Why would I bother doing that?” Remember that the primary and secondary coils are insulated so they don’t physically touch, and thus they are electrically isolated. In the real world, that means you can use a transformer as a safe way to solve a ground loop. All you need to do is flip the ground lift switch on a DI or mic splitter. (See Fig. 1.)

Whirlwind’s ISOXL puts an isolation transformer into a convenient in-line XLR-to-XLR barrel connector.

‡‡         Slick Transformer Tricks

Another thing a 1:1 transformer can do is block DC or RF. Radio frequencies are in the MHz range; DC is 0 Hz. Let’s suppose we design an audio transformer to have a frequency response of 5 Hz to 100 kHz. For audio applications, that’s a very wide frequency response, probably more than we need. Our transformer would block DC and also would block radio frequencies. This is a good way to keep the audio signal clean. The fact that DC cannot pass between the primary and the secondary is good news for ribbon microphones: put a transformer on the output of a ribbon mic and the ribbon is protected from accidents involving phantom power that might otherwise blow up the ribbon.

Unfortunately, there’s some bad news. If the secondary is connected to a mic input and we turn on phantom power, phantom won’t be passed to the primary and the microphone that’s connected to it. That’s why many mic splitters have a “direct” output and an “isolated” output. The direct output is connected straight to the mic input on one mixing console (let’s say, the monitor desk), and the isolated output passes audio from the mic through the transformer to another console (let’s say, the house console). If we turn on phantom at the monitor desk, the condenser mic is happy. If we turn on phantom at the FOH console, the phantom never reaches the microphone so no harm is done to any of the gear — but the mic doesn’t work. If we hear hum from a ground loop, the ground lift switch at the iso output lets us lift the ground to break the loop, thus restoring order to the space and time continuum.

You may have noticed that high-quality direct boxes tend to be expensive, and that’s often due to the price of the transformer. A good audio transformer can easily cost $65 or higher, so when you drop $150 on a DI a good portion of your hard-earned cash is paying for the transformer.

Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.

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