"Explain Like I'm 5": Audio Transformers May 25, 2017 12:28

What are transformers?

Transformers are those huge, heavy, primitive-looking parts you’ll see in both vintage and modern audio gear.

They look something like this:

Or this:

Or this:

What do audio transformers do?

A lot of things! I guess that’s why we keep them around, primitive and expensive as they are. 

Transformers do a lot of different jobs in audio gear, including:

  • Stepping voltages up or down: increasing output level of microphones, bringing instruments down to mic level, etc.
  • Providing balanced inputs and outputs
  • Impedance matching
  • Eliminating ground loops
  • Blocking DC while passing audio signal

How do transformers work?

A transformer is just two long wires wrapped around one magnetic core. Signal passes from one wire to the other, but the wires don’t touch. What sorcery is this?! It’s a  funny feature of our universe called “electromagnetism,” where electric current creates magnetic fields and vice versa.

Modified and reused under the GNU Free Documentation License

The electrical current running through first coil of wire (the “primary”) creates a magnetic field in the core. This magnetic field then induces a corresponding voltage in the second (“secondary”) coil. Voila! We now have the same signal at both sides of the transformer without a single electron making the journey from one side to the other.

How do transformers step voltages up or down?

We only get that same exact signal on both sides when both coils have the exact same number of turns around the core. By changing the number of turns in each coil, we can directly change how much signal is transferred between them.

For example, if the primary coil has 200 turns and the secondary has 100 turns (we call this a “turns ratio” of 2:1), only half the signal will be transferred. This is called a “step down” transformer. However, we could turn that same transformer around and use it as a “step up” transformer to double the signal!

If that sounds too good to be true (free gain!), it’s because we’ve only considered half of the equation—if a transformer steps up voltage, it steps down current by that same amount, and vice versa. Another way of saying this is that transformers can’t create or destroy power (power being the product of voltage and current). You could think of transformers as trading voltage for current, while power stays the same.

For example, let’s say a 1:2 transformer sees an input of 1 Volt at 1 amp current. At the secondary, the voltage will be stepped up to 2V, but with only 0.5 amp of current available. So a 1:2 transformer for voltage is a 2:1 transformer for current.

How do transformers match impedance?

Sometimes it’s important to match the impedance of two devices that are being connected (see "ELI5: Impedance" for more on that). For example, your guitar won't sound very good if you plug it right into a mic preamp—the output impedance of the guitar is just too high to transfer all its signal to the preamp. So we use  a DI box to step the guitar’s output impedance down to mic level.

And guess what's in a passive DI box—a transformer! Transformers can step impedance up or down in the same way they do with voltage and current. Except whereas they change voltage by the turns ratio and current by the inverse of the turns ratio, they change impedance by the square of the turns ratio.

So let’s look at a DI box as an example. A typical passive DI box transformer has a turns ratio of 12:1, which means it will step down the guitar’s output impedance by 144:1 (12 squared). A typical output impedance for a single coil pickup is around 20k Ohms, which our DI box will step down to 138.8 Ohms, which is typical of a microphone. Now we can run that guitar directly into our mic preamp with no impedance issues. Transformers win!

Why do transformers sound so good?

Of course, we don’t just keep transformers around to do technical jobs—they also sound really good. There are a couple reasons for this, mostly having to do with the unique ways in which they "fail" to be perfectly clean and linear.

Like all analog components, transformers clip when given too much signal. Transformer clipping happens when the core saturates and can't contain any more magnetic flux. This sets a hard limit on the amount of signal the transformer can pass and generates harmonic distortion.

What makes transformer saturation so lovely is that the distortion it creates is inversely proportional to frequency. Which is a fancy way of saying transformers create more warm, gooey, low-frequency distortion and less harsh, bright, high-frequency distortion.

Transformers also exhibit another distortion phenomenon called "hysteresis." This is where the core, after getting magnetized by a signal, stays magnetized for a short period of time after the signal is removed. Hysteresis creates low-frequency, harmonic distortion at all signal levels, not just when the core is saturated. This same effect is a large part of the desirable sound of analog tape.

Why is transformer inductance important?

Inductance is a measure of how well a component converts voltage into magnetic flux. We're concerned with inductance in audio transformers because higher inductance in the primary coil translates to better low-frequency response. Inductance can be increased with either more windings or a more permeable core material (see below).

How does core composition affect the sound? 

Different materials have different abilities to contain magnetic flux—this is called “permeability.” Core materials with higher permeability create higher primary inductance, and therefore better low-end response. However, more permeable core materials will also saturate faster than less permeable ones. Ah, nature, where everything's a tradeoff!

The most common core materials for audio transformers are M6 steel (steel with a bit of silicon) and nickel/iron alloys. Cores with high nickel content are more permeable and more expensive, with less hysteresis than steel cores. 

In general, steel will have higher distortion at normal signal levels due to hysteresis, while nickel will have higher distortion at higher levels, due to saturation. For this reason, you'll often find high-nickel cores in high-quality transformers designed for lower signal levels, and steel cores in cheaper transformers or those designed for high signal levels.

What’s a zobel network?

Transformers, like all other parts that exist in the real world, have unintended effects called “parasitics”; ie, extra resistance, inductance, and capacitance that an ideal transformer would not have.

Sometimes these parasitics will combine to cause the transformer to ring in the audio range. That is, if fed a certain frequency, the transformer will keep ringing at that frequency even after the signal is removed.

A Zobel network is a simple, passive filter consisting of a resistor and capacitor placed after the transformer to eliminate ringing.

What'd I miss?

Thanks for reading! Let me know in the comments below if you have any questions about transformers I didn't answer.