What Are Microphone Transformers
Introduction
When audio technology was just starting out, transformers played a crucial role in the first generation of electronic circuits, which relied entirely on tubes. Around 1920, radio broadcasts were just beginning to reach the public, and the demand for broadcast audio systems grew quickly. All of these systems depended on transformers.
Transformers became essential when live sound reinforcement equipment was introduced because they were the only way to match the impedance of microphones to vacuum tube preamplifiers.
Besides being used as interstage devices and line output drivers, transformers were also important for matching a power amplifier’s output stage to the voice coil impedance of a loudspeaker—just as they are today in audiophile tube amplifiers and musical instrument amplifiers.
As transistor-based preamps and power amplifiers became more common, transformers became less necessary. Still, many electric guitarists insist that tubes “just sound better.” However, transformers do more than just match impedance. At the source, they can differentially balance microphone signals or lines, and at the destination, they can de-balance those signals.
In this article, we’ll discuss the role of transformers in microphones and the differences between microphones that use transformers and those that do not.
What are Transformers?
When most people hear “transformers,” they think of the large power transformers that deliver electricity from power lines into our homes and buildings. But a more fun description might be “robots in disguise.” The transformers that supply mains voltage to buildings handle much higher voltages and currents than the small transformers used with microphones. Still, both types work on the same fundamental principles.
In electrical terms, a microphone transformer is a passive device that physically separates two circuits while still allowing them to communicate electrically. Inside a mic transformer, two conductive coils—one for each circuit—are wound around a shared magnetic core. These transformers adjust voltage, current, and impedance as needed.
Transformer-coupled microphones use basic transformers made up of two circuits. An electrical transformer has a magnetic core with conductive wire windings wrapped around it. Typically, the primary winding connects to the circuit carrying the microphone capsule’s output signal, while the secondary winding connects to the microphone’s output circuit.
To simulate the frequency response of different microphones and their dynamic behavior with high-end amplifiers, Microphone Transformer uses Sound Magic’s award-winning modeling technology. Transformers can only pass alternating current (AC); they block direct current (DC). This means they provide DC isolation between circuits.
History and Theory
Transformers are among the oldest technologies still widely used in electrical power networks and audio circuits around the world. Since their invention in 1885 by three Hungarian engineers—Ottó Titusz Bláthy, Miksa Déri, and Károly Zipernowsky—their basic design has remained largely unchanged.
In a transformer, voltage, current, and impedance are increased or decreased between the primary input coil and the secondary output coil, much like how a gearbox changes gears. Transformers are commonly used as buffers between circuits, such as between the output of a microphone preamp and the input of an equalizer in a console channel strip.
By electrically isolating the two circuits, matching their impedances, and sometimes adding a bit of gain, the transformer serves as an electrical isolation device.
One advantage of transformers is that they are passive devices, requiring no special maintenance or treatment. They don’t need additional circuitry or power supplies to function, are relatively inexpensive to produce, and can even work when connected in reverse—a step-down transformer wired backward becomes a step-up transformer.
Transformers, whether for audio or electrical purposes, operate according to Faraday’s principles of electromagnetic induction. An alternating current flowing through one coil winding induces a voltage in another nearby coil.
When to Use and How Does it Work?
In many high-quality audio processors, transformers play a key role in creating an analog sound. All of the most sought-after vintage equipment make use of transformers. While the design of a transformer greatly affects its sound, every detail plays a part. When a transformer receives louder signals, its core can become saturated, which leads to clipping.
This type of distortion produces more low-frequency harmonics than high-frequency ones, resulting in a warm, dense sound. Because of hysteresis, the magnetic properties of the core can change slowly even at lower levels, below the point of obvious clipping. This also leads to an increase in low-frequency harmonics.
Each transformer design offers its own unique harmonic color, which is why Neve gear sounds so different from API gear.
Much like compression, a transformer that is close to saturation tends to fill in sonic gaps, creating a more cohesive sound with added character and density. Sometimes, to add a bit of analog “glue,” we run mixes through transformers at unity gain, without boosting the level or causing saturation.
If you’re working “in-the-box” but want the color of real transformers, you can use hardware inserts in your DAW or patch the mix output from the DAW into an analog mix bus.
Do Transformer Mics Make a Difference?
Transformers help balance the unbalanced output of tube microphones. They can also reduce the high impedance that often comes from tube circuits, which can be problematic for cables that are left connected for long periods of time.
If a microphone capsule produces a low output voltage, a step-up transformer can boost it. The real question isn’t, “Are transformers designed better?” Instead, it’s, “Is this microphone designed well?”
We believe that most microphone manufacturers don’t use transformers just to create a “transformer sound.” Instead, transformers are installed to serve a specific electronic function or purpose. Most transformers are designed to achieve a particular technical goal rather than to intentionally color the sound.
Transformers can help solve certain design challenges. For example, audio-frequency condenser microphones need a balanced output—and, in the past, a transformer was the only way to achieve this. Along with providing balance and isolation, transformers can also add some character and distortion to the signal.
We’re not entirely sure that high-voltage tube microphones or ribbon microphones can be designed without transformers. A good transformer can add significant coloration to the sound, and high-quality transformers are not inexpensive.
Building a high-quality small-signal transformer can be an expensive and specialized task. However, transformers have the advantage of being simple devices, and in audio, simplicity done well is almost always better.
Conclusion
Audio transformers are commonly used to match microphone impedances to mic preamp input circuits. They are especially useful for matching the high-impedance tube circuits found in tube microphones to standard output impedances. In ribbon microphones, they convert the very low voltage, very low impedance signals from the ribbon motors into normal microphone output levels.
When choosing a transformer for your microphone, as with any engineering decision, you should consider the specific purpose you have in mind. It’s important to understand the capabilities of your microphone’s internal preamp tube so that you can make an informed choice.
Even though transformers might seem simple, they are always interesting. Of all the passive components, transformers are perhaps the most fascinating, yet many people overlook them. A lot of myths and misunderstandings exist, often because writers don’t fully grasp the basic principles. Part of our job is to clear up as many of these misconceptions as possible.
We recommend choosing a transformer that is specifically designed for your application, and using a source with low impedance (not negative impedance). While negative impedance drivers can potentially improve performance, they also come with a number of risks.
