What Are Microphone Transformers And What Is Their Role?

Some microphones have transformer-coupled outputs, while others are deemed transformerless. Some microphones even have transformers in the middle of their signal chains rather than at their outputs.

What is a microphone transformer? A mic transformer is a passive electrical device that physically isolates two circuits while maintaining an electrical relationship between the circuits. Mic transformers have two conductive coils (one for each circuit) wound around a common magnetic core. They adjust voltage, current, and impedance.

In this article, we'll discuss the role of the transformer in microphones as well as the microphones that use transformers and those that do not.

What Is A Transformer?

When we hear the term transformer, we often think of the power transformers that feed electricity to our buildings from power lines. That, or the “robots in disguise.”

The power transformers that provide our buildings with regulated mains voltages deal with much higher voltages and currents than microphone output transformers. Still, they are both designed on the same principles.

Transformers are passive electromagnetic devices that transfer energy from one circuit to another by means of inductive coupling.

Basically, each circuit has its own conductive wire that wraps around a common magnetic core. This effectively couples the two circuits.

Inductively coupled circuits, like those in transformers, are configured so that a change in current through one circuit's conductors induces a voltage across the ends of the other circuit's conductor.

The most simple transformers, which are common in transformer-coupled microphones, are made of two circuits. Each circuit has a conductive wire, known as a winding, that wraps around the transformer's magnetic core.

Typically, the primary winding connects to the circuit that features the microphone capsule's output signal, while the secondary winding connects to the output circuit of the microphone.

Here is a simple diagram of a transformer:

• P = primary winding: the primary winding is on the side of the mic capsule. This does not necessarily mean it is in the same circuit as the mic capsule. Rather, it means that the winding is between the transformer and the capsule rather than between the transformer and the mic output. The primary winding can be thought of as the “input” of the transformer, bringing the mic signal from the capsule to the transformer.
• S = secondary winding: the secondary winding is on the side of the mic output. This means the winding is between the transformer and the output rather than between the transformer and the capsule. The secondary winding can be thought of as the “output” of the transformer. As the signal passes through the primary winding, the transformer effectively produces a proportionate but altered signal across the secondary.
• MC = common magnetic core: in order for inductive coupling to work, we need electromagnetic induction. The voltage across the primary winding causes a changing magnetic field within the magnetic core which then induces a voltage across the secondary winding.

So basically, a voltage across the primary winding (due, ultimately, to the mic capsule) causes a change in the magnetic core's magnetic field. This change in the magnetic field then induces a voltage across the secondary winding. This is all due to electromagnetic induction.

Note that transformers only pass alternating current and block direct current. This is known as DC isolation. A direct current is, by definition, not an alternating current. Therefore, it would not produce a changing magnetic field which is necessary for electromagnetic induction to induce a voltage across the other/secondary winding.

It's also critical to note that power input to the transformer (at the primary) and power output from the transformer (at the secondary) is the same (except for conversion losses). I'll repeat this later when we discuss step-up transformers, step-down transformers, and the roles of transformers in microphones.

General Equations For Microphone Transformers

Now that we've covered the basics of microphone transformers, let's look at some concrete calculations we can make with transformers. This will help us in the upcoming section about the role of transformers in microphones.

Here are some transformer formulas with explanations. Note that these are ideal formulas and do not take into account the naturally occurring conversion losses.

Power Formula For Transformers

As previously mentioned, in ideal situations (with no conversion losses), the power at the primary winding of a transformer should be equal to the power at the secondary winding:

P_p=P_s

P = Power

This happens regardless of the ratio of turns between the two windings.

It's useful to note that power is equal to the product of voltage and current:

P=IV

I = Current
V = Voltage

With this in mind, let's look at how to calculations a transformer's effect on voltage and current.

Voltage Formula For Transformers

The number of turns in a conductive coil is a major factor in determining the voltage that can be electromagnetically induced across the coil. All else being the same, a coil with more turns will have a great voltage induced across it.

Transformers have a common magnetic core, and the windings are nearly always made of the same conductive material with the same gauge. The turns ratio, then, becomes a critical part of the equation.

Here is the formula for calculating the voltage difference in a transformer:

V_s = \frac{N_s}{N_p}•V_p

V_s = Voltage across the secondary winding
V_p = Voltage across the primary winding
N_s = Number of turns in the secondary winding
N_p = Number of turns in the primary winding

So as we can see here, if the secondary coil has more turns than the primary, then the secondary coil will have a greater voltage than the primary.

Another way of writing this is:

\frac{V_s}{V_p}=\frac{N_s}{N_p}

As a reminder, this formula is true for ideal circumstances and does not take into account any conversion losses.

To learn more about voltage and microphone signals, check out my article What Is A Microphone Audio Signal, Electrically Speaking?

Current Formula For Transformers

So far, we know that transformers do not alter power from one winding to another. We've also discussed that the winding with more turns will have a greater voltage.

So if P = I ⋅ V, then an increase in voltage would mean a proportionate decrease in current. That is exactly what happens in transformers.

Here is the formula for calculating the current difference in a transformer:

I_s = \frac{N_p}{N_s}•I_p

I_s = Current through the secondary winding
I_p = Current through the primary winding
N_p = Number of turns in the primary winding
N_s = Number of turns in the secondary winding

If the secondary winding has more turns than the primary, it will also have less current than the primary.

Another way of writing this is:

\frac{I_s}{I_p}=\frac{N_p}{N_s}

Impedance Formula For Transformers

Another important electrical value that transformers affect is impedance.

Note that this isn't on the impedance of the transformer itself, but rather the difference between the impedance in the primary winding's circuit and the secondary winding's circuit.

Substituting resistance (a DC quantity) for impedance (an AC quantity) in Ohm's Law, we get:

|Z| = \frac{V}{I}

|Z|= Impedance (absolute)

This isn't a perfect substitution necessarily, but it leads us to the following equation for transformer impedance:

Z_s = (\frac{N_s}{N_p})^2•Zp

Z_s = Impedance of the secondary winding
Z_p = Impedance the primary winding
N_s = Number of turns in the secondary winding
N_p = Number of turns in the primary winding

Note that we got to the above equation by comparing the secondary winding to the primary winding for each value (Z, V, and I) and then swapping the V_s/V_p for N_s/N_p and the I_s/I_p = N_p/N_s.

So what does this equation mean?

If a transformer's secondary winding has more turns, it will have higher impedance than the primary. This impedance value will be equal to the square of the turns ratio multiplied by the primary winding impedance.

If a transformer's secondary winding has fewer turns, it will have lower impedance than the primary. This impedance, again, will be equal to the square of the turns ratio multiplied by the impedance of the primary winding. In this case, the square of the turns ratio will be less than one.

General Types Of Microphone Transformers

So now that we know how transformers work, let's look at the types of transformers found in microphones and the roles they play.

First, let's note that microphone transformers are audio transformers and work within the audible frequency range of 20 Hz – 20,000 Hz. Poorer-quality transformers may have smaller ranges than this. Proper audio transformers will work within this range.

When discussing the roles a transformer plays in a microphone, it's important to know that there are different types of transformers that are used in microphone design. These transformer types are:

• Step-up transformer
• Step-down transformer
• Isolation transformer

Step-Up Transformer

In the diagram shown above, we described a step-up transformer.

A step-up transformer is designed to increase or “step up” the voltage from the primary to secondary windings. This requires the secondary winding to have more turns (the number of times the wire is wrapped around the magnetic core) than the primary.

With all else being equal, a conductive wire with more turns will have a greater voltage induced across it when subjected to the same change in the magnetic field.

The diagram above shows the primary winding (capsule side) on the left and the secondary winding (output side) on the right.

As the capsule's AC voltage (mic signal) is applied across the primary winding, it produces a changing magnetic field in the magnetic core. This changing magnetic field, then, is common to both windings. Since the secondary winding has more turns, the voltage induced across it is greater than the voltage across the primary.

This increase in voltage is accompanied by an even greater impedance increase and a proportionate decrease in current.

Step-Down Transformer

A step-down transformer is essentially a step-up transformer wired in reverse. Its primary coil has more turns than its secondary coil.

The diagram above shows the primary winding (capsule side) on the left and the secondary winding (output side) on the right.

The AC voltage produced by the mic capsule is ultimately applied across the primary winding. This voltage induces an alternating magnetic field within the magnetic core of the transformer. This, in turn, induces a smaller voltage across the secondary winding (due to the secondary winding having fewer turns).

This decrease in voltage is accompanied by an even greater decrease in impedance along with a proportionate increase in current.

Isolation Transformer

The lesser-known isolation transformer has a 1:1 turns ratio and is designed not to affect the primary and secondary windings' voltage, current, or impedance. Rather, its purpose is to electrically isolate the main microphone components and circuitry (including the capsule) from the microphone output connector.

It's important to note that all transformers act to isolate two circuits. Though there is an electromagnetic relationship between the two windings of a transformer, each winding is in its own isolated circuit.

These isolation transformers are built to block DC and protect microphones without affecting the mic signal at the output.

The Roles Of Transformers In Microphones

To wrap things up, let's look at the many roles of transformers in microphone design:

• Block DC voltage (including phantom power) from entering the sensitive microphone components
• Balance unbalanced audio signals
• Adjust microphone output impedance
• Adjust microphone output voltage (signal strength)
• Colour the microphone's output signal

A quick note on colouration: transformers often add soft distortion to a mic signal, especially when the turns ratio is large. This is because of the natural imperfections in the conductive wires and magnetic cores, as well as the conversion losses that naturally occur with electromagnetic induction.

It's important to note that transformers are not always needed in microphone designs. The high-quality transformers are expensive, and the inexpensive transformers have detrimental effects on microphone audio.

Many moving-coil dynamic microphones do without them to decrease costs. These mics are typically durable enough to bypass transformers.

Many condenser microphone manufacturers have opted for cheaper transformerless output circuit designs that use transistors to perform the same functions as the transformers.

To learn more about microphones and the relationships between transformers and transistors, check out my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).

With that, let's look at some common microphone types that use transformers in their designs:

Moving-Coil Dynamic Microphone Transformers

The Shure SM57 is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top 11 Best Dynamic Microphones On The Market
• Top 12 Best Microphones Under $150 For Recording Vocal

Step-up transformers are sometimes put at the output of moving-coil dynamic microphones to help boost their relatively weak mic signals. These mic signals can afford the increase in impedance that comes with an increase in voltage.

Some transformers colour the mic signal more than others, and all transformers protect the cartridge from DC voltage.

For more information on moving-coil dynamic microphones, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.

Passive Ribbon Dynamic Microphone Transformers

The Royer R-121 is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top 12 Best Passive Ribbon Microphones On The Market

Step-up transformers are always used at the outputs of passive ribbon microphones. Ribbon diaphragms are very fragile and need the protection for DC powering that transformers provide.

To learn more about ribbon microphones and microphone powering options, check out the following My New Microphone articles:
• Will Phantom Power Damage My Ribbon Microphone?
• Do Microphones Need Power To Function Properly?
• Do Microphones Need Phantom Power To Work Properly?

The step-up transformers increase the voltage of the relatively weak ribbon mic signals without increasing the impedance to unusable levels.

As with all microphones, some transformers will colour the microphone signal more than others.

Active Ribbon Dynamic Microphone Transformers

The Royer R-122 MKII is featured in My New Microphone's Top 11 Best Active Ribbon Microphones On The Market.

Active ribbon microphones will generally have step-up transformers right after their baffles (diaphragm/element).

These step-up transformers often have higher turns ratios and work to boost the mic signal (voltage) at the expense of increasing the impedance.

These transformers also protect the sensitive diaphragm from the DC power required to run the active circuitry of the ribbon mic (notable the impedance converter).

The impedance converter (FET) takes the higher-voltage/higher-impedance signal from the transformer and effectively decreases the impedance to usable levels without lowering the signal's voltage. Some impedance converters even act as pseudo-amplifiers and increase the voltage of the signal even more.

These microphones typically have transformerless output circuits.

Active Tube Ribbon Dynamic Microphone Transformers

The Royer R-122V is featured in My New Microphone's Top 11 Best Active Ribbon Microphones On The Market.

Tube ribbon microphones generally require two transformers in their design.

The first transformer is a step-up transformer and acts to:

• Increase the relatively weak mic signal level (voltage) from the ribbon diaphragm. This is done at the expense of increasing the impedance, which the vacuum tube will properly convert.
• Protect the ribbon diaphragm from any potential DC voltage that could damage it.

The second transformer is a step-down transformer and acts to:

• Further lower the impedance to a professional useable level.
• Balance the microphone signal from the vacuum tube.

To read more about ribbon dynamic microphones, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.

FET Condenser Microphone Transformers

The Neumann KM 84 is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top 12 Best Vintage Microphones (And Their Best Clones)

Some solid-state condenser microphones (particularly the early ones) utilize step-down transformers at their outputs.

These step-down transformers act to adjust the output impedance of the mic signal properly.

Tube Condenser Microphone Transformers

The Sony C-800G is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top 11 Best Microphones For Recording Vocals
• Top 11 Best Tube Condenser Microphones On The Market

Tube condenser microphones typically have step-down transformer-coupled outputs.

These step-down transformers effectively adjust the signal impedance to a useable level. The signal strength from the vacuum tube can afford the voltage decrease inherent in a step-down transformer.

Output transformers also balance the output signal and protect the microphone from DC voltage that could be coming in on pins 2 and 3 of the audio cable (phantom power).

Related Questions

What is a class A amplifier? Class A amplifiers are considered the best-sounding and are the most common class of amps. They use a single output transistor and are always on. They offer excellent linearity, high gain and low signal distortion levels when designed correctly.

What is impedance matching? Impedance matching is the practice of connecting a source to an input for maximum power transfer. The load impedance of an electrical input must be matched appropriately to the sound impedance of an output device. With mics, we're more concerned with impedance bridging for maximal voltage transfer.

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