What Are The Differences Between Tube & FET Microphones?


Tube condenser microphones are commonly cherished for their character while non-tube (FET) microphones are often described as more accurate and are much more popular on the market today. All subjective preferences, there are other major differences between the tube and the FET microphone.

What are the differences between tube and FET microphones? Vacuum tubes and field-effect transistors both act as impedance converters and pseudo-amplifiers in active microphones that require that sort of processing. Tube mics have vacuum tubes while FET mics have FETs. In general, tube technology adds more colour and is more fragile than solid-state FET.

In the following paragraphs, we’ll dig deeper into uncovering more differences between tube microphones and FET microphones.


Tube Vs. FET Microphones

Tables are an easy way to disseminate information. Let’s look at the differences between tube and FET mics in the following table:

 Tube MicrophonesFET Microphones
Impedance ConverterVacuum tube (at least a triode)Field-effect transistor (often JFET)
Power SourceExternal power supply unitsPhantom power or DC bias voltage
Self-NoiseMoreLess
Audio QualityTypically warmer (tube saturation and high-end roll-off)Typically colder (accurate sound capture)
Transformer-coupled outputYesSometimes
DurabilityFragile tube componentsMore durable solid-state components
PriceVery expensiveLess expensive

To read a bit more about microphone transistors (and transformers), check out my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).


Similarities Between Tube And FET Microphones

Before getting into the differenced between Tube and FET microphones, it’s a worthwhile exercise to note the similarities.

Tube and FET microphones are not two different transducer types, but rather differentiate between different active electronic topographies. So by, that we can say that tube and FET microphones are both active.

The AKG C 12 VR is a tube condenser microphone

Tube and FET typically make up the two types of impedance converters in condenser microphones, though they are present in some ribbon microphone designs as well.

The Rode NT1-A is an FET condenser microphone

In fact, vacuum tubes and field-effect transistors both do the same job in a microphone design: impedance conversion and pseudo-amplification. So although these microphones are different from one another, their signal processing is nearly the same.

AKG and Rode are both featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.


Impedance Converter

We had mentioned that vacuum tubes and FETs fulfill the same role in active microphone design. They act mainly as impedance converters and, to some extend, pseudo-amplifiers.

This is the obvious difference: tube mics have vacuum tubes while FET mics have field-effect transistors.

In this section, we’ll assume that the tubes and FETs are within condenser microphones, which is usually, but not always, the case.

Tube microphones utilize vacuum tubes as impedance converters.

Condenser microphone capsules naturally output an audio signal with extremely high impedance. With such high impedance, the electric signal will not be able to travel through any significant length of wire/circuitry without serious degradation.

Electro-Harmonix 6072A 12AY7EH dual-triode vacuum tube

Designing the microphone with a triode vacuum tube immediately after the capsule helps to drop the signal impedance.

Here is a simple diagram of a triode vacuum tube:

  • H: heater
  • K: cathode
  • A: anode
  • G: grid

A triode vacuum tube requires heat to function properly. A heater or filament is part of the design and draws electricity from an external power source to effectively heat the tube.

Once heated, the cathode will begin emitting electrons. These negatively charged electron naturally flow up from the cathode to the positively charged anode plate. This flow of electrons is essentially an electric current and the tube can be incorporated into a circuit.

This flow of electrons is unbalanced and has relatively low impedance.

The above description tells us how a diode vacuum tube works, but how do triodes fit into microphone design? This is where the grid comes into play.

The grid can be thought of as a high-impedance input. It is electrically connected to the mic capsule and receives the AC voltage (mic signal) from the capsule.

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

The grid then modulates the flow of electrons from the cathode to the anode. When the AC signal at the grid is at a peak, the flow between the cathode and anode will also be at its highest. When the AC signal at the grid is at a trough, the flow between the cathode and anode will also be at a its lowest.

So a low-level high-impedance signal from the microphone’s capsule effectively modulates a lower-impedance signal with greater amplitude. In other words, the tube acts as an impedance converter and amplifier of the mic signal.

Note that vacuum tubes aren’t truly amplifiers since they do not apply gain directly to the signal. Rather, they use one signal to modulate a greater signal.

The vacuum tube was invented in 1904. The triode, which is the most basic vacuum tube for microphone applications, was brought to the market in 1906 under the name “The Audion.”

The first tube microphone to hit the commercial market was the Neumann CV3 “The Bottle,” released in 1928. It was a large-diaphragm tube condenser microphone.

FET microphones utilize field-effect transistors as impedance converters.

Field-effect transistors provide the same practical function as the triode tube, only differently. Let’s discuss.

Here is a simple diagram of a field-effect transistor:

  • S: source
  • D: drain
  • G: gate

With the FET, a biasing voltage is applied to the source/drain and an electrical current is produced between the source and drain.

Transistors are semiconductors that are typically made of doped silicon. They have an active channel through which charge carriers (electrons or holes), flow from the source to the drain.

So here we have an electric current flowing from the source to drain.

The conductivity of the active channel from source to drain is a function of the voltage applied at the gate (across the gate and source terminals). This voltage is the AC voltage or mic signal from the mic capsule.

As with the grid of the triode, the gate of the FET can be thought of a high-impedance input.

So the mic capsule’s output signal is sent straight to the gate. This effectively modulates another signal at the “output” of the FET. This outputted signal is of lower-impedance and often has a greater amplitude.

So, in effect, the FET acts as an impedance converter and pseudo-amplifier, just like the aforementioned vacuum tube.

FETs aren’t truly amplifiers because they do not apply gain directly to the signal. Rather, they use one signal to modulate a greater signal.

The transistor was invented in 1947. The theory behind the field-effect transistors began developing in the early 1920s and the first practical working JFET (junction-gate field-effect transistor) was created in 1953.

The first commercially sold solid-state FET microphone was the Schoeps CMT 20, which was released in 1964.

Note that tubes and FETs perform the same functions in active ribbon microphone designs. Of course, mic signals from ribbon elements have lower impedances than those from condenser capsules. Therefore, the impedance conversion may not be as regulatory (though sometimes the ribbon signal is boosted to high-impedance via a step-up transformer before entering the tube or FET.

To read a detailed account of microphone history and innovation, check out my article Mic History: Who Invented Each Type Of Microphone And When?


Power Source

As we talked about in the similarities section, both tube and FET microphones are active. This means they both require external power to function properly. The source of said power, however, is different between the two mic types.

Tube microphones require external power supplies.

The heating of a vacuum tube typically requires more power than any other active components within the mic. With tube condenser microphones, an external power supply effectively heats the tube and charges the capsule.

Neumann U 47 tube mic with its power supply unit

Neumann is also featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.

Tube microphones came before phantom power and DC bias voltages were common methods of powering active mics. Even with that, modern tube mics are designed with external power supply units because they require more power than can be supplied by phantom or biasing.

FET microphones typically get their power from DC bias voltage or phantom power.

Solid-state microphones require much less power than their tube counterparts.

Let’s talk about condenser microphones. We will separate them further into true condensers and electret condensers.

True condensers require an external voltage to properly charge their capsules while electret condensers utilize electret material to hold a quasi-permanent charge in their capsules.

So with true condensers, they require external power for their capsules, FET impedance converters, and sometimes their printed circuit boards. This power most often comes from phantom power, which is supplied via microphone preamplifiers, standalone units, or batteries (in certain mic designs).

With electret condensers, external power is required only for their FET impedance converters and, in some cases, their printed circuit boards. Depending on the microphone design, this can be done through phantom power or DC bias voltage. DC bias voltage is often supplied to miniature electret mics via wireless transmitters.

Note that with active ribbon mics, no amount of the power is sent to the ribbon element/baffle. Still, tube ribbon mics require external power supplies while true and electret ribbon mics can run on phantom power (or DC bias).

For more information on powering microphones, check out my articles Do Microphones Need Power To Function Properly? and Do Microphones Need Phantom Power To Work Properly?


Self-Noise

Active microphones naturally have what is referred to as self-noise. This self-noise is the inherent noise added to the mic signal by the active components even when no sound is present.

Tube microphones typically have higher self-noise than FET microphones.

Vacuum tubes, with their heaters and glass encapsulation, naturally produce a bit more noise than semi-conductive FETs. This due, in part, to the greater voltage that tubes require in order to function properly.

Of all active microphones, modern solid-state large-diaphragm condenser microphones typically have the lowest low self-noise values (even below 10 dBA). This is largely because the FET is quieter than a tube and the large-diaphragm produces a greater signal than a small-diaphragm, which improves the signal-to-noise ratio.

Tubes typically also add “tube sound,” a slight colouration to the mic signal that is cherished by audiophiles, engineers, and musicians alike.

To read deeper into microphone self-noise, check out my article What Is Microphone Self-Noise? (Equivalent Noise Level).


Audio Quality

High-quality vacuum tubes and high-quality field-effect transistors both sound great and help to produce excellent audio quality in their microphones.

However, there is much debate about how tube and FET microphones sound different from one another. Let’s discuss this briefly here.

In general, tube microphones sound:

  • Warmer and richer.
  • Larger, Bigger, and more dimensional.
  • Smoother top-end.

In general, FET microphones sound:

  • Detailed, honest, and accurate.
  • Colder and brighter.

It’s worth noting that not all tube and FET mics produce high-quality audio.

That being said, if a manufacturer is going through the trouble of putting a vacuum tube microphone on the market today, it’s likely that they’ll do their best to produce a high-quality microphone.

The same cannot be said for FET microphones, since FETs are relatively easy and cheaper to manufacturer. For instance, the cheap electret mics in consumer electronics have FETs in their design.


Transformer Or Transformerless?

Audio transformers are sometimes included in the output design of microphones. They act to balance the mic signal; step the voltage (mic signal strength) up or down; adjust the output impedance of the signal to proper levels; and to protect the mic from DC voltages like DC bias and phantom power.

Tube microphones always have output transformers.

Step-down transformers are typically designed into tube microphone outputs. These transformers were absolutely necessary when solid-stated electronics were not available and are still overwhelming used today.

The step-down transformers act to drop the impedance to usable levels while also balancing the tube’s outputted audio signal at the microphone output. These two steps are necessary for the microphone to output consistent high-quality audio.

Although tube mics today could opt for transformerless output circuitries, they by-and-large do not.

FET microphones sometimes have output transformers.

All early solid-state condensers also had transformers to help balance their outputs and adjust the impedance of their output signal.

The Neumann KM 84 (1966) has an output transformer

However, in the late 1970s, manufacturers began opting for more affordable transformerless output circuit designs.

The Neumann KM 184 (1993) has a transformerlss output

These output circuits were made of solid-state electronics that had multiple benefits over audio transformers: namely noise, cost, and size reduction. High-quality transformers have not dropped much in price over the ages, but transistors and other solid-state components continually become cheaper.

Today, there are many condensers on the market with and without transformers.


Durability

Durability and longevity often go hand-in-hand. Choosing a mic that will last a long time is essential, especially when some FET and tube mics cost so much money (we’ll get to the prices in the next section). So how do tube and FET mics differ in terms of durability?

Tube microphones are generally less durable than FET microphones.

Vacuum tube electronics are, by default, less durable than transistor electronics. Therefore, all other things being equal, tube mics are less durable than FET mics. Why is this?

Tubes are made of glass while FETs are made of solid-state semi-conductive material like silicon. If bumped or dropped, for example, the glass of the tube is more likely to shatter than the transistor.

Tubes require heaters, which have a limited lifespan before they burn out. Transistors do no have the same issues of getting used up.

Vacuum tubes are more sensitive to humidity and much more sensitive to temperature than solid-state FETs. For example, if you were transporting microphones in the cold to another studio, it would be wise to allow the tube mics time to naturally warm up to room temperature before using them while the FET mics would more than likely fine to start using right away.


Price

Tube microphones are known for their high price points. FET mics include such a wide range of products that their price range mirrors this width.

Price range of tube microphones:

The most expensive condenser and ribbon microphones on the market have tube electronics. Though there are some consumer-grade tube mics on the market for cheap, you’ll be looking at at least $500 for an entry-level prosumer tube microphone.

Tube mics, especially the highly sought after vintage models, command price tags above $10,000!

Active ribbon microphones that feature tube electronics are also quite expensive. The AEA A440, for example is a whopping $5,800.

Price range of FET microphones:

As mentioned, FET mics run the gamut of cheap consumer electret mics all the way up to high-quality professional studio mics.

Prices of FET mics range from less than $0.01 (for bulk orders of cheap electret mics) to several thousands of dollars.

For more information on microphone pricing, check out the following My New Microphone articles: How Much Do Microphones Cost? (With Pricing Examples) and Top 20 Most Expensive Microphones On The Market Today.


What are the differences between condenser and dynamic microphones? The main difference between dynamic and condenser mics is that dynamics convert sound to audio via electromagnetic induction while condensers do so via electrostatic principles. This leads to differences in design and overall sound. Condensers are active while dynamics are usually passive.

To learn more about the differences between condenser and dynamic mics, check out my article Differences Between Dynamic & Condenser Microphones.

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

What are the differences between large-diaphragm condensers (LDCs) and small-diaphragm condensers (SDCs)? LDCs generally have a diaphragm diameter greater than 1″ while SDC diaphragm diameters are typically less than 1/2″ (this means there’s a grey area in between). LDCs are often quieter and have more character while SDCs benefit from more accurate/consistent frequency, transient, and polar responses.

To read more about the differences between LDCs and SDCs, check out my article Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones.

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