While reading over a microphone spec sheet, you’ve undoubtedly come across values for either output impedance, rated load impedance, or both! Understanding these impedance values is crucial to truly master microphones.
So what is microphone impedance? Microphone signals are AC voltages. Impedance is the “AC resistance” of audio signal voltages. Impedance controls the flow of the audio signal. In order for a mic signal to travel optimally, the microphone output impedance must “match” or “bridge” the input impedance (load impedance) of its mic preamp.
Microphone impedance bridging is critical for the optimization of your microphones. If you’re interested in getting the most out of your microphones and your preamps, this article is for you!
As an added bonus, we’ll also discuss the internal impedance of microphones and touch on condenser microphone design to make this article a full, in-depth guide to microphone impedance.
Be sure to check out My New Microphone’s article How Do Microphones Work? (The Ultimate Illustrated Guide)!
Related article: The Complete Guide To Understanding Headphone Impedance
Table Of Contents
- What Is Microphone Impedance?
- The Output Impedance Of A Microphone
- The Load Impedance Of A Microphone
- High-Impedance And Low-Impedance Inputs And Outputs
- Impedance Bridging Between Ribbon Microphones And Preamps
- Internal Impedance Of Condenser Capsules
- Microphone Impedance Converters
- Related Questions
What Is Microphone Impedance?
Let’s start with a general description of impedance:
- Electrical impedance is a measurement of the opposition/resistance to an alternating current in a circuit when a voltage is applied.
- Impedance is measured in ohms (like resistance) and can be thought of as a type of “AC resistance” in an AC circuit.
Impedance, as it applies to microphones:
- Audio signals are AC voltages since they have both negative and positive voltages (the positive and negative amplitude of the signal). Audio signals are, therefore, alternating currents (having both negative and positive currents).
- Microphone impedance controls the flow of alternating current in an audio circuit when an audio signal voltage is applied.
- All microphones (like any electronic devices that generate an AC voltage) have an output impedance.
- A microphone creates a circuit with the preamplifier (or another audio device) it’s connected to. These preamps have input impedance (known as the microphone’s load impedance).
- The output impedance of the microphone must be a fraction of the preamp input impedance (load impedance) in order for the mic signal circuit to function optimally.
Microphones are complex. Resistors, capacitors, inductors, transformers, tubes, transistors, and other electronics all play a role in determining a microphone’s overall “resistance.”
To simplify things for ourselves, we consider the total “resistance” of a device as one number.
This number is the output impedance, measured across the microphone’s output terminals.
A microphone signal is only passed once the mic is connected to a preamp (or the next audio device in-line). The microphone output must create a circuit with a load in order for the mic’s audio signal to move!
It is, therefore, important to not only pay attention to the inherent impedance of the microphone but to the impedance of the preamp or inline device as well!
The two impedance values critical to proper microphone function and signal transfer are:
- Output impedance: the inherent impedance of the microphone across its output connection.
- Load impedance: the input impedance of the audio device next-in-line (typically a microphone preamplifier) that creates the circuit with the microphone output.
Some microphone manufacturers will make note of both the output impedance and the rated load impedance (ideal load impedance) values on their spec sheets. Others will not.
As a general rule of thumb, load impedance should be at least 10 times higher than the microphone’s output impedance. Some other sources state 5 times higher. So long as the load impedance is magnitudes higher than the microphone output impedance, the mic signal should flow efficiently from the mic to the preamp.
Nearly all professional-grade studio/live microphones are designed with low output impedances. Professional preamplifiers are typically designed with high enough input impedances to obey the 10x rule of thumb.
So there’s not too much to worry about. However, knowing the relationship between mic output impedance and mic load impedance is practical information in many professional cases!
To learn more about mic signals, check out my article What Is A Microphone Audio Signal, Electrically Speaking?
Let’s dive deeper.
The Output Impedance Of A Microphone
On any given microphone’s spec sheet, you’ll find a value for its output impedance.
Any professional microphone is deemed “low-impedance,” which roughly means an output impedance in the range of 50 ohms – 600 ohms. Most professional mics have an output impedance between 150-250 ohms.
The general ranges of output impedance in microphones are as follows:
- Low-impedance mics: <600 Ω
- Medium-impedance mics: 600 Ω – 10,000 Ω
- High-impedance mics: >10,000 Ω
The Shure SM57 (link to check the compare prices on Amazon and select online retailers) is an example of a professional microphone with a low output impedance of 150 Ω
Note that microphones don’t have “input impedances” because they do not receive AC. They have output impedances because they output AC voltages (analog mic signals).
Having said that, there are plenty of devices within active and passive microphones that convert the impedance of the signal within the mic. These components effectively have input and output impedances. Examples include amplifiers, tubes, transistors and transformers.
To learn more about microphone output impedance ratings, specifically, check out my article What Is A Good Microphone Output Impedance Rating?
Microphone output impedance is actually frequency specific. However, rather than providing graphs, manufacturers typically provide microphone output impedance specifications as nominal impedance.
Nominal impedance is the approximate output impedance of the microphone averaged across the audible frequency spectrum (20 Hz – 20,000 Hz).
There is no one number that can truly tell us the impedance of a microphone. However, the nominal impedance value has become the standard and we are usually able to infer what we need from this single value.
Why Are There No Professional High-Impedance Microphones?
The only advantage of high-impedance microphones is the low cost of manufacturing. The disadvantages, however, are grave indeed! In fact, I’d argue that there’s never an application for high-impedance microphones in any professional recording or sound system.
A good example of a high-impedance microphone would be a typical consumer karaoke microphone.
These karaoke microphones have a very high output signal level and therefore need a high-impedance output. This high output means no gain staging or amplifier within the mic, which drastically decreases manufacturing costs. These microphones don’t need preamplifiers to bring their signals from mic to line level.
The big downside is that high-impedance microphones do not perform well over long cable runs. The longer the cable, the worse the result.
This is due to the inherent capacitance in a microphone cable. When a signal with high-impedance is sent through a microphone cable, a low-pass-filter is essentially created. The longer the cable, the lower the “filter cutoff” and the more muffled the sound.
To add insult to injury, the higher the impedance, the more susceptible the signal is to external noise and interference. Electromagnetic and radio interference worsens the signal-to-noise ratio of the signal and worsens the quality.
For more info on signal-to-noise ratio, check out my article What Is A Good Signal-To-Noise Ratio For A Microphone?
This is not good at all, and so low-impedance outputs have become the standard for professional audio.
Why Are Professional Microphones All Low-Impedance?
It all comes down to protecting the quality of the audio. What good are expensive capsules and circuit boards if the microphone’s audio gets degraded in the cable as it travels to the preamp?
Low-impedance microphones allow for long cable runs (anything practical) without any noticeable degradation of the audio signal.
This audio degradation would, in theory, be in the form of the attenuation of high-end frequencies.
Like their high-impedance counterparts, low-impedance mics do in fact have an attenuation of high frequencies that is dependent on cable length. However, the attenuation cutoff happens at frequencies far beyond the audible spectrum. In other words, we won’t hear the difference.
The Load Impedance Of A Microphone
We’ve covered microphone output impedance and why low output impedances are better. To fully understand mic impedance, though, we must comprehend the other type of impedance involved with microphone signals: the load impedance.
The load impedance of a microphone is the input impedance of the audio device that follows the microphone in the signal chain. Typically the device is a microphone preamplifier but could also be a filter, pad, or another audio device.
We want the load impedance to be higher than the microphone’s output impedance. As previously mentioned, we generally want it at least 10 times higher for the best results (maximum voltage transfer).
This maximum voltage transfer is called “bridging,” although it is sometimes confused with the term “impedance matching.”
The Neumann KM 184 (link to compare prices on Amazon and select online retailers) has a low nominal output impedance of only 50 Ω. It’s rated load impedance is 1kΩ.
The recommended load impedance is 20 times (at least 10 times) that of the mic’s output impedance.
True impedance matching involves having the same impedance at the microphone output and the preamplifier input. Impedance matching actually degrades the audio signal. We are concerned with impedance bridging.
Load Impedance And Output Impedance
Why does the load impedance need to exceed the microphone’s output impedance?
To better understand, we’ll change our reference point from the microphone to the microphone preamplifier. From this perspective, the microphone becomes the “source” and the preamp becomes the “input”:
- The microphone output is now “source output.”
- The microphone output impedance is now “source output impedance.”
Without getting into metaphors of water pipes and traffic lanes, I’ll show you the formula for voltage transfer between a source impedance and input impedance:
V(input) = Z(input) • V(source) / [Z(input) + Z(source)]
- V(input) = Voltage at the preamp input.
- V(source) = Voltage at the microphone output.
- Z(input) = Input impedance of preamp (microphone load impedance).
- Z(source) = Microphone output impedance.
If everything else was to stay constant, increasing the load impedance “Z(input)” results in a closer matching between the output voltage of the microphone and the input voltage of the preamp.
Once again, this maximum voltage transfer is called “impedance bridging” or, alternatively, “voltage matching.”
We’re interested in bridging and not true impedance matching. Once again, if we were to match impedances, the voltage difference would result in a significant signal loss at the preamplifier.
Load Impedance On The Specs Sheet
Sometimes the specs sheet of a microphone will feature a certain value for its “recommended load impedance” or “rated load impedance.”
The mic input at the preamp must match or exceed the recommended load impedance in order for the mic to function optimally. In fact, all the other specs of the microphone are dependent on the rated load impedance being met or exceeded!
Note that this is rarely ever an issue with professional audio equipment but it’s worth knowing.
High-Impedance And Low-Impedance Inputs And Outputs
As discussed, we want low microphone output impedance and a relatively high load (preamp input) impedance.
Modern preamps have been “standardized” to have high input impedance, so there’s usually nothing to worry about with microphone-to-preamp impedance bridging.
However, some mixers and preamps offer both mic inputs and line inputs. It’s important to know the difference between the two, including their input impedances in order to correctly bridge the microphone and the mixer channel.
Microphone inputs are nearly always female XLR while line inputs are usually 1/4″ TRS jacks. Simply plugging a microphone via XLR to a mic input shouldn’t cause any issues!
We’ll touch on mic level versus line level a bit here.
- Mic level sources are in the range of -60 dBV to -40 dBV (0.001 V to 0.010 V) with output impedances typically between 150 Ω to 200 Ω.
- Mic level inputs (often XLR) on preamps and mixers typically have an input impedance of at least 1500 Ω and expect a voltage typical of a mic level signal.
- Line level sources are usually around 0 dBV (1 V), which is 100x to 1000x the voltage of a mic level.
- Line level output impedances are oftentimes between 100 Ω to 600 Ω.
- However, line level inputs (often TRS) have an impedance of more than 10,000 Ω and expect a voltage typical of a line level signal.
- Instrument level sources find themselves somewhere between mic and line level and often need a preamp to boost them up to proper line level.
- Instrument level output impedances can get very high. An electric guitar, for example, can have an output impedance of 7,000 Ω to 15,000 Ω or even higher!
- For this reason, line inputs on preamps and mixers may come with a “Hi-Z” or “line/instrument” switch.
As long as we connect our microphones into mic inputs, we shouldn’t ever have to worry about microphone input or load impedance!
For an in-depth read into microphones and the various signal level types, check out my article Do Microphones Output Mic, Line, Or Instrument Level Signals?
Plugging A High-Z Output Into A “Low-Z” Input
A high-impedance output will often overload a preamp if it is inputted into a low-impedance mic level input. This will result in distortion and could potentially even damage the circuitry of the preamp!
If you need to, for example, plug a keyboard (high-Z) into a mixer channel mic input (low-Z) via an XLR cable, it is advised to put a DI box (Direct Inject box) in-line to convert the high-impedance signal to a low-impedance signal before the mic input.
From what we’ve discussed, it’s best to plug the DI box in-line as close to the high-impedance source as possible. The longer cable run should carry the converted low-impedance signal to the preamp. This will help conserve more of the high-end frequencies of the audio signal.
Plugging A Low-Z Output Into A “High-Z” Input
This is where it gets kind of confusing.
Let’s say our microphone is Low-Z, which is ideal. Similarly, the mixer input is Hi-Z, which is also ideal. So what’s the issue?
The issue is that Hi-Z line or instrument inputs expect a line or instrument signal. Line and instrument signals have much higher voltages than mic level signals.
Plugging a Low-Z mic into a High-Z instrument or line input results in a low-level signal. The audio signal will be close to the noise floor and so any boost at the preamp will also bring up the noise.
This poor signal-to-noise ratio is undesirable, to say the least!
Once again, you probably won’t run into this problem if you stick to simple XLR cable connections between the microphone and the mic input!
To learn more about the potential inputs for microphones, check out my article What Do Microphones Plug Into? (Full List Of Mic Connections).
Impedance Bridging Between Ribbon Microphones And Preamps
Microphones require their load impedances to be upwards of 10 times their output impedances for optimal performance. Passive ribbon microphones are the most sensitive to this requirement. So let’s talk about them as a good example of impedance bridging.
What happens when the load impedance is too low?
When the input impedance of the preamp is too low, the connected passive ribbon microphone suffers in several ways:
- The ribbon itself can become damped, resulting in restricted movement and muffled sound.
- More preamp gain is needed, which may worsen the signal-to-noise ratio.
- Low-end clarity of the mic signal becomes compromised.
- Distortion and artifacts can be introduced into the signal.
- Transients are dulled.
- The top-end response is compromised.
None of the above are what we necessarily want from an expensive ribbon microphone so ensuring we have high enough preamp input impedance is crucial!
For more information on dynamic ribbon mics, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.
Frequency-Specific Output Impedance
Yes, microphone output impedance is frequency-specific.
Ribbon microphones often have a frequency-specific spike in output impedance that greatly exceeds their nominal impedance value.
At the resonant frequency of ribbon mics, the output impedance can spike to several times the nominal output impedance.
For example, AEA’s R84 (link to compare prices on Amazon and select online retailers) has a nominal output impedance of 270 Ω and a rated load impedance of 1.2 kΩ.
The rated load impedance is just shy of being 5 times that of the nominal output impedance.
However, at the R84’s resonant frequency (16.5 Hz), the actual output impedance reaches 900 Ω and ramps downward as we move into the audible spectrum. To fully unlock the low end of an AEA R84, a preamp with an input impedance as high as 9kΩ would be necessary.
AEA recommends its TRP2 preamp (link to check the price at select retailers) to go with its passive R84 ribbon microphone.
The TRP2 has plenty of clean gain (63 dB) to boost the low-level signals from the passive R84 ribbon mic. This microphone preamp is also designed to accept and boost mic level signals.
What we’re more interested in in this article is the input impedance of the TRP2:
- 63 kΩ when phantom power is disengaged. This is more than enough to get the full bottom end from AEA’s passive ribbon mics.
- 10 kΩ when phantom power is engaged. This is, once again, more than enough to get the full bottom end. This input impedance is lower since active ribbon mic signals require less gain from the preamp.
Microphone output impedance nearly always increases at lower frequencies.
Choosing The Right Preamp
The wide variation of impedance across the frequency response of a ribbon mic may cause some issues.
It takes a very high-impedance preamp to handle the true potential of a ribbon mic. As discussed above, several issues will arise when the preamp does not have a high enough input impedance. Most notably among these issues is the loss of low-end, where the actual microphone output impedance is very high.
However, the alteration to the “true” sound of the ribbon mic can be put to use, creatively.
Many studio professionals actually utilize various preamps to achieve different sounds out of a single ribbon microphone!
Variable-impedance preamplifiers, like the Cloudlifter CL-Z inline preamp (link to compare prices on Amazon and select retailers), are an easy and fun way to alter the sound of ribbon microphones and all other microphones for that matter.
For more information on microphone preamplifiers, check out My New Microphone’s articles What Is A Microphone Preamplifier & Why Does A Mic Need One? and Best Microphone Preamplifiers.
Internal Impedance Of Condenser Capsules
There are other microphone impedances besides output impedance and load impedance that we should know about.
The internal impedance of condenser microphone capsules is a major factor in their design and should be a part of our greater discussion on mic impedance.
The condenser capsule design is based on a parallel-plate capacitor. The movable diaphragm acts as the front plate and the backplate is stationary.
This capacitor functions by holding a fixed electrical charge which is supplied by external means (phantom power or external power supply) or via electret material as is the case with electret mics.
With a fixed charge, any change in capacitance causes an inversely proportionate change in voltage. The capacitance is dependent on the distance between the two plates. Therefore, as the diaphragm moves back and forth and the distance between the plates varies, an AC voltage (aka mic signal) is produced.
That is a very basic explanation of the electrostatic condenser capsule/transducer. The main point we need to understand is that the capsule must hold a constant electrical charge to function properly.
In order for the capsule to hold this charge without leakage, it must have an incredibly high impedance. This high impedance keeps the electrical charge from dissipating out of the capsule.
However, this high impedance also applies to the electrical lead wires that carry the mic signal out of the capsule. Therefore, an impedance converter must be incorporated into the condenser microphone design in order to effectively use the signal from the capsule transducer.
To learn everything you need to know about microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).
This brings us to the next section in this guide to microphone impedance.
Microphone Impedance Converters
In order to actually utilize the extremely high-impedance signal from its capsule, a condenser microphone must have an impedance converter immediately after its capsule in its design.
Microphone impedance converters (ICs) can either be vacuum tube-based (as is the case with tube condenser microphones) or transistor-based (as is the case with solid-state/FET condenser microphones).
Vacuum tube ICs and transistor ICs essentially work in the same way. They both require an external voltage to be applied in order to cause an electrical current to run through them. This current will ultimately be the converted signal.
While the IC is running, the high-impedance signal from the capsule is applied and effectively modulates a higher-level lower-impedance signal. If we think of an impedance converter as having an input and an output, then the IC acts as both an amplifier and an impedance converter.
The study of microphone impedance converters goes much deeper than this explanation but that is for a different article. The goal here was to let you know that tubes and transistors are required in condenser microphones due to the high impedance of the condenser capsule.
For more information on condenser microphones and their impedance converters, check out the following My New Microphone articles:
• What Is A Tube Microphone And How Do Tube Mics Work?
• What Is A Solid-State Microphone? (With Mic Examples)
• What Are FETs & What Is Their Role In Microphone Design?
• What Are The Differences Between Tube & FET Microphones?
How does preamp gain affect a microphone signal? Preamp gain boosts quiet mic level signals (-60 dBV to -40 dBV) to the power of line level signals (o dBV). Therefore, preamp gain must be able to boost microphone signals by up to 60 dB! Increasing preamp gain may add noise, gentle saturation, or alter the frequency response of the audio signal.
To learn more about gain and my recommended mic preamps, check out What Is Microphone Gain And How Does It Affect Mic Signals? and Best Microphone Preamplifiers, respectively.
Is all professional gear low-impedance? All professional microphones are considered low-impedance, but not all professional gear is low-impedance. Electric guitars, keyboards, and effects pedals are all considered to be “high-impedance” and even output unbalanced audio!
For information on all the possible microphone specification, please continue to my article Full List Of Microphone Specifications (How To Read A Spec Sheet).
This article has been approved in accordance with the My New Microphone Editorial Policy.