Microphones, like all electric devices that output electrical signals, have output impedances. I’ve you’ve ever checked out a microphone specs sheet, you likely would have spotted an output impedance rating.
What is a good microphone output impedance rating? Professional microphones are low-impedance. A professional or “good” mic output impedance is in the range of 50 Ω – 500 Ω, though some pro mics have impedances slightly outside this range. So long as the output impedance is much lower (1/10th or less) than the load impedance, it is considered good!
Let’s talk more about microphone impedance and why low output impedances are preferred with professional microphones.
For a full list of microphone specifications, check out my article Full List Of Microphone Specifications (How To Read A Spec Sheet).
What Is Microphone Impedance?
Electrical impedance is the opposition an alternating current experiences in a circuit when a voltage is applied.
To simplify things, I like to think of impedance as electrical resistance, but for AC signals rather than DC signals. Just like resistance, impedance is measured in ohms (symbol: Ω).
A microphone outputs an AC voltage (otherwise known as the mic signal). Once the microphone is connected to a preamp (via and XLR cable or another type of cable), a circuit is completed and the electrical signal experiences impedance.
Within this circuit there are two impedances we need to be aware of:
- Microphone output impedance: this is the impedance of the signal as it is outputted from the microphone.
- Load impedance: this is the input impedance of the device (almost always a preamp) that is in-line after the microphone.
For an in-depth read into microphone impedance, check out my article Microphone Impedance: What Is It And Why Is It Important?
To better understand why 50 Ω – 500 Ω is the typical range for professional microphone output impedances, we must understand load impedance and impedance bridging.
For proper voltage transfer from our microphone to our preamp, we want a low mic output impedance and a high preamp input (load) impedance.
- Generally speaking, we want the load impedance to be at least 10 times that of the microphone output impedance.
- In an ideal world, we’d want an infinite load impedance and absolutely no output impedance, but that’s not realistic in the real world.
Low output impedance allows the relatively weak mic level signal (low voltage) to create some current through through the mic cable.
The high load impedance allows the preamp to provide amplification without attenuation or distortion.
Ensuring this difference is essential for proper voltage transfer and is known as impedance bridging (often confused with impedance matching).
Impedance bridging for voltage transfer can be understood by the following equation:
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.
Although there are no standards per se, most professional mic preamps have input impedances between 2 KΩ and 10 KΩ.
This means that most pro preamps will work perfectly fine with most professional microphones (output impedances between 50 Ω – 500 Ω) so there’s nothing to worry about!
Rated Load Impedance
It’s worth mentioning that professional microphones with good output impedance ratings will also typically have a rated load impedance specification.
The rated load impedance of a microphone is the recommended minimum load impedance for that microphone.
In fact, if this load impedance is not met or exceeded, the rest of the microphone specifications will become untrue. The microphone performance will be hindered.
Pro tip: altering the load impedance (by using different preamps or with a variable-impedance preamp) may lead to interesting and creative results. Dropping lower than the rated load impedance will change the sound of the microphone, but that’s not always necessarily a bad thing.
In the vast majority of cases, the rated load impedance will be 10 times the microphone’s output impedance.
In order to design a microphone with a consistent and low output impedance, manufacturers often use impedance converters. These converters are built into the microphone design and include the following:
- Vacuum tubes.
Transformers are often used at the output of a microphone to bring the microphone impedance to a consistently low value.
Step-up transformers bring the voltage of passive dynamic mics up to mic level. They also bring the impedance up, but within the “good range” in professional mics.
Step-down transformers are often used in active microphones to bring the impedance down to good professional levels at the mic output.
Transistors (FETs and JFETs) are often used in condenser microphones to immediately drop the ultra-high impedance from the condenser capsules.
By bringing the impedance down, the mic signal is able to travel through the internal circuitry of the microphone (and mic cable) without experiencing severe degradation.
For more information on microphone transformers and transistors, please click through to my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).
Vacuum tubes, like the aforementioned transistors, act to boost the voltage and convert the impedance of mic signals (typically from condenser capsules).
The tube accepts a high-impedance signal at its grid and effectively outputs a lower-impedance signal from its anode.
Low Vs. High-Impedance Microphones
So a “good” (professional) microphone output impedance rating is in the range of 50 Ω – 500 Ω. Most pro mics find themselves within the 100 Ω – 350 Ω range.
These professional mics work well with professional preamps, which bring their signals up to line level for use with professional mixers, recorders, etc.
High-impedance microphones are available on the market, though typically for consumer applications (think karaoke machines). These microphones have poor signal transfer but are very inexpensive to manufacturer.
Mic Examples With Good Output Impedance Ratings
Here are 3 examples of professional microphones to give you a solid idea of good output impedance ratings:
Schoeps MK 4 / CMC 6
Output impedance: 35 Ω
With an output impedance of only 35 Ω, the Schoeps MK 4 / CMC 6 modular microphone will work perfectly fine with any practical microphone preamp. Its rated load impedance is 1 KΩ.
Output impedance: 150 Ω
The output impedance of the Electro-Voice RE20 is 150 Ω, though it can be changed to 50 Ω or 250 Ω by soldering the output connection differently. Though the microphone does not have an explicit rated load impedance, the RE20 will work perfectly with any preamp with an input impedance above 2.5 KΩ.
Output impedance: 350 Ω
With an output impedance of 350 Ω, the Sennheiser e906 is on the higher end of the professional mic output impedance range. A preamp with 3.5 KΩ input impedance will handle this microphone signal with ease, though the microphone’s rated load impedance is only 1 KΩ.
What are the characteristics of a good microphone? Although there are many variations between microphones, professional mics typically have the following characteristics:
- Low output impedance.
- Consistent polar response.
- Balanced output.
- Low self-noise.
For in-depth articles on the important microphone characteristics listed above, please read the following My New Microphone articles:
Microphone Impedance: What Is It And Why Is It Important?
The Complete Guide To Microphone Polar Patterns
Do Microphones Output Balanced Or Unbalanced Audio?
What Is Microphone Self-Noise? (Equivalent Noise Level)
How do I choose the best microphone for my application? Choosing the best microphone for an application requires consideration of the sound source and the acoustic environment. Choose a microphone with specs (frequency and polar response; sensitivity; max SPL; etc.) that will compliment the sound source in the specific application.
For in-depth reads on frequency response, sensitivity, and max SPL, please click through to the following My New Microphone articles:
Complete Guide To Microphone Frequency Response (With Mic Examples)
What Is Microphone Sensitivity? An In-Depth Description
What Does A Microphone’s Maximum Sound Pressure Level Actually Mean?