What Is Amplifier Impedance? (Actual Vs. Rated Impedance)

Impedance is a confusing topic when it comes to amplifiers, speakers and pretty much all other audio devices. If you've ever wondered what the ohm (Ω) rating meant on your amp and how it affects the amp's performance in a system, this is the article for you!

What is amplifier impedance? Audio amplifiers have inputs and outputs that have impedance values. Mic, line and speaker inputs have certain impedances to act properly as loads for inputted devices. Amp outputs have rated impedances to help match the nominal impedance of speakers though the actual impedance is rarely specified.

In this article, we'll define impedance and its role in audio and then get into detail about the various impedance values of amplifiers and what they tell us about our amplifier units.

Table Of Contents

• What Is Electrical Impedance?
• Impedance And Audio
• Input & Output Impedance
  • The Input Is A Load
    • Preamp Input Impedance
    • Power Amp Input Impedance
    • Headphone Amp Input Impedance
  • The Output Is A Source
    • Preamp Output Impedance
    • Power Amp Output Impedance
    • Headphone Amp Output Impedance
• Actual Impedance Vs. Rated Impedance & Matching Loudspeakers
• Other Amplifier Types & Their Impedance Specs
  • Microphone Preamplifiers
  • Phono Preamplifiers
  • Headphone Amplifiers
  • Instrument Preamplifiers
  • Power Amplifiers
• Active Units & Built-In Amplifiers
  • Active Speakers
  • Integrated Amplifiers
  • Powered Mixers
• Related Questions

What Is Electrical Impedance?

Electrical impedance is the measurement of a system's opposition to alternating current when a voltage is applied. In a way, it can be thought of as AC resistance.

Impedance has a real DC resistance component and an imaginary reactance component. It possesses both magnitude and phase and is frequency-dependent.

Impedance is measured in ohms (Ω) and is noted in equations by the letter Z.

Impedance And Audio

Audio is loosely defined as an electrical representation of sound. Sound is defined as mechanical wave energy between 20 Hz and 20,000 Hz.

Analog audio is electrical energy (whether the audio is stored or in motion). This electrical energy is made of alternating current between 20 Hz – 20,000 Hz (sometimes more and sometimes less) and is subject to electrical impedance.

Digital audio represents analog audio in digital formats and is also subjected to impedance as it travels. However, the discussion of impedance often has to do with analog signals.

Amplifiers, like all other audio devices that deal with audio signals, have inherent impedances.

Related articles:
• Microphone Impedance: What Is It And Why Is It Important?
• The Complete Guide To Understanding Headphone Impedance

Unlike mics and headphones, amplifiers have both inputs and outputs and, therefore, have input and output impedances. We'll get to each of these ratings shortly.

I'll state again that impedance is frequency-dependent. This is apparent in the impedance graphs of audio transducers (microphones, headphones and loudspeakers) that have natural resonances that affect their impedance values across their frequency responses.

Therefore, a nominal impedance value is typically given for audio devices. Nominal impedance refers to the “average” impedance of the audio device across its frequency response. It is used to garner a general understanding of how the device will act in a system.

The impedance values of amplifiers, too, are measured as nominal values. However, you'll rarely, if ever, find an impedance graph to show the dips and valleys in the actual impedance of the amplifier across its response.

Amplifier impedance is much more consistent and can be assumed to be flat across its response.

Input & Output Impedance

We've discussed previously that audio devices have inputs and outputs and that amplifiers have both.

This is because amplifiers are designed to amplify an inputted signal. They take a lower-level signal at their inputs, apply gain, and output a stronger version of that signal (ideally with no distortion other than an increase in amplitude/voltage).

Therefore, input impedance and output impedance values are important to understand with audio amplifiers.

These values will tell us how the amplifier (or preamplifier) will perform when various devices are connected at the inputs (mics, instruments, line sources) and when various devices are connected at its outputs (speakers, headphones, processors, etc.).

Typical nominal impedance values of various audio devices can be found in the following table:

In terms of impedance, when connecting audio devices together, the device receiving signal at its input is considered the load and the device outputting signal is considered the source.

Let's talk more about the load and source.

The Input Is A Load

The amplifier inputs act as an electrical load for the connected source. It accepts the audio signal (AC voltage) from the source.

When it comes to impedance, the load impedance must always be significantly higher than the source impedance for optimal voltage and signal transfer.

This can be explained in the diagram below:

The source and load circuit can be simplified as voltage divider. Therefore:

\frac{V_L}{V_S}=\frac{Z_L}{Z_L+Z_S}

And:

V_L=V_S\frac{Z_L}{Z_L+Z_S}

Let's say that Z_L (the load impedance or amplifier input impedance) was equal to Z_S (the output impedance of the connected device). In this scenario, V_L (the voltage or strength of the signal received at the amplifier's input) would be 1/2 of V_S (the voltage or strength of the connected device's output signal). Half the signal strength was lost!

Let's now say that Z_L was 9 times Z_S. In this scenario, V_L would be 9/10 of V_S. 90% of the signal strength was transferred!

So then, a much higher load impedance is required for optimal signal transfer. As a general rule, the load Z should be at least 10x that of the source Z.

This is important information to understand, but generally speaking, the amplifier manufacturer will ensure its inputs have high enough impedance values to work well with the devices it's designed to accept at its inputs.

Preamp Input Impedance

Preamplifiers are often designed to accept the following signals/devices at their inputs:

• Microphone preamp: mic level signals (from microphones or mic outputs)
• Phono preamp: low-level signals (from turntables)
• Instrument preamps: instrument level signals (from electric pickups or other low-level electric instrument outputs)

The Heritage Audio 1084 is a single-channel microphone preamplifier and EQ. Its impedance specifications are as follows:

• Microphone input impedance:
  • HI: 1200 Ohm minimum
  • LO: 300 Ohm minimum
  • Higher gain positions gradually have greater impedances, optimum for low gain ribbon mics. Input is transformer balanced and floating
• Line input impedance: 10KOhms bridging, transformer balanced and floating

Power Amp Input Impedance

Power amplifiers are often designed to accept the following signals/devices at their inputs:

• Line level signals:
  • From preamps – either separate or integrated
  • From passive audio mixers and interfaces
  • From other line level sources

The Crown Audio XLi 2500 is a stereo power amplifier with both RCA (unbalanced) and XLR (balanced) inputs. Its input impedance values are as follows:

• Balanced: 20 kΩ
• Unbalanced: 10 kΩ

Headphone Amp Input Impedance

Headphone amplifiers are also typically designed to accept line level signals at their inputs.

The AKG HP4E is a headphone amp with two stereo inputs and four stereo output channels of high-quality audio.

Input impedance: 100k Ohms

The Output Is A Source

An amplifier takes a signal at its input and outputs a stronger version of that signal. The output of an amplifier is connected to another device that accepts this amplified output signal.

In other words, the amplifier's output acts as a source, and the connected device acts as the load. Let's look at our simple voltage divider circuit once again:

In this scenario, the amplifier's output impedance must be lower than the connected device(s) input impedance for optimal signal transfer. The general rule of thumb, again, is a difference of 10x or more.

Preamp Output Impedance

Preamplifiers are typically going to output signals into:

• Mixing consoles or recorders (either integrated or separate)
• Audio interfaces (either integrated or separate)
• Power amplifiers (either integrated or separate)

The aforementioned Heritage Audio 1084 preamp has the following output impedance specification:

Output impedance: Less than 75 Ohms, transformer balanced and floating, to drive a load of 600 Ohms

Power Amp Output Impedance

Power amplifiers are typically going to output signals into loudspeakers.

Power amplifier manufacturers rarely, if ever, specify an output impedance. We'll discuss this more in the following few sections.

Headphone Amp Output Impedance

Headphone amplifiers are designed to output signals into headphones.

The Rupert Neve Designs RNHP is a single-output desktop headphone amplifier. Its input impedance ratings are:

Output Impedance: 0.8 Ohms @ 1 kHz, 16 to 150 Ohms load, 0 dBu input

Actual Impedance Vs. Rated Impedance & Matching Loudspeakers

Matching power amplifiers are loudspeakers is a topic that many audio professionals and hobbyists have an interest in.

One of the important factors in matching amps and loudspeakers is selecting units that have compatible impedances (the amplifier's output impedance must work well with the input impedance of the loudspeaker(s)).

Because this factor is so important, many power amplifiers will have rated impedance specifications.

Rated amplifier output impedance is a specification that basically states the lowest load impedance the amplifier will drive effectively. In other words, it tells us the lowest nominal speaker impedance the amplifier is capable of driving properly.

The main takeaway here is that rated amplifier impedance is not the actual output impedance of the amplifier. The actual output impedance will be much lower than that rated impedance to achieve ideal voltage/signal transfer from the amp to the speaker.

In fact, we won't find any power amplifiers that have output impedance values above 0.1Ω.

That being said, we won't typically be told what the actual output impedance of the amplifier is. Rather, we'll be told the speaker input impedances the amplifier can perform at and/or how the amplifier will perform with different speaker impedances.

Remember our basic voltage divider circuit and signal transfer equation.

V_L=V_S\frac{Z_L}{Z_L+Z_S}

Suppose a speaker has a lower nominal impedance Z_L (let's say 4Ω rather than 8Ω). In that case, the amplifier must output a greater V_S in order to achieve the same V_L (which will produce the same outputted sound pressure level, all else being the same).

So then, an amplifier must produce more power to drive a lower impedance speaker. This increase in power generally comes with an increase in heat, which could potentially cause damage to the amplifier and/or the speaker. The increase in power may also lead to amplifier clipping and distortion if the power requirements are above the amp's power limit.

Looking back at the example of the Crown Audio XLi 2500, we can see the amplifier is designed for the increased power requirements and has rated output impedance values of 4Ω and 8Ω:

Power:

• 4Ω Dual: 750W
• 8Ω Dual: 500W
• 8Ω Bridged: 1500W

The Anthem STR is a stereo integrated amplifier with a built-in DAC. Its rated output impedance goes as low as 2Ω with the following power ratings:

Output (per channel, continuous, 20 Hz – 20 kHz, <1% THD):

• 200 W (8Ω)
• 400 W (4Ω)
• 550 W (2Ω)

Other Amplifier Types & Their Impedance Specs

To further our understanding of various audio amplifier types and their typical impedance values, let's look at some examples.

In particular, we'll look at the following amplifier types:

• Microphone Preamplifiers
• Phono Preamplifiers
• Headphone Amplifiers
• Instrument Preamplifiers
• Power Amplifiers

Microphone Preamplifiers

Microphone preamplifiers are designed to boost mic level signals of microphones to line level for use in other audio devices.

Microphones typically have balanced outputs (XLR) with output impedances between 50Ω and 600Ω. More often than not, a microphone's output impedance is between 150Ω and 250Ω.

The famous Shure SM57 is an example of a professional microphone with a low output impedance of 150 Ω.

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

Mic preamp inputs generally have impedances between 1 to 10 kΩ.

Mic preamp outputs generally have impedance values below or around 100Ω.

The aforementioned Heritage Audio 1084 has the following input and output values:

• Microphone input impedance:
  • HI: 1200 Ohm minimum
  • LO: 300 Ohm minimum
• Output impedance: Less than 75 Ohms, transformer balanced and floating, to drive a load of 600 Ohms

To learn more about microphone preamplifiers, check out the following My New Microphone articles:
• What Is A Microphone Preamplifier & Why Does A Mic Need One?
• Best Microphone Preamplifiers

Phono Preamplifiers

Phono preamplifiers are designed to boost the level and improve the signal-to-noise ratio of a turntable (record player) before the signal is sent through a power amplifier and to loudspeakers.

Let's begin with the input impedance specs of phono preamplifiers.

Most phono preamp inputs have the standard 47kΩ impedance and accept MM and MC cartridge outputs. Many of these preamps offer selectable input impedances as low as tens of ohms to hundreds and thousands of ohms for moving-magnet (MM) cartridges.

Phono preamps designed for ceramic cartridge output will have input impedances in the MΩ range (1,000,000 ohms).

The Pro-Ject Phono Box MM is a simple phono preamplifier with the following impedance rating:

Input capacitance/input impedance, MM: 47kohms/120pF

The Phono Box MM has line outputs but does not specify the exact output impedance.

Record players have varying output impedances depending largely upon the type of cartridge (the transducer device that holds the stylus) used. The actual impedance is rarely, if ever specified, but the following points hold true:

• MC (moving-coil) cartridges offer very low impedance, allowing them to work with preamps well below the 47kΩ standard.
• MM (moving-magnet) cartridges have much higher impedances and require the standard 47 kΩ loads to function as designed.
• Ceramic cartridges, which are used in older cheaper turntables, have incredibly high impedance and require preamps with inputs in the MΩ ranges.

The Ortofon 2M Red is an example of a moving-magnet phono cartridge with the following impedance specifications:

• Internal impedance, DC resistance: 1.3 kOhm
• Recommended load resistance: 47 kOhm

Headphone Amplifiers

Headphone amplifiers boost line level signals and adjust signal impedance to drive headphones appropriately.

Typical headphones have nominal impedances in the range of 8 Ω to 600 Ω though their actual impedance values can exceed these numbers at certain frequencies.

Electrostatic headphones work a bit differently and can have nominal impedances in the hundreds of kΩ with dedicated high-impedance amplifiers to drive their high-voltage, low-current signal requirements.

The popular Sennheiser HD 280 Pro has an impedance of 64 Ω.

Headphone amplifiers, then, have their headphone jacks/outputs designed with are designed with very lower output impedance.

The aforementioned Rupert Neve Designs RNHP, for example, has an output impedance specification of .08 Ω @ 1KHz, 16-150 Ω load, 0dBu input.

Note that specialized electrostatic headphone amps will generally have higher output impedances.

To learn more about headphone amplifiers, check out my article What Is A Headphone Amplifier & Are Headphone Amps Worth It?

Instrument Preamplifiers

Electric instruments also require amplification before they can be mixed and played back through headphones and/or speakers.

Some instruments, like keyboards, will output line level signals and will only really require a power amp to get to speaker level (or a headphone amp to get to headphones). Many keyboards even have built-in speakers and headphone jacks.

Other instruments, like electric guitar, require preamps. These preamps offer much of the tone in a guitar amplifier setup.

The transducer of an electric guitar is called a pickup. Pickups range quite a bit in their output impedances, but the ratings are typically in the tens of thousands of ohms or more (>10 kΩ).

An example of a guitar pickup is the Seymour Duncan JB Humbucker that has a direct current resistance (DCR), which we can take as “impedance” of 16.60 kΩ.

The Lehle Sunday Driver is a great example of a simple guitar preamp. Its impedance ratings are as follows:

• Impedance, Input D (Driver mode): 1 MOhm
• Impedance, Input S (Sunday mode): 4 MOhm
• Impedance, Output: 150 Ohm

Power Amplifiers

Power amplifiers are designed to amplify line level signals (from audio mixers, playback devices, interfaces, etc.) to speaker levels capable of properly driving loudspeakers.

Once again, power amplifiers tend to have recommended/rated load impedance values that suggest the appropriate input impedance of the connected loudspeakers. However, the actual output impedance of a power amp is rarely specified.

Loudspeakers typically fall into one of the following nominal impedance values:

• 1 Ω
• 2 Ω
• 4 Ω
• 6 Ω
• 8 Ω
• 16 Ω

Manufacturers will often define their loudspeaker as fitting into one of the above categories to help keep with the standard and allow for easier reading and comparing of the specification.

The Electro-Voice ZLX-15 is a 2-way 1000W passive PA speaker with a 15″ woofer. It has the following impedance values:

• Nominal Impedance: 8 Ω
• Minimum Impedance: 7 Ω

As for input impedance, power amplifiers are generally tasked with amplifying line level signals and will have input impedances that are appropriate for line level signal transfer (either balanced or unbalanced).

The aforementioned Crown Audio XLi 2500 has the following impedance values:

• Input Impedance (nominal):
  • Balanced: 20 kΩ
  • Unbalanced: 10 kΩ
• Power:
  • 4Ω Dual: 750W
  • 8Ω Dual: 500W
  • 8Ω Bridged: 1500W

Active Units & Built-In Amplifiers

It's critical to note that some units have built-in amplifiers, so their impedance values (either input or output) will reflect that. The impedances will be in the appropriate range for the intended input or output signal transfer.

Let's consider a few examples:

• Active Speakers
• Integrated Amplifiers
• Powered Mixers

Active Speaker Input Impedance

Active loudspeakers will not have the typical 1Ω to 16Ω input impedance (though their drivers may). Rather, their input impedance will be appropriate (and much, much higher) for the input (whether it's a mic input, balanced line input, unbalanced line input, etc.).

The QSC KW153 is an example of a 3-way active PA speaker with a 15″ woofer and multiple inputs.

Input Impedance (Ω):

• Channel A XLR /¼”:
  • Mic gain setting:
    • 0 dB: 38 kΩ (Balanced) 19 kΩ (Unbalanced)
    • +12 dB: 10 kΩ (Balanced) 5 kΩ (Unbalanced)
    • +24 dB: 2.66 kΩ (Balanced) 1.33 kΩ (Unbalanced)
    • +36 dB: 660 Ω (Balanced) 330 Ω (Unbalanced)
• Channel B XLR /¼”: 38 kΩ balanced / 19 kΩ unbalanced
• Channel B RCA: 10 kΩ

To learn about all the differences between active and passive loudspeakers, check out my article What Are The Differences Between Passive & Active Speakers?

Integrated Amplifier Impedance

Integrated amplifiers combine a preamplifier and an amplifier into a signal unit. Therefore, we can expect the inputs (though perhaps not all) to have mic level impedance ratings while the outputs should generally have impedances that would drive loudspeakers.

The aforementioned Anthem STR is a stereo integrated amplifier with a built-in DAC. Let's have a look at some of its specs:

Phone preamplifier

• Input Impedance: 100 Ω (MC), 47 kΩ || 270 pF (MM)

Output (per channel, continuous, 20 Hz – 20 kHz, <1% THD):

• 200 W (8Ω)
• 400 W (4Ω)
• 550 W (2Ω)

Powered Mixer Impedance

A powered audio mixer has a built-in power amplifier. It mixes audio at line level and includes preamps for microphone and instrument inputs. However, it is designed to output speaker level audio signals rather than line level (as is the case with passive mixers).

The Mackie PPM608 is a popular example of a powered mixer. It has the following impedance specifications:

• Impedances:
  • Mic mono input: 3.6 kΩ balanced
  • Line mono input: 20 kΩ balanced
  • Line mono input, Ch 5, 6 Hi-Z: 500 kΩ balanced
  • Main and monitor preamp outputs:
    • 240 Ω balanced
    • 120 Ω unbalanced
• Loudspeaker outputs:
  • Recommended load impedance: 4 – 8 Ω per channel

Related Questions

Are 4-ohm or 8-ohm speakers better? Though the term “better” is subjective, some would argue that 4-ohm speakers, though being more difficult to drive and requiring more powerful amplifiers, do sound better. This may be due to the fact that most consumer-grade speakers are 8-ohm for better compatibility, while high-end speakers are more likely to be 4-ohm.

Can I mix speaker impedance? Speakers with different impedances can be mixed and matched so long as the connected amplifier is capable of driving them. This is true of mixing and matching different speakers with a single amplifier and connecting multiple speakers to a single amplifier.

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