What Is A Microphone Preamplifier & Why Does A Mic Need One?


So you’ve got your microphone and you’re ready to go. Not so fast! You’ll also need some sort of microphone preamplifier to effectively use the signal from your microphone.

What is a microphone preamplifier and why does a mic need one? A mic preamp is a type of amplifier with the purpose of bringing mic level signals up to line level for use with professional equipment. Microphones output mic level signals and need preamps if they are to be used with mixing consoles, recording devices or digital audio workstations.

There’s a lot to know about microphone preamplifiers and their role with microphones. In this article, we’ll discuss mic preamps in great detail to better your understanding of these largely necessary audio devices.


Table Of Contents


What Is A Microphone Preamplifier?

A microphone preamplifier is an active electronic device that is designed primarily to supply gain to a mic signal in order to output the signal at line level.

To preface this lengthy discussion on mic preamps, let’s first define mic and line levels:

  • Mic level is the typically nominal level outputted by microphones. It ranges greatly between -60 dBV and -20 dBV.
  • Line level is the professional standard for recording/mixing audio with a nominal level of +4dBu (1.78 dBV)

Note that there is a “consumer line level” with a nominal level of -10 dBV but we will not be discussing this line level in this article.

A microphone preamplifier is designed to take a microphone signal at its input; apply an appropriate amount of gain (which is typically adjustable), and output a line-level signal.

With the levels mentioned above, a good microphone preamplifier should be able to apply at least 60 dB of gain in order to bring low-level mic signals (typically from dynamic or ribbon mics) up to line level.

Ribbon microphones, which notoriously output very low-level signals, sometimes require their own specialty preamplifiers. These preamps are able to provide a great amount of clean gain so that the ribbon mics can be used effectively. Skip to the section on ribbon mic preamps by clicking here.

Impedance is another concern with ribbon mics since it is frequency-dependent. Therefore, some preamps will actually alter the sound of a ribbon microphone (often negatively and particularly in the low-end).

To learn everything you need to know about microphones and mic and line levels, check out my article Do Microphones Output Mic, Line, Or Instrument Level Signals?

Professional microphones, in general, use XLR connections and cables and so for this reason, mic preamp inputs are generally female XLR connectors.

Microphone preamps can be single-channel (having one input and one actual amplifier). More often, though, a microphone preamp audio device will have multiple channels, each with its own amplifier.

So to answer the big questions “what is a microphone preamplifier and why does a mic need one?” A mic preamp is a necessary device needed to amplify mic output signals to a level where the signals can be effectively used in other audio equipment.

Mic preamps are, therefore, very important. Let’s discuss them and their importance in more detail here.


What Is Gain?

In electronics, gain is the measurement of an amplifier’s ability to increase a signal’s amplitude from the amp input to the amp output. An amp “applies” gain to an input signal to make it stronger at the output of the amplifier. Gain works by adding energy to the signal. This energy is converted from an external power source of some sort (whether that’s an AC wall plug, phantom power, batteries, or another source).

Microphone preamplifier gain increases the amplitude of a microphone signal. Gain boosts signal strength from mic level to line level so the microphone signal is compatible with professional audio equipment. Mic preamps control gain and are typically the first circuits a signal passes through after the microphone output.

To learn all about microphone gain, check out my article What Is Microphone Gain And How Does It Affect Mic Signals?


The Difference Between A Preamplifier And An Amplifier

Although a microphone preamplifier and an amplifier both act to boost signal level by applying gain, there is one key difference between the two:

A preamp boosts a weaker mic level signal to line level while an amplifier boosts a line level signal to speaker level.

Basically a preamplifier brings a microphone’s output signal up to par with other signals in recordings and other audio equipment. At line level, an outputted signal (of a recording, mixing console, etc.) can be further amplified by an amplifier before being outputted from a speaker.


XLR Cables, Balanced Signals & Differential Amplifiers

XLR cables are used to carry the balanced audio signal from microphones to mic preamps.

A balanced signal requires 3 individual pins/conductors. The XLR is balanced and is set up in the following format:

  • Pin/line 1: this is the shield/ground wire that acts to protect the audio lines and as a reference point against the audio lines.
  • Pin/line 2: this is the positive polarity audio wire.
  • Pin/line 3: this is the negative polarity audio wire.

As we can see from the above description of XLR design, there are two audio signals in a balanced cable that have opposite polarities. This means that if we were to combine these two signals together, they would cancel each other out and we would be left with silence.

But the engineers that designed the balanced audio cable are clever. The balanced preamp doesn’t simply sum the audio signals together. Rather, a differential amplifier within the preamp sums the differences between the two audio conductors (pins 2 and 3), effectively “bringing the mic signal back.”

More clever still is that any interference or noise picked up within the balanced XLR cable is common on both pins and the differential amplifier with effectively eliminate this noise via common-mode rejection.

Finally, balanced cables are capable of carrying phantom power which is a smart and safe method of powering active balanced microphones without affecting the audio signal whatsoever. Phantom power sends a standard +48 V DC on pins 2 and 3 and so this DC voltage is also eliminated at the differential amp.

To learn more about balanced audio and XLR cables, check out my articles Do Microphones Output Balanced Or Unbalanced Audio? and Why Do Microphones Use XLR Cables? respectively.


Supplying Phantom Power

Speaking of phantom power, this excellent powering method is often supplied straight out of the microphone preamplifier. Let’s talk about how.

Preamplifiers are active devices. They themselves require power to function and typically plug directly into the wall (unless they’re battery powered). This power allows the active components of a preamp to work including the differential amplifier and the phantom power source.

So phantom power originates from the power mains.

Phantom power is activated via a switch on the mic preamp. There are no microphone preamps that have permanent phantom power (this can be dangerous if a power surge occurs and P48 is connected to a mic is not designed to handle it).

Once activated, a switch closes, creating two identical circuits with a voltage and resistor. Typically this voltage is +48 V DC and the pair of resistors have a resistance of 6.8 kΩ. These voltages send direct current to pins 2 and 3 or the preamp input (relative to pin 1) and once a mic cable and mic are connected, the phantom power completes a circuit with the microphone. Microphones are, in general, designed to “take what they need” from phantom power and reject the rest.

Phantom power is required for modern balanced condenser microphones and even for some active ribbon microphones.

For everything you need to know about phantom power and microphones, check out my in-depth article What Is Phantom Power And How Does It Work With Microphones?


Other Preamplifier Features

Phantom power is a basic function shared by many preamplifiers. However, there are many features and functionality that may be included in a microphone preamplifier.

These additional functions include:

  • Polarity flip
  • High-pass filter
  • Passive attenuation device (pad)
  • High-impedance mode
  • Channel strip (equalizer, compressor, limiter, etc.)
  • VU meter

Polarity Flip

A polarity flip switch will effectively change the polarity of the mic signal within the preamplifier. This will often aid tremendously in fixing certain phase issues though effective mic placement should be practiced to avoid phase issues in the first place.

To learn more about polarity in microphone signals, check out my article Microphone Polarity & Phase: How They Affect Mic Signals.

High-Pass Filter

A high-pass filter allows the higher frequencies to “pass” and rolls-off frequencies below a certain cutoff point. A high-pass filter helps to do the following:

  • Reduces low-end rumble and handling noise within the mic signal.
  • Reduces the proximity effect in directional microphones.
  • May help to reduce the effect of plosives in the mic signal.

To learn more about high-pass filters and microphones, check out my article What Is A Microphone High-Pass Filter And Why Use One?

Passive Attenuation Device

Pads work to reduce the level of the signal by a certain amount. With a preamp applying gain to a signal, you may wonder why a pad would be necessary since it seems counterintuitive to boost a signal just to reduce its level.

The first scenario would be that a microphone is subjected to such a high sound pressure level that the mic signal is too hot for the preamp to handle. In this case, a pad can effectively bring the signal down so to not overload the preamp. This scenario makes the most sense but is rarely ever the case.

The truth of the matter is that even the cleanest preamps colour the sound somewhat. Applying more gain can help add character and weight to the sound but also comes with the risk of signal overload. With a pad, we can apply more colourful gain to the signal without as much risk of overloading the preamp circuitry.

For more information on microphones and pads, check out my article What Is A Microphone Attenuation Pad And What Does It Do?

High-Impedance Mode

Some microphone preamps double up as instrument preamps as well. Instruments such as electric guitar output signals with higher impedance and benefit from an input/preamp with higher input impedance.

High-impedance (Hi-Z) mode works much better with these instruments. Note that inputs with a Hi-Z mode are typically combo jacks (having both XLR and TRS connections) since instruments are typically sent through TS or TRS cables rather than XLR (like microphones).

Channel Strip (EQ, Compressor, Limiter)

Some microphone preamps are part of a bigger audio device unit. These “preamp” units may also include audio devices such as equalizers, compressors, limiters, and more.

Equalizers work to adjust (boost or cut) the frequencies of an audio signal. The high-pass filter mentioned previously is an aggressive form of equalization.

Compressors work to reduce the dynamic range of the audio signal, which yields the result of bringing up the lower-level sounds and “fattening up” the signal, making it sound closer and heavier.

Limiters are like compressors set to an extreme and do not allow the audio signal level to exceed a certain point. Limiters are often used as safety precautions so to not overload a signal path.

VU Meters

Some preamps with “channel strip” functionality will provide VU meters in their design so that the user can see the signal strength represented by a needle on a gauge.


Preamp Input Impedance & Microphone Load Impedance

The mic inputs in a microphone preamp have their own input impedance. Microphones, similarly, have an output impedance.

Impedance is essentially electrical resistance in AC signals. The impedance of an audio signal, then, is the difficulty the signals have when travelling through a cable.

For optimal voltage transfer from the microphone to the mic preamp, the input impedance of the preamp must be significantly higher than the output impedance of the microphone. A good rule of thumb is having the preamp input impedance be 10x that of the mic output impedance.

Note that some microphones have a rated load impedance specification. This refers to the minimum input impedance a mic preamp must have in order for the microphone to function properly. In fact, all other mic specifications are based on this recommended load impedance being met or exceeded.

For everything you need to know about microphone impedance, check out my articles Microphone Impedance: What Is It And Why Is It Important? and What Is A Good Microphone Output Impedance Rating?


Ribbon Microphones And Specialty Mic Preamps

Ribbon microphones excel in terms of low-end response; neutral/natural high-end response, and accuracy in their transient response. The catch to this, unfortunately, is a very low output level.

Many standard preamps simply cannot provide enough gain, or enough clean gain to properly bring out the beauty of a ribbon microphone’s sound.

A high-impedance, high-gain ribbon-specific preamp is often the solution to really bringing out the charm and character of a ribbon microphone.

AEA is a world-class ribbon microphone manufacturer and has its own line of optimized ribbon-specific preamplifiers. The AEA TRP2 (link to check the price on Amazon) is one example of a ribbon mic preamp:

AEA TRP2

AEA is featured in My New Microphone’s Top 13 Best Microphone Preamplifier Brands In The World and Top 11 Best Microphone Brands You’ve Likely Never Heard Of.

The TRP2 boasts an amazing 85dB of quiet JFET gain; switchable phantom-power, polarity flip, and high-pass filter and a 63 kΩ high-impedance input. All of these specs help tremendously in bringing out the transient and frequency response and the true character of any ribbon mic.

I’ll note here that active ribbon microphones, which have internal amplifier components (that we’ll discuss shortly), actually have their impedances and levels set within the mic circuitry so that they may be used with “standard” microphone preamplifiers.

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


Mic Preamp Types & Examples

Microphone preamps and inputs are featured in many different audio devices.

These devices include:

  • Standalone preamps
  • Audio interfaces
  • Mixing consoles
  • Field recorders

Standalone Preamps

Standalone preamps are simply preamps that are separate from other audio components. This unofficial title essentially refers to mic preamps that are not built into mixing consoles or recording devices. I’ve also made a point, in the examples provided, to separate standalone preamps from audio interfaces (which we’ll get to shortly).

These preamps work to boost mic level signal to line level with gain. Some are more complex and have greater functionality than others. Some have more inputs and outputs than others. However, each of these preamps is its own audio unit.

Let’s look at some wildly different examples:

  • BAE 1073DMP Desktop
  • Cloud Microphones CL-1
  • Warm Audio TB12 Tonebeast
  • Heritage Audio 1084
  • Manley MMSLAM

BAE 1073DMP Desktop

The BAE 1073DMP Desktop Mic Pre (link to check the price on Amazon) is a the first portable 1073 microphone preamplifier. This preamp is based on the design of the legendary Neve 1073 preamp and even has the same pre-amp circuit and the same Carnhill transformers as the 1073 and 1084.

BAE is synonymous with high-quality production and attention to detail in its replication of classic Neve hardware. Note that the DMP1073 lacks the EQ section of the original Neve 1073 (though the BAE 1073D does).

This single-channel microphone preamplifier features a built-in power supply unit in its solid steel chassis. It boasts 71 dB of gain and has phantom power and phase flip switches.

Additionally, the DMP1073 features a DI (direct inject) with 1 input and 2 throughs). This means we can run instruments through the DMP1073 as well, which makes this unit very flexible.

Cloud Microphones CL-1

The Cloud Microphones CL-1 (link to check the price on Amazon) is about as simple as it gets when it comes to mic pres.

This single-channel preamp is designed with dynamic and ribbon microphones in mind. Dynamic and ribbon mics often have low outputs and require quite a bit of gain to become line level. Many standard preamps are capable of providing enough gain to these signals, though the gain required is often near their limits where colouration/distortion becomes an issue.

So the CL-1 can provide an additional boost of 25 dB to your dynamic mic before the signal reaches the main preamp. This gain is completely transparent and does not colour the signal whatsoever.

There are zero controls on this simplistic preamp and the preamp is actually powered via phantom power from the main mic preamp.

Warm Audio TB12 Tonebeast

The Warm Audio TB12 Tonebeast (link to check the price on Amazon) is a great example of a rack-mounted microphone preamp (19″ rack-mountable). This relatively large preamp actually only has a single channel. However, the internal circuitry (including top-notch CineMag transformers) takes up significant space in order to allow the incredible flexibility and functionality of this preamp.

Warm Audio is featured in My New Microphone’s Top 11 Best Microphone Brands You’ve Likely Never Heard Of.

The TB12 is capable of supplying 71 dB of gain and has multiple options for maximal flexibility within one preamp unit. Let’s go through the options here.

  • 2 switchable Cinemag transformers (steel and 50% nickel) for varying character
  • Transformer bypass switch
  • 2 switchable operation amplifiers: 1731 (vintage) and 918 (clean/modern)
  • 2 switchable capacitors for varying character (tantalum and electrolytic)
  • Switchable input impedance (150 Ω and 600 Ω) for varying tone
  • 2 MΩ Hi-Z 1/4″ input for instruments (guitars, bass, keyboards, etc.)
  • Output attenuation (allowing for saturation flexibility with opamps and transformers.
  • 1/4″ insert for inline effects units (compressors, EQs, etc.)

On top of all that, additional features include:

  • +48V phantom power
  • 80 Hz high-pass filter
  • Polarity flip switch
  • -20 dB pad
  • LED VU Meter

All of this functionality makes the TB12 an exceptional example of a microphone preamp with all the “bells and whistles.”

Heritage Audio 1084

The Heritage Audio 1084 (link to check the price on Amazon) is a Class A mic preamp/EQ featuring custom polystyrene and polypropylene film tone capacitors. It provides up to 80 dB of gain and is designed to fit in the input module slots of vintage 80 Series sound mixing consoles. This pre/EQ can also be fitted into the Heritage Audio Frame 8 and Rack 2 powered enclosures.

Class A amplifier design is the least efficient but has the highest sound fidelity.

This solid-state mic pre is modelled after the legendary Neve 1084 preamp. It provides just over 80 dB of gain with less than -100 dBu of noise and less than 0.025% total harmonic distortion at 1Khz and <0.05% at 100Hz.

The input impedance is switchable between a “hi” option (1200 Ω) and a “lo” option (300 Ω). The line (instrument) input has a 10 kΩ impedance rating. So regardless of the input source, the 1084 will be able to handle it and make it sound beautiful.

Manley MMSLAM

The Manley “MMSLAM” (link to check the price on Amazon) is a mastering stereo limiter and mic pre (hence the name “MSLAM”).

This unit features 2 individual tube (2 x 12AT7A NOS GE)microphone preamplifiers with selectable phase reverse, high-pass filter, and switchable phantom power. These functions are available on the back panel.

The outputs of this unit are also tube-based (2 x 6414W NOS USA dual triodes).

Each of these channels has two distinct limiters: an electro-optical levelling amplifier circuit (ELOP) and a FET circuit (field-effect transistor).

The ELOP limiter option has optional ratios of 10:1, 5:1, 3:1, 2:1 and AutoHF, which starts off as a 1.5:1 ratio and increases gradually to 10:1 for high-frequencies. This last ratio is similar to a de-esser.

The FET limiter options include 50%, NORM, LP Lim, BOTH, and CLIP, offering ultimate flexibility in the super-fast brick wall-capable solid-state limiter.

To learn about My New Microphone’s recommended preamps, check out the Best Microphone Preamplifiers.


Audio Interfaces

With the popularity of digital recording, many modern preamplifiers are built into audio interfaces. These I/O interfaces have analog-to-digital converters and digital-to-analog converters to effectively allow communication between analog devices (notably microphones, instruments, headphones and speakers) and digital devices (notably computers and their digital audio workstation software).

The microphone preamplifiers, again, boost mic level signals to line level before the interface converts the signal into digital data to be used with the computer.

Let’s look at a few examples of audio interfaces with mic preamps:

ART Tube Opto 8

The ART Tube Opto 8 (link to check the price on Amazon) is a rack-mountable audio interface that features 8 individual mic preamps and connects to computers via ADAT (optical). Its digital I/O works at 24-bit at 44.1 or 48kHz.

ART is featured in My New Microphone’s Top 11 Best Audio Interface Brands In The World.

Each of the 8 channels features class-A tube amplifier circuitry. The first two also double as hi-Z instrument/line inputs.

Each preamp boasts 70dB of gain; full 48V phantom power; a pad; phase flip switch; an 80 Hz low-frequency roll-off option, and an output level control.

Phantom power is switchable across inputs 1-4 and 5-8 but is not available on an input-to-input basis.

UAD Apollo Twin Duo

The UAD Apollo Twin Duo (link to check the price on Amazon) is an audio interface that features 2 premium unison mic/line preamps; 2 line outputs; front-panel Hi-Z instrument input, and low-latency headphone output. It has world-class ADC and DCAs or 24-bit/192 kHz digital audio conversion. This interface connects via Thunderbolt.

UAD Apollo Twin

UAD is also featured in My New Microphone’s Top 11 Best Audio Interface Brands In The World.

With the Apollo Twin DUO core processing, we also have access to emulation plugins of vintage compressors, EQs, tape machines, mic preamps, and guitar amp plug-ins with near-zero latency. Rather than running on the computer’s CPU, these plugins run within the Apollo Twin, effectively freeing up processing for actually recording and monitoring. These plugins run as VST, RTAS, and AAX 64 in all major DAWs and in all operating systems.

In addition to all these featured, the Twin also has a built-in talkback mic for communication with studio talent and recording slate cues.

Focusrite Scarlett 2i2

The Focusrite Scarlett 2i2 (link to check the price on Amazon) is my favourite “budget” audio interface. It’s pretty simple with limited functionality but is more than enough for many project studios.

Focusrite is yet another brand featured in My New Microphone’s Top 11 Best Audio Interface Brands In The World.

This interface has two combo preamps for both mic and instrument signals with a button switch. Each mic pre has 3 kΩ impedance and has 56 dB of gain available. Phantom power is switchable between both inputs and no inputs.

To learn about My New Microphone’s recommended audio interfaces, check out the Best Microphone Audio Interfaces.


Mixing Consoles

Mixing consoles have multiple inputs for microphones, instruments, and line-level signals. Because mixing consoles utilize line level signals within their circuitry, the mic inputs on mixing consoles have microphone preamplifiers.

Let’s have a look at a few mixing console examples.

Mackie Mix8

The Mackie Mix8 (link to check the price on Amazon) is an 8-channel compact audio mixer.

Of these 8 channels, the first 2 are mic/line inputs. The mic preamps feature phantom power, panning, overload indication, and 50 dB of gain.

The channels of these mic preamps feature multi-band EQs and 75 Hz high-pass filters.

Allen & Heath AH-SQ-7

The Allen & Heath AH-SQ-7 (link to check the price on Amazon) is a 48-channel 33-bus digital console. It features 32 exceptional onboard mic preamps. Thanks to its SLink intelligent port, the SQ-7 can take on an extra 16 mic inputs for a total of 48 microphone preamplifiers.

The AH-SQ-7 is an incredibly powerful audio tool and it would take another full article just to go over its basics.


Field Recorders

Field recorders generally recording with microphones and so mic preamps are necessary for their design. Let’s have a look at two final examples of mic preamps in two field recorders.

Zoom H4n Pro

The Zoom H4n Pro (link to check the price on Amazon) is a 4-channel digital recorder (up to 24-bit, 96 kHz digital audio). It features a built-in stereo X/Y microphone setup but also has 2 combo mic/line inputs (each with its own mic preamp).

Each mic preamp has −16 dB – +43 dB of gain and an input impedance of 3 kΩ. They also have 3-selectable phantom power options: 48 V, 24 V, and off.

Sound Devices MixPre 6

The Sound Devices MixPre 6 (link to check the price on Amazon) is a 4-preamp, 6-channel, 8-track digital audio recorder. Its digital audio can run from 44.1 to 96 kHz sample rate with 16/24-bit bit-depth.

Each mic preamp has adjustable limiters, bit depth, sample rates, gain, pan, low-cut, phase inversion, and phantom power. All functionality can be accessed through the digital menu.

Each mic preamp has +6 dB to +76 dB gain and 4 kΩ.


Internal Microphone Amplifiers

Besides external microphone preamplifiers, many microphones have amplifiers built-in to their designs.

Though true signal amplifiers are active and require power to function, there are other passive means of effectively boosting a microphone signal. In the context of this article, I’ll refer to these passive devices as “pseudo-amplifiers.”

So the internal amplifiers (and pseudo-amplifiers) commonly found in microphones are:

  • FET impedance converters
  • Vacuum tubes
  • Step-up output transformers
  • Amplifier circuits

FET Impedance Converters

FET impedance converters are solid-state transistor-based impedance converters found mostly in solid-state condenser microphones (though they are also found in some active tube microphones).

Condenser microphone capsules essentially function as parallel-plate capacitors and require a fixed charge in order to function properly. A byproduct of holding this charge is that the AC voltage (mic signal) produced and outputted by the capsule has extremely high-impedance. It also has a remarkably low amplitude.

In order to actually use the signal from the capsule, a condenser microphone must have an impedance converter.

While the FET impedance converter in solid-state microphones effectively drops the signal impedance to a usable level, it also acts to boost (amplify) the signal (even though the impedance convert itself is not an amplifier).

To learn more about microphone FETs, check out my articles What Are FETs & What Is Their Role In Microphone Design? and What Is A Solid-State Microphone? (With Mic Examples).

Vacuum Tube

A vacuum tube acts as an impedance converter as well. Before transistor technology began being utilized in microphone design, vacuum tubes were a necessary part of a condenser microphone.

Today, tube microphones are still in production and are cherished for their sonic character which often adds a pleasing amount of saturation and compression to the mic signal.

The vacuum tube, like the FET IC, effectively boosts the voltage of a mic signal without being a true amplifier.

To learn more about vacuum tubes and their roles in tube microphones, check out my article What Is A Tube Microphone And How Do Tube Mics Work?

Step-Up Output Transformers

Output transformers are commonly found in passive dynamic and ribbon mics as well as in condensers.

A step-up transformer effectively couples two independent circuits around a magnetic core and uses electromagnetic induction to boost the voltage (while decreasing the current) of the AC mic signal passing through it.

Note that a step-up transformer also increases the impedance of the signal in the secondary winding (circuit) relative to the primary winding (circuit).

Passive transformers are a perfect example of “pseudo-amplifiers.”

To learn more about microphone transformers, check out my article What Are Microphone Transformers And What Is Their Role?

Amplifier Circuits

Amplifier circuits (with actual signal amplifiers) are present in some microphones.


How do you connect a microphone to a speaker? Some active speakers have microphone preamps and amplifiers built into their circuitry and so a microphone may be plugged directly into the speaker via the mic input. Other active speakers require a preamplifier signal (ie: from a mixer to the input of the speaker) while passive speakers require the mic signal to be amplified to speaker level before reaching the speaker.

Can I plug a microphone directly into speakers? Some active speakers have mic inputs with microphone preamplifiers along with their internal amplifiers. Simply plug the microphone directly to the speaker and the speaker will effectively boost the signal from mic level to line level and from line level to speaker level.

To learn more about connecting microphones and speakers, check out my article How To Plug A Microphone Into A Speaker.

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