Passive Amplifiers Vs. Active Amplifiers (Sound & Audio)


Amplifiers can be found all around us if we choose to look for them. They’re in our phones, televisions, entertainment systems, vehicles, shopping malls, and many other places. Though we may think of active amps when we think of amplifiers, there are also passive amplifiers that are common enough in our daily lives.

What is the difference between a passive and active amplifier? Amplifiers are essentially devices that increase the amplitude of a quantity. In sound and audio, passive amps do not require electrical power and work by increasing/focusing sound pressure levels. Conversely, active amps require power and work to increase the amplitude of electrical audio signals.

The focus of this article is to define passive and active amplifiers and share examples of each to further our understanding of sound and audio. Let’s get to it!


Table Of Contents


Passive Amplifiers

A passive amplifier amplifies sound (increases the amplitude of acoustic power, sound intensity and sound pressure level) by passive means. In order words, it does so without the use of external electrical power or additional energy of any sort.

There are different ways in which sound can be “amplified”. The two most significant are the amphitheatre and the horn.

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The Amphitheatre

Perhaps the first type of passive amplification to be used effectively was the amplification provided by amphitheatres.

A natural amphitheatre is a location near a tall rock face that naturally amplifies sound by reflecting sound waves back to the original location.

So rather than having the sound waves dissipate into open space (free field far field) and only hearing the initial sound wave, the amphitheatre provides reflection so that some of the sound wave is redirected back to the listener (semi-diffuse field), thereby increasing the perceived loudness.

Illustration Of An Amphitheatre

Though amphitheatres may certainly enhance the perceived loudness of the original sound, they come at the cost of time delay and phase issues that may affect the sound negatively.

The distance travelled between the sound source and the reflected surface, in addition to the distance between the surface and the listener, will cause some change in the sound quality. Therefore, the “amplification” process is less precise. This can be thought of as distortion.

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The Horn

A more common and truer method of passive amplification comes from the concept of a horn.

A horns is a flared tube that attaches to a speaker driver and conduct the air vibrations caused by the driver to the open air.

Many woodwind and brass instruments utilize horns to increase their sound power. Speakers of many sorts utilize horns to amplify their sound output as well.

The phonograph (which became the gramophone and was the precursor to the record player) is a historic use of speaker horn technology.

The phonograph (invented in 1877 by Thomas Edison) predates electrical recording (which came about in 1925). With the advent of electrical recording and playback came electrical active amplification.

Before then, it was all mechanical passive amplification. Most of which was achieved by the horn.

The wood and metal gramophone (link to check the price on Amazon) pictured below is a modern production by Benzara:

Benzara Gramophone

The passive horn amplifier is also used in megaphones; indoor and outdoor PA systems; some modern tweeter designs, and is even the basis of some speaker enclosure designs, which we’ll get to shortly.

So the question is: how do horn-type “passive amplifiers” work?

Let’s start by looking at a simplified illustration. To the right is the horn structure and to the left is the driver structure (with its diaphragm, coil and magnetic structure drawn):

Passive Amplifier Speaker Horn Illustration

So we can see that the driver is essentially closed off from the exterior environment except through the horn.

At the driver end, the horn is very narrow. This is called the throat.

At the other end, the horn is very wide and open to the exterior environment. This is called the mouth.

In order for the speaker driver to produce sound, the following happens:

  • An electrical audio signal is passed through the coil
  • The coil moves within the magnetic field due to electromagnetic induction.
  • The diaphragm oscillates back and forth because it’s attached to the coil.
  • The diaphragm pushes and pulls the air around it, producing sound waves.

Speakers are transducers. They convert electrical energy (audio signals) into mechanical wave energy (sound waves).

The big issue is that they are incredibly inefficient at doing so. That is to say that most of the electrical power in the speaker driver is lost as heat rather than being converted into acoustic power (which translates into sound pressure level and perceived loudness).

The horn-type passive amplifier improves this efficiency, thereby “amplifying” the speaker’s acoustic output.

This has to do with acoustic impedance. Acoustic impedance is essentially the opposition that a system presents to the acoustic flow resulting from an acoustic pressure. In other words, the ease with which a sound wave propagates through a particular medium.

Acoustic impedance is typically measured in Pascal (pressure) seconds (time) per cubic metre (volume): P • s / m3.

However, in confined spaces like a tube where the same medium (air) is at either end of the tube, we prefer specific acoustic impedance, which is measured in Pascal seconds per metre: P • s / m.

There is a significant impedance mismatch between the driver’s moving diaphragm (very high density) and the air around it (very low density). Therefore, most of the energy put into moving the driver’s diaphragm is lost as heat rather than transferred to the medium (air) around it.

The small cross-sectional area of the horn’s throat acts to somewhat restrict the passage of air, thereby providing a relatively high acoustic impedance (relative to the free field outside of the horn).

This allows the driver to produce higher amounts of pressure per unit displacement and improves the efficiency of the driver.

So then, the sound waves at the throat of the horn are of relatively high pressure and low displacement. As the sound waves make their way to the mouth of the horn, they gradually decompress to a state of lower pressure (ambient pressure) and higher displacement.

Simply put, the horn improves the efficiency of the driver to make it louder.

The drawback is that the horn works best to amplify plane waves (ideal high-frequency waves) and actually reduces low-end frequencies with wavelengths below the circumference of the mouth.

This is why we’re more likely to see horns used in conjunction with tweeters (or even full-range speakers) rather than with woofers.

The Pyle PH391 (link to compare prices at select retailers) is a great example of a “horn tweeter”:

Pyle PH391

Megaphones, like the Pyle PMP52BT (link to compare prices at select retailers) and some PA speakers, like the Pyle PHSP5 (link to compare prices at select retailers) also use horns to passively amplifier their sound output.

Note that the Pyle megaphone also utilizes active electronic amplification. Note, too, that a “megaphone” can be as simple as rolling up a piece of paper into a cone shape (think back to the days in elementary school).

Pyle PMP52BT (left) & Pyle PHSP5 (right)

In addition to improved loading, horns also work to direct the sound waves in the medium, making them relatively louder on-axis (in front of the horn) and relatively quieter off-axis (in a direction away from where the horn is pointing).

There are many different types of horns that provide varying amounts of filtering and amplification. These designs are beyond the scope of this article. Just know that the horn is one method of passive amplification.

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The Speaker Enclosure

Have you even noticed that most speaker units have their driver(s) housed within a box? This box is called an enclosure and, it too, acts as a passive amplifier.

Let’s begin with the basics and describe a speaker without an enclosure.

We’ve already described the speaker driver as a transducer and know that the diaphragm(s) of the speaker move inward and outward to pull and push air and produce sound waves.

So, then, the speaker driver will push sound in one direction and then the other. It does so many times a second (20 Hz to 20,000 Hz is the audible frequency range).

When the speaker pushes forward and creates a change in sound pressure, it simultaneously creates and equal but opposite sound pressure to its rear. The sound wave that would be produced, then, is largely cancelled out by the rear “anti sound wave.”

This phase cancellation is particularly bad at lower frequencies, which are more omnidirectional by nature.

That isn’t to say that there would be a complete phase cancellation that would yield complete silence. It is to say, however, that the speaker driver, by itself, is actually pretty bad at producing full and loud sound waves.

So by closing off the rear of the driver (at least partially) from the ambient medium, we can mitigate this cancellation. This is done with an enclosure and, in a sense, is a way to “passively amplify” the speaker’s output.

The simplest type of enclosure is sealed. In this design, the rear of the driver is completely sealed.

This, in theory, eliminates all phase cancellation from the rear and, therefore, increases the amplitude of sound that is propagated from the speaker.

Sealed Enclosure

The bass-reflex is perhaps the most common ported enclosure type that has a small port in the enclosure to allow sound from the rear of the driver to eventually get out to the listener.

The front port(s) and tube(s) of the bass-reflex enclosure are carefully designed to cause the rearward sound waves to emanate from the speaker in-phase with the frontward sound waves.

When done correctly, this design yields a louder speaker than the sealed enclosure mentioned above.

Bass-Reflex Enclosure

The aforementioned concept of the horn can also be incorporated into speaker enclosures. These enclosures are commonly referred to as transmission line enclosures.

The rearward sound waves from the driver are effectively sent through an acoustic labyrinth that gradually expands its cross-sectional area (link a horn).

These rear sound waves can be carefully tuned and passively amplified to reduce intensity loss throughout the distance travelled. Transmission line systems are among the best in terms of high fidelity, particularly at low frequencies.

Transmission Line Horn-Style Enclosure

For more information on speaker enclosures, check out my article Why Do Loudspeakers Need Enclosures?

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The Passive Smartphone Amplifier

Most of us are consistently walking around with a speaker on our person. This speaker (speakers, actually, in most cases) are in our phones.

With the rise of portable audio players and smartphones has come the rise of passive amplifiers.

Any smartphone can get louder without headphones or speakers. The built-in speaker can be amplified passively with any of the above-mentioned techniques.

Sticking the phone in a bowl can make it louder thanks to the reflections discussed in the amphitheatre section of this article.

There are also plenty of DIY and pre-built enclosures/horns available for a variety of different smartphones.

For example, Edison Amp has an awesome old-school horn-style passive amplifier (link to check the price on Amazon) pictured below:

Edison Amp Passive Smartphone Amplifier

Fanshu has a simpler but effective Passive Smartphone Amplifier (link to check the price on Amazon) as well:

Fanshu Passive Smartphone Amplifier

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What About Passive Preamplifiers?

Perhaps you’ve landed on this article because you’re confused about passive preamplifiers. I know I certainly was when I first heard the term.

Preamplifiers are often discussed as microphone preamplifiers: active devices that apply gain to mic level signals to boost them up to line level.

Alternatively, they’re thought of as gain controls that we can put in-line before the power amplifier that drives our passive speakers or the input of our active speakers. These preamps can also act as switchers with one or more inputs and one or more outputs with switches that allow us to mix and match the inputs (audio sources) and outputs (amplifiers and speakers).

To learn more about active and passive speakers, check out my article What Are The Differences Between Passive & Active Speakers?

Passive preamplifiers are similar to the latter definition above in the fact that they are used to control the signal level going into a power amplifier that will ultimately be driving our speakers.

These preamps are commonly used but are not absolutely necessary in the signal chain from the playback source and the speaker(s).

However, the signal level control is volume only. The passive preamp has no power and, therefore, no gain to offer the signal. It can simply act as a volume control (and one that can only “turn down” at that).

The most common benefit a passive preamplifier has over an active amplifier commonly is a cleaner sound. In other words, simpler components and no power allow the output to more accurately resemble the input.

All in all, the passive preamplifier can be thought of as a glorified volume control and switcher device.

The Nobsound Mini NS-05P (link to check the price on Amazon) is an example of a passive preamplifier. It has a single volume control and 2 switches to switch its input (RCA or XLR) and output (RCA or XLR).

The NS-05P does not require power nor does it supply any gain to the signal.

Nobsound Mini NS-05P

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Active Amplifiers

In general, and especially in the world of audio, the term amplifier refers to an active amplifier.

Aa active amplifier amplifies audio signals (electrical AC voltage and power) with gain supplied by an external power source.

Note that active amplifiers are electrical amplifiers that amplify audio signals while the aforementioned passive amplifiers amplify sound.

For more information on the differences between sound and audio, check out my article What Is The Difference Between Sound And Audio?

Energy cannot be created nor destroyed and so active amplifiers require a power source in order to supply gain and boost the level of the signal.

Active amplifiers are found throughout the signal chain and work to boost the signal level from one device to the next.

Therefore, there are plenty of amplifier types to discuss, including:

All of these active amplifier types work the same. They take an input signal, apply gain (via external power), and output a stronger signal.

Ideally, these amps will do so in a clean fashion. That is to say that the quality of the output signal will be exactly that of the input signal and only the amplitude of the signal will be different.

However, the components of these amplifiers will inevitably colour the sound in the process of amplification. Sometimes this effect is cherished and other times it’s unwanted.

All amplifiers have limits and can only accept so much input level and output so much output level without adding significant distortion to the signal. Overloading amplifiers can even cause irreversible damage due to overheating.

With that said, let’s briefly touch on the active audio amplifier types mentioned above.

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Power Amplifier

The power amplifier is the most common type of amplifier. Many of the other active amplifier types on this list are considered to be power amplifiers.

An audio power amplifier is an electronic amplifier that amplifies a low-level signal (typically near line level) to a speaker level signal that can properly drive speakers (and sometimes headphones). Power amps are the “heavy lifters” of the signal chain that are put in-line right before the loudspeakers.

In powered loudspeakers, the power amplifier is built-in to the main speaker and is used to drive the main speaker along with any of its satellites.

With active speakers, each driver has its own power amplifier and each amp is put in-line after the cross-over (which deals with line level signals rather than speaker level signals).

The amp types in the following list are also power amplifiers. I’ll mention each individually:

Therefore, there are plenty of amplifier types to discuss, including:

And they can be of the following types:

The Crown Audio XLi 2500 (link to compare prices at select retailers) is an example of a 2-channel power amplifier with 33 dB of gain and power ratings of 750W Per Channel at 4 Ohms; 500W Per Channel at 8 Ohms, and 1500W at 8 Ohms Bridged Mono.

Crown Audio XLi 2500

Crown Audio is featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.

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Preamplifier

An audio preamplifier (also referred to as a preamp or simply as a “pre”) is an electronic amplifier that boosts weak signals (mic and instrument level) up to a signal strong enough (typically around line level).

This boosting makes the audio signals noise-tolerant and strong enough for further processing, recording, mixing and playback.

A preamplifier is need to bring microphone and electric pickup levels up to usable levels. For this reason, microphone require microphone preamps and electric guitars (and basses and other instruments) have preamps in their amplifiers.

Preamps are also used to boost the levels of recorded audio from record players and other sources before the signal is sent to the power amplifier.

Because preamplifiers deal with low-level signals, they can also have contouring controls (EQ) and other nifty processing built-in at relatively low cost.

It is, therefore, common to have some control over the sound of the signal at the preamplifier stage.

Think of the bass, mids, treble, gain, presence and reverb controls on a guitar amp; the high-pass filters, EQ and pads on a microphone preamp; the contour controls on the preamp of your sound system. These extra processes are often found on preamplifiers.

Preamps in entertainment systems allow for ease of control over the volume/loudness of the system.

Preamps are also often used as switchers and distribution amplifiers. These allow us to switch between multiple inputs and/or outputs to route signals to where they need to go.

The passive preamplifier mentioned earlier fall into the switcher category and allows for some control over volume though they do not supply any gain.

Preamplifiers are also extremely common and can be found in the following active amplifier types that we’ll touch on in this article:

And they can be of the following types:

The OSD Audio Pre-1 (link to check the price on Amazon) is an example of a preamplifier with multiple inputs and contour control.

OSD Audio Pre-1

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Integrated Amplifier

An integrated amplifier combines a preamplifier and power amplifier into a single unit.

Integrated amps have all the controls of the preamplifier with the large gain supply required to drive loudspeakers.

We’ll find integrated amplifiers in receivers, car amplifiers, and instrument amplifiers. They can be solid-state, tube or digital varieties.

The Denon PMA-800NE (link to compare prices at select retailers) is an example of an integrated amplifier.

Denon PMA-800NE

Denon is featured in My New Microphone’s Top 11 Best AV Receiver Brands In The World.

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Receivers

A receivers is essentially a power amplifier (or an integrated amplifier) with a built-in radio receiver.

Receivers, therefore, when set to pick up a local radio station, can be plugged into a power source and the loudspeaker (or headphones) and still work. There is no need to plug in an audio source.

The Sony STR-DH190 (link to compare prices at select retailers) is an example of a receiver.

Sony STR-DH190

Sony is also featured in My New Microphone’s Top 11 Best AV Receiver Brands In The World.

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Car Amplifier

Car amplifiers, as the name suggests, are for use in automobiles.

These amps are typically simple power amps that receive the low-level signals from the car stereo; amplify them, and drive the car’s loudspeakers with them.

That being said, there are many car amps with tone controls and a preamp section.

Aftermarket car amplifiers are common for car audio fanatics in order to drive power-hungry subwoofers.

The power is taken from the vehicle’s electrical system.

The BOSS Audio Systems R1100M (link to compare prices at select retailers) is an example of a car amplifier.

BOSS Audio Systems R1100M

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Instrument (Guitar, Bass, Etc.) Amplifier

An instrument amplifier is a special type of an integrated amplifier.

For ease of explanation, let’s take a guitar amplifier as an example.

A typical guitar amp will have its preamp circuit. This preamplifier acts to boost the low-level signal from the electric guitar’s pickup to a healthy level.

It also typically provides some options to process the sound. The option include but are not limited to:

  • Gain
  • Bass Control
  • Mids Control
  • Treble Control
  • Presence
  • Master (typically controls the gain of the power amplifier)
  • Reverb

The amp may even have a switch to alter the circuitry from “clean” to “dirty” and beyond.

After this stage, the signal is sent to the built-in power amplifier that supplies enough gain to effectively drive the large speaker driver.

Some guitar amplifier are considered combo and have combined amplifier circuits and cabinets.

Note that different guitar amps have different circuits for their knobs to control. It is not always the way I’ve explained it above.

The Orange Crush12 Combo (link to compare prices at select retailers) is an example of a combo guitar amplifier/cabinet.

Orange Crush12

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Headphone Amplifier

A headphone amplifier is a special kind of power amplifier designed to properly drive headphones.

Headphones have much higher impedance values than loudspeakers and are made with much small drivers (in many cases).

Many headphones fit within the same general sensitivity and impedance and can be driven by a typical headphone amp. These “typical” headphone amps are often found inside headphone jacks (just after the digital-to-analog converter DAC in digital devices like smartphones and mp3 players).

Some special types of headphones (like electrostatic headphones) have vastly different impedance and sensitivity ratings than “typical” headphones and, therefore, require special headphone amplifiers.

Some USB microphones also have built-in headphone amplifiers.

The Rupert Neve Designs RNHP (link to compare prices at select retailers) is an example of a headphone amplifier.

Rupert Neve Designs RNHP

The HiFiMan Shangri-La Sr (link to check the price at B&H Photo/Video) is an example of a specialty electrostatic headphone amplifier.

HiFiMan Shangri-La SR

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

For more information on electrostatic headphones, check out my article The Complete Guide To Electrostatic Headphones (With Examples).

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Microphone Preamplifier

A microphone preamplifier, as the name would suggest, is used to boost mic level signals (typically from microphones) to strong signals near line level.

Once boosted, the audio signals can be more easily processed, recorded, mixed, and eventually sent to a power amplifier to drive speakers and/or headphones.

Many microphone preamplifiers have built-in EQs, filters, pads and phase flips. These additional options help tremendously in using the many different microphones on the market.

Mic preamps also typically have a phantom power supply to properly power the active microphones that require them.

Some USB microphones also have built-in microphone preamplifiers.

The Golden Age Project Pre-73 (link to check the price at B&H Photo/Video) is an example of a single-channel microphone preamplifier.

Golden Age Project Pre-73

The Cloudlifter CL-1 (link to compare prices at select retailers) is another example of a microphone preamplifier.

Cloudlifter CL-1

For more information on microphone preamplifiers, check out my articles What Is A Microphone Preamplifier & Why Does A Mic Need One? and My New Microphone’s Best Microphone Preamplifiers.

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Impedance Converter Amplifier

Impedance converters are required in condenser microphones in order to properly use the capsule transducer’s captured audio signal.

The high impedance of the condenser mic capsule makes it difficult to take the audio signal from it. An impedance converter put in-line immediately after the capsule is required to drop the impedance and allows for a usable signal.

The impedance converter can be either transistor-based (solid-state) or tube-based.

The AKG C414 XLII (link to compare prices at select retailers) is a condenser microphone with a solid-state impedance converter.

AKG C414 XLII

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

For more information on solid-state condenser microphones, check out my articles What Is A Condenser Microphone? (Detailed Answer + Examples), What Is A Solid-State Microphone? (With Mic Examples) and What Are FETs & What Is Their Role In Microphone Design?

The Warm Audio WA-47 (link to compare prices at select retailers) is a tube condenser microphone with a vacuum tube impedance converter.

Warm Audio WA-47

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

To learn more about tube microphones, check out my articles The Complete In-Depth Guide To Tube Condenser Microphones and What Is A Tube Microphone And How Do Tube Mics Work?

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Distribution Amplifier

An audio distribution amplifier (ADA) is a device that accepts a single audio signal at its input and provides this same signal to multiple isolated outputs.

Each output can then be processed, amplified, monitored, etc. as its own isolated signal.

The RDL EZ-ADA4 (link to compare prices at select retailers) is an example of a stereo distribution amplifier.

RDL EZ-ADA4

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Solid-State Amplifier

Solid-state amplifiers utilize solid-state (transistor-based) electronics to amplify audio signals.

The Crown Audio DSi (link to compare prices at select retailers) is an example of a solid-state power amplifier.

Crown Audio DSi-2000

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Tube Amplifier

Tube amplifier utilize vacuum tube electronics (specifically triodes or higher) to amplify audio signals.

The Rockville BluTube (link to compare prices at select retailers) is an affordable tube power amplifier.

Rockville BluTube

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Digital Amplifier

A digital audio amplifier received a digital audio signal at its input and digitally converts it to a PWM (Pulse Width Modulation) format.

The Pulse Width Modulator then produces a high voltage signal which is filtered to replicate the original audio signal in analog form.

The AudioSource AD508 (link to compare prices at select retailers) is an example of a digital power amplifier.

Audio Source AD508

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A Note On Amplifier Classes

Though beyond the scope of this article, I’d like to list out the different amplifier classes we’ll encounter in the world of audio:

  • Class-A
  • Class-AB
  • Class-B
  • Class-C
  • Class-D
  • Class-E
  • Class-G
  • Class-H
  • Class-S

There are variations of the above classes in amplifiers today but these are the most common types we’ll find on the market.

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Can I use active speakers with an amplifier? Active speakers have internal amplifiers for each crossover band. Sending speaker level signals to an active crossover may very well overload it and cause severe distortion or damage. However, if the levels of both devices are low, an external amplifier could be connected to an active speaker.

What is a passive speaker and how does it work? A passive speaker does not have an internal amplifier and thus requires an external amp to drive it properly with speaker level signals. Matching the speaker to an appropriate amp, then, becomes paramount. In multi-driver passive speakers, the crossover is designed to handle speaker levels signals.

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