What Does A Car Amplifier Do And How Do They Work?

Wondering how a car amplifier works and what they’re used for? Ready to upgrade your car audio system you’d like to upgrade but need to know more first? You came to the right place!

I’ll cover everything you need to know:

  • How a car audio amplifier works
  • What they do and why they’re helpful for sound
  • The differences between the different audio amplifier classes

…and much,much more!

What does a car amplifier do?

diagram showing a car amplifier installed in car with speakers

A car amplifier boosts a very small electrical audio signal from the head unit to a higher voltage. It can drive loudspeakers connected with speaker wire with more power, volume, and sound clarity. Most car audio in-dash stereos have only about 15 to 18 watts available and begin to distort very badly at their maximum output.

An external amplifier not only solves this problem but also can greatly improve the reproduction fidelity of the music you enjoy.

For powerful bass, subwoofers need lots of power which is impossible to get from a car stereo alone. This is also a problem when you’d like to use 4 ohm or even 2Ω aftermarket speakers that need more power for maximum performance when upgrading your sound system.

An external amp provides higher sound quality as it can offer lower noise and distortion due to more advanced electronics included for that purpose. Car stereos often use a more basic design and cheaper components that limit what you can hear.

Amplifier power ratings explained

The continuous or RMS watt rating describes the power output an amp can realistically deliver reliably. Although it sounds impressive, the “peak” or “maximum” power specification is more of a technicality and not what you’ll get during normal use.

RMS power refers to the root mean square measurement used in electrical engineering to describe alternating current (AC) signals like those used in sound electronics. As AC voltage is different than direct current (DC), the math is a bit different. RMS measurements take this into account.

CEA logo

To help buyers get more accurate information the Consumer Electronics Association (CEA) 2006 standard was created to encourage providing accurate specifications. Products sold that list the CEA-2006 specifications have demonstrated they’ll deliver what the manufacturer advertises.

How does a car amp work?

how does a car amplifier work diagram

An internal power supply, supplied by the battery positive and negative power wiring, generates a higher output voltage – and more power – than is otherwise possible from a 12 volt power source. Due to the large amps drawn, thick gauge power wire and ground wire connections are made to the vehicle battery and metal chassis directly in most cases.

A car stereo sends an audio signal via an RCA cable or other connection where the boosted electrical signal is delivered to speaker terminals for use with wire.

Nearly all aftermarket car audio power amps have the following:

  1. A DC-DC (direct current) step up switching power supply
  2. Noise prevention (“ground loop”) circuitry
  3. Speaker crossover circuitry
  4. Bridging circuitry
  5. Extra convenience functions (ex.: speaker-level input stages or bass boost)

Car amp power supplies explained

Illustrated car amp switching power supply section

Car audio presents a really tough challenge: using only a +12V supply, we can’t directly create a large power output to use for both factory speakers or upgraded speakers.

To help better understand this, we can look at Ohm’s law which states that power is related to the resistance of a load (speaker impedance) and the supply voltage. The load resistance must decrease or the voltage must increase to get more power.

Car amplifier power output formulas image

Ohm’s Law is a set of mathematical formulas used to calculate the power delivered to a load. In order to get more power you’ll either have to (1) lower the Ohms load (impedance) or (2) increase the volts available.

A switch-mode power supply (SMPS) inside makes this possible as it takes the +12V battery supply and increases it several times to about 28 volts to 32 volts for example. Both positive and negative polarity supply paths are provided as well (also called supply “rails”).

The switching integrated circuit (IC) chip drives high-current transistors connected to a transformer on and off thousands of times per second. The transformer, based on the number of turn of copper wire on it, produces higher voltages at its output. These are smoothed and stabilized to produce a power source for the speaker channels.

Car amp SMPS waveforms example image

Snapshot of the alternative waveforms (on/off voltage signals) used to drive the transistors in an SMPS supply several thousand times per second.

DC-DC switching supplies like those in a car amp are called step-up as they’re able to take a lower voltage and multiply it several times to a higher voltage. Step down power supplies do the opposite.

Some components such as high-power switching transistors are attached to the heavy metal chassis of the amplifier to remove and dissipate excess heat generated. Otherwise they would eventually fail due to heat destroying them.

How amps turn on and off

A low current signal (often less than .025 amps)called the “remote” wire is used to avoid the amplifier staying on and draining the battery when not in use. The remote wire is connected to an output at the car stereo or ignition wiring. When a +12V signal is removed the amp turns off and stops drawing battery power.

Input stages, amplifier sections, and output stages

Illustrated image of car amplifier audio output stages

Shown are two sections that make great car audio possible: the amplifier & output section and the input stages. The input stage board consists of many smaller circuits to do things like providing crossover functions, making bridging for more power possible, providing gain control, and blocking ground loop noise. The amp stages take the audio signal from the input stage board, use the power supply’s output, and boost it to create a powerful output.

Input stages

The input stages have a number of jobs to carry out:

  • Allow the amp to connect to stereos with no RCA jacks (speaker-level inputs, where provided)
  • Prevent ground loop noise from getting into the signal path
  • Provide crossover functions
  • Allow adjusting the gain level of the amp

Input stages use an extremely versatile electronic component called an op amp (operational amplifier). Op amps are tiny amplifier circuits built into miniature chips used for all sorts of audio functions.

Noise prevention

Ground loop noise is a common and very frustrating car audio installation problem. Also called alternator whine, it’s a type of low-level electrical noise in the audio signal path that appears as a terrible whine that increases and decreases with the engine speed.

This type of noise is due to electrical currents that flow within a vehicle’s body and the connections of the audio components. When there’s a difference of electrical potential in 2 or more points in the system, a small voltage difference is created and can be amplified. This appears as a high-pitched and annoying whine you can hear from your speakers.

A filter design called a differential amplifier circuit is used on the front end (input side) to effectively block and cancel out this noise before it can get into the musical signal pathways.


Electronic crossover functional diagram showing the basic blocks of operation

Electronic crossovers offer a way to control the frequency range sent to car speakers. For example, when driving subwoofers, a low pass filter blocks everything except bass to give a great bass sound. Likewise, you can prevent bass from distorting smaller speakers such as tweeters or door speakers by using the high pass crossover.

When used correctly crossovers allow you to get more volume, protect your speakers, and provide better sound clarity. For more advanced systems like those using component speakers they’re especially helpful.

Electronic crossovers in amps perform the same function as passive speaker crossovers but without bulky components like inductors and capacitors they require. They’re also easy to adjust or turn on/off at the flick of a switch.

Amplifier gain

As the name implies, amplifiers boost an input signal from a stereo. Because there’s no standard output voltage used by different car stereos, a gain adjustment is necessary for the best results including sound quality, sufficient power, and preventing distortion called clipping.

The amplifier gain adjustment provides a way to control the output level based on the input signal level. As some car stereos have stronger output signals the gain control can be reduced to better match it to the output level & volume.

The gain level affects the headroom which is the amount of amplification still available before the limit is reached. All types from car to home stereos and even home DJ equipment often include a gain adjustement.

It’s also a way to reduce noise. Because as all amps add some level of noise (like a “hiss” or similar) to their outputs, lowering the gain also reduces the noise produced and heard.

Amplifier sections and output stages

Car amplifier output stage illustration

Simplified illustration of the amplification stages and output that most car amps use. Beefy output transistors capable of handling large amounts of current are connected to the switching power supply and drive the speaker. They’re controlled by some smaller components needed to split the audio signal into positive and negative halves needed to reproduce the musical signal.

Power amplifiers (as seen in the image above) have main sections dedicated to modify or manipulate audio signals as well as for delivering more current and voltage. The first sections contain small components like miniature transistors that divide the audio signal into two halves.

Next, after passing through the crossover filter and other stages, they’re directed to large high current transistors supplied with higher volts from the SMPS. The input signal is magnified to an identical but much larger waveform connect to the speaker outputs.

Car audio amplifier classes explained

Teacher discussing car amplifier classes

The class listed by a car audio amplifier manufacturer is the type of technology used for the audio signal and to deliver power. Every audio amp uses a class A, class D, or class A/B design.

  • Class A: The most inefficient but provides audiophile quality sound. (Used in some past designs like from Rockford Fosgate)
  • Class A/B: Until recently the most popular because they offer reasonable cost and good sound quality. Class A/B amps work similar to class A but switch off when the signal reaches the zero voltage threshold, reducing power consumed slightly.
  • Class D: These amps are increasingly more common as they use newer technology to rapidly switch the power circuitry on and off, reducing waste and heat. Sometimes mistakenly called a “digital amplifier” although they don’t use digital technology for their basic functions.

Class D vs class A/B differences

Class A/B vs class D amplifier operation diagram

Class A/B designs conduct, follow, and amplify the analog input signal most of the time, using some power even when nothing is being delivered to the speakers. This is because even when you’re not driving speakers some power is wasted as heat due to losses. That’s one reason this type gets fairly warm especially during hard use.

They have an efficiency somewhere around 65% meaning they draw about 35% higher amps from the battery than is delivered as power to speakers. This is wasted purely as heat!

By contrast, a class D amp uses pulse width modulation (PWM) technology that switches power components on only a portion of the time. Unlike class A/B amps they convert the input signal to square waves used to control the on/off timing. These are converted back to smooth audio signal waves before being delivered to speakers.

Class D amplifier models are about 85% efficient meaning they run cooler and can be much smaller than A/B designs. As they offer more power in less, they’re really common for very high power subwoofer amplifier (mono amp) models sold today.

Class A/B amps offer lower noise levels for less money, although today’s D models have caught up fairly well especially for brand name designs.

What are channels on an amp?

Channels are independent audio paths that are used to create a sound output from an amplifier. In stereo recordings, the left and right audio sources differ and provide left vs. right audio sound.

In car audio, left and right audio channels are separate outputs from a car stereo or an independent signal path with an amplified output. They’re used to give a fuller, more realistic sound when listening. Additionally, some stereos provide front and rear outputs pairs, although these are simply the same signals provided to drive an extra pair of speakers in the rear.

In many cases stereo amplifier channels can be “bridged.” This means that 2 channels can work together in a push-pull fashion to speakers with more power than one channel alone (usually about twice the power of a single speaker output).

Car audio amp connections, inputs, and controls

Crunch PD2000.4 car amplifier ends image

Most car amplifiers, whether expensive or budget-priced, are very similar in how they’re designed. Generally power connections are located on one end and audio inputs and controls are found on the other end.

In image above you can see adjustable crossover dials and the switches to enable them. The point after which sound frequencies are blocked, called the cutoff frequency, is adjustable by the user with a small dial in some cases. In other models, it’s a fixed setting operated with a switch and can’t be changed.

However, fixed crossover frequencies are normally set to the most commonly used values for convenience.

Power connections

Pioneer GM-D9605 wiring terminals close up image

Power and speaker connection terminals on a Pioneer GM-D9605 car amplifier.

Power connections are typically made using larger-gauge copper wire and connectors, often included in an amp wiring kit. RCA cables are used to connect the amplifier to the audio signal outputs of the head unit from which music is played.

Note that to bridge 2 channels on the amp and deliver more power as a single channel, they’re connected in a particular way. The bottom image above shows the polarity and wiring connections required. They’re marked as “Bridged” as you can see.

In bridged mode, one channel provides the positive speaker connection and the other provides the negative.

Speaker level inputs

Pioneer GM series car amplifier high level input harness image

A speaker level input wiring harness is shown. Speaker outputs from a factory stereo can be connected to the wiring then plugged into an amp to provide a signal when RCA jacks aren’t available.

Upgrading a factory installed car audio system can be challenging as RCA jacks aren’t usually available. In that case speaker-level (also called “high level”) inputs, if available, can be used to avoid buying additional parts.

These take the higher voltage speaker level signal directly from speaker wiring and reduce it to a smaller level the amplifier can use.

The other option is to use a speaker-level adapter to do the same thing.

How are car power amps installed?

image of car amplifier having wire connected

A typical installation for a car amplifier. Large-gauge power wires are connected to the power terminals and fastened using screws. Speaker wiring is connected similarly. After installation, the sound system is tested and the amp’s gain control and crossovers are adjusted as needed.

In order to work properly, car amplifiers have to be installed with sufficient size and type of power wire. The most important reason for this is because a vehicle’s original wiring cannot handle the high-current demands of an amplifier.

For example, some may draw 50 to 80 amps or more or when driving subwoofers very hard. Factory wiring isn’t rated for these kinds of demands and a loss of voltage (and consequently, power) to the amplifier would occur. Therefore we route a large enough power cable to the battery and make sure a good, clean electrical connection is made.

A sufficiently rated fuse, held in a fuse holder, protects the positive power cable. If a problem like a short circuit occurs the fuse would blow and protect against causing a fire.

There are several wiring connections used on all car amplifiers:

  1. A fused large-gauge power wire to the battery (+12V)
  2. Negative connection to the “ground”: negative battery terminal or car’s metal body
  3. Remote-on wire to switch the amp on and off with the stereo or ignition switch
  4. Audio input signals: RCA cables or speaker-level inputs
  5. Speaker wiring connections

Fortunately, this wiring is often easily found pre-packaged and ready for use by buying an amplifier wiring kit.

Car amplifier installation diagram

Notated diagram showing typical car amplifier installation

How are car amplifiers helpful?

Alpine MRV-F300 installed under seat

Shown: Some modern car amps are small enough to fit under a seat. Larger or multiple amps can be mounted to an amp rack for a cleaner installation.

Today’s amplifiers can power a whole car audio system with excellent sound quality and volume. Some are very compact and can be installed under a seat or even inside your dashboard! A good amplifier provides several times the power output of an average car stereo. Also, they’re much more clear and crisp sounding.

Car amplifiers have many benefits as well as being a necessity in some cases. Typical car stereos, even today, can only produce about 15 to 18 watts of power per channel at most.

At higher volumes and when attempting to drive speakers that need more power, the sound from a stereo becomes distorted and terrible. Driving speakers with higher volume is basically impossible with only a car stereo.

Car amps offer much better sound, especially for bass-heavy music. There are also some special situations where they’re the only option for upgrading the sound in a vehicle:

  • Factory stereo systems with no woofer
  • Factory-installed amplifiers that have died or are weak
  • The desire for powerful, clear sound when enjoying higher-quality music
  • Vehicle owners who want heavy volume
  • Boat and other outdoor vehicle owners (outdoor vehicles need extra power for better sound)

Additional benefits

As I mentioned earlier, many offer features like built-in crossovers that can prevent distortion and allow you to play speakers at higher volumes with enhanced clarity. Factory systems normally can’t do this and the sound will “break up” early when turned up to higher volumes.

Installing an aftermarket amplifier resolves this problem and allows more control over your audio system. Additionally, a system can easily be expanded to add a subwoofer for missing bass by either adding a 2nd amplifier or using 2 channels of a 4 channel amp to drive it.

2 channel vs 4 channel amps – what are the differences?

2 channel vs 4 channel amp diagram

4 channel amplifiers are essentially a 2 channel amp with an additional stereo channel pair designed in. They offer several benefits:

  • You can drive front and rear speakers
  • Can drive front full-range speakers and use rear channels for a subwoofer
  • More compact than 2 separate stereo amps

Most 4 channel amplifiers today are bridgeable so they’re very flexible in how they can be used. However, be aware that a 2 or 4 channel amp normally requires a minimum of 4 ohms when bridged, unlike stereo mode.


About the author

Marty is an experienced electrical, electronics, and embedded firmware design engineer passionate about audio and DIY. He worked professionally as an MECP-certified mobile installer for years before moving into the engineering field. Read more »

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