How-to guide

How To Hook Up A 4 Channel Amp To Front And Rear Speakers

Whether you’re wanting more advanced sound or just need a good basic system, adding a 4 channel amp is a great idea. I’ve been enjoying clear, fantastic sound in my own vehicles for years using my 4 channel amps. It feels great to hear music the way it’s supposed to sound!

In this guide I’ll show you how to hook up a 4 channel amp to front and rear speakers. After installing hundreds of amps in vehicles just like yours. I want to share with you the basics you need to do it yourself.

And hey – don’t worry. In most cases you can do it yourself and you can get professional results on a budget!

Below you’ll also find recommendations for amps, installation kits, signal adapters, and accessories you’ll need for the least amount of money (and the least amount of headache!)

Infographic – How to hook up a 4 channel amp (tips and general guide)

Hook up amp 4 channel amp front rear speakers infographic diagram


Basics first

If you’re reading this it means there’s a good chance you’re not already familiar with a lot of topics about installing an amp, connecting wiring, and much more.

Let’s start with the basics and I’ll cover as much as I can help you understand the whats, hows, and whys of hooking up a 4 channel amp to your vehicle’s speakers.

Not everyone has used car stereo electronics before, so I’m going to be as thorough as possible.

What is a 4 channel amp?

4 channel car amplifiers may seem obvious, but there’s more to it. Additionally, there are some interesting (and good) ways they differ from 2-channel amps.

In fact, there are actually a few benefits you’ll get using one 4 channel amp instead of 2 stereo ones to power your front and rear speakers.

Holding Alpine MRV-F300 amp in my hand

Today’s 4 channel amps offer newer technology, better sound, and more compact size than in the old days. An excellent example is the Alpine MRV-F300 50W x 4 model. It uses Class D technology to run extremely cool and yet it’s small enough to fit under a car or truck seat. Very nice!

A 4 channel amplifier is a stereo amplifier with 2 additional channels built in that boosts (amplifies) a weak signal to a higher voltage signal. This drives loudspeaker voice coils and produces sound.

4 channel amplifiers integrate additional channels into a more compact and efficient design than separate amplifiers.

Additionally, they provide added flexibility, as most can be configured for “bridged” operation which can provide more power to 2 channels.

Bridged mode is a special design which is basically a “push-pull” set up: one channel (normally used for the left speaker) produces a signal that’s the opposite of the second channel (normally used for the right speaker). This causes the speaker to receive a voltage audio waveform that is the difference of the two channels – which can produce more power to the same speaker.

Essentially, bridged mode is a flexible way to get more power if you’re not driving more speakers. It means 2 channels are sharing the workload of one speaker between them (and so they can deliver extra power).

2 channel vs 4 channel amp diagram

2 channel vs 4 channel amp diagram

A 4 channel car amp is basically an expanded version of a 2-channel amp. However, because they’re built together rather than as 2 separate 2-channel amps, they’re more compact. This saves installation space and makes it easier too. Additionally, most can be bridged to use 2 channels (or 3, depending on your needs) so you’re not restricted to using them with only 4 speakers.

The benefits of using an amp to drive speakers

Whether you have a factory stereo or a great aftermarket (non-factory) one, adding an amplifier is one of the best decisions you can make.

In-dash stereos are limited in how much power they can produce. They cannot drive speakers with the same clarity and low distortion like a good amplifier can. The volume you’ll be able to get from your speakers is limited, likewise.

There’s simply no way around it – most in-dash stereos are limited to about 15W-18W RMS of power for each speaker channel. That’s because they’re running directly from the +12V supply. Amplifiers are unique in that they take the +12V electrical supply and boost it to a higher voltage.

When a signal is boosted and sent out to your car’s speakers the voltage is much higher and the speaker can receive much more power.

That’s why tiny amplifiers are rarely worth bothering with – if there’s no special power supply inside, it’s simply not capable of producing much power.

Getting great sound

Powering speakers from an amp makes a big difference, and I’ve enjoyed excellent sound for years this way.

When an amplifier drives your vehicle’s speakers it’s often not even pushed to its limits. The sound produced at the speaker has lower distortion, doesn’t “bottom out” when heavy bass is played, and you can get a lot more volume, too!

Additionally, using an amplifier with built-in high-pass crossovers means you can block out lower-end bass that causes your speakers to distort and attempt to play music tones they’re not suited for.

The result is cleaner sound, less distortion, and great volume – you can crank your music even higher!

Just imagine driving down the road with the windows open and finally being able to blast the music you love. I’m confident you’ll love it as much as I do.

Things you’ll need

Shopping list image
It only takes a few minutes to make a list of the parts, wire, tools, and other bits and pieces you’ll need. Planning ahead can mean the difference between getting your system going without major problems or having a frustrating time – or complete failure! I always get organized and get my items together before I start a job.

Planning ahead is very important. You don’t want to run out of wire or discover you don’t have the rights parts, for example. That will mean you can’t finish your project.

It’s even worse when you have to drive around town searching for items or you’re not able to do anything after the stores close. Believe me, I’ve been there, and it’s terrible!

Notes about wire, tools, and a few other things

When it comes to installations, always plan to have more, rather than not enough, wire. This goes for speaker wire as well as RCA cables.

The amplifier kits I recommend have the right length for your amp installation, but speaker wire & RCA cables are another matter in this case.

What length and size speaker wire do I need?

AmazonBasics 16-Gauge Speaker Wire - 100 Feet

There’s no need to spend an excessive amount of money on speaker wire. 18 gauge is sufficient for many installations, but 16 gauge is a great choice too, if the price is right. A great example is this AmazonBasics 100 foot roll. I recommend a 100 foot roll for many installations with a 4 channel amp (see why below).

Here’s a rough estimate of the worst-case scenario for the length of speaker wire required. I’ll use the example of installing an amp using speaker-level inputs, with the following typical installation:

  • Amp is located in the trunk
  • Speaker level signal is connected near the radio (center console)
Let’s use roughly a 15′ length of distance from the radio to the amp. That’s a good estimate in my experience.

So we have:

  • Wire from radio to amp (signal wire): 4 channels x 15′ = 60 feet
  • Wire from amp to speaker wiring near radio: 4 channels x 15′ = 60 feet

Total estimated wire required: 120 feet.

That means you need 2 100 ft rolls of wire. Or at the least, 1 100 ft roll and 1 50 ft roll. If you’re planning to use a line-level adapter, expect to pick up a 100′ roll.

If your installation is using RCA jacks, expect a 100 ft roll also (4 channels x 15′ length estimate for the speaker wire from the amp).

What about RCA cables?

KNU Conceptz KCA-K4 4 gauge amp wiring kit RCA cables image

If you’re installing a 4 channel amplifier and using RCA cable connections, you’ll need to buy a 2nd pair along with your amp wiring kit, as most only include a 2-channel cable.

For most installations I recommend 18′ length cables. That’s usually long enough for most vehicles and you should usually have enough length to hide the cables inside the interior and under the rear seat, etc.

There’s no reason to spend an excessive amount of money. Just pick up some good quality, well made cables. Even a pair like these inexpensive ones will be fine in most cases.

Tools you’ll need.

Crimp tool with crimp caps
Crimp tools are great for installing your amp and speaker wiring with professional results. If you’re doing your own installation, you can get buy with an inexpensive tool like this Tekton crimp tool. Crimp connectors are sold separately in many automotive parts stores or general stores and are very affordable.

I recommend a few tools. If you shop carefully, you can avoid getting ripped off on tool prices. When connecting speaker wiring to factory wiring, it’s easier to use crimp connectors than solder.

Never simply twist wire together and wrap it in electrical tape. Always use a reliable connection.

During warm weather electrical tape adhesive can fail and the tape can come off of wire. This exposes it to possible short circuits and potential damage to your radio or amp.

If you have access to a cordless drill, that’s fantastic! They’re great for drilling holes in the vehicle’s metal for mounting your amplifier or connecting the ground wire to bare metal.

I also recommend the following:

  • Wire cutters (some crimp tools have this built in)
  • Roll of quality electrical tape
  • Wire ties (“zip ties”), 6″ length, bag of 100
  • A digital test meter for voltage measurement

Etekcity MSR-R500 digital test meter example

A test meter is often incredibly helpful when installing an amplifier. However, you don’t need to spend much money! A basic but good budget model like this one at Amazon will work great.

I recommend getting an affordable but good digital test meter in order to locate a switched +12V wire for getting a remote-on signal for the amp.

They’re also extremely helpful when troubleshooting power problems when something isn’t working.

Shopping lists

Here’s a general but pretty accurate list of what you’ll need for connecting a 4 channel amp to front and rear speakers.

Installation types 1 or 2: Factory radio or no RCA connections

  1. 4 channel amplifier with speaker level inputs or amp and line-level adapter
  2. 120 feet or more speaker wire, 18 gauge or larger
  3. Amp wiring kit
  4. Crimp tool and butt (wire crimp) connectors (25 or more minimum)
  5. Cutting pliers
  6. Electrical tape
  7. Wire ties, 6″, bag of 100
  8. Test meter

Installation type 3: RCA connections

  1. 4 channel amplifier with speaker level inputs or amp and line-level adapter
  2. 100 feet roll speaker wire, 18 gauge or larger
  3. Amp wiring kit
  4. Additional RCA cables, 18′ minimum
  5. Crimp tool and butt (wire crimp) connectors (25 or more minimum)
  6. Cutting pliers
  7. Electrical tape
  8. Wire ties, 6″, bag of 100
  9. Test meter

Be sure to plan ahead and estimate the amount of speaker wire you’ll need . For the amp installation itself I strongly recommend using a pre-made amp wiring kit like you’ll find here in my amp kit buyer’s guide.

You’ll also need to get a 2nd pair of RCA cables. I recommend 18 ft length minimum. Don’t spend too much money, but do get good quality ones.

How to get a signal to your amp

Image of an aftermarket car stereo outputs labeled

Image of an aftermarket (non-factory) stereo showing the RCA jacks and speaker output wiring. Either one can be used for getting a signal to an amp, but RCA jacks provide a better option. They’re normally lower distortion and allow using plug-in RCA cables. If those aren’t available, either an amp with speaker-level inputs or a line level (speaker level) adapter can be used.

In order to install a 4 channel amp and drive all 4 speakers, in many cases the biggest obstacle is getting a signal to the amp. Once that’s done, the rest is usually a standard amp installation.

There are 3 basic ways to get a signal to your 4 channel amplifier:

  1. Connect speaker outputs to your amp’s speaker level inputs
  2. Connect a line-level adapter to the radio then use RCA cables to the amp
  3. Connect your radio to the amp using RCA cables directly
NOTE: I won’t be covering factory sound systems that are “premium” and have a factory amplifier. Those such as Bose, JBL, and Mark Levinson, often found in luxury vehicles or special-edition models, are much more complex and harder to interface with. In that case my advice is to speak with a good installation shop first and do your research.

If you feel that factory amplified systems should be here as well, send me a message or comment and let me know.

In a few cases adapters are available to connect an amp to a factory amplified system’s audio wiring, but it’s often difficult or there are obstacles you won’t be aware of beforehand.

One of the reasons why is that factory amplified systems often have non-standard wiring connections for the audio path and are prone to bad noise problems if you connect an amplifier without the proper adapter or wiring.

Which type of connection do I need?

If you have a radio with RCA jacks, skip on down to the next section.

However, if you have a stereo with no RCA jacks (which is always the case for factory-installed stereos) you’ll have to buy one of the following:

  • A “line level” converter
  • An amplifier with speaker-level (“high level”) inputs

1. Line level converters

PAC LP7-4 4 channel line level converter

Line-level converters like this PAC LP7-4 4-channel model are designed to take speaker-outputs from a stereo with no RCA jacks and adapt them to RCA jacks. Using this, you can run RCA cables to your amplifier.

Line level converters are designed to allow connecting to an amplifier’s RCA inputs by adapting speaker outputs from a stereo to a low-level signal it can use.

It’s very important to buy a quality, well-designed line-level adapter for avoid noise, poor sound quality, and other problems. Don’t get the cheapest – instead, get a name brand model you can rely one (like the one above).

2. Speaker level inputs

Car amplifier speaker level input example
Amplifiers with high-level (speaker-level) inputs like this one allow connecting to speaker wiring for a signal source. This avoids having to buy a separate adapter.

Speaker level inputs are common on many 4 channel amplifiers. These amps contain electronics that scale down speaker wiring signals to a lower signal safe for the amplifier’s input circuitry.

They’re simple to connect: normally it’s just a matter of connecting both positive (+) and negative (-) wiring for each speaker channel on a small wiring harness included. This then plugs into the speaker level input connector.

4 channel amp speaker level harness example

A typical speaker-level input harness for a 4 channel amp. The wires are color coded to make installation easier. White = left front, gray = right front, green = left rear, and purple = right rear.

While it can save money (you won’t need a line-level adapter in this case) I often recommend that people consider buying a line-level converter anyway.

This allows an easier upgrade for your stereo later, which is very common for people to do. Using the line-level converter now will allow you to run RCA cables to your 4 channel amp to be used later if you purchase a better stereo (which will include RCA jacks, almost always).

3. RCA jack (line-level) connections

RCA jacks provide a clean, lower-noise connection than speaker-level adapters do, but honestly it’s not noticeable to the average person. RCA cables (line-level connections) are the preferred way to connect a signal to your amp if you have that option.

Rear of Pioneer car stereo showing RCA jacks

RCA jacks on the rear of a Pioneer head unit. This is the ideal way to connect your amplifier’s signal inputs, if available. For a 4 channel amplifier you’ll need 2 stereo RCA cables to do so. White represents the left channel white red represents the right. These are standard colors for audio outputs for both car and home stereo.

If your stereo has RCA jacks, then congratulations. Things just got a bit easier – and potentially better sounding, too!

You’ll need 2 stereo RCA male-to-male cables (4 audio channels total) to run from the radio to your 4 channel amp. That’s 4 signal channels: left & right front and left & right rear.

4 channel amp signal connection diagram

Here’s a helpful diagram showing the most common connections you’ll need to make one of the 3 most common cases I mentioned earlier:

  1. Connecting to your amp’s speaker level inputs
  2. Using a line-level converter
  3. Connecting your amp to the radio’s RCA jacks

4 channel amp signal connection diagram

You can also click here to view the .pdf document for print or download.


Connecting and running signal wiring

Speaker-level connections

As mentioned above and as shown in the diagrams, if you’re using speaker-level outputs to get a signal from the radio, you’ll need to connect wire. Ideally you’ll do so near close to the radio, then run the wire together as a bundle.

You can bundle speaker wire together with wire ties in order to keep it neat and make the installation easier.

Estimate the length of speaker wire you need to reach the amp (or line level converter) for each audio channel. To do so, run a length of wire from the radio to where the amp will be installed, then allow a little extra and enough length to run around curves and interior part.

Cut 7 more lengths of wire, for a total of 8:

  • 4 channels (4 pairs of wire) going to the amp’s speaker level inputs
  • 4 channels from the amp to the radio’s factory speaker wiring

Image of blue butt connector

I recommend connecting to speaker-level outputs using crimp connectors and a crimp tool for a reliable, solid connection. Blue connectors are normally the right size for 18-16 gauge wire.

Factory stereo color codes

If you have a factory stereo, you’ll need to find the wiring colors in order to find the speaker wiring.

A great resource for that is The12Volt.com, where you’ll find wiring diagrams for your vehicle and color codes listed.

Making connections

Image of factory stereo wiring harness

After removing the radio you’ll find connectors similar to this for the factory stereo wiring harness. You’ll need to separate the speaker wires, cut them, and attach wiring to run to the amp.

Remove the radio and disconnect the factory wiring plugs or aftermarket radio’s wiring harness.

Cut the speaker wires, leaving sufficient length to move the wire and to have enough length to connect to the wire freely.

Strip a small portion on both the stereo’s speaker wire and your amp speaker wiring. If using a line-level adapter, connect to the stereo’s speaker output side. Then connect the 4 pairs of wire to the speaker wiring in the harness.

Insert the stripped wire (about 1/4″ of bare wire) into the connectors and crimp them carefully using a crimp tool if you have one. Alternately, you can twist together wire, solder it, and carefully wrap it with electrical tape or use heat shrink tubing for insulation.

If using speaker level inputs on your amp, also connect 4 pairs of wire to the output of the stereo.

Wire bundle with zip ties example

To make a neater, more professional installation, bundle the speaker wiring similar to this using wire (“zip”) ties. I recommend using 6″ ties which often are sold in packs of 100.

Once all wiring is connected, bundle it up using wire ties or, optionally, a little bit of electrical tape wrapped around. In both cases spacing out wire ties or tape about ever 1 or 1.5 along the length of the wire works well.

Connecting RCA cables

Example of connecting RCA cables

Connecting RCA cables to an aftermarket (non-original) stereo for running to an amplifier.

If you’re using a line-level converter or have a stereo with RCA jacks, connect all 4 cables plugs to the front and rear outputs.

RCA cables are sometimes marked with left and right symbols (“L” and “R”). In some cases, white, clear, or some lighter color can be used to represent the left channel.

Connect the cables consistently so you’ll be able to recognize which is which. If the front and rear RCA cables are identical, you might want to mark front and rear using some masking tape and a marker or pen.

Connect the remote-on amp wire

Don’t forget the remote wire! Amp wiring kits include a small wire that’s used to connect the amp so that it switches on and off with the accessory position of the ignition.

Locate a +12V wire that has power when the ignition is switched to “ACC” or similar, but turns off with the key. You may also have good luck finding an existing wire color from vehicle wiring diagrams I mentioned earlier or from a Google search.

I recommend checking the wiring even if you have already located it online, just to be sure.

Before re-installing the radio connect this wire and run it along side the speaker wiring.

Installing the amp

Belva BAK82BL amp wiring kit all included imageAn amp wiring kit like this one will make installing your 4 channel amp much easier. A good-quality one like this Belva 8-gauge complete kit includes not just wiring but much more. You’ll also need to pick up a 2nd pair of RCA cables (if using them) and sufficient speaker wire.

Your amplifier needs a good solid metal connection to ground and you’ll need to run the positive battery wire to the engine compartment. Your amp wiring kit will also include a fuse holder that should be installed near the battery as well (most kits include instructions, by the way).

You’ll also need to connect the amp’s speaker outputs to the wire you ran from the radio.

As it also applies to 4 channels amps, for the amplifier installation you can follow my guides here:

Here’s a basic diagram as well to help:

How to install a 4 channel amp diagram

Setting up your amp

Image of MRV-F300 amplifier end - controls side

Once installed, you should set up your amp’s gain levels and crossovers for best sound. In this image you can see the adjustable crossovers for both front and rear channels. Turn on the high-pass crossovers and adjust to a setting close to 50-60Hz, to allow sufficient bass for music but block low-end bass that distorts.

Once installed, you’ll need to set up your amp’s gain levels and crossovers, if available. Most sold today have that. (See my recommendations at the end for some great models)

Gain control is the amount of signal amplification the amplifier performs. Ideally, with a good input signal it can be kept low to minimize any hiss or noise that can appear when it’s turned up high.

Here’s a great rule of thumb for how to adjust the gain for this type of system:

  1. Turn down gain controls on the amp
  2. Turn the stereo’s volume to 2/3 of maximum
  3. Slowly raise the gain controls until the volume is sufficient

When finished you should have sufficient volume available from the stereo, but noise should be minimal. You’ll still need to tweak it a bit if volume is too high or too low.

Setting the crossover

As I mentioned at the beginning of this guide, using high-pass crossovers will allow more volume with less distortion and will help protect the speakers from heavy bass.

For both front and rear channels turn on the high-pass feature and, if an adjustable dial is available, set it near 50 to 60Hz. Some models don’t offer an adjustable frequency for the cutoff, but are likely preset to a good level.

Test and tweak

Once installed, test and tweak your amplifier as needed. A great way to mount your 4 channel amp is by using a board mounted to the car, covered with speaker box carpet or other material.

Play some music you’re very familiar with and adjust things like bass, treble, and the fader as needed. Using music you’re very familiar with (of high quality) means you’ll be able to notice any problems with the sound fairly easily.

If you don’t already have the option, you might consider later upgrading to a head unit with built-in equalizer (EQ) functions to help tailor the sound.

Summary and recommended products

Hopefully you’ve found this post helpful. Hooking up a 4 channel amp to your front and rear speakers takes some work and time, but it’s a great way to get sound you’ll love.

Considering buying an amplifier? You can find some great 4 channel amps (including the Alpine MRV-F300 pictured here) in my 4 channel amp buyer’s guide.

You’ll also need a good amp wiring kit – I’ve got a good amp kit buyer’s guide here.

If you find anything missing or have suggestions, just leave a comment below or send me a message!

Your comments are welcome!

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