How To Wire A 4 Channel Amp To 4 Speakers And A Sub: A Detailed Guide With Diagrams

4 channel amps are great and have a lot of flexibility for the most part. However, they’re intended for use with 4 speakers or 2 speakers and a sub – not both.

However, it is possible to wire a 4 channel amp to 4 speakers and a sub! I’ve put together some really detailed information to help make this as painless (and inexpensive) as possible.

Read on to find out how! There are plenty of detailed diagrams to guide you, too.

What you need to know first

Clip art image of a face thinking - Things to know content image

Let’s get a few things out of the way before we go any further. To drive 4 speakers and a sub with a single 4-channel amp you’ll have to make a few compromises.

I’ll explain here what I mean:

  • You’ll likely have to give up front-rear fader control ability as you’re giving up 2 channels for the rear speakers to drive a sub (2 front/2 rear).
  • Most but not all car amps today can handle speaker loads down to 2 ohms. If yours doesn’t, you’ll need a work-around I’ll show you. You may need a few extra parts.
  • Driving 4 speakers from 2 channels means the speakers are sharing the amp’s power, so you’ll have less power available to each of those.

That being said, don’t worry too much! You can still enjoy the music you love with a nice-sounding 4 speaker + subwoofer system.

What does the minimum impedance (Ohms) rating mean for an amp?

Image showing the minimum speaker Ohms rating for Rockville RXA-F1 4 channel amp as an example

Image showing a typical 4 channel amplifier’s minimum speaker load (Ohms) rating. Most car amps today have a minimum of 2 ohms per channel in standard (stereo) use and 4 ohms minimum when bridged. Never run an amplifier with a speaker load that’s lower than the rating! Your amp will run hot and possibly become damaged.

Today’s 4 channel (and other) car amplifiers have a minimum speaker load they can handle. The speaker impedance or “Ohms” rating of a speaker (also called the speaker load) is the resistance to the flow of electrical current that the amplifier sees at the speaker terminals.

Amplifiers are designed to handle a minimum speaker load. The rating shown on the amp or in the owner’s manual serves as a warning.

If you connect a speaker load below the amp’s minimum rating it can get hot and even become permanently damaged at some point.

I’ve seen this happen when people try to “get more power” by incorrectly wiring speakers to an amp. Don’t do it! Burning out your amp’s output stages is not a nice surprise.

Minimum speaker loads for wiring a 4 channel amp to 4 speakers and a subwoofer

Diagram showing series and parallel speaker Ohms calculation examples

Diagram showing how a car amplifier’s Ohm rating works with speakers. Speakers are usually connected in series or parallel (well, really, most often in parallel) which affects the total resistance an amp will see. That’s important because amps are designed for a certain minimum speaker load (Ohms rating).

As I mentioned above, most car amps today can handle a 2 ohm load per stereo channel (left & right channels) and 4 ohms minimum when bridged to drive an amplifier (called “mono”).

The real answer, however, is that it depends on your amp’s ratings. Always be sure to check to be sure. To keep it simple, I’ll summarize what will work for almost all systems & amplifiers you’ll come across.

The amp wiring systems I’ll cover here

In this post I’ll cover 3 types of systems as that should cover almost all amps you’ll find:

  1. 4 channel amps with a minimum speaker load of 2 ohms in stereo, 4 ohms bridged (for the subwoofer)
  2. 4 channel amps with a minimum speaker load of 2 ohms in either stereo or bridged
  3. 4 channel amps with a minimum speaker load of 4 ohms

#3 is less common but it’s one you’ll run across. Maybe you’ve got an older amp that’s been sitting around unused. If so, you’ll be glad to know there’s a work-around that I’ve come up with that will let you wire your 4 speakers up without damaging your amp.

4 Channel Amp Wiring Examples

Here are the speaker wiring and speaker (Ohms) loads possible for nearly all amps you’ll run across. I’ll describe 3 main system setups which I’ll cover in detail.

System 1: 2 speakers (parallel wiring) to each front channel = 2 Ohms x 2 + 1 subwoofer wired for 4 Ohms on the rear channels bridged for more power. This is the first and best choice for most modern 4 channel amps.

System 2: 2 speakers in parallel to each front channel = 2 Ohms x 2 + 1 subwoofer wired for 4 or 2 Ohms on the rear channels bridged for more power. 4 channel amps that handle 2 ohms bridged are less common but they are out there.

System 3: 2 speakers on each front channel (parallel wiring) wired with extra parts for 4 ohms per channel + 1 subwoofer wired for 4 ohms minimum. Because these kinds of amps can’t handle a 2 ohm load, it’s a bit harder and needs a different approach.

Testing speaker ohms with a multimeter

Image showing an example of how to test speaker ohms with a multimeter

Shown: An example of how to measure speaker impedance (Ohms) with a multimeter. It’s a great way to know for 100% sure what kind of speakers you’re dealing with to avoid problems with your 4 channel amp.

Car stereo and home speaker speakers are very similar except for the impedance rating they use. A speaker’s impedance value, measured in Ohms, is just the total measurement of electrical resistance the amp will see from the speaker’s voice coil.

Partly due to tradition in the electronics world and partly due to various other electrical reasons car stereos are commonly rated at 4 ohms and home stereo speakers around 8 ohms.

The good thing is that all you really need to know is roughly what the resistance of a speaker is. If you can measure that you can tell what Ohms rating to go by!

How to measure speaker Ohms with a multimeter

Example image showing how to use multimeter probes on a speaker

To measure the Ohms (resistance) of a speaker’s voice coil, hold the meter probes to the speaker terminals, making sure to keep firm contact to bare metal Paint, insulation, dirt, and solder flux can mess up your reading otherwise.

For example, we usually have a label on a speaker telling us if it’s 2 ohms, 4 ohms, and so forth. However, as crazy as it sounds, some speakers don’t!

That’s why it’s excellent to have a multimeter handy – you can find out 100% for sure.

Measuring speaker Ohms with a multimeter
  1. Set the multimeter to the Ohms setting. If the meter has an auto-ranging function you should be fine. Otherwise, set it to the lowest setting like the 10 Ohms or 200 Ohms range, etc.
  2. With the speaker disconnected, touch the speaker terminals with the meter probes. Be sure to touch bare metal on the terminals and make good contact.
  3. Read the measured value. The general range will tell you the Ohms rating of the speaker (Example: 3.6 ohms resistance would mean a 4 ohm speaker)

It’s important to be sure you’re not measuring across substances that can interfere with your measurement.

Things like the following can cause problems (I have seen this happen many times!):

  • Leftover solder flux or solder coating from manufacturing
  • Heavy oxidation
  • Heavy dirt, dusty, or other contaminants that build up over time
  • Paint or other coatings that don’t conduct electricity

If in doubt, you can rub them gently with a bit of sandpaper or even scratch the meter probes against the terminals to make better electrical contact.

Note: If a speaker is “blown” or burned out from abuse or physical damage to the voice coil you’ll never get a reading. That’s because for blown speakers the voice coil no longer has a complete electrical path you can measure.

Multimeters show an open circuit condition as “infinite” Ohms, which just means there’s no reading to be made.

Speakers don’t measure exactly 4 or 8 ohms!

Car and home speakers are rated by their general Ohms (impedance) rating. For example, 2, 4, and 8 ohm speakers are never measure exactly with those Ohm measurements.

That’s because each speaker’s design is a bit different from the next. The resistance you measure from a speaker is due to the voice coil’s resistance thanks to the long wire it’s made of.

Here’s an example chart to help you know what to expect when measuring speakers.

Speaker RatingTypical Measurements
2 Ohms1-1.8 Ohms
4 Ohms3.2-3.6 Ohms
8 Ohms6 Ohms or more
16 Ohms12 Ohms or more

As you can see, you won’t measure exactly 4 ohms for a 4 ohm speaker. It will be in the general range and close to its advertised rating, however.

What is “bridging” an amp? Why is this best for driving a subwoofer?

Diagram showing a 4 channel car amplifier bridged to 2 channels

Shown: Example of a 4-channel amp bridged to 2 channels.

Bridged mode (mono mode) is a built-in amplifier feature in which a “push-pull” set up is created: 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).

When this happens the result is that you’ll get substantially more power with them working together than you would with one channel alone.

Bridged mode is a flexible way to get more power from 2 channels (in this case the rear channels, for example). That’s especially important because subwoofers are big, heavy speakers that need more power than small speakers to produce the bass sound you want.

Diagram – How to wire a 4 channel amp to 4 speakers and a sub

Note: Most standard aftermarket car speakers are 4 ohms each so I’ll use that assumption for my diagram. Always be sure to check your speakers before you wire them to your amp to be sure they’re compatible.

(Or click on the image to enlarge & zoom)

Detailed diagram for how to wire a 4 channel amp to 4 speakers and a subwoofer

Using power resistors for harder installations (4 ohm min. amps)

Image showing examples of higher power resistors (4 ohms) for use with speakers

Shown: High power resistors that can be used with a 4 channel amp when adding more speakers. Using these will allow you to keep the total speaker load (speaker Ohms the amp sees) to a safe level so the amp won’t get damaged. They’re inexpensive and are easy to use.

Power resistors are larger versions of the resistors used in many electronic products. Unlike their smaller siblings, they’re designed to be able to handle a large amount of heat.

Because of this, they’re a great way to deal with more difficult speaker and amp systems. In example #3 I showed you earlier, it’s not possible to use two 4 ohm speakers in parallel with an amp that can’t handle 2 ohm loads.

We can use 1 large resistor for each speaker as a work-around for this.

This workaround won’t be necessary for most modern car amps. However, since a few amps out there can’t handle 2 ohm loads (especially older amps you might have that you’d like to use) I’ve included this just in case.

Diagram showing how to wire speakers with power resistors for 4 ohms total

In the case of a car amplifier that can’t go below 4 ohms per channel, there’s no other way. However, if we add one 4 ohm power resistor in series with each speaker we can use the math to our advantage.

By doing so, we’ll end up with two 8 ohms measurements in parallel which will give us a nice, safe 4 ohms per channel!

However, you’ll want to buy the right kind of resistor. I recommend at least 25 watt (25W) 4 ohm resistors. While you won’t find them in many local stores, the good news is that they’re affordable. I’ve used them many times for speaker problem-solving.

I’ve used some like these over at Amazon.

How to get around the lack of a front/rear fader

As I mentioned at the beginning, if you’re using a 4 channel amp to drive both 4 speakers and a subwoofer, you’re going to have to make compromises. There’s simply no way around it.

The biggest one that comes to mind is losing the front/rear fader control. However, I’ve come up with a sort of solution that helps a bit!

By adding inline resistors to the rear speakers you can drop their volume and it acts as a “fader” already. You can use an L-pad (speaker volume attenuation) site like this one here to calculate resistor values for you.

I’ve already done the work for you, and here’s a great example. If you’re wiring 4 speakers and a subwoofer to a 4 channel amp, by adding 60 ohm resistors to the rear speakers you’ll drop the volume by 24dB (decibels).

The result will be that the sound is “faded” to the front and the rear speakers won’t be too loud.

Image showing to how to create a fader to rear speakers using resistors

Following the example I’ve provided here in the diagram above it’s pretty easy. You’ll need to pick up some 15W-20W (or higher) resistors. I recommend about 60 Ohms as it will give a volume reduction of 24dB.

That should be enough in most cases: Not totally silent in the rear, but most of the volume towards the front speakers.

What crossover settings should I use on my 4 channel amp?

Close up image of a car amp crossover controls

As most modern car amplifiers include very nice optional crossovers, for good sound it’s smart to take advantage of those.

Here are some good recommendations for the 4 main speakers and the subwoofer:

SpeakerCrossover Settings
4 main speakersFlat (crossover off) or 56-60Hz high pass
Subwoofer80Hz low pass

By using a high-pass filter for the 4 main speakers you can crank the volume when you want for more power & sound before distortion happens. When using a subwoofer, it’s really important to block vocals and upper-frequency music from getting to your sub.

The idea is to get “pure” bass sound as that’s what subwoofers are best suited for: Just purely producing great-sounding bass you’ll love.

What if my amp can’t be bridged?

Image with diagram of how to bridge an amplifier

While it’s true that nearly all amps with any “real” power today can be bridged from 2 channels to 1 channel (called “mono”, or bridging) for more power, some can’t. That’s especially the case for some older “old school” amps you might have in your closet but still like to use.

In that case, unfortunately, you’re really limited. Sorry! You have just a few options:

    1. Buy another small amp just for the subwoofer/subwoofers
  1. Run one small subwoofer from each of the rear channels

The problem with option #2 is that subwoofers need a lot more power to drive them well and sound good. There’s just no way around it.

8″ subwoofers are an option or a dual 4 ohm voice coil subwoofer could be used. That way you can safely drive each channel with a 4 ohm load even older amps can handle. These days, though, it’s usually easier just to get a cheap budget amp and avoid the headaches.

How to connect a 2 ch. car stereo to a 4 channel amp

Diagram showing a 2 channel car stereo connected to a 4 channel amp

If your stereo doesn’t have output channels that’s ok! You can still connect a head unit car stereo with only 2 channels (left and right) to a 4 channel amp easily. In most cases, you only need 2 RCA Y adapter cables. The head unit’s left channel RCA jack is connected to the left front and left rear amp inputs. Then the same for the right channels. If using speaker level inputs on the amp, use the connections shown above. NOTE!: Use only ONE of the two connections shown. Never connect both the speaker level and RCA jacks at the same time – speaker level signals can damage your head unit’s RCA outputs.

If you’re wanting to install a 4 channel amp for 4 speakers and a subwoofer but only have 2 stereo channels available, that’s ok.

As shown in my diagram, you can connect 2 channels to a 4 channel amp using either the speaker level inputs wired in parallel or by using RCA “Y” adapter cables.

RCA y adapter cable image

All you need is a decent quality pair (2 total) 1 female to 2 male RCA “Y” adapters like these inexpensive ones from Amazon.

The sound quality will be exactly the same. Modern car amps are designed in a way such that all 4 inputs will get the same signal and there won’t be any problems.

After connecting the stereo to the amp, you’ll need to adjust the gain controls for both the 4 main speakers and the subwoofer output. My advice is to use a high-quality music track you know well already so you can easily tell when music sounds correct.

Additional suggestions & parts you may need

An amp wiring kit like this one will make your life easier! A a great (but affordable) amp wiring kit like this Belva 8-gauge complete kit includes not just the basics but a lot more. You’ll also need to pick up a 2nd pair of RCA cables (if using them) and maybe some extra speaker wire, too.

I can’t stress enough how important it is to be ready to install your 4 channel amp. While there are several ways you can go about it, wasting gas, time, and getting stressed out isn’t worth the headache.

It’s a lot better to start on the right foot and be ready at installation time. My advice is to pick up a good amp wiring kit and the tools you need beforehand.

I’ve also got some great guides ready, too, if you’d like more ideas:

Need advice on finding the right installation wiring? Check out my post with some of the best wiring kits for the money here. You’ll not just save a few dollars but avoid getting ripped off on fake wire gauges.

Comments, questions, or more? Let me know!

I’d love to hear from you and make this post even more helpful. Let me know in the comments below if you have questions or comments.

You can also reach me here via my contact page.

Your comments are welcome!

  1. I understood everything until I got to the bottom when connecting the head unit. My head unit has 4 RCA out puts. L & R Rear and 2 for the sub. So I wired the front 2 speakers off the head unit, then I used the RCA Rears to wire the 4 6×9 speakers as you described in parallel using channel 1 & 2 and bridged the sub to channels 3 & 4 like described. Just not sure about the RCA connection from the head unit to the amp. I would guess one RCA to channel 3 and one RCA to channel 4 then from sub woofer positive speaker wire to channel 3 and negative speaker wire to channel 4 on amp? My head unit is a sony cdx-1200u if that helps. I am afraid to turn in on so not to fry anything.

    • Hi Barry. You would do the following:

      – Connect the rear channel RCAs from the Sony to the amp front or rear channels just as you mentioned. I usually use ch. 1 & 2 on the amp for that.
      – Connect the 2 subwoofer RCAs from the Sony to the amp, let’s say ch. 3 & 4.
      – The connections for the bridged subwoofer wiring depend on the amp. Since you didn’t specify the amp brand & model, I can’t say for sure.

      However, usually the bridged wiring is labeled on the amp itself. Otherwise, you could hopefully find the owner’s manual online. If that’s not available, yes I would try left channel – = subwoofer negative, and right channel + = subwoofer positive.

      Hopefully this helps. Thanks!

  2. Hi Marty,

    So Im planning on upgrading my 2008 CIvic SI stock system. Ive bought the KenWood (Dpx303mbt) good price has 6 channels But The Rms is at 22watts like most decks. However, this will still boost my stock speakers. So i originally had the 4 Channel amp in the car the Clarion XR2420 Running just the extra Sub woofer in the trunk. [..]

    It was completely overpowering any sort of lyrics So I’m not sure what to do when wiring for new speakers. Should I just keep the Factory Sub wired to the Harness when i’m wiring the aftermarket harness together?. I dont think i can add that to your system 1?.

    Furthermore, i haven’t Bought speakers for the front or rear yet. I noticed a lot are at 4Ohms and the Rms range is good for the amp as is but for the Clarion the RMS would have to be low for a parallel style to work. I’m good at wiring and Soldering. but i am lost when its coming to wiring the speakers to this amp. [..] . If you can guide me to a system setup where i can listen to clear music with strong clean bass would be greatly appreciated .

    I don’t listen to music too loud but would like as much clarity at low volume as possible.

    Kind Regards,

    • Hi Jordan. I edited your comment down to the essentials as it was very long.

      Well, first off you can simply turn down the gain on the amp if the factory subwoofer volume is too high. Additionally, I see that the Kenwood head unit has subwoofer outputs so you may be able to turn that signal down as well. (I’m assuming you’re using the low-pass subwoofer output from the Kenwood but I’m not sure). Definitely at least use the low-pass crossover in the Clarion amp if you haven’t already, by the way.

      For your system questions based on what your wrote, here’s what I think:

      – You’re better off not using the factory subwoofer at all. You’d be better off using even a budget 10″ or 12″ subwoofer in a sealed or ported box than using the factory one. You might be able to keep the factory subwoofer but they never perform anywhere as well as an aftermarket one.

      Ideally you’d buy one already matched to the enclosure.

      – If you absolutely must use all 4 speakers and drive everything from only 1 amp, you’ll have to give up the front/rear fader control as I mentioned in my article. So the system #1 example is still the best way to do it in that case.

      If it were me, however, I’d just leave off the rear speakers as you’ll probably find them to be overbearing without a fader control.

      – There’s another option: Use the front and rear channels for the 4 front and rear speakers, then pick up an affordable powered subwoofer. You can find one for around $100-$120 or so, and they’re pretty good. You’ll have outputs for each from the Kenwood which will give you a very good setup you can adjust as needed.

      Here’s an example of a good budget powered subwoofer: Rockville RVB12.1A 12″ powered subwoofer.

      Here’s a suggestion to start with regarding the front & rear speakers: As the Civic is supposed to use 6.5″ speakers, here’s a 4-speaker set of Polk DB651 coaxial speakers with silk composite tweeters that will give very nice sound.

      I recommend avoiding speakers with mylar/plastic dome tweeters as the sound just isn’t as nice as those with a better tweeter. Plus you’ll often pay close to the same for the better ones, too.

      I hope this helps! :)

    • Hi George! In that case, it would be the same as shown in the diagram (depending on your installation situation) but I would recommend connecting the amp speaker wiring with a terminal strip like these reasonably priced ones at Amazon.

      That will make it easier to connect multiple speakers if you need to. It’s easier than using crimp (“butt”) connectors or other methods, plus you can change it later if you need to without any problems. :)

      • Question when installing a 4 channel amp are u connecting the amp to the speakers at the door or to the harness that plugs in the radio

        • Hi Marcus. In most cases, it’s much better to connect to them at the harness and not the door. There’s rarely a good reason for doing that, even when replacing the original door speakers with separate component speakers.

          Getting wiring through the door is often a terrible job and in my experience the factory speaker wiring going to the doors is good enough.

  3. Hi Marty,

    My car Audio configuration is as below:
    – Kenwood HU with 2 P/O
    – Infinity Components speakers (front)
    – JBL Co-axial (rear)
    – Infinity Dual-coil Subwoofer
    – Audior RIP 4 Channel Amplifier.

    After adding the sub to the system I’m missing Left to left sound and Right to right sound effect. I think entire right channel is dedicated for SUB and entire Left channel is dedicated to the 4 speakers.

    What would be the best combination to get best sound output and “left to left/right to right” sound effect?

    Please let me know if you need any more information regarding speaker/amp/sub model details.

    Thanks and regards,

    • Hi Saptarshi. You would use system example #1 in the diagram above.

      If you’re having problems with the left to right stereo signals, then it sounds like either the RCA inputs or speaker connections (or both) are incorrect.

      By “2 P/O” I assume you mean 2 RCA outputs, since you didn’t list the model number. Also the specific Auditor model number is not listed either so I’ll go by the standard suggestion here:

      1. Kenwood head unit to amp: 2 RCA jacks to 2 Y adapters at the amp: L channel to ch. 1 & channel 4. R channel to ch. 2 & ch. 4 inputs.

      2. Subwoofer bridged to channels 3 & 4 (assuming these channels have a low-pass crossover)

      3. Left ront & rear speakers in parallel to the ch. 1 output. Likewise for the right channel speakers on ch. 2.

      That should take care of it. Have a good day. :)

  4. Thank you for the reply Marty. ?

    Yes, 2 P/O is 2 RCA I meant.

    Amplifier model: Auditor RIP 4280
    (Focal(dot)com doesn’t have the manual now, if you need, please check from manualzz(dot)com – 4x90W RMS is the tagline)
    HU Model: Kenwood 2DIN DPX-U5120

    You said ch.5, but I do not have 5 channel ☹️, let me know if I fail to understand it correctly.

    And also I’m getting less sound o/p from Components than Co-axial. Too much sound is coming from Co-axial.

    Also one question, can Co-axial and Subs share power from amp(I was thinking to have 4 speakers on 4 channels and rear Co-axial to share power with sub. I don’t know if it is really possible.)?


    • Hello again, Saptarshi. When I wrote “ch. 5” that was a typo (error). I’ve corrected it, sorry.

      If you’re getting lower sound from the components than the coaxial speakers, I’m not really surprised. Likely they need more power to drive them than the coaxials, that’s why. The coaxial speakers probably have a higher efficiency (decibels per watt) than the components.

      You have to decide which speakers you’d like to use, as you’re trying to drive too many speakers from one amp. There aren’t enough channels to have both the proper power & channels you need and drive a subwoofer. The best thing to do in my opinion is just disconnect the coaxial speakers.

      Otherwise, add a 2nd amp just for the subwoofer and use the Focal 4 channel amp to drive the speakers with a left/right, front/rear set up.

  5. Hi Marty,
    I have a 2 channel head unit without rca output. Also, I have a 4 channel amp and a sub and i want to have 2 front speakers and i sub. Can i connect factory unit with amp?

    • Hi John. Yes you sure can! I’m not sure which amp you have but here’s how:

      1. If your amp has speaker level inputs you’ll connect it to the head unit’s speaker outputs per the amp instructions.
      2. Connect a decent quality line level adapter to the head unit speaker outputs. From that, you’ll go to the amp with RCA cables (2 channel) where you’ll use a pair of RCA Y adapters to go to the 4 channel inputs.

      Left channel to front left, rear left. Right channel to front right, rear right.

      Then bridge the rear channels to drive the subwoofer.

      • Dear Marty thank you for your reply. My amp is Magnat 560 4 channel. With the second method do i have stereo sound? Also with which way can i connect REM of amplifier to stereo unit?

        • Yep, you’ll have stereo sound as long as it’s connected correctly.

          For factory systems I use one of several things, depending on what’s more time-efficient & reliable:

          1. Use a fuse tap adapter to connect a +12V if the fusebox is fairly easy to access.

          2. Tap off of a +12V accessory/switched wire behind the head unit or cigarette lighter socket (if switched with the ignition).

          3. Some factory systems make it hard to find a +12V standard wire. Those may need a data line-to-remote wire adapter. (Usually though this isn’t necessary)

          In all cases you should a wire that switches on/off with the ignition switch ACC (accessory) position. You don’t need a large wire as a remote signal uses less than 0.5 amps.

      • I have factory 2 ch. car stereo (2017frontier) to a 4 channel amp. My amp is pioneer GM-A4704, if I use for speaker level connections as your diagram. How do I connect sub? Do I use B channel outputs, 2 channel bridged?

        • Hi Stu. For your amp you can just set the 2/4 channel amp switch to the “2” position, then bridge 2 channels and set the low-pass crossover. Connect the ch. 1 & ch. 2 speaker-level inputs.

          Most amps would need to be connected to a signal for the rear channels also but since yours has a 2/4 ch. input switch that shouldn’t be needed. You’ll need to bridge 2 channels in order to have enough power to drive a subwoofer well.

  6. I just got a Pioneer head unit and a 2 channel power amp 400.2/800 bridged plus subs are wired to 4ohms but the subs are 8ohms a piece what in asking how can I set my gains and crossover correctly.

    • Hi Stephen. Actually 8 ohm speakers aren’t a good idea, as you’ll never develop full power to them vs using 4 or 2 ohm versions. It won’t hurt anything but 8 ohms subwoofers are best suited for home amps & not car amps.

      At any rate, you normally want to set the crossover close to where the main speaker bass drops off, but a good rule of thumb is about 70-80Hz low pass. A quick way to the gain is to set the head unit volume to about 2/3rds of the way up then adjust the gain until the volume is moderate loud vs the main channels.

      There are better ways to do it but it involves using a test signal and checking amp levels etc, so that’s a bit more complicated.

  7. Hi Marty, I have a kenwood DNN990 HD with 3 out rca, 1st- front 2nd- rear & 3rd- SW. also I have a DS-18 gen 1200.4 AMP. How can I connect radio to amp with rca? And then how I connect speakers and 1 sub? Thanks.

    • Hello Manny. You would use the setup shown in system #1 in my diagram. Connect the from RCA outputs to the front amp channels then the subwoofer output RCAs to the rear channels.

      Then bridge the rear channels and use the low pass crossover.

  8. Hey Marty!
    What is an ideal power resistor for a 4 speaker of375w connected to an amplifier of channel of1000w with a subwoofer of 1800w many watts resistor should I go for and what resistance??

    • Hello, Marwin! You should be fine with 50W rated resistors for most 4 channel amps. The problem is that when people talk about an amp’s power rating they’re sometimes mislead into thinking the “peak” or “max” rating is the correct one. (For example “1000W” may actually be only about 250-300W or etc).

      The RMS rating is what you should go by. For typical amps that’s around 50W-100W/channel for a 4 channel amp. As a general rule, though, divide the RMS power of each channel by 2 and use that as minimum resistor power rating. If you can send me the amp model number I can check.

      For 4 ohm speakers, use a 4 ohm resistor. As I explained in the article, that allows us to keep the speaker load at 4 ohms total when using 2 speakers in parallel.

      You can find power resistors at Amazon & eBay. They’re not expensive if you shop around.

  9. Hi Marty, my radio has rear and front channels (4 in total).
    I would like to buy a 5 channel amplifier, to get sound to 4 speakers and one subwoofer.
    How would the connection radio to amplifier be if the radio doesn’t have a channel output for the sub?
    Would I need to have a RCA y from the radio to the amplifier?

    • Hello there, Nuno! 5 channel amps nearly always have a provision to get the bass (subwoofer) signal from the main channels if you don’t have a separate subwoofer output on the radio. So don’t worry you should be ok.

      You’ll need at least one pair of RCA cables, or 4 if using a front & rear fader if the radio has 4 outputs. (Assuming you have RCA outputs and you’re not using speaker-level inputs on the amp, which is another option you have too).

    • Marty, no need to respond. Just letting you know, I went with the 5-channel Pioneer GM-D9705. Your write ups are a lot different than others. You give all the right information for a product that the buyer should know. Most of the stuff online is either not enough or just plain BS that a seller would write. Thank you, I’ll definitely spread the word about your site on my forums.

  10. Hi. First I want to say, thank you for this site! I replaced my factory HU and all 6 speakers, 2 pillar tweeters, 2 front door, 2 rear deck speakers. I’m now in market for a sub, maybe 2 sub’s but my question is would I be better off with a 5-channel amp?

    • Hi Rob and thanks for dropping by! All in all, yes, if you get a good one a 5 channel amp is a great way to power your whole system (plus it’s easier than installing 2 separate amps).

      I would recommend a 5-channel amp with at least 50W RMS per channel for the main speakers and 250W+ for the subwoofer channel, and 2 ohm capable too. Most good 5 channel amps today meet or exceed this, though.

      The Pioneer GM-D9605/GM-D9705 I reviewed here is a great example.

  11. Thank you Marty! One more question. Let’s say I left my 2 tweeters hooked up to run off my HU, that would leave my 4 speakers + 1 sub.
    Would a 4 channel amp be loud enough & clear enough for the average person?
    Thanks again!

    • Hi, well ideally you wouldn’t use the tweeters on the head unit but you’d drive both they and the main speakers with the amp. Otherwise, there’s a good chance the amp would have to be turned down to match the head unit’s more limited output. But you can always try it and see how it works for you.

      Well, you’d need a decent amp (and a 2 ohm stable one) to run all 4 speakers and a sub off on just one. It can be done, though, yes.

      If you can, 75W & up per channel would be better. Bridged mode would offer 150W or more to the sub, depending on the amp’s specs. That’s enough for decent bass depending on the subwoofer, box, and vehicle you have.

  12. Hi Marty,
    I have a Pioneer GM D8604 amp, a pair of 6×9’s TS 6965v3 (450w peak) and a pair of TS S20 Pioneer Tweeters (200w peak). How actually can i install them so that i can get loud and also best sound quality.

    (I was thinking of parallel a speaker with a tweeter and hook them in bridged mode, but i am unsure whether the wattage will be too high for the tweeters). Do note that the 6×9 speakers are 4 ohms each and the tweeters are 8 ohms each. Hope to hear from you soon. Thanks.

    • Hello, Shubby. Your situation is a bit different than most because of the 8 ohm tweeters which won’t match your 6x9s.

      Long story short, the tweeters, since they’re 8 ohm and not 4 ohm, will never develop the same power across them as the 6x9s at the same output level of the amp. So once you start cranking up the volume the tweeter output level won’t match that of the other speakers unfortunately.

      I recall seeing the TS S20s before and wondering why they used an 8 ohm driver – that’s odd.

      Anyway, in this case there’s no “perfect” solution, unfortunately. For everyday listening it will probably be fine, but if you’re wanting the best musical clarity especially at higher volumes you would do best to use 4 ohm tweeters instead.

      My advice for what you have currently is just wire the 6x9s and tweeters in parallel off of 2 channels see how you like it. Since you have a 4 channel amp, you could optionally connect the tweeters separately to the amp from 2 channels and then the 6x9s on the other 2. This way you can adjust the gain/outputs separately to have them match in terms of volume.

      The problem with that, however, is you’re giving up 2 channels that could be used later for a subwoofer or other speakers.

      The GM-D8604 has plenty of power so there’s no need for bridging. Personally I would recommend spending $50 or so for some good quality silk or metal dome 4 ohm tweeters instead of using the TS S20s.

      (Note: Go by the RMS or “nominal” power ratings for the speakers, not the “peak” rating to know their continuous power ratings. That’s 80W for the 6x9s and ~50W for the tweeters, according to the specs).

  13. Hi Marty, great write up. Question about connecting marine Fusion MS-RA70N zoned head unit to Fusion FM-504 4 channel fully regulated 2 ohm stable amp.

    Plan is to use Setup#1 as shown with (4)Alpine SPS-M601W 4 ohm speakers in parallel. Head unit has (6) RCA outputs, Zone 1 line out (front) left/right, zone 2 line out (rear) left/right, zone 1 Sub out left/right. What is best way to connect RCAs to Amp?

    Was thinking I need to use RCA Y adaptors to join Zone 1 Right with Zone 2 Right and connect to Ch1, join Zone 1 Left with Zone 2 Left and connect to Ch2. Basically paralleling the RCA inputs like speaker output. Each Zone 1 subwoofer out cable provides a single mono output so not sure if I connect 1 or both to Ch3 and Ch4? Thanks (Sub is Fusion MS-SW10)

    • Hi Steve. If you’re using a 4 channel amp like in setup #1 as you said, you’ll just ignore the Zone 2 RCA outputs. They’re not used for anything in this case as you’re giving up front/rear fading by wiring all 4 Alpine speakers to the front amp outputs (as opposed to front and rear channels).

      However, should you add another amp later, according to the Fusion manual you can link Zones 1 & 2 so they’re controlled by the same volume adjustment. That’s something useful to keep in mind.

      In the case of the sub RCAs, you should only need to connect one to the amp, although since you’re probably running a pair there too you can connect both. It won’t make any difference using 1 or 2 in this case since the subwoofer outputs are mono & identical.

      Thanks for stopping by!

      • Awesome, thanks. Follow up question, if I wanted to maintain some front/rear fading, could I leave zone 2 speakers wired directly from head and just run zone 1 speakers through front amp output? Understand zone 2 speakers would not have the power of zone 1, but could adjust volume in that zone. Sub would still be bridged through rear amp channel and head manual says sub volume is controlled through zone 1. Thanks for the great advice.

        • Hi Steve, yes you can do that if you like without any issues, aside from the volume which you’re already aware of.

          You’re welcome. :)

        • Initially connected amp to 1 zone and sub leaving zone 2 speakers connected to head unit. After playing, decided to go ahead and connect all 4 speakers to zone 1 leaving zone 2 available for either another amp or future move to a 5 channel amp. Sounds great and I’ll work with crossover High and Low filters to refine. Thanks for your help.

  14. Hi. I’m from Malaysia. My English is not so good. I have an active subwoofer Pioneer Ts-WX120a. I want to connect it to the amp Kenwood KAC PS404. The amp is 4 channel. The amp output to FL, FR, RL, RR. Can I connect to RL and RR. The subwoofer have rca red and white. It included rca to line input cable converter. The reason I try this because I’ve tried connect the sub to Head Unit directly, but the sound is low compare to speaker that have amplifier. Actually I’ve tried connect it to the amp.And it work. But I want to know if it will damage the amp or the active subwoofer.

    • Hello Azizi. In this case, you should be able to connect the line-level adapter to the Kenwood amp’s speaker outputs to get a signal if you like.

      Usually that will not cause a problem (at least it shouldn’t at regular listening levels). The Pioneer’s internal circuitry scales down the speaker level signals to line level.

  15. Marty, I have been searching all over the internet for help with my amplifier in my boat. Your article is the first that has helped an amateur like me start to understand this stuff…so thank you so much for that.

    I’m replacing a 6-channel amplifier with a new 6-channel amplifier. Here is my question:

    To channel 1, I’ve attached the lower left side speakers
    To channel 2, I’ve attached the lower right side speakers
    To channel 3, I’ve attached the upper left side speakers
    To channel 4, I’ve attached the upper right side speakers
    I’ve used channels 5 and 6 to bride the speaker wires coming from the subwoofer.

    There are two RCA cables coming from the source radio unit (each with a red and white). I’m not sure where to plug those into the RCA ports. There are 6 RCA ports. I’ve tried every arrangement/combination and the sub doesn’t work.

    Upon thinking it through, it seems there is no input signal for the sub, but it worked with the previous amplifier and I’m almost certain I’ve set it up the same way as the old amplifier was connected. I’m completely stumped.

    • Hi Cody & I’m glad to hear that you found my info helpful as I really do work hard on my articles.

      In this case it would help a lot if I knew the model number of your amplifier so I can look into it. Generally speaking, though, if there aren’t dedicated subwoofer input RCA jacks then it is derived from one (or more) of the input pairs. It’s hard to say more without more info in this case.

      Also, be 100% sure you’ve got it bridged correctly just to remove any doubt there too.

  16. Thanks for the quick reply. The amplifier is Pyle PLMRA630BT.

    It has always been confusing to me that there aren’t dedicated RCA inputs coming from the radio unit for the subwoofer, but because I’m a novice when it comes to audio, and because it’s always worked, I’ve just taken it for granted.

    Upon further inspection of the radio unit, I see 4 sets of RCA coming out. Two sets lead over to the amplifier (the ones I mentioned previously). The 3rd goes toward the front of the boat away from the amplifier. Not sure where it leads….perhaps to the controls on the driver cockpit area. The 4th set is capped with little rubber caps. I know those were not used previously but should I try connecting those to the amp as well?

    I guess the amplifier does not work like a receiver, correct?….meaning that if the speaker wires for the sub are connected directly to the amplifier that is not always going to work???

    Does it work to test the ohms of the sub with a multimeter as you described in the article or is that a different beast? Perhaps the sub is blown. I would think not, but I’m not sure.

    Sorry to jump around like that with the questions. Just trying to figure this out. Seems like it shouldn’t be that complicated…but I’m stumped.

    • Hi Cody. Unfortunately the amp manual isn’t helpful.

      – This is probably an issue with not having a signal to the last 2 channels (5 & 6) where you would have the subwoofer bridged
      – I would try connecting the additional 2 RCAs at the amp to channels 3 & 4 and set the switch to 1/2/3/4. Hopefully the last 2 channels will derive a signal from the 4 main inputs, but it’s a guess since the info is very poor.
      – The extra RCAs in the boat may be AUX inputs, but I can only guess without seeing it myself.
      – The subwoofer is *probably* fine, but it’s always a good idea to play music on a speaker to be 100% sure it’s working to eliminate that as a possibility.
      – If channels 5/6 do not have sound with 1-4 connected, you may need to get some “Y” RCA adapters to drive a signal to the additional channels.

      So my advice is to try connecting channels 1-4, set the switch as mentioned, and see the results. If not, you’ll need to try connecting the other channels as I mentioned. Unfortunately there’s some trial and error involved since the information is poor.

      Hopefully this helps!

  17. OH. MY. GOSH!!!!!! You figured it out!!! I would have NEVER tried that. Works like a charm now. You just saved me so much heartache, time spent, and money. Thank you, thank you, thank you.

    How can I repay you? Do you have a Facebook page or somewhere I can leave a review? Can I give you good press somehow? Gift card? You name it. I am SO grateful.

    • That’s awesome, Cody, & thanks for letting me know! You don’t have to repay me, but I do appreciate that offer.

      Maybe just pass my website along to anyone else who might find it helpful. :)

      Enjoy your music!

  18. Hi Marty,
    Very informative description for the 4 channel amp and bridged sub. I have a similar question but with a two channel amplifier. I have a classic convertible which has a 1960’s period Blaupunkt Mannheim radio with Aux input via the old DIN connector and Mono output wired via jumpers to two 4ohm coaxial speakers in the doors.

    Love the period look but it’s almost inaudible after 40mph due to the road and wind noise so need more volume. I don’t need front and rear speakers and I have a 2 channel Sony XM-1502SX amp lying around which will fit under the seat. I would like to use this to run 2 hidden 75W RMS component speakers with separate tweeters (including crossovers) from one channel and then run a passive sub in the trunk from the other channel. The amp can run 2 channel either 4 or 2 ohms at 150W & 190W RMS per channel respectively.

    Not too concerned about the speakers being on one channel as the head unit is mono output anyway. Any ideas how I might do this?

    Many Thanks

    • Hi Andy. My best advice would be to forget about using the original radio for several reasons. Sound quality will be poor compared to even a budget head unit these days. Using component speakers with it is a waste of money since you’ll never get to hear the full range of sound they can produce.

      Also, it doesn’t really make sense given you have other options.

      What I would normally do with an older vehicle is one of two things: 1. Replace the original radio with an aftermarket one (this is harder and may require cutting the dash), or 2. use an underdash kit like this one from Metra to allow you to add an aftermarket one fairly easily.

      You’ll get far better sound & volume that way for your effort. Assuming you do that, you might consider either replacing the Sony amp with a 4 channel class D as they have great power & sound in a compact size.

      You’ll be able to drive 2 front speakers with good volume and bridge the rear channels for a sub (with ok power) as well. Or add a small mono amp just for the sub if you have room.

      With the underdash kit, you can remove the stereo if you sell your car later.

      Hopefully this helps!

  19. Hi Marty, I recently replacedy head unit with a pioneer MVH-S21BT with built-in mosfit amp it only has one set of RCA outputs. I also replaced the 4 door speakers with Pioneer TS-A652F 6.5 3 way speakers, and a kicker pt250 BassStation 10″ powered sub. I have the sub wired in with the rest speakers and RCA y adapter splitting the 1 output from the head unit plugged into a 1000 watt Jensen xda94rb 4 channel amp
    My problem or question is, why don’t my new speakers alund near as good as the factory speakers that came in my Tahoe?

    • Hi Ryan. It’s a little bit tough since I can’t hear your truck myself, so I’ll do the best I can. Also I couldn’t find much information about the Pioneer MVH-S21BT. It doesn’t look like it has an EQ built-in, which would have been helpful.

      So, here are my thoughts:
      – You said you’ve got a 4 channel amp but I assume you’re using it with the pair of RCAs from the Pioneer? You might consider using a 4 channel RCA to line-level converter to get front/rear fader control
      – I don’t know what year model & trim level your Tahoe is, does it have dashboard factory tweeters? If so you’ll probably want to connect those or replace with aftermarket ones & add them too.
      – For sure, make sure all the speaker connections are wired the same way (each speakers positive and negative connections are the same from the amp) or you can have cancellation which hurts the sound a lot.
      – How’s the bass from the Kicker PT250?

      It would help to know what the difference is you’re hearing between what you have now and the factory speakers. Thanks.

  20. Hi Marty,

    I wonder how I can connect the front speakers together with their tweeters (wich an in-line crossover in their wire). Can I connect the tweeters directly as shown in “System 1” together with the front and rear speaker so that there are basically three wires connected to the output?

    Thanks in advance!

    • Hello Andi. If you’re not using 2-way crossovers for both the tweeters and the other speakers (but instead, a single one to the tweeters only) then it’s a different situation. I would recommend only using 1 tweeter in parallel with 1 speaker in that case.

      Note that system #1 shows using a 2 ohm capable amplifier. If you wire 2 speakers and the tweeters at the same time it will end up with less than 2 ohms total speaker load, which isn’t ok.

      Personally I would recommend sticking with 1 speaker + 1 tweeter (or 2 speakers instead) unless you use 2-way crossovers. Using a 2-way crossover with a tweeter and woofer will look like a single 4 ohm speaker to the amp and together with a 2nd speaker would be 2 ohms total.

      • Hi Marty, thanks for your kind and quick reply! I now connected the front speakers and tweeters to a 2-way-crossover. My final question would be if I now need/can remove the in-line-crossover of the tweeters, since I now, without removing them, I hear some noise from them. Thanks again!

    • Hi Eben. No, in most cases you can’t as monoblock amps usually only produce a low-pass (bass) output. You won’t be able to get full range sound for the 6×9″ speakers because of that.

      Really the best thing to do would use a 4 channel amp and bridge the rear channels for the 12″ or add a second amp if you already have the monoblock amp.

  21. Hi Marty,
    I’ve got a 70 Mustang, & will be leaving the original radio in its place to keep it all looking original, I’m looking to put a aftermarket sound system in, a freind has a Rockford Fosgate P400-4 punch I can have, I was hoping to run a sub in the trunk & replace to front speakers with decent ones, can this amp be installed & possible adding a bluetooth device while using my iPhone as the head player, this way I’m avoiding installing any head unit, or would I need to buy a different Bluetooth amp, any suggestions or hep in which is best way to go would be greatly appreciated
    Regards Neil

    • Hi Neil. Yes you can use that amp and use a Bluetooth receiver, but the ones that will work ok for car use are generally not that great sounding. An amp with built-in Bluetooth receiver will be better (Rockville and some other brands have some good ones that can run your whole system).

      However, neither of those options are the best. I’ve installed aftermarket radios in muscle cars including 1960s Mustangs, Chargers, and so on. The best thing to do for most of my customers was to use an under-dash kit like this Metra 99-9000 kit I use for installations. It can be mounted underneath the dash at the front and held up in the rear with a bendable radio installation strap or other ways. (You can also get a double DIN version of the kit and use ANY head unit.)

      They work great and look good. You’ll get a LOT more for your money using a good head unit rather than an amp with Bluetooth built in. Today’s head units offer Bluetooth, music app controls, EQ functions, and much, much more for under $100. It’ll also be less hassle to deal with than using only a phone & amp.

      Bluetooth amps are better for motorcycles, boats, and so on where you don’t have as many options.

      • Marty thats great feedback thanks, so would you suggest get the Rockford Fosgate P400 with the under dash head unit like you mentioned, or start again, im looking to have a decent sound system (like everyone else lol) so any recommendations on what head unit, front speakers & sub or even the lot as a package deal would be much appreciated, will purchase the Metra 99 you mentioned above thanks heaps

        • Hi Neil – yes if the Rockford works fine I would keep it. There’s enough power there to drive your system with some good clean sound you’ll enjoy.

          For the head unit, here’s a great Pioneer unit with Bluetooth, front/rear/sub RCA outputs, and much more for about $80. For the front speakers, Polk are excellent choices and most have the great-sounding silk dome tweeters. Pioneer, Alpine, and Kicker also have some very good affordably priced coaxial speakers that are good, too.

          You can save money on the subwoofer and still have excellent bass, so have a look at the Skar or Rockville subwoofers. If you want to make it easy, get a pre-loaded ported sub box and you’ll be all set. If you want a more specific recommendation let me know. One 12″ decent subwoofer in a ported box will jam pretty hard. :)

          Unfortunately I don’t (currently) know of an entire system package to recommend. Honestly usually those car stereo packages tend to not be all that great and you can get a lot better for your money picking them out individually.

  22. awesome Marty, btw I’m in Oz & just clicked link for pioneer on amazon, that item they don’t ship down under :( – but Im sure I can source it locally, or other ones similar, I know this might sound silly sorry as I’m not upto date with the tech side of audio, with the hidden head unit being bluetooth does this still mean the operation is all controlled by iPhone/App or remote that comes with head unit if supplied?

    I might pass on mates Rockford as its 6yrs old & its NOT free lol, I would only be installing front speakers in doors can’t fit any in rear, thats why I was looking for best option for trunk whether it be bigger speakers or Sub, if you could also send some links as to speakers or or Sub that’ll be awesome, keep up the awesome work Marty doing a great Job Mate.

    • Ah ok I didn’t know you weren’t in the USA. My mistake.

      For the Bluetooth question(s), really I would say you should read the owner’s manual of which ever model you’re thinking to buy. Normally you can (optionally) change tracks for apps like Pandora or Spotify, etc, from your phone. But it depends on the model of the Bluetooth device and what it offers. The volume and other adjustments can be often be made from the phone or the head unit, but it depends on the unit and what media source you’re using too.

      Ok, these may not be available where you are, but they’re good examples of what I’m referring to:
      Main speakers (I’ll pick 5.25″ as an example as you didn’t specify):
      Pre-loaded subwoofer enclosure: 12″ ported subwoofer enclosure example (under $100 USD)
      5.25″ coaxial speakers, silk dome: Polk here and Alpine here

  23. Hi Marty,
    I replaced all 6 speakers in my 2017 Subaru Outback with coaxial aftermarket speakers rated at 4 ohms each. I’m still using the factory head unit (Fujitsu Ten, no Harmon Kardon). If I add a Pioneer GM-D1004 4 channel mini-amp to get a little oomph , then how would I connect everything? The little amp can’t run below 4 ohms. Should I just delete the rear door speakers? They don’t add much excitement to the system. I’ve also ordered a Rockville SS8P 8″ self powered sub to add to the mix but it won’t need power from the Pioneer amp. Many thanks for your thoughts on this. I’m 72 and car stereos have become more complicated as my brain ages.

    • Hi Bill. The GM-D1004 is rated for down to 2 ohms in stereo mode, actually, but that shouldn’t matter in this case. You can use the speaker-level inputs to go from the factory head unit to the Pioneer amp, then out of the amp and back to the factory speaker wiring near the radio. It would essentially be wired in between the speaker out of the radio and the factory speaker wiring.

      You would wire it according to the factory wiring colors (you can usually find these online) and match them to the amp’s speaker wiring labels.

      Since it’s so compact you can very likely install in or around the dashboard. That model doesn’t need a remote-on wire usually as it has auto-sensing for the speaker inputs. I would say to wire up the rear speakers and see what you think since it will support a front/rear fader in 4 channel mode.

      You can always change it later if you’d like.

  24. Hey great write up! I really appreciate finding this information here. My current setup is Sony XAV-AX5500, Focal Integration ISS 165, Focal RCX-165, and 1 Pioneer TS-SW2002D2 subwoofer. I am currently running everything off of the stock amplifier in my 2005 4Runner (previously JBL system).

    I may replace the factory amp with the JL Audio JD400/4. Run the speakers in parallel on channel 1&2 and bridge channel 3&4 to power the sub. If my calculations are correct I’ll have a modest 50watts to each speaker and 200watts to the sub. Good enough for me.

    I’m a little confused about the RCAs still. The sub is obvious enough. Sub output on headunit goes to channel 3&4 on amp. Do I then just connect RCA for the front to channel 1&2? What if I did the rear to channel 1&2? Would there be any difference? Thanks!

    • Hello there & thanks for the feedback, Joe. Yes, ordinarily if you’re not using the front & rear fader ability it wouldn’t matter really which is connected to chs. 1 & 2 or 3 & 4 with that amp. Adjusting the fader would affect the volume to the main speakers vs the subwoofer but doesn’t serve any purpose in that case so you’d leave the fader set to the middle.

      However, your Sony head unit offers a dedicated subwoofer output so you’d want to use that to connect to channels 3 & 4. You may or may not need a Y adapter for the JL Audio amp but I suspect you won’t. Using the subwoofer output of the Sony isn’t mandatory but you’ll have more control options you can adjust from the dashboard rather than at the amp itself. Then the front outputs to 1 & 2 as usual.

  25. Adding subs to a boat. Currently have polk audio pa4a head unit, wet sounds 4 channel amp and 4 6.5 wet sounds speakers. I was reading and sounds like I can bridge the amp to add a single sub as Marty described above, but was hoping could add 2 subs?? Is that possible? If so how do I wire them ? Any help appreciated. Thx!

    • Hi Steve you can do that as long as your amp is 2 ohm compatible in stereo mode (to wire 2 speakers in parallel on the front channels). You can use 2 subwoofers, but the total load will have to be 4 ohms (unless it supports 2 ohms in bridged mode. You didn’t specify the amp model so I’m not sure which one you have.)

      You can use two 8 ohm subwoofers wired in parallel or two 4 ohm DVC subwoofers each wired for 8 ohms, then connected to the amp in parallel. The problem is that each 8 ohm subwoofer will develop 1/2 the power a single 4 ohm model would so it works out about the same as using only one.

  26. Hey Marty,
    Your write ups are very informative. I am very thankful that I found this site of yours.
    I bought a Pioneer GM-A4704 and planning to connect it to my 2012 Hyundai Elantra stock stereo. Set up would be 4 speakers and 1 sub.
    In your expert opinion, which of the following would you best suggest? ( Note: Speaker wire input and not RCA)
    Option 1
    Ch A – Connect all 4 speakers, front and rear speakers in parallel ( speaker ratings 4 ohms each )
    Ch B – Connect sub
    Option 2
    Ch A – Connect 2 front speakers
    Ch B – Connect sub
    Leave the 2 rear speakers connected to the stock stereo. ( Original connection )
    Option 3
    Ch A – Connect 2 front speakers
    Ch B – Connect sub
    Leave the 2 rear speakers connected to the stock stereo. ( But connected on the front channel of the stock stereo. In this way the rear channel will be dedicated to the Sub)
    I will be appreciative on your earliest reply.



    • Hi Jeff. Of the three choices, I think #2 and #3 are the ones you’d (most likely) be happy with. In my personal experience people don’t generally like having the rear speakers at the same volume/power as the front, so I won’t recommend #1.

      There are some things to know, however:

      • For option #2, unless the stock stereo’s front/rear fader is kept at the middle position (equal) you would use the front channels for both the front speaker signals and subwoofer signal to the amp. If the rear radio channels have the volume decreased the sub’s input signal will be lower than it should be and we don’t wan that

      • Option #3 may or may not work out. It will depend on (1) the amp’s power output capacity and (2) what the amp’s gain is set to. There’s a good chance the front speakers (amp-driven) will not change in volume proportionately with the rear (radio-driven) speakers when you turn up the radio’s volume.

      That’s because the amp’s signal output is higher than the stock radio. However, it’s worth a try to see what you think & if it works for you!

      I hope this helps & have a good day. :)

      • Your a legend Marty. I really appreciate your expert opinion. Not to mention your speedy replies.

        I have an additional query if you don’t mind. Below is the specification of the amp. Does this mean that the amplifier supports 4Ω and 2Ω connection? If I consider doing Option #1, will it be fine considering all my speakers are rated 4Ω. So if I do Option #1, impedance will be 2Ω. Will this be fine?

        Pioneer GM-A4704 Specs
        Max power output…80W x 4(4 Ω) / 130W x 4 (2 Ω) / 520 W Total ( Bridge )
        Load Impedance…4 Ω (2 Ω to 8 Ω allowable )

        Anticipating your earliest response



        • Thank you Jeff that’s very kind of you. Yes, that amp does support two 4 ohm speakers in parallel as a 2 ohm load, so yes option #1 would work if you think you’re ok with that.

          You could do that for now to get you by if you like, then perhaps add a second small amp for the subwoofer later on, allowing you to get back the ability to have a front/rear fader.

  27. Thank you so much Marty,

    The usage of power resistor to fade the rear speaker is a good idea as well. I’ve read also, you can connect capacitor to kill the base.



      • Hi Marty, I have tried option #1 for a start just to try if it will work. Your analysis was on point.

        There is an ON and OFF low humming noise on the 4 speakers though. It is just a steady low hum which goes on and off. I tried disconnecting the input signal from the amp and it goes away. But after reconnecting there it is again. I have utilized the speaker level input.

        I need your expert advice.

        Appreciating your usual swift reply.


        • Hi Jeff. Hmm, that’s an odd issue you’re having. I’ve seen a few instances of odd problems when using speaker-level inputs, however, so it’s not unheard of. In order to verify it’s coming from the head unit, you can optionally use an RCA to 3.5mm cable to connect audio from your phone’s headphone jack and play some music through the amp. The noise shouldn’t be present in that case.

          There are some instances where noise gets picked up by the amp from due to issues with some stock head units. Honestly, the best thing to do at this point is to get a good quality line-level converter. If connected properly the issue should be eliminated.

          They’re available in 2 channel and 4 channel outputs. I recommend a model with adjustable level outputs (PAC and Scosche make some very good ones).

          I would be 100% sure you have a very good ground connection and there are no connection issues before spending the money, however. It’s always ideal to double-check.

  28. Hey Marty thanks so much for all the great info! I do have an odd question perhaps. Sorry if its been asked already. Say I’m using set up #1. However I want to use a DVC 4 ohm woofer. The amp can only do 4 ohm bridged, and sub can only be wired for 2 ohm or 8 ohm. Is it possible or feaseable to add resistors to the woofer, like you suggested for speakers running on a 4ohm minimum load amplifier? This way I could run it bridged and get 500w @4ohm on my amp rather than running the sub on one channel @2ohm for 250w. Im opposed to only running one coil on a DVC sub. Worst case I would just run it at 2ohm until I get different gear.
    I look forward to your response,

    • Good morning, Jason! Good question, actually. First, regarding the resistor question, it’s not practical to use them with subwoofers because of the larger power levels. That takes a huge resistor. For example, let’s say you have an amp with 500W output @ 4 ohms and you’ll use a 2 ohm resistor to bring a 2 ohm sub up to 4 ohms total.

      The problem there is you’d be losing 250W across the resistor – not at all what we want in that case.

      You really don’t have many options, but running it on one 4 ohm coil would be the best thing to do for now. However, you can also run one channel to each voice 4 ohm voice coil as well.

      Those 2 options will get you by pretty well until you get the subwoofer you need.

  29. Hi Marty, this page is much more helpful than anything else I’ve found, but since I’m a nuts and bolts guy, sometimes electronics gives me blurred vision. I like my 1989 vintage car audio, and it currently sounds great, but I was interested in adding two more speakers to a 2 channel amp. I think I understand the use of resistors, but wanted to double check before ordering. The amp is 4 ohm, and the current 6X9’s are 100W RMS @ 4 ohms, but the new speakers are 60W RMS @ 3 ohms(though Infinity states “they have come up with a safe way to get a little more power out of your system – these 3-ohm speakers combine with the speaker wire in your car so your car stereo sees actual 4-ohm impedance”). If I treat these as 3 ohm speakers, should I add 25W 2.5 ohm resistors to the circuit to get my 4 ohms to make the amp happy? Should I also bump up the wattage of the other two resistors for the 6 X 9’s to 50W 4 ohm? Thanks for your help.

    • I looked at the diagram again and am convinced I did it wrong. You added a 4 ohm resistor to each 4 ohm speaker, then hooked them in parallel to get your 4 ohms. That would mean I would have to add a 5 ohm resistor to each 3 ohm speaker to get the 8 ohms/ halved if hooked in parallel to get the 4 ohms.

      • Good morning, Doug. Yes, that’s right, in this case I would play it safe and use a 5 ohm or a 4 & a 1 ohm as those are often easier to find when shopping. The 3 ohm speakers are interesting but (adequate, and properly connected) speaker wire ordinarily has very little resistance between the head unit and the speakers, so that sounds somewhat like a gimmick.

        Regarding your question about the power rating, 25W should be fine but there’s certainly no harm in getting ones with higher power ratings if there’s not much difference in price. That’s especially true if you think you might upgrade your amp to a higher power one at some point.

        You can also use multiple resistors like this: 2Ω 25W + 2Ω 25W = equivalent of a 4Ω 50W.

  30. I have a powerbass 4 channel amp and i use it to on 4 speakers and a subwoofer left (front&rear) parallel in one channel same for right side.i bridged channel 3&4 for a subwoofer 8ohms.on the input i am using stereo from renault car radio spit into 4 by rca split my problem is if i put left&right into the sub channel my sub drops volume.i have to remove one rca cable in order for bass volume to be up.what am i doing wrong? I know its mono but i thought in the input i can also have 4channels

    • Hi Rebza. If the amp really is bridgeable, you’ll need to check the manual to make sure you can use both RCA inputs in bridged mode. Not all amps use both when 2 channels are bridged.

  31. Hi Marty, I have a old Sony MEX-BT2500 deck. It’s 4 channel with only speaker line out (no rca). I want to run a 4 channel amp (Pioneer GM-D8704) to the front and back speakers. And run a separate amp. Old coustic 2 channel amp to a speaker box in the trunk,it’s a box with 2 -10” Speakers with tweeters I want to use as a (sub) so to speak. Is this possible?? It’s a 96 Honda Accord
    Thanks Brooks

    • Hi Brooks. The good news is that not only can you do it, but it’s also not hard to do. I also remember that Sony head unit, by the way. :)

      The GM-D8704 has speaker level inputs built in that will let you connect directly to the speaker level outputs of the Sony head unit. However, in this case you’re wanting to add a second amp which I’m not sure if it has that or not.

      My suggestion in this case would be to use a line level converter like I used in my old Accord, the Scosche SLC4 seen here, which is a really good one. Since your Pioneer amp has the added bonus of RCA output jacks, here’s how you’d do it:

      Sony MEX-BT2500 speaker outputs -> line level adapter -> RCA to inputs on Pioneer amp -> RCA outputs to Coustic amplifier. You’ll then use the low-pass filter on the Coustic amp if it has one. Hopefully the 10″ speakers will sound like you hope; if not, you may need to get some dedicated subwoofers and/or a better amp with a low-pass crossover.

  32. Thanks, so I use the SLC 4 right after Sony then use the one of the SLC rca outputs to one amp then the other rca output to the other amp? How about those line out gain & factory amp gain on SLC? Any suggestions on how to set those?and one more thing sorry. Is FAI-3a the same as the SLC?
    I like this deck basically because it has separate bass and treble & Blue tooth. Is there any newer decks that have separate B & T?? Thanks so Much

    • Hi there. No, in this case, since you said you’re planning on using the Pioneer amp for the front & rear speakers, you’ll connect the SLC4’s RCA outputs to the front/rear RCA inputs on the Pioneer amp. Then you can just run an RCA cable from the RCA outputs of the Pioneer amp to the 2nd amp, called “daisy chaining” them sometimes.

      I would set the output levels on the SLC4 to about 75% and see how that works out for you. More than likely it won’t be a problem. Regarding your question, a Google search resulted in this: “The SLC-4 is the EXACT same things as the FAI-3… its just that one is almost exclusively sold in Wal-Mart, while the other is available to all retailers.

      Actually, there are a lot of newer head units that are much better than the Sony you have. I recently picked up a Kenwood KMM-BT522HD and there are a number of other Kenwood KMM head units you can find out there. Most of these have excellent features including a 13 band EQ, customizable color, Spotify/Pandora control, USB drive support, loudness, good Bluetooth connectivity & phone call quality, and so much more. I got one off of eBay for under $100 new in the box.

      There are some versions available in double-DIN also, as well as some models by Pioneer that are pretty good too. The Kenwood units offer a better EQ however.

  33. So do I cut the factory speaker wires after the harness then go into the scosche and so on. Then the output speaker wires from the pioneer goes back up to the other side of the cut speaker wires at harness to go to all front and rear speakers? Thanks for all the help

    • Yes that’s right, or just use a wiring harness adapter so you don’t have to cut the factory wiring. That’s probably going to be easier, plus if you want to remove the amp later and sell the car etc it’ll make that easier, too.


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