How-to guide

How To Bridge An Amp – Info, Guide, and Diagrams

Bridging an amplifier can seem like a strange and almost “magical” concept. If you need help figuring out how to bridge an amp, you’re in the right place!

Let me help you understand what bridging is, why it matters, and how to bridge your amp.

It’s easy to make mistakes and – unfortunately – you can permanently damage your amplifier and even your speakers. Don’t worry though as I’ll cover what you need to know before that happens.

Contents

What is car amp bridging?

Diagram showing a 4 channel car amplifier bridged to 2 channels

A 4 channel car amplifier bridged to 2 channels. This is a very common wiring use of a 4 channel amp for situations in which you’d like more power available and don’t need 4 separate amplifier channels.

Bridging is a special feature in car amplifiers which lets you get the maximum amount of power they can produce by using a built-in channel-sharing design.

It means using 2 amplifier channels working together to drive a speaker or a set of speakers with by using the power that normally is split between 2 separate amplifier channels.

It’s actually a pretty cool subject, and very useful! For example, I have owned many car amplifiers and normally used 4 or 5-channel amplifiers. I used 2 channels, in bridged mode to drive a single subwoofer or a pair of subwoofers in the trunk.

This meant having more power available at my disposal and more flexibility even if I changed my speaker setup later.

How does an amp make more power when bridged?

Pioneer GM-D9605 internal components and circuit board images

Internal view of a car amplifier. The output sections have one channel per pair designed to provide the bridging option when needed. In normal use with 1 speaker per channel, you won’t notice a difference as they’re wired normally and have separate audio signals.

This is actually a very cool and fascinating subject. If we do the math, we’ll discover something very interesting!

How about we take a real-life example and I’ll show you exactly how this works?

Example #1

A 2 x 50 watts/channel amplifier is connected to two 4 ohm speakers. With some math, we would find out that our little example amplifier puts out 14.14 volts when it reaches 50W into a 4 ohm speaker.

We can find the power using this formula: Power (W) = (Volts x Volts) / Ohms, or P = V^2 / R.

So (14.14)^2/4 = 50W for each amplifier channel.

Example #2

We connect the same 2 x 50 watts/channel amplifier in bridged mode across one 4 ohm subwoofer.

The voltage available across amp’s bridged channels working together in a push-pull fashion is:

Total voltage: 2 x 14.14V = 28.28V

Power across the 4 ohm subwoofer: V x V / R = (28.28)*(28.28) / 4 = 800 / 4 = 400W in bridged mode. (Assuming the amp is designed to support that much power output)

So by bridging the amplifier in this example, we can get close to 400W – yes 4 TIMES – the normal available power when in bridged mode, depending on the connected speaker.

Note: It’s important to remember that we’re assuming a few things, like that our amplifier is rated to provide that much power.

Not all amps can deliver that much – it depends on their limitations and how they’re designed.

Also, note that in this mode each channel is handling several times the electrical current it had to before. Therefore it will draw more power from the car battery.

How do car amps make this possible?

The reason this is possible is that today’s car amps have a design in which one of each 2 audio channels is actually inverted (you can also say 180 degrees out of phase) but is normally connected at the output in a non-inverted fashion.

You’re normally not aware of this as it has no effect on the end user.

This means that in bridge mode the bridge connection is made so that the amp channels have a difference of the available voltage at each output. This voltage difference is twice that of one channel alone.

Connecting to a bridgeable amplifier – how to bridge an amplifier

Image with diagram of how to bridge an amplifier

Bridgeable amplifiers normally have the terminals labelled accordingly. In that case, bridging is easy to do as long as you observe speaker impedance (Ohm rating) warnings from the manufacturer

The good news is that many car amplifiers sold today can be bridged. Note that some (especially smaller, lower-cost products) may not have the feature built in so it’s very important check before buying one. 

Never assume you can bridge an amp.

To bridge the amp, connect the subwoofer or bridged speaker positive (+) terminal to the positive amplifier bridged terminal label, and the speaker negative (-) terminal to the negative bridged amplifier terminal also.

Use good speakers & reliable connections

Image showing example of speaker terminals with crimp connectors used

It’s important to use a good reliable connection for speakers. Car and trucks are subject to vibration and lots of movement, so a poor connection can cause problems over time.

Since bridging means that much more power is now available to you speakers (more often than not this is used for subwoofers) it’s best to use a quality speaker wire and make secure, tight connections on both ends.

Before doing so, verify that you have met the required minimum impedance (speaker load, in Ohms) as specified on your amplifier or its instruction manual.

For today’s car amplifiers this is a minimum of 2 ohms when bridged, usually. Some do support less or need 4 ohms but for best results and to make sure you are setting up your system to the best of your requirements it pays to be sure.

Quick Tip
Some older model amps and a few modern amplifiers have a “bridge mode” switch that must be used or may require only one RCA audio connection is made. Always make sure you’ve checked!

Don’t forget the crossover

As long as you have the correct speaker impedance in use (see my additional info below) you should be ready to use your bridged amplifier! Note: it may be necessary to set the built-in low-pass crossover also, if available. I’ll cover more about that below too.

Note! You must be sure your amplifier can handle a bridged connection. Not all amplifiers are designed to do so, and most home stereo receivers and home theater amplifiers cannot support a bridged connection.

ALWAYS VERIFY BEFORE ATTEMPTING TO BRIDGE AN AMPLIFIER!

See my additional information below if your amplifier doesn’t support this by design.

Understanding the minimum speaker load (Ohm rating)

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

Shown: An example of measuring the speaker impedance (total resistance) in Ohms for a speaker’s voice coil.

Your amplifier should state in the owner’s manual the minimum required “impedance” (the resistance load a speaker has) for use. This includes both for normal stereo usage as well as a rating for bridge mode too.

If you cannot find the documentation anywhere a good rule of thumb for car amplifiers is to use 4 ohms.

Often you’ll see the phrase “stable to 2 ohms” or something similar to describe what the amplifier can handle.

Here are the basic rules for correctly connecting speakers in bridged mode:

  • You can connect a SINGLE speaker of the required minimum impedance or higher
  • You can connect MULTIPLE speakers if the total adds up to the required minimum impedance or more (see diagram)
  • Dual-voice coil speakers can be used if they can be wired to give the correct amount

Bridging examples

For an amp that requires a 4 ohm load minimum when bridged use:

  • One 4 ohm subwoofer
  • TWO 8 ohm subwoofers wired in parallel ( 8 / 2 = 4)
  • ONE dual 8 ohm voice coil subwoofer wired in parallel (gives 4 ohms)
  • TWO 2 ohm woofers connected in series (2 + 2 = 4 ohms)

For an amp that requires a 2 ohm load minimum when bridged use:

  • One 2 ohm subwoofer
  • TWO 4 ohm subwoofers wired in parallel ( 4 / 2 = 2)
  • ONE dual 4 ohm voice coil subwoofer wired in parallel (gives 2 ohms)
  • FOUR 8 ohm woofer wired in parallel (8 / 4 = 2)

Amp bridging wiring diagram

Here’s a diagram showing the most common wiring setups most people will use for typical amplifiers.

(Click to enlarge or click here to get the Adobe .pdf file you can download and print)Infographic diagram for how to bridge an amp and connect to speakers correctly

Quick Tip
For most systems, if using 2 speakers, connect them in parallel to ensure each receives the maximum amplifier power vs. connecting the same speakers in parallel.

Setting the crossovers on a bridged amp

Close up image of a car amp crossover controlsClose up of a modern typical car amp with built-in crossover features. This allows high-pass, full-range, or low-pass use with adjustable sound range controls in this case.

For most bridged amp situations where the amp is connected to a subwoofer or set of subwoofers, this is what I recommend:

  1. Set the crossover switch to low-pass filter (LPF) or similar on your amplifier
  2. Adjust the LPF dial, if available, to the maximum level
  3. Play music contain bass you’d normally listen to
  4. Begin turning the LPF dial down until almost no voice or upper-range music is heard (note: for reference, a good rule of thumb is 80 Hz or lower in case you’re able to know the actual cut off frequency of your amp)
  5. Adjust the gain if more higher-volume power is needed when the volume dial of your stereo is turned up

Can you bridge an amp without bridging built in?

Amplifiers without the feature built in are unfortunately difficult to bridge, but it can be done. Several solutions exist:

  1. Build your own bridging module (read more here)
  2. Find a bridging module (these are likely very old and hard to find, but may be possible on eBay)
  3. Use an electronic crossover with adjustable phase (0-180 degree control) for each channel and set 1 of the 2 to be 180 degrees out of phase

Unfortunately, unless you’re a hobbyist and are good with electronics (and have the right tools and parts), ideas #1 & #2 aren’t very practical.

It’s much easier to buy an older electronic crossover or an inexpensive crossover that has a bridging or mono feature provided.

Tri-mode capable amplifiers

Diagram showing a car amplifier with tri-mode wiring connections

Example of wiring speakers with a tri-mode capable amplifier.

Amps that have a “tri-mode” or “tri-bridgeable” feature are the same as other bridgeable amplifiers but can also be in bridged mode and wired to 2 speakers at the same time. This makes it possible to keep a regular stereo pair of speakers connected while the amp is bridged and connected to a woofer or other speaker.

This is normally not used by most people, however, it does have some benefits as well as drawbacks.

  • Pro: This allows a 2-channel amp to connect to 3 or more speakers
  • Con: for woofer use on the bridged output, an external passive crossover is needed – these are often large, heavy, and expensive for low-frequency speakers like subwoofers

Summary

Bridging your amp should be a fun, easy, and enjoyable way to get more power for your money. It’s a nice way to get system flexibility as well.

When it comes to amplifiers it’s important to have a good, solid wiring connection. Make your installation easy – check out this great post with the top recommended amp wiring kits for your money.

Questions, comments, or suggestions? I’d love to hear from you! Let me know in the comments below or you can reach me here.

16 Comments
  1. Please help, I have 4 channel 75 watt each bridgeable amp. It has no information or switches etc, just basic layout. It says it is 2ohm stable and I want to run 4 ohm sub bridged but when I connect the way everyone describes it sounds identical. Have you any suggestions?

    • Hi Hector. What’s the brand and model number of the amp? Also, do you mean there are no “BRIDGE” labels near the speaker terminals? Most bridgeable amps have those printed on them.

      Let me know :)

      • Marty, I have an issue maybe you can help with. I have an 8” sub dual voice coil connecting it to a kenwood KAC-5206. I also have 4 6 1/2” Kicker 4ohm speakers I want to connect to it. Model number is CS65’s on the kickers. Is this possible and if so, how would it be done.

        • Hello Jeff. The main issue here is that the Kenwood KAC-5206 is only a 2 channel amp. If it were a 4 channel it would be possible.

          The short answer? No, you won’t be able to use this one for all 5 speakers. You can, however, run 2 6.5″ per channel in parallel (2 ohms total). You’ll need to add a 2nd amp for the subwoofer in this case.

          Here’s one that’s pretty cheap (about $50 or below) and will work well for your 8″ subwoofer: Boss R1100M mono amp.

          Thanks for dropping by! :)

  2. Hi Marty,

    Thanks for the detailed technical knowledge.
    Can you please guide me for the following :-

    When I make the Subwoofer(dvc 4ohm each VC) wiring in parallel and add to bridged mode in amp. It clips as I guess sub is having 2ohm impedance now.
    But if I go series wiring the output is lower but works just fine.
    So what type of wiring you suggest with 2 stock speakers and a sub ?
    Pioneer 6704 Amplifier
    Pioneer Subwoofer 1211d4
    Can you confirm if adding the Sub with parallel wiring to the 6704 amp Can work or not?
    I read it is 2 ohm stable !

    • Hi Mickey. You’re having problems with your amp because it’s not 4 ohm capable when bridged:

      190 watts RMS x 2 in bridged mode (4-ohm stable in bridged mode).” I would recommend using only one 4 ohm voice coil. That subwoofer isn’t really a good match for your amp in this case, since the amp isn’t 2 ohm capable.

      When wired as an 8 ohm setup (series) it can’t develop the same power as a 4 ohm, so that’s not a good choice.

      • Thank you Marty,
        If I may ask for a favor, can you please suggest better amplifier to match with pioneer 1211d4 sub. With room to add 1 pair component speakers later on.

        Thanks for the technical knowledge.

        • Hi there, sure thing. Have a look at the Pioneer GM-D8704 4-channel. It’s capable of handling 2 ohm loads and also has more power, too. The GM amps are very nice for the money.

          You’ll be able to wire your dual 4 ohm sub in parallel to use it in bridge mode.

  3. Thank you so very much
    It took me a while to search the amp in my area. So finally I have ordered this one.

  4. I have a question. So say my amp is 4ohm bridge and my sub is wired at 2ohm. What will happen? Is that real bad or what is the the possible bad scenario there?

    • Hi Devon. If you try to drive a speaker that’s a lower impedance than the amp is rated for best case it will start to get very hot and most likely shut off (protection mode).

      In the worst case scenario it could be damaged permanently.

  5. Im getting a Skar EVL18D2 sub. What would you recommend as far as a good 1ohm amp that’s real cheap? I just dropped $400 for the sub and box so money for my project is pretty tight and I just want a good amp that’s ideally below $200.

  6. Hi Marty.Hello and thank you for the tons of info you are sharing with us.
    I have opted recently for an entry SQ Build and i really apreciate if you could advise me on
    how to get this work properly.
    HU :Pioneer MVH-S520 BT
    Zapco DSP-Z8 IV
    3 Way Conponents: K2 Power 165 KRX3 6-3/4″
    4 Ch.Amp. Audio System HX Series 85.4
    4 Ch.Amp. Audio System HX Series 175.4
    Focal 12″ p30 performance
    The hx 85.4 is for the Tweeters and Mids
    as for the hx 175.4 goes for the mid base and bridged sub.
    as for now im not getting anything from low frequencies.can you assist on how to set the gains and frequencies
    on both amps and eventual the dsp ?
    Cheers,
    Ralph

    • Hi Ralph.

      1. Regarding the low frequency issue: If you have the Pioneer subwoofer RCA outputs enabled (if that’s an option) and connected to the DSP then it’s most likely a configuration in the DSP that needs to be changed.

      I would check this by first bypassing the subwoofer inputs to the DSP and instead run them to the HX 175.4 for the subwoofer. If you have a bass signal at that point then you know the DSP isn’t set up right yet. (Like dedicated vs derived bass output, or the low-pass output isn’t enabled, etc).

      I’m not sure about the DSP-Z8, but for my Alpine DSP, for example, I have to switch the subwoofer output on and use a dedicated RCA jack.

      2. Regarding setup: It’s a bit of an open-ended question regarding how to set your system because it always depends on what you’re after and etc. Also if you need or don’t need time delay makes a big difference, too. It’s too much of a big topic to go into here so I’ll just covers some basics.

      – It sounds like you would like to bi-amp the channels, which allows time delay adjustment to be used between the different speakers. You can just use the high & low-pass crossover frequencies provided by the K2 Power crossovers to set the DSP accordingly.

      But if you’re not planning on using time delay, honestly you’re probably fine use the included speaker crossovers and driving them with the HX 85.4, maybe bridging it to 2 channels if you like. Then out to the speaker crossovers.

      – For best results you’ll want to use a real-time analyzer (RTA) & a pink noise test track to find the peaks and dips in the sound response once you’ve got it all working so you can adjust the EQ on the DSP. You can use several different kinds but you could use one like the Audio Tool app for smartphones and a better plug-in microphone like the Dayton Audio iMM-6 to help.

      It won’t be high-end but will probably be good enough to fix the biggest sound response issues. There are more accurate ways to do it, but they’re expensive and a lot less convenient to use.

      Also, you’ll need to set the Pioneer’s EQ, boost, or any other sound features to off or flat. If you’re mixing settings on both the Pioneer AND the DSP, you’ll never get it set right.

      Set the Pioneer flat (no EQ, no boost, etc) and do everything on the DSP.

      – I use high-pass for my main speakers at around 56Hz (-18dB/octave) and a low pass on my subwoofer for about 70Hz (-18dB/octave). If you have an RTA you can see the bass response in your vehicle and see what needs adjustment. Then once it’s setup right you can adjust it to what sounds best to you, too.

  7. Thank you so much for the information.I will definitely let you know the result.

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