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.

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 = 200W 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 200W – yes 2 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.

Your comments are welcome!

  1. I read with interest but stil not clear on whether my idea os workable.

    I have a power amp. 1200W per channel. Speaker output is 4 pole speakon (NL4) with1+ and 1- on pins 1 ands 2, and 2+ and 2- on 3 and 4. I have only one sub and dont want to leave one channel of the amp idle so thought:

    1. Rewire the sub so each channel goes to a different driver. Tricky and not ideal.

    2. send 1+ and 2+ to the same pin at the speaker end and 1- and 2- to the same pin at the speaker end. This would effectively mean both channels supplying one load.

    All the values are good so its just a case of would it work?

    sub 2×15″. rated around 1500 program @8ohm which it is as there drivers are both 16ohm in parallel. The amp has trim so will be fine level wise.

    I rad you warning about damaging amps and this has no bridged mode, but this would effectiveky be a bridge-bodge :-).

    Am i making sense?

    Reply
    • Hello, no I didn’t really understand your comment/question. The English was very hard to follow and I could not understand it fully.

      So you have two 15″ woofers of what Ohms rating each? What the is the minimum Ohms rating of the amp? These details are important in order to help you.

      If the amp is not bridgeable you’ll use one woofer per channel normally.

      Reply
      • The amp is not bridgeable. My idea was to route both channels of the amp to the parallel input of the sub. The sub is a twin 15″ rated at 8 ohm, so the drivers will be 16ohm each, which in parallel is 8 ohms.

        The question really is if both channels of the amp can drive one single load – which is two drivers in parallel.

        From what you said this should work if the load is consistently both drivers simultaneously connected to both channels, which being a single 2 channel amp is always going to be the case.

        so, both channels of the amp are driving a single load – of two parallel drivers.

        To me it seems logical that the amps two channels will simply double the voltage at the load stage (speaker), so i get double the power but spread over the two drivers at a combined 8 Ohms.

        Though pushing 2400W into a 1500W program enclosure, there is a 40dB trim, so i can bring the level down to what is needed in relation to the full range speakers.

        Or, put more simply, can a two channel amp with both channels routed together run one cabinet ?

        Reply
        • Ok, that’s helpful to know. I didn’t have that information before. Ok so if this is an 8Ω rated dual voice coil (DVC) subwoofer, it’s 8Ω per voice coil, not 16Ω.

          You can just use one channel per voice coil which ideally would be with a mono signal to have the same signals driving the voice coils in tandem. Not a deal-breaker but would be ideal.

          Each voice coil will receive up the the amp’s maximum RMS (or “continuous”) power rating at 8Ω per channel. Voltage can’t be increased unless two channels are bridged so that isn’t a factor here.

          The subwoofer will receive 2x the power of using a single channel with an 8Ω load which will increase the volume by about 3dB – but it’s shared between them . However, you can drive them harder and heat will be shared between two coils as opposed to driving a single coil with the same amount of power.

          So yes, you can drive a single dual voice coil speaker, one channel per voice coil, with a stereo amplifier.

          Reply

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