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How to Bridge a Car Amplifier

Bridging an amplifier refers to combining two amplifier channels into one to drive a single load (speaker) with more power than the sum of the two original channels combined. A two channel amp can be bridged to one channel, and a four channel amp into two channels.

Amp bridging is a common practice in the car audio industry as it increases the amp’s power output.

Bridging a four-channel amplifier for example will effectively double its output (watts). However, the main disadvantage of bridging is that bridged channels can’t drive impedance loads as low as they can when unbridged.

Bridging an amp isn’t rocket science. It’s relatively easy as long as you have some basic knowledge of car audio wiring and how to connect an amp to your car’s audio system.

So, let’s find out how you can bridge your amplifier…

Before we move forward, a quick note:

It should be noted that not all car amplifiers are bridgeable. Bridgeable amplifiers are featured with an inverted channel or an on-board switch (to invert the output of the amp) as part of their design to allow bridging. Unbridgeable amp lack that part which is why you can’t obviously bridge them.

Also, keep in mind that mono and bridging are not necessarily the same. In other words, you cannot bridge a monoblock amplifier. Remember that bridging refers to combining two opposing amplifier channels into one, and a monoblock amplifier has only one channel.

Disclaimer

Before attempting to bridge a car amplifier, there are a few things you need to keep in mind.

First and foremost, always ensure that your amplifier is bridgeable. This should be indicated somewhere in the user’s manual. If you’ve lost the manual, google is your friend.

Additionally, always ensure that your amp can handle the increased power load once it’s bridged. On the same note, do not bridge a car amplifier that will be unstable at the bridged load, or if your speakers cannot handle the increased power.

Furthermore, it goes without saying that bridging an amplifier cuts its impedance load in half. This can cause overheating problems if the amp is not properly mounted.

Is Bridging an Amp Better?

This is a great question, for which the answer is an emphatic “Yes”. Bridging a car amplifier is really useful and appropriate in some situations.

In this article, I’ll describe a couple of different scenarios where bridging an amplifier is a great way to go.

  • Bridging a 2-channel amp to power a subwoofer: Let’s say you’re on a tight-budget and looking for an amplifier to power a 150 watts RMS rated 4-ohm sub. You could get a small 2-channel 4 ohm amp that normally puts out 50 watts RMS per channel, and bridge it, so it could produce an output of 150 watts RMS, which would be ideal for driving the sub.

 

  • Bridging a 4-channel amp to power a pair of component speakers: It goes without saying that no matter what kind of car receiver you have, it wouldn’t do justice to power hungry speakers. This is especially true if you have a pair of component speakers.  That said, powering a pair of high-performance component speakers is another common scenario where amp-bridging is required. You can get a 4-channel amplifier that normally puts out a mere 40 watts RMS per channel, but can deliver up to 120 watts RMS per channel when bridged.

 

  • Bridging a 4-channel amp to power a sub and a pair of speakers: “Can you bridge only 2 channels on a 4-channel amp?” is a common question that we frequently get asked. For which the answer is a resounding “Yes”. It’s actually a pretty common scenario in the industry to use a 4 channel amplifier with its two channels running their normal power for pair of front speakers, and the other two channels bridged to power a subwoofer.

Bridging a 4 Channel Amp

First of all, consult the amplifier manual or research your amp model online to be sure it’s bridgeable.

On a four-channel amp, you should see 8 terminals with a screw-down clamping system to hold the speaker wire securely in place. Each terminal will be labeled as follows:

  • Channel 1
    • “A” (positive)
    • “B” (negative)
  • Channel 2
    • “C” (positive)
    • “D” (negative)
  • Channel 3
    • “E” (positive)
    • “F” (negative)
  • Channel 4
    • “G” (positive)
    • “H” (negative)

How to Bridge a 4 Channel Amp to 2 Sub

Bridging a 4 channel amplifier to 2 subs is no different from the bridging it to 1 subs. What you need to do is to simply connect the positive lead of the first subwoofer to terminal A (the positive for channel 1) and connect the negative subwoofer lead to terminal D (the negative for channel 2).

Following the same method, take the wires coming from the second subwoofer, but this time, connect the positive lead of the second subwoofer to terminal E (the positive for channel 3) and connect the negative lead to terminal H (the negative for channel 4).

How to Bridge a 4 Channel Amp to 1 Sub

To bridge a 4 channel amplifier to 1 subwoofer, hook up the positive subwoofer lead to terminal A (the positive for channel 1) and hook up the negative subwoofer lead to terminal D (the negative for channel 2).

In order to tightly secure these wires, unscrew the screw in the amp terminal, and place the speaker wire in between the top and bottom portions of the terminal, and then screw the screw down tightly.

Now, since we’re using a 4 channel amplifier, only 2 out of its 4 channels are used. The remaining two channels could be used to drive a pair of speakers or you could bridge them as well to power a second subwoofer.

Bridging a 2 Channel Amp

On a two-channel amp, you should see four terminals. A positive and a negative terminal for channel one, and likewise for channel two. Each terminal will be labeled as follows:

  • Channel 1
    • “A” (positive)
    • “B” (negative)
  • Channel 2
    • “C” (positive)
    • “D” (negative)

How to Bridge a 2 Channel Amp to 1 Sub

To bridge a 2 channel amplifier to 1 subwoofer, connect the positive subwoofer lead to terminal A (the positive for channel 1) and hook up the negative subwoofer lead to terminal D (the negative for channel 2).

How to Bridge a 2 Channel Amp with 2 Subs

If there is a question we get most often – about amp bridging – it’s “Is it possible to bridge a 2 channel amp to 2 subs instead of 1?”.

The answer to this question is “Yes”. You can definitely bridge a 2 channel amp to power 2 subs, and here’s how to do it:

1. Wire the Amp to the Subwoofers

Start by running a wire from the positive terminal on the amp to the positive terminal on the first subwoofer.

2. Wire Both Subs Together

Next, you need wire the second subwoofer to the first one. There are two ways to do that: either you wire them in series or in parallel.

If you wish to wire your subs in series, run a single wire from the negative terminal of the first subwoofer to the positive terminal of the second subwoofer.

If you wish to wire them in parallel, you will need to run two wires between the two subs. The first wire will connect the two positive terminals, and the second will connect the two negative terminals.

3. Complete the circuit

Last but not least, you need to complete the circuit by connecting the wire from either the first or the second sub’s negative terminal to the negative bridged terminal on the amp. This will complete the circuit whether the subs are wired in series or in parallel.

How Does Bridging Work?

Bridging an amplifier increases the power that can be supplied to one speaker, but it does not increase the amplifier’s total available power.

If so, then where does all the extra power come from when you bridge an amplifier?

Well, in the very beginning of this article, we’ve mentioned that bridging an amplifier refers to combining two of its channels into one with twice the voltage.

In other words, using the negative signal of one channel with the positive signal of the other channel effectively doubles what each channel alone could put out at half load, which is usually the maximum output the amp can put out.

Bridgeable amps are featured with an inverted channel(s) which produces voltage that is generated at the opposite polarity of the regular, un-bridged channel. In other words, when you bridge a two channel amplifier, what happens is you give both channels the same audio signal with the polarity of one channel reversed.

Bridging an amplifier doubles the available voltage swing at the load compared with the same amplifier used without bridging.

Speaking of voltage swing, the sine output signal is essentially swinging between the positive and negative supplies within the amplifier. And the maximum positive- and negative-voltage capability of a car amplifier is limited by its power supply’s limits, i.e., its voltage rails, which are also referred to as the DC (Direct Current) supply.

According to Ohm’s Law, power is calculated as the product of voltage and resistance. In other words, to figure out the maximum power output of a car amp, take the voltage produced by the amplifier times itself and divided by the rated loudspeaker load.

Power=(voltage across speaker*voltage across speaker)/(resistance of the speaker)

or P = V^2 / R

For example, if we have an amplifier with a power supply output of 40 volts, the maximum instantaneous voltage that can be applied across the non-bridged speaker’s terminals is 20 volts. This amp will only be able to produce 100 watts (peak power) into a 4 ohm speaker.

Power=(20*20)/4 ohms
Power=100 watts

Alex Brown

Hey There, my name is Alex Brown, I'm an LA-based sound engineer with over 10 years experience installing, troubleshooting, and repairing commercial, automotive, and household sound equipment. I've installed highly competitive car audio systems, and everything from navigation systems to full car stereo systems, remote starters, alarms and beyond. I enjoy creating solutions and simplifying everyday needs. I also love helping people get great sounding gear, thereby, saving the world from bad sound one customer at a time.

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