Bridging an amplifier can seem like a strange and almost “magical” concept. If you need help figuring out how to bridge an amp, then welcome! Let me help you to better understand the important basics as well as get the most power out of your amplifier.
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!
How to bridge an amp – the basics
Part 1 – What does bridging do?
Bridging is a special feature in audio amplifiers which is used to get the maximum amount of power out of the amplifier under certain conditions. It is basically bridging – or using – two amplifier channels together to drive a speaker or a set of speakers with additional power that normally is split between individual 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 could always use 2 channels, in “bridge mode”, to drive a single subwoofer or pair of subwoofers. This meant having more power available at my disposal and more flexibility regardless if I changed my speaker setup later in time.
The science of bridging and getting more power – how does an amp make more power?
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?
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
We will get the following power available:
The voltage available across the speakers, since the amplifier’s bridged channels are working together sort of in a push-pull fashion is:
Total voltage: 2 x 14.14V
Power across the 4 ohm subwoofer: (28.28)*(28.28) / 4 = 800 / 4 = 400W in bridged mode
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. It’s important to remember that we’re assuming a few things, like that our amplifier is rated to provide that power we estimated and so forth. Also note that in this mode each channel is handling several times the electrical current it had to previously when in stereo mode.
The reason all of this is possible is because 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 difference is twice that of one channel alone.
Part 2 – Connecting to a bridgeable amplifier – how to bridge an amplifier
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 to be aware of this when 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. Since bridging means that much more power is now available to you speakers (more often than not this is used for subwoofers) the it’s best to use a decent 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 require 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.
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.
ALWAYS VERIFY BEFORE ATTEMPTING TO BRIDGE AN AMPLIFIER!
See my additional information below if your amplifier does not support this by design.
Part 3 – How to determine the speakers and Ohm load (impedance) you can use for a bridged amp
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
Let’s check out a few examples to help explain these ideas:
For an amplifier that requires a 4 ohm load when bridged I can 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 amplifier that requires a 2 ohm load when bridged I can 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)
Here’s a diagram with the most often used setups to help you better understand.
(click to enlarge or click here to get the Adobe .pdf file you can download or print)
Part 4 – Setting your crossover on your bridged amp
Close 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 in which the amp is connected to a subwoofer or set of subwoofers, this is what I recommend:
- Set the crossover switch to low-pass filter (LPF) or similar on your amplifier
- Adjust the LPF dial, if available, to the maximum level
- Play music contain bass you’d normally listen to
- 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)
- Adjust the gain if more higher-volume power is needed when the volume dial of your stereo is turned up
How to bridge amps that do not have a built-in bridge design
Amplifiers without the feature built in are unfortunately difficult to bridge, but it can be done. Several solutions exist:
- Build your own bridging module (read more here)
- Find a bridging module (these are likely very old and hard to find, but may be possible on eBay)
- 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
Understanding Tri-mode capable amplifiers
Amps that have say they have a “tri-mode” or “tri-bridgeable” feature are simply the same as other bridgeable amplifiers with the exception that you can also 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 peopls, 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
In the 4 points above I’ve explained what bridging an amp does, how to bridge an amp, how to determine the right speakers to connect, and I provided diagrams for this as well.
Bridging your amp should be a fun, easy, and enjoyable way to get more power for your money and a great way to get more enjoyment out of whatever car audio system you have. Using the same method you can also add a 2nd or 3rd amplifier and bridge them as well for even more flexibility and power!
If you still have any questions or suggestions about the things you’d like to see covered please let me know and I’ll be happy to help.