For decades, stereo and multichannel amplifiers have offered the option of bridging to deliver significant power to a single speaker. Whether you refer to it as bridging, a bridge-tied load (BTL) or strapping, the process is the same. Curious how it works? Read on and we’ll explain.

How Most Stereo Car Audio Amplifiers Work

If you were to look at the signals on the positive and negative terminals of one channel versus the other, you are likely to find that they aren’t the same.

In amplifiers that are bridgeable, a popular configuration is to have the Left Positive and Right Negative speaker connections connected to the ground plane (a virtual ground, not the same as the chassis ground) inside the amp. The other two connections receive audio signals from the output devices.

When the signal on the Left Negative speaker terminal on the amp in the image above moves positive, the speaker will cone will move inward. When the signal on the Right + speaker terminal on the amp moves negative, that speaker will also move inward.

Common Amplifier Bridging Configuration

When your installer wires a single speaker to your amplifier in a bridged configuration, he or she only uses the terminals with signal on them. Because the signal on one channel is inverted relative to the other channel, the voltage present across the speaker terminals has the potential of being twice what is available on a single channel.

How Much Power Does a Bridged Speaker Receive?

The amount of power an amplifier produces is determined by the voltage the amp can produce and the impedance of the connected speaker. This statement assumes that the power supply in the amp can provide adequate current to the load.

In our example amplifier, let’s make the assumption that the positive and negative voltage rails are set at +21 volts and -21 volts. The output voltage on each channel can swing between plus and minus 20 volts relative to ground. The maximum power this amplifier can produce per channel when connected to a 4-ohm speaker is calculated using the maximum output voltage squared, divided by the load impedance. In this example, we have 20 volts squared divided by 4 ohms. This works out to 100 watts.

If you have a 4-ohm speaker wired in a bridged configuration, it can receive a maximum of 40 volts. This is because the signal on the positive speaker terminal goes in the opposite direction as the signal on the negative terminal. If we apply the same math, we get 40 times 40, divided by four, which is 400 watts.

What’s With Amplifiers ‘Seeing’ a Load?

You will find that most people in the audio industry refer to amplifiers “seeing” a load. In our original two-channel stereo wiring configuration, each channel has a single 4-ohm speaker connected to it. As such, each channel sees a 4-ohm load. When we bridge an amplifier, you will note that the minimum impedance for this configuration is double that of each individual channel. Our model amp can drive a 4-ohm load for each channel, but only a single 4-ohm speaker when bridged. Have you ever wondered why?

While amplifiers are designed to increase voltage, the amount of current their power supply can pass to the output devices is often a limiting factor. If we look at a single channel of this amp when connected to a 2-ohm load, the amp needs to provide 10 amps of current to the 2-ohm speaker when the output is at 20 volts. If we were to wire that 2-ohm speaker in a bridged configuration, we now have a maximum of 40 volts across its terminals, and we’d need 20 amps of current from each side of the amp. If the amp isn’t capable of delivering 20 amps per channel, it will either distort, overheat, go into protection or become damaged. If we wire a 4-ohm speaker in a bridged configuration, each channel only has to produce 10 amps of current.

Audio Signals When Bridging an Amp

For a speaker connected in a bridged configuration to function optimally, you will want to make sure that the audio signals going to the left and right channels are identical. Many high-quality amplifiers will have a mode selection switch that will let you select from Stereo, L+R or Mono operation. In the Stereo setting, the signal from the left RCA input goes to the left output channel, and the right RCA input goes to the right output channel. When you select L+R, the signals from both the left and right inputs are mixed together inside the amp and sent to both channels. Finally, in most applications, the Mono option uses only one of the input terminals and feeds that signal to both speaker outputs.

Bonus Amp Information

Back in the ’80s and ’90s, most car audio amplifiers were quite large. They were designed to produce lots of current because, well, size and heat weren’t an issue. When you connected a single 4-ohm speaker in a bridged configuration, most of these amplifiers would produce roughly four times as much power as that speaker would receive if connected to a single channel.

As amplifiers have become smaller, the size of their power supplies has decreased dramatically. As such, many compact amplifiers can’t deliver the current required to quadruple power. You’ll see ratings like 65 watts per channel when connected to a 4-ohm speaker, but only 95 watts per channel when driving 2-ohm speakers. In this example, the amp would only produce 190 watts when bridged as opposed to the 260 watts (four times 65 watts) you might have hoped for. This doesn’t make the amp necessarily sound worse; it’s simply a trade-off based on the desire for smaller chassis sizes and lower prices.

Upgrade Your Car Audio System with a Bridgeable Amplifier

If you’re considering upgrading your car audio system with a subwoofer and you have a two-channel amplifier, your installer is likely to choose to wire that sub in a bridged configuration. In most cases, with a high-quality amp, you’ll get all the power you need to crank your music up to concert volume levels. If you’re shopping for an amp, drop by your local specialty mobile enhancement retailer to get some advice on the solution that is best for your application.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

When it comes to upgrading your car audio system to deliver amazingly realistic performance, many people choose what’s known as a three-way front stage. In the simplest of terms, this refers to there being three speakers on each side of the vehicle. As we explained in our article about the need for different speaker sizes, your audio system requires woofers and tweeters to reproduce your music properly. The question is, why add dedicated midrange drivers to the mix? Read on to learn the answer.

The Source of Sound

One key advantage of using a dedicated midrange speaker with a woofer and tweeter is that the small speaker can be placed on the dash or at the top of the door. These locations can deliver the perception that the voices are coming from a point higher in the vehicle than if you were relying solely on a woofer mounted in the door. Ideally, your audio system should — when designed, installed and calibrated properly with a DSP — seem to reproduce a soundstage that spans the middle of your windshield from pillar to pillar, or beyond.

In a two-way speaker system with a woofer and a tweeter, audio frequencies below 3,500 Hz may seem to come from the doors. This causes the height of the soundstage to be stretched vertically in the vehicle, and it detracts from the sense of realism. Adding a set of midrange speakers can raise the soundstage.

Consistent Frequency Response

Every speaker in the world, from every manufacturer, experiences a phenomenon called directivity. Directivity refers to the shape of the sound being created by the speaker with respect to the frequency of the sound. Take, for example, a 6.5-inch woofer. At low frequencies, the sound created by a speaker emanates in a spherical pattern that would, if possible, extend behind the speaker itself. This characteristic is part of why the shape of the mounting surface for a speaker is so crucial to its performance.

As the output frequency increases, the shape of the sphere moves from being equal in a 360-degree circle around the speaker to a ball in front of the speaker. The sound created outside this ball doesn’t cease to exist, but it is definitely attenuated (quieter).

A further increase in frequency results in the ball narrowing to what would be perceived as a spotlight shape. Unless you are in front of the speaker, these audio frequencies will be quieter.

Directivity is an issue because, in almost every vehicle, the speakers aren’t aimed on-axis with the listener. For a door-mounted woofer, we may be at as much as a 70-degree angle. If this speaker is operated with a tweeter and filtered at 4,000 Hz, we may be sitting in a position where there is very little output.

The chart below shows, in general terms, the maximum frequency you want to use for a given size of speaker in order to maintain smooth frequency response throughout the listening environment.

Increased Low-Frequency Output

For the product specialist designing your audio system, selecting a woofer, midrange and tweeter for a three-way front stage may, depending on the brand being used, allow for a slight improvement in bass performance from the woofers. Since there’s no need for the 6.5- to 8-inch woofers to produce midrange frequencies much above 400 Hz, the cones can be a little heavier, so they play deeper than a similarly sized speaker that needs to play to 4,000 Hz.

Some companies switch from using a phase plug on their midrange drivers that play higher frequencies to adding a dust cap. The dust cap adds mass to lower the resonant frequency of the driver.

Another advantage, especially for those audio systems without a subwoofer: Your installer can turn up the output of the woofer relative to the midrange to increase bass output.

Though the image below doesn’t exactly follow the laws of physics, it shows how the output of a three-way speaker system delivers even sound distribution at the listening position.

The image below shows, in the same artistic fashion, how upper-midrange audio information from a door-mounted woofer may not be clearly audible at the listening position.

Three-Way Front Stage System Design

Many companies offer three-way speaker sets that include passive crossover. Passive crossovers allow all three speakers on each side of the vehicle to be powered by a single amplifier channel. In most cases, the crossover points are fixed with passive networks, though some do include options for midrange and tweeter output levels.

To optimize the performance of a three-way front speaker set, it’s best to use a six-channel amplifier along with a digital signal processor. The features of the processor allow your installer to fine-tune the output of each speaker to deliver accurate and natural frequency response at the listening position.

Upgrade Your Car Audio System Today!

If you’re after the most realistic, detailed and accurate car audio system performance available, drop by your local specialty mobile enhancement retailer and ask about upgrading your vehicle with a three-way speaker set. Once it’s installed and calibrated, the results will be amazing!

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

A while back, we took a look at the difference between single and dual-voice-coil subwoofers. In a nutshell, there is no benefit or drawback to either choice. The options exist so you can present the amplifier you have chosen with a load impedance that allows it to make power reliably. There have been many instances where consumers bring a subwoofer into a car stereo shop in hopes of using it in their vehicle only to find out it’s not the right impedance. They need a 4-ohm sub instead of a dual 4-ohm. The question then becomes, can you use just one of the voice coils? That’s a fair question, and it deserves a detailed answer.

Can You Use Only One Subwoofer Voice Coil?

If you look at the voice coil former and the winding of a single-voice-coil sub, you’ll find a single positive and negative connection. On a dual-voice-coil sub, there are two pieces of wire and two sets of connections. The coils can be wired in series, in parallel or individually to power the subwoofer.

Car audio installers can use a set of specifications called Thiele/Small parameters in conjunction with modeling software to predict the performance of a subwoofer in a specific enclosure. All subwoofer manufacturers provide these specifications with both coils in use.

The short answer about using one coil is no. Using one coil not only affects the thermal power handling capabilities of a subwoofer, but it also changes the strength of the magnetic field and alters the Thiele/Small specifications. Rather than explain the math involved in the change, let’s use an example.

Thiele/Small Parameter Changes

Let’s start by measuring the parameters of an inexpensive 12-inch dual-voice-coil subwoofer. We measured the subwoofer with both coils wired in parallel, then we disconnected one coil and repeated the test. The results are in the chart below:

Sealed Enclosure Performance

We modeled the subwoofer performance using both voice configurations in VituixCAD 2 software. We started with a simple sealed (acoustic suspension) enclosure with a net internal volume of 1.08 cubic feet. Graph 1 shows the output of the subwoofer with 2.83 V (which is 8-watts into a 4-ohm load) applied to the coil. The response is nice and smooth, with a good transition from the midbass region and very little ripple. The Qtc (Total Q Factor) for the system with both coils connected is 0.8. This is a good balance between low-frequency extension, efficiency and low-distortion operation. The black trace that slopes down on the left shows predicted SPL.

Graph 2 shows the response of the same driver when only one voice coil is connected. You can see that the overall output of the system has decreased significantly and that there is a lot more ripple (a big bump in the response around 57 Hz. It’s also worth noting that the total system Q has jumped to 1.18. This Qtc value is quite high and will add resonance and distortion to the system.

Vented Enclosure Performance

Next, we designed a vented (bass reflex) enclosure using the specs taken with both coils in parallel. We simulated a 2-cubic-foot enclosure with a vent tuned to 35 Hz. As you can see from Graph 3 below, using both voice coils results in relatively smooth performance across the entirety of the operating range. The system efficiency has increased dramatically as compared to the sealed enclosure, with a jump in predicted output from 87 dB to 94 dB with the same amount of power.

Finally, we can see in Graph 4 how poor the performance of the subwoofer is with only one coil connected. Output at 40 Hz has dropped by 4.5 dB, and the overall response definitely isn’t as smooth.

Make Sure Your Subwoofers are Wired Properly

The above simulations clearly demonstrate the importance of using car audio subwoofers as they were designed. Your installer will need to wire both voice coils to your amplifier to ensure that the subwoofer maintains its power rating and functions the way the manufacturer designed it. If you have any questions, work with your local specialty mobile enhancement retailer.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.