A few weeks ago, we were browsing a few car audio related Facebook groups and Reddit subreddits. We saw the same question come up time after time: “I have two dual 2-ohm voice coil subs; how do I show the amp a 1-ohm load to make max power?” It wasn’t just once or twice that this question was asked – we saw it and versions of it at least a dozen times in less than an hour of browsing. Clearly, this is a topic that consumers struggle to understand. Subwoofer wiring configuration is crucial information if you’re shopping for an amplifier to mate to an existing set of subwoofers, or vice-versa.

Subwoofer Voice Coil Impedances

We’ve talked about single- and dual-voice coil designs in the past. In essence, they exist to provide the Product Specialist designing your car audio system with different wiring options so that they can maximize the performance of your subwoofer amplifier.

We’ll be clear: A single voice coil subwoofer isn’t “better” than a dual voice coil sub, or vice versa. The only benefit of a DVC sub is that the coils can be wired in series, in parallel, or each can be fed directly to an amplifier channel. There are benefits in terms of installation flexibility, but the number of connections to the voice coil doesn’t improve the driver’s performance or sound quality.

In subwoofers available from professional mobile enhancement retailers, you’ll typically see either single 2-ohm or single 4-ohm drivers and dual 2-ohm or dual 4-ohm offerings. There are some unique offerings as well. The JL Audio W7AE drivers have a single 3-ohm coil, so three drivers can be wired together to present a 1-ohm load to an amp. Audiotec-Fischer has some quad 3-ohm-coil woofers designed to extract maximum power from its line of non-bridgeable amplifiers.

When you get into some competition-level subwoofer products, you may see some dual 1-ohm coil configurations. If we think long and hard, we seem to recall hearing about a quad 0.7-ohm coil competition-level subwoofer. Nevertheless, there are a lot of options available.

Picking the Final Load Impedance

When it comes to wiring subwoofers to an amplifier, your installer has a few wiring options: parallel, series and series-parallel. The voice coils of a single dual-voice-coil woofer can also be wired in series or parallel. The goal of the options is to present the amplifier with a net impedance that allows it to make the best use of the available power.

You see, most subwoofer amplifiers produce more power as the load impedance drops. If we look at an amp like the ARC Audio X2-1100.1, it’s rated to produce 350 watts into a 4-ohm load, 600 watts when connected to a 2-ohm load and 1,100 watts when the subwoofers are wired to present a 1-ohm load. If you’ve purchased this amp, it only makes sense to present it with a 1-ohm load so that you get the most power available for your investment.

Here’s a small aside about amplifiers: Many subwoofer amps don’t make more power into lower impedances yet are still designed and listed as capable of running these loads. For example, the Rockford Fosgate T1500-1bdCP is rated to produce 1,000 watts when connected to a 4-ohm load, then 1,500 watts when wired to a 2- or 1-ohm load. The same goes for the Hertz Mille Power 1. It produces 600 watts into a 4-ohm load and 1,000 watts when connected to a 2-or 1-ohm configuration.

We highly recommend wiring these amplifiers to 2 ohms. The higher impedance will improve the amplifier’s efficiency, allowing you to deliver more power to your subwoofers with less draw on your electrical system.

Subwoofer Wiring Options

Let’s start by looking at wiring pairs of subwoofers to an amplifier. The simplest would be two 4-ohm single-voice-coil subwoofers wired in parallel. Two identical loads wired in parallel present a single load that is half of the single-load impedance. In this case, two 4-ohm subwoofers present the amplifier with a net 2-ohm load.

If you have an amplifier like the Rockford Fosgate T1500-1bdCP that we’ve used in the drawings that include a pair of terminals, you can wire one sub to each terminal. The two positive terminals are wired together inside the amp, so are the negative terminals. Having two sets of terminals can help keep the wiring neat and tidy in installations like this.

In both of the above scenarios, the load the amp “sees” is half of the impedance of one woofer. The chart below shows you the net load for common subwoofer options.

A less common configuration is to use low-impedance drivers and wire the subwoofers in series. Series wiring results in the sum of each voice coil impedance being added. For a pair of 4-ohm woofers wired in parallel, the amp would “see” an 8-ohm load.

A quick note about wiring subwoofer voice coils in series: Sometimes it’s necessary when multiple drivers are involved. With that said, there is a small drawback. The inductance of the individual voice coils adds and can result in a very reactive load for the amplifier. This configuration can cause instability in lower-quality amplifiers and reduce mid-bass performance from the system. The latter can result in difficulty getting the sound from the subwoofers to blend with the midbass woofers so that the bass appears to be coming from the front of the vehicle. It’s more important to present the amplifier with the correct load, but keep this in mind.

Dual Voice Coil Subwoofer Wiring

Let’s start by looking at the options to wire a single DVC subwoofer to an amplifier. We have the choice of wiring the individual coils in series or in parallel.

Wiring multiple dual voice coil subwoofers to a single amplifier presents a whole gamut of options. For example, we can wire all the coils in series, all in parallel, or mix them in a combination of series and parallel. It’s crucial to note that each DVC subwoofer in the system needs to be wired the same way. If one has its coils wired in series, all of them need to be wired in series. The same goes for wiring them in parallel. We’ll explain with a few examples.

Let’s start with an example of two dual-voice coil subwoofers with all four coils wired in parallel. This is a common configuration for dual 4-ohm subs wired to an amplifier optimized to make power into a 1-ohm load.

In this example, the load seen by the amplifier is one-quarter the value of each coil. So, for 4-ohm voice coils, the amp will see a 1-ohm load.

It is also possible to wire all four voice coils in series. This might be an option for drivers with very low impedance coils, like a dual 1-ohm design. Just as with our single voice coil subwoofers, wiring in series adds the impedance of each coil. This is a very unlikely configuration in a car audio system.

Now that you’ve had a taste of how subwoofers and their voice coils can be wired in series or parallel, let’s combine both to create what is easily the most common configuration: two dual-voice-coil subwoofers with their coils wired in series, and both drivers wired in parallel with each other. In this configuration, the amplifier “sees” a load that’s equivalent to the impedance of a single coil.

You can wire more than just two subwoofers together in series, parallel and series-parallel wiring. Four, eight or even 16 subwoofers can be driven from a single monoblock amplifier, so long as the net impedance is correct.

Two Amplifiers on a Single Subwoofer

If you have a dual voice coil subwoofer and would like your installer to use two amplifiers or all four channels of a four-channel amplifier, the electrical connections are quite easy. In either scenario, your installer will need to use a digital scope with a voltage readout or a high-quality digital multimeter to ensure that each coil is receiving exactly the same voltage. In addition, the subwoofer will need to be chosen such that each coil’s impedance maximizes the power available from the amplifier.

If you want to use a four-channel amp, then the connections to the subwoofer are the same. One coil will be wired to channels one and two in a bridged configuration and the other coil to channels three and four when bridged. In almost all cases, a four-channel amp will want each pair of bridged channels to see a 4-ohm load, so a subwoofer with dual 4-ohm voice coils is the only choice.

A Few Rules To Keep in Mind

There are a few guidelines to keep in mind as you and the Product Specialist you’re working with discuss your system design. First, all subwoofers connected to an amplifier should have the same impedance. For example, you can’t use a dual 2-ohm and a dual 4-ohm sub on the same amplifier.

The second rule is that all the voice coils of a DVC or quad-coil subwoofer must be used. If you need a 2-ohm load but have a dual 2-ohm subwoofer, you need to change the amp or the sub in the system design. Using one voice coil changes all the parameters of the woofer and doesn’t promise good sound or reliability.

Last, all the subwoofers should be installed in enclosures with the same air volume. We once heard a “story” where a vehicle owner told us one 12-inch sub was in a small enclosure for fast bass, and the other was in a large enclosure so it would produce deep bass. No. Just No. Please don’t do this. They all need to have the same amount of air behind them in the enclosure to deliver predictable results.

Oh, I suppose we should mention that mixing different size woofers or different models of woofers in one system is also less than ideal. That whole “fast” and “deep” stuff is silly. If you want to use some smaller woofers for midbass, that makes complete sense. This is popular in system applications where high-efficiency PA-style speakers are being used. With that said, very few car audio subwoofers are designed to play up to 250 or 300 Hz without some significant amounts of cone resonance distortion.

Anyway, please don’t mix and match different size subwoofers in a single system. We recommend saving your money and having the subwoofer system designed and installed properly. Your local specialty mobile enhancement retailer can help you choose the best amplifier and subwoofer combination, then design an enclosure that will make the best use of the space in your vehicle, so your stereo system sounds 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.

For car audio enthusiasts who’ve invested time studying the Thiele/Small parameters of speakers and subwoofers, you’ll know there is a specification called Xmax. This variable, typically expressed in millimeters or inches, is intended to describe the one-way excursion capability of the driver. Put another way, it’s the distance the cone can move forward or rearward from its rest position before distortion overwhelms the signal being produced.

Mechanical Xmax Calculations

The simplest way to calculate the Xmax specification is to subtract the thickness of the top plate from the height of the voice coil winding and divide it by two. This calculation describes how far the woofer (or midrange or subwoofer) cone can move forward while keeping the voice coil inside the magnetic gap. If and when the coil attempts to move out of the gap (determined by the thickness of the top plate), the strength of the magnetic field is dramatically reduced and the output of the speaker becomes non-linear. If the audio signal being produced by the speaker isn’t the same as the signal sent from the amplifier, significant distortion is being added to the signal, and your music won’t sound right.

It’s worth having a baseline for values that are considered normal and impressive in terms of Xmax. Tweeters seldom have an Xmax specification, but that doesn’t mean it’s not a value that can be calculated. For example, a linear excursion of plus or minus 0.2 mm (0.0079 inch) would be an excellent specification for a tweeter. For a small 3- or 4-inch midrange driver, an Xmax of plus and minus 2 mm would be good, and greater than 4 mm would be excellent. For a 6.5-inch mid-woofer intended for a door speaker application, an Xmax value of 4 mm would be good, and anything above 8 mm would be exceptional.

Subwoofer excursion capability increases dramatically as the price of a driver increases. For entry-level 10- and 12-inch subwoofers, rated for 300 to 450 watts, an Xmax of 10 to 12 mm is good. Stepping up to mid-level woofers with power ratings between 500 and 600 watts typically results in the Xmax spec jumping up to 14 or 16 mm in each direction. The cream of the crop in subwoofers offer Xmax capabilities beyond 18 mm, with some drivers capable of more than 24.5 mm (1 inch) of excursion in each direction. Shallow-mount subwoofers typically scale these values down a bit, with mid-level drivers capable of 8 to 10 mm of excursion and premium solutions capable of more than 15 millimeters.

Those who care less about sound quality and are interested only in achieving maximum sound pressure levels can continue to increase the power to a speaker beyond the Xmax value. In this scenario, peak SPL measurements are typically all that matter, so sound quality and accuracy take a back seat.

Xmax Isn’t Always a Calculation

A second way to specify the Xmax parameter involves using acoustic test equipment and laser-based cone excursion measurement. Using this method, a signal is fed to the driver being tested at an increasing level until the speaker’s output reaches a distortion level of 10%. These performance-based measurements are crucial as they take into account non-linearities caused by limits in the compliance of the speaker’s suspension components.

For example, if a driver has a 16 mm top plate and a 32 mm voice coil height, the calculated Xmax would be 8 mm. Suppose the surround on the driver is very narrow or stiff and starts to limit cone travel at an excursion level of 5 mm. Once again, the output audio signal won’t replicate the signal from the amplifier, and significant harmonic and intermodulation distortion will be added to the output. The speaker also won’t play any louder. The surround or the spider can cause compliance-based excursion limiting, and these characteristics are usually the limiting factors in how loudly a speaker or subwoofer can play. In the car audio industry, very few manufacturers use the Xmax+10% specification method as investing in the test equipment is very expensive.

Though it isn’t as common as it was years ago, another mechanical measurement affects how loudly a speaker can play. If the bottom of the voice coil former runs into the bottom plate of a speaker, you’ll hear a very sharp snapping sound. Of course, this limits excursion and can damage the former very quickly. This situation happens when the suspension components aren’t stiff enough or if the driver is designed to be extremely shallow and there isn’t adequate space at the bottom of the driver.

One Last Thought on Xmax Specifications

If you’re shopping for a subwoofer, the Xmax spec isn’t the be-all, end-all in determining how loudly the subwoofer system will play. The way the driver interacts with the enclosure plays a huge role in determining the audio system’s effective efficiency and low-frequency extension. Check out our articles on speaker specifications and subwoofer enclosure designs for more insight into this topic.

When it’s time to choose the right subwoofer for your car or truck, drop by your local specialty mobile enhancement retailer and talk with them about your expectations and the speakers and subwoofers that are the best for your application. Be sure to audition one of their demo vehicles to see if the subwoofer solution meets your performance goals.

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.

For car audio enthusiasts, especially those interested in competing in SPL competitions, amplifier efficiency should be one of their primary concerns. In this first of three articles on efficiency, we take a close look at the Rockford Fosgate M5-1000X1 Class-D subwoofer amp and analyze how well it makes use of power at different output levels.

What Is Car Audio Subwoofer Amp Efficiency?

In the case of car audio amplifiers, or this specific analysis of a Class-D subwoofer amp, some of the voltage and current from your alternator and battery are converted into heat. This is wasted energy. Don’t fret, and it happens with every electronic device – even if the heat produced is minuscule.

When we talk about efficiency, we look at how much power is sent to the load (speakers) compared to how much is consumed by the amplifier. We set up the Rockford Fosgate M5-1000X1 that Dave MacKinnon reviewed for BestCarAudio.com in early 2021 on the bench. First, we used our D’Amore Engineering AMM-1 to confirm that we had a 1.0-ohm resistive load. Next, we measured output voltage, input voltage and input current at 60 points between idle and an output level of 800 watts where the CLEAN circuit put the amp into protection.

A few things to note. The amplifier idles at about 1.5 amps of current. That’s the power it takes to drive the circuitry without the amp producing any power. At output levels below about 1 watt, this idle current significantly outweighs the current required to deliver power to the speaker. As such, the efficiency in this range is well under 5%.

If you analyze the graph above, you can see that the efficiency increases very quickly to over 70% by the time the amplifier produces 140 watts. At this level, the current draw is 17.45 amps. Beyond that, efficiency increases to a maximum of 76% while delivering 400 watts. The current draw at that level is 49.78 amps.

Graphs like this have been floating around the car audio industry for many years and are often combined with a graph of a Class-AB amplifier to highlight the efficiency benefits of the Class-D design. One problem with this way of looking at the data is that the power delivery scale is linear. In the real world of reproducing audio, we know that we need to double the power to a speaker every time we want to increase the output by 3 dB. That’s a logarithmic requirement. Let’s analyze this same data with the percentage of maximum power on a logarithmic scale.

Suddenly, our amplifier doesn’t look quite as efficient. The numbers haven’t changed, though; it’s just the way we’re analyzing them. You may only need 5 or 10 watts to drive your subwoofer at very low volume levels. At these levels, the amplifier wastes a LOT of power as heat. The efficiency in this range is only 18 to 27%. This means that around a quarter of the power going into the amp is going to your speakers; the rest is running the amplifier’s signal processing and is being converted to heat.

How Manufacturers Specify Efficiency

When you’re looking at amplifier efficiency specifications, the odds are very good that they’ve been taken with the amp at full power. In almost every case, this is the level at which a Class-D amplifier makes the best use of the power it’s drawing from your electrical system.

What matters much more in the real world is what the amp is doing at 10% or 25% of its rated output. This is where 95% of users spend most of their time listening to music. More efficiency in this range means less draw on the electrical system. Efficiency is paramount for marine applications where we might be running on batteries for several hours. It’s not uncommon to find one amplifier that draws half as much current to produce the same output as another model with worse efficiency, especially when the amplifier is loaded down.

Efficiency and Load Impedance

One last item we’ll look at is efficiency versus the current required to produce power in the load. It would be impossible to count the number of times we’ve heard someone suggest loading an amp down with additional speakers or the use of lower impedance speakers to produce more power. If the power supply in the amp can deliver the current, sure, you get more. The question is, is there a cost to this power? Let’s see how our test amp behaves in terms of efficiency when driving a 2-ohm load.

As you can see, the amp makes better use of power when connected to a 2-ohm load. The peak efficiency is up to 84.7% at an output level of 691 watts, with the amp drawing 58.1 amps of current. Compare that to the 1-ohm load, and the amp was drawing 71.5 amps to make the same power. Those extra 13 amps of current are wasted as heat.

It’s also worth noting that the amp produced more power in this 2-ohm load configuration before the CLEAN output distortion indicator was illuminated. Instead of 800.3 watts, it produced 865.3 watts. That’s 8.1% more power, for free, with less current draw and less heat. Sooo … if the amplifier you’ve chosen is rated to produce the same or even remotely similar power levels into 2- and 1-ohm loads, please don’t load it down.

Does Amplifier Temperature Effect Efficiency?

While we had the Rockford back on the bench, we took one more set of measurements. We wanted to know if the amplifier would become less efficient as it got hotter. Now, it’s crucial to keep in mind that this particular test will be specific to this amplifier from this company. There’s no way to predict if other designs work the same way. So we started the 2-ohm testing with the amplifier’s heat sink at 19 degrees Celsius and measured its efficiency. Then, we repeated the measurements as the temperature rose to 45, 50, 55 and 60 degrees. We’d have gone farther, but our load resistors were starting to make some strange crackling noises.

If someone had asked us to predict how the amp would behave, we’d have put all our chips on predicting that efficiency would decrease as the amp heated up. It only makes sense as wires, components and traces on the circuit board would increase resistance with temperature. The findings are very interesting and say a lot about the work the designers at Rockford Fosgate put into their products.

With Great Power Comes Increased Efficiency

There are three takeaways from this experiment. First, if you need a small amplifier for your midrange speakers or tweeters, choose one that only makes as much power as you need. A 150-watt amp is overkill when you’ll only use 10 or maybe 20 watts on a midrange driver. Second, if you compete in SPL competitions, choose a speaker load impedance that balances power production with amplifier efficiency. You may be throwing away power in the form of heat by loading your amplifiers down too much. If the limiting factor in your audio system is the ability to feed the amp, choosing the most efficient solution is crucial.

Last and certainly not least, amplifier efficiency depends on output level. Our testing shows that efficiency increases as power output approaches the rated output level when it comes to name-brand Class-D car audio amplifiers. Posting a single value on a website or in a brochure (do companies still make brochures?) doesn’t tell you much about the design or how efficient the amp will be when used at low power levels.

When it comes time to shop for an amplifier for your car or truck, drop by a local specialty mobile enhancement retailer and talk with them about the options for your vehicle. They’ll help you choose the right solution and integrate it into your vehicle so that it sounds excellent and functions reliably for many decades.

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.

If you’ve been around the car audio scene for a few decades, then you’ll recall a time when almost all amplifiers doubled their output power when the load impedance was halved. For example, an amplifier might have been rated to produce 75 watts per channel into a 4-ohm load and 150 watts into a 2-ohm load. It wasn’t unheard of to see low-power amplifiers like the infamous Orion 225 HCCA continue to double their power, right down to 0.5 ohms per channel. Do modern amplifiers behave like this? If not, why?

Making Power from an Amplifier

Let’s start with the basics. Your amplifier is designed to boost the voltage of the signal coming from your radio. By way of an example, let’s say you have a 1-volt RMS signal and you want the amplifier to produce 50 watts of power into a 4-ohm load. The signal would need to be increased to 14.14 volts out of the amp. That’s a signal gain of just over 5 decibels.

Because car audio speakers have a relatively low voice coil impedance (4 ohms), the amplifier will also deliver a significant amount of current to the speaker. In our 50-watt, 4-ohm example, the current flowing through the speaker is 3.536 amps.

One last piece of math. We’ll call this hypothetical amplifier a Class-D design and say that it has an overall efficiency of 80% when driving 4-ohm speakers. To produce 200 watts of power, the amp will need to consume 250 watts of power from the vehicle electrical system. If there’s 13.5 volts at the amplifier power terminals, it will draw 18.52 amps of current from the alternator and battery.

Old-School Amps Were Huge

Back in the day, nobody cared if a 200-watt amplifier was 20 inches long, 12 inches wide and had to be mounted in the trunk. Today, installers want that power from a package that will fit behind the radio or under a seat. Why does the size of the amplifier matter? Well, in order for it to function reliably, the heat sink needs to be large enough to keep the amplifier cool while it produces full power. In this case, our amp needs to shed 50 watts of heat. This number is the difference between the 250 watts it consumes and the 200 watts it delivers to the speakers. Dissipating 50 watts isn’t a significant issue.

Let’s add a second speaker to each channel of the amplifier so that we have a net load impedance of 2 ohms. The amplifier will still attempt to produce 14.14 volts on each output, but it will now flow 5 amps of current to each pair of speakers. We’re up to a total output current of 20 amps from our 14.14 amp draw at 4 ohms. When you load an amp down, its efficiency drops. Let’s say this Class-D amp offers 72% efficiency when driving 2-ohm loads. If the amp is to produce 50 watts to each of the eight speakers (400 watts), it needs to draw 555.6 watts from the car. At 13.5 volts, that would entail 41.15 amps of current flowing into the amplifier. The heat sink will need to dissipate 155.6 watts of thermal energy. That’s a LOT of heat.

When designing this amplifier, the engineer will need to come up with a way for it to manage this 155.6 watts of heat without allowing the components inside the amp to overheat. If the amp has to be very small, this might be a significant problem. Large heatsinks help radiate thermal energy into the air that surrounds the amplifier. Another cooling method is to add a fan to the amplifier design. Fans can dramatically reduce the size of an amplifier and help ensure that they run at cool temperatures. If you choose an amp with a fan, make sure it flows air across the heatsink where the output and switching devices are located. Blowing air into the middle of a circuit board does almost nothing.

The last option, and one that has become quite common, is to limit the amount of current the power supply will pass. Limiting current directly limits the amount of heat energy the heatsink needs to manage. Let’s reverse-engineer how much power our amplifier can produce if we limit the heat sink’s thermal capabilities to 120 watts.

If the energy wasted by the amp is 120 watts at an efficiency of 72%, then the total power it can consume is 430 watts, with 310 watts going to the speakers. In this scenario, each channel is producing 77.5 watts of power and each of the eight speakers would be receiving 38.75 watts when everything running at its maximum output capability.

Is There Anything Wrong with Current-Limited Amplifiers?

As we’ve demonstrated, limiting current controls the maximum amount of power an amplifier can produce in order to ensure that it doesn’t overheat when pushed to its limits. Are there any drawbacks to a design like this? Not really. Simply, you don’t get as much power when you load the amplifier outputs down further. This design decision isn’t directly detrimental to sound quality, distortion or the addition of noise to the audio signal.

If you’re shopping for an amplifier for your vehicle, drop by your local specialty mobile enhancement retailer and ask them about the best solution for your system design and budget.

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.

It’s time to dispel another unsubstantiated car audio myth! This time, we’re talking about claiming that you can’t ground an amplifier directly to the battery in a vehicle. Perhaps it would add to the clarity of the statement to say that many inexperienced installers think the ground wire for the amp has to go to the chassis of the vehicle. Many of these statements also add, “Connecting directly to the battery will cause noise.” All of these statements are nonsense. Let’s check them out.

How Electricity Flows

Whether you subscribe to the conventional current theory that electricity flows from the positive side of a power source through a load and back to the negative terminal, or to the electron theory that holds that electrons flow from negative to positive, what matters is that an electrical circuit is a loop. Your load, be it a light bulb, radio or amplifier, needs to have two electrical connections. Power flows from the electrical source through the load and back to the source again. The current in both conductors is equal.

Our vehicles have two power sources: the battery and the alternator. The battery is there to start the vehicle. It feeds a high-torque electric motor that spins the engine when you turn the ignition key to start. Once the engine starts, the alternator takes over the operation of the vehicle. Technically, you can remove the battery from the car and it will continue to run. Don’t try this, though, as many new vehicles monitor current flow in and out of the battery to control the alternator’s function. Once the engine is running, the alternator recharges the battery.

Having an Amplifier Installed in Your Vehicle

In the “good old days,” installing an amplifier in a car or truck involved having your installer run a large-gauge wire from the battery’s positive terminal through a fuse holder or circuit breaker to the positive terminal of the amplifier. The wire from the amplifier’s ground terminal would be bolted to the chassis of the vehicle.

Until about a decade ago, most cars and trucks were built with steel, and the chassis or unibody components were spot-welded together. As a result, there was often enough surface area to allow current to flow from the ground of the amp back to the negative terminal of the battery or alternator.

What’s Wrong with a Chassis Ground?

A few things could go wrong when using the chassis as a ground point for a high-power amplifier. First, the connection to the chassis needs to be secure and reliable. Second, your installer needs to brush or grind away any paint, rustproofing or undercoating from the metal before they drill a hole to make the connection. Third, the chassis may not be a direct electrical return path to the battery.

The ground connection needs to be secure. Because the cable is of a reasonably large gauge, it can experience moderate loads during acceleration and braking and from the vibrations associated with regular driving. This stress can loosen the connection and cause intermittent behavior or damage.

You’d think removing paint, primer and coatings would be easy, but the number of poor grounds we see is amazing. In some cases, especially with cars that are painted silver, some primers and corrosion-resistant coatings can look similar to bare metal.

In many modern vehicles, lighter metals such as aluminum are used in vehicle construction. These materials don’t conduct as well as steel. What’s more concerning is that many new vehicles are assembled with high-strength adhesives and even two-sided tapes from companies like 3M, Dow Chemical and Henkel. These materials are not electrically conductive.

Ohm’s Law in 20 Seconds

Whenever current flows through a device or conductor with resistance, a voltage is produced across that device. In the case of a piece of wire, the resistance is typically low, so very little voltage is wasted. When the current flow is significant, we can see several volts across the wire, and it will heat up. To calculate the voltage drop across a resistance, multiply the current flow (in amps) by the total resistance of the device (ohms). For example, 2 amps of current flowing through a 4-ohm resistor results in 8 volts being produced across that resistor.

Proper Amplifier Grounding

Feeding your high-performance amplifier with the power it needs means delivering as much voltage as possible to the power terminals. If the amplifier draws a significant amount of current, then your installer will need to use large-gauge conductors to prevent voltage losses due to the resistance in those conductors. This same logic applies to the ground return path. A test performed several decades ago by one of the engineers at JL Audio in Miramar, Florida, showed that most vehicle chassis have a resistance equivalent to a piece of four AWG cable. Modern vehicles are much worse.

Let’s say you have a good quality 1,400-watt amplifier like the ARC Audio ARC1000.4 DSP we tested a few months ago. This amp produced 1,406 watts of power (at 1% distortion) when provided with 13.37 volts at the power terminals. It drew 137.1 amps of current to deliver this power (at an impressive 76.7% efficiency). If your vehicle had a 16-foot run of all-copper, CTA-2015 compliant, four-AWG power cable to feed the amp, there would be a voltage drop of 0.588 volts across the length power cable. The ground return path (assuming it has the same resistance) would result in a similar loss of voltage at this current level. Your electrical system would need to be able to supply the battery with 14.546 volts and have a spare 137 amps of current capacity available. For a full-sized sedan or truck, these numbers are quite reasonable. So, you might be able to get away with using the chassis as a ground – if you know it’s all-steel from front to back.

When Grounding Goes Wrong

Now, what happens when we run into a vehicle where the chassis isn’t a good ground? Perhaps it only has the current-carrying capacity of a piece of 12-AWG wire. At low to moderate volume levels, our amplifier won’t draw much current. If the amp is a modern Class-D design, it will likely have a driver IC that includes low-voltage support for start-stop vehicles. If so, the amp won’t shut off even if the voltage drops below 6 volts for a moment.

Let’s run the math again. We want about 1,400 watts and will attempt to draw 137 amps of current. With a chassis with the current-carrying capacity of a 12-AWG conductor, 3.74 volts would be lost across our chassis connection. The amp will likely stay on, but it certainly isn’t going to produce 1,400 watts when it only sees 10.8 volts. In our experience, aluminum and bonded-construction chassis have even more resistance, and the voltage drops are significantly worse.

The Parallel Ground

Going back to the testing done by the folks at JL Audio, they propose that your installer run what they refer to as a parallel ground. The installer can bolt the ground lead from the amp to the chassis, then run a conductor of the same size as the power leads to the battery’s negative terminal.

In this scenario, the worst-case condition is that the new ground wire carries all the current from the amp, and you have a minimal voltage drop. More likely, some current will also flow through the chassis to the battery, and this results in less voltage being wasted than if it wasn’t grounded at the rear of the vehicle.

Stop The Noise

We can’t fathom where the myth about grounding to a battery and getting noise came from. Perhaps there was an installation that had a ground loop of some sort. Ground loops can (and do) happen when the chassis is used for grounding.

Let’s dispel this myth with one sentence. Think of all the marine audio systems and stereos in Corvettes that you’ve seen over the years. Fiberglass certainly can’t be used as a ground return path. These vehicles don’t have any noise problems associated with how power is distributed.

When it comes time to have an amplifier installed in your vehicle, you can improve its efficiency by providing it with as much voltage as possible. This means minimizing voltage losses due to power cable or ground return path resistance. Upgrading to larger or better wiring and having the installer at your local specialty mobile enhancement retailer run a parallel ground is a great way to ensure that your system will function reliably.

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