If you’ve asked your local specialty mobile enhancement retailer about upgrading your car stereo in recent years, they may have told you that they will need to add load resistors. While seemingly new in concept, the purpose of these resistors is simple. They’re often needed to ensure that the factory-installed amplifier in your car, truck or SUV continues to function the way it was designed.

Modern Car Audio Systems and Class-D Amplifiers

Most modern car audio amplifiers, especially those installed by a vehicle manufacturer on an assembly line, use Class-D output circuitry to provide a balance of sound quality and amplifier efficiency. Automakers are fanatical about reducing fuel consumption. If an electronics supplier can deliver an amplifier that draws less current, there’s less load on the alternator, and the engine has to do less work.

All Class-D amplifiers use a filter circuit on the switching devices’ output to remove high-frequency noise caused by the MOSFETs switching off and on. The output of the filter is the original audio signal. These filters typically comprise a capacitor and an inductor. The value of these components is based on the load (speaker) impedance the amplifier is designed to drive. If the values are incorrect, high-frequency audio information may be attenuated, or too much noise might be allowed into the output.

Upgrading a Mobile Audio System

If you visit a local stereo shop and tell them you want to upgrade your audio system, they’ll likely suggest new speakers and an amplifier. In many cases, the shop will use the factory amplifier’s output to feed to the new amp. All good so far.

When the speaker is disconnected from the factory-installed Class-D amplifier, the filter circuit that was designed for a 2- or 4-ohm load isn’t going to function correctly. The signal presented to the capacitor and inductor will cause an oscillation, and a great deal of high-frequency noise may be added to the signal.

Another issue is that these oscillations in the output filter network may become quite significant in terms of their voltage. These voltages can damage components in the amplifier. Companies like Dodge, Chrysler and Ram have circuitry in their radios and amplifiers that prevents them from producing any output if their original speaker isn’t connected.

Upgrade Your Car Stereo Today for Better Sound!

If you can’t turn the volume on your factory-installed car stereo system up without the system distorting, drop by a local car stereo retailer and ask about upgrading your audio system with new speakers and a more powerful amplifier. If they mention that the upgrade will require load-resistors to prevent noise or hiss, now you know why.

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 look at some amateur car audio forums and Facebook groups, you’ll see a disturbing trend of people using speakers that are intended for home applications in their cars. While these speakers initially seem to offer impressive performance for their price, they simply aren’t designed to withstand the challenges associated with a mobile audio system. In this article, we’ll delve into what makes a “real” car audio speaker the only choice for your next sound system upgrade.

Why Is the Car Audio Environment Different From Your Home?

We’ll get into the environmental challenges of car audio systems shortly, but the biggest difference between listening to music in our cars or trucks and our homes is the volume level. If you are really into a song and decide to crank up the music in your car, volume levels can easily exceed 110 to 120 dB. This volume level taxes the power production capabilities of your amplifiers and, of course, gives your speakers a thorough workout.

Trying to listen to music at that volume level at home would quickly result in your neighbors calling the police to register a noise complaint. Quite simply, we listen to our car stereos at much higher volume levels than we do at home, and each speaker in the system needs to be able to handle a lot more power.

Power Handling Specifications

Marketing departments often take advantage of the topic of speaker power handling to inflate the perceived quality or value of a speaker. Printing a “peak power” number that is four or five times the continuous power handling number is functionally useless.

Useful power handling specifications define two numbers. The continuous power handling specification, also called the RMS power handling spec, is the amount of power that the speaker can handle indefinitely. Most companies test the power handling abilities of their speakers using pink noise that approximates the musical energy distribution found in music.

The peak power handling specification takes into account the dynamic nature of the music we listen to. Modern rock or pop music has a crest factor of 10dB. The crest factor means the quiet parts of the track are, on average, 10dB quieter than the loudest parts.

It’s important to note that the operating frequency range is a fundamental and important part of this specification. We wouldn’t feed a midrange driver bass frequencies below 60Hz and likewise, wouldn’t feed a tweeter midrange and bass information below 1,000Hz.

The number one reason speakers fail is that they can’t withstand the heat created by the power sent to the speaker. One relatively easy way to compare the thermal power handling capabilities of a speaker is to look at the diameter of the voice coil. Larger coils can handle more power because the heat is spread over a larger area.

High-quality speakers with accurate power handling specifications will typically have relatively large voice coil formers for the given speaker size. A 6.5-inch midrange designed to handle a true and genuine 100 watts of power will need a voice coil that’s at least 1.5 to 2 inches in diameter to dissipate that thermal energy.

Cone Excursion Considerations

You can tell a lot about the knowledge and experience of a speaker designer by analyzing the specifications and features of an entire series of speakers. How one speaker is intended to work with the models that operate in adjacent frequency ranges is quite telling. For mid-range and mid-bass drivers, power handling is a significant consideration, but so is excursion capability. In terms of speaker specifications, the Xmax spec describes how far forward or backward a speaker can move linearly. If you double this value and multiply it by the effective cone area, you get the total volume of air a speaker can displace.

In most cases, more excursion capability means two things: more output capability and, when designed properly, improved linearity when operated at lower excursion levels. A speaker that can handle a lot of power but can’t move very far without distortion isn’t much use as a mid-bass driver in a high-power system. Most 6.5-inch midrange and mid-bass drivers have Xmax specifications of at least 4.5 mm in each direction. With that said, some exceptional designs offer twice that specification.

In home audio, the crossover point between a woofer and a midrange driver is typically much higher and as such, home audio drivers simply can’t keep up. Sure, at low volume levels, this isn’t an issue. When you want to crank things up, hearing a voice coil former crash into a bottom plate is cringe-worthy.

Environmental Testing Considerations

For a true car audio speaker, the challenge of ensuring the driver design’s reliability means that the speaker needs to be tested in the same environmental conditions in which the speaker will operate. Companies that have detailed testing procedures and own or work with experienced and properly equipped factories have extensive processes that involve both extremely low and high temperature and high humidity tests. We are talking about low temperatures that would result in an extreme cold weather warning in most cities and high temperatures that exceed world-record setting conditions. Likewise, extensive testing will include both low and high humidity conditions.

North America doesn’t rank as the most humid place in the world, but states like Florida, Louisiana and Texas do see their fair share of humidity. Southeast Asia, on the other hand, registers 100 percent humidity almost all the time.

A speaker design engineer needs to take humidity into account, especially when the design uses a pulp or pressed paper cone that can absorb moisture. Coatings must be applied to ensure the speaker parameters don’t change, and adhesives must be chosen to ensure reliable bonds between components. Humidity can also cause corrosion on exposed carbon steel components in the motor structure and chassis design if they are not coated.

Ultraviolet Exposure Considerations

Face it: Speakers designed for car audio applications are essentially outdoors and exposed to ultraviolet radiation. You can imagine the importance of using designs and materials that can withstand UV in a marine application, but conventional car audio speakers that are mounted in the dash or rear deck of a vehicle see similar amounts of sun exposure.

UV exposure will cause what is known as UV degradation. Synthetic polymers like polypropylene and synthetic rubbers that do not include UV stabilities will discolor quickly, develop a chalky surface, crack or, in some cases, disintegrate completely. A change in color does not necessarily affect the performance of a speaker. If the damage continues, though, the weight of the speaker cone may be affected, and the compliance (stiffness) of the suspension may change. The result will be unpredictable performance and eventually complete failure.

Physical Assembly Testing

Loudspeaker drivers designed for home audio applications only have to withstand the vibrations that the speaker system itself creates. In a mobile environment, our cars and trucks impose huge physical forces on the speaker. Bumps in the road can yield high g-forces that multiply the weight of the motor structure several times. Likewise, an out-of-balance wheel and tire can cause vibrations that are felt through the entire vehicle chassis.

The fasteners and materials used to assemble these speakers and the mounting and installation hardware must withstand these vibrations at both low and high frequencies. Manufacturers of high-quality speakers will use industrial vibration tables to test the suitability of the design and materials.

How to Choose a Speaker For your Car

Most companies guard their quality control and qualification testing standards closely, making it effectively impossible to search for that information. When you are shopping for speakers, look around the retailer’s showroom to see what they have on display. Ask to see what a new speaker looks like and hold it next to one that has been on a display board for a few months or years. If you see discoloration, that could be a sign that the materials chosen may not be up to the challenge of the automotive environment.

Next, choose speakers from reputable car audio brands that focus on engineering rather than marketing. If you don’t have time to do the research for yourself, ask your local mobile electronics specialist what makes the brand you are interested in better than its competitors. If they can’t provide an answer based on physical features and designs, it might be worth continuing your shopping. To be clear, “They sound great” or “Lots of competitors use them” aren’t valid answers. Oh, and don’t be fooled by “price marketing.” Just because one speaker is more expensive doesn’t mean it has more features and technologies that make it sound good, or that it has undergone proper design testing.

Last and most certainly not least, make sure you audition the speakers you intend to buy and compare them to other offerings. The key to a quality speaker lies in its lack of distortion and its clarity, especially at high volume levels. If you find a solution that is well-designed and sounds great, you will be able to enjoy your music for decades to come. Visit your local mobile enhancement retailer today to find out what automobile-specific options are available. They’ll be a worthy investment!

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 almost as long as there have been car audio enthusiasts, they have been adding amplifiers to their vehicles to increase the power available to drive their speakers. More power means we can turn the volume up higher without distortion. When it came to connecting amplifiers to speakers, the first power boosters connected to the speaker wires of the radio. Now, modern head units offer dedicated RCA preamp outputs to make things easier.

Manufacturers specify how much voltage their source units can produce on these preamp outputs. Most radios offer at least 2 volts of signal, but some offer 4 volt, 5 volt or even 8 volts. Does this preamp voltage matter? Does more voltage make your system louder? Read on to find out.

Signal Chain

When we install an aftermarket radio in your dash and connect it to the amplifier, we have to make at least one adjustment to ensure everything will work properly. We refer to this step as “setting the gains.” This procedure involves matching the output voltage limit of the source unit with power production capabilities of the amplifier to ensure that both achieve maximum output when the volume is cranked all the way up.

Adjusting an amplifier’s sensitivity is, in theory, a simple process. That said, there are many things to take into consideration. How loud is the source material recorded? Does the head unit distort at full volume? Has anyone adjusted the radio’s tone controls or equalizer? Does the amplifier have any tone controls? If any one of these criteria are overlooked, the sensitivity adjustment may not be accurate.

What is the drawback to an improperly configured sensitivity control? If it is set too low, then you cannot get all the power available from the amplifier to your speakers. If the sensitivity control is set too high, then you can easily distort the output of the amplifier. A second side effect of adjusting the sensitivity control too high is that you increase the noise produced by the amplifier. Nobody wants to hear a hiss in the background of their music, so setting things properly is critical.

What Does High Preamp Voltage Do?

Some intensive research among several of us “old” car audio enthusiasts revealed that there were even a few twin-shaft cassette receivers with high-voltage preamp outputs. Some sales and marketing folks decided that more voltage meant more volume. In the early ’90s, several head unit manufacturers started marketing their radios as having voltage preamp outputs. If nothing else changed, sure – in theory, more voltage means more output. That said, if you swap from a 2 V source unit to a 4 V, and then readjust the sensitivity control on your amplifier down to compensate for the extra voltage, the maximum output level should stay the same.

If more voltage does not make your system louder, what is the benefit of this extra voltage? The answer is a reduction in gain of the amplifier and, thus, a reduction in potential noise. If your amplifier is set up to produce full power with a 2 volt signal and has a subsequent signal to noise ratio of 85 dB, then it is not unreasonable to expect that the noise would reduce by about 3 dB when we turn the sensitivity down by the same amount.

To the Test

We set up a premium consumer-grade amplifier on the lab test bench. It had an S/N Ratio specification of 89 dB when producing 1 watt of output and connected to a 4 ohm load. This is a pretty good rating these days. We set the amp up to produce 1 volt of output with a 1 kHz sine wave at a reference level for our function generator at -30 dB relative to full signal. We then took a long frequency response measurement.

The next step was to reduce the output signal of the function generator by an arbitrary amount – we chose 6 dB. We readjusted the sensitivity control of the amplifier so the output level was once again 1 volt and took another long frequency response measurement.

The results of the two measurements are shown in Figure 1.

In Figure 1, you can see the large spike in frequency at 1 kHz on the right side of the screen. We zoomed in to fill the screen with as much information as possible below this frequency. The gold line shows the background noise produced by the amplifier with the sensitivity control at the lower of the two settings. The green line shows the background noise when we increased the sensitivity of the amplifier by 6 dB.

Figure 2 shows the difference in noise level at 100 Hz. Not surprisingly, the difference in noise is just under 6 dB.

Conclusion

When it’s time to go shopping for a new source unit, among the dozens of cool connectivity features, options for display technologies and different brand names, paying attention to specifications is still important. If you are planning to add an amplifier to your mobile electronics system, make sure your source unit can produce at 4 volts of output on the preamps. This extra voltage will allow your installer to reduce both the sensitivity controls on your amplifier and the background noise level of the system.

Visit your local mobile electronics specialist retailer today for more information on which source units have high-voltage preamp outputs.

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.