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Basics of OEM Integration in Your Newer Vehicle

OEM IntegrationInstalling a mobile audio system involves much more than just hooking up some wires and driving in a few screws. Almost all vehicle manufacturers have realized that some amount of audio system tuning will dramatically improve their customers’ experience. When you decide it’s time to upgrade your factory audio system, we need to take into account any tuning that may have been done by the factory. It’s not easy, but at the same time, it’s not impossible. This article provides a brief look at what is involved in connecting a new amplifier and speakers to your factory source unit or amplifier, or as it is called, OEM integration.

It’s Not Flat

No, we aren’t talking about the Earth. One of the first steps in designing a premium audio system upgrade is to measure the electrical signal that is being sent to each of your speakers. Why? As we mentioned, more and more factory source units and amplifiers include advanced signal processing. Even some of the most basic audio systems with only four speakers have custom equalization for each speaker. Once our installers know what they are dealing with, they use that information in designing your new system.

OEM IntegrationThe signal measurement process should be done before system design. If the signal is flat (no equalization), then the system design may take a different direction from when the signal is equalized. Compensating for factory equalization requires that some kind of equalizer be installed in the new system. Either way, we have to allot time to measure the acoustical response of the new system and make appropriate adjustments.

Think of this process as though your installer were a surgeon preparing to solve a medical problem for you. The surgeon will order x-rays, scans and many tests long before you’re on the operating table. Nobody wants to start work on a project without knowing what they are dealing with. Modern car audio systems are no different.

Front and Center

OEM IntegrationAutomobile manufacturers strive to make all their customers happy – within a certain budget. This goal includes providing an enjoyable audio experience for everyone in the vehicle. Many mid- and high-level factory audio systems use a center channel to help create a listening experience that is enjoyable for both occupants of the front seats. Configured properly, a center channel can help create a realistic soundstage from both seating positions. Trust us when we say that some of the people who tune these factory audio systems have a lot of experience doing what they do.

When we want to upgrade the audio system in a vehicle with a center channel, we have to reverse-engineer how that speaker is functioning. Is it a mono signal that is the sum of the left and right signals? Is it an up-mixed signal that plays sounds that are not reproduced by the left and right speakers? It could also be a little bit of both. Your installer will have to do some tests to decide what is happening and how to deal with it.

Do We Undo or Up-do?

OEM IntegrationIf you have a vehicle with a genuine up-mixed center channel, one of the best ways to upgrade the audio system is to leave the processing alone and add better speakers, amplifiers and signal processing. With this method, we can create a sound system that sounds amazing from both the driver and passenger seats.

The basic concept is to take the left, center and right signals; recombine any frequency filtering; then send those three signals to whatever speakers you choose for each side of the vehicle and the new center channel. Often, the new system will be a three-way speaker set in the doors and maybe the A-pillars, and a two-way system in the center. We will most certainly use a digital signal processor and/or an integration processor to do the signal filtering and tuning for the new system.

How We Undo What They Do

OEM Integration
The JL Audio Fix 82 can “fix” a number of factory signal problems.

Several processors on the market can sum filtered channels together and undo factory equalization and time alignment. Some of these processors require manual adjustments and some work automatically with a custom setup track. After using one of these processors to provide a full bandwidth signal, we can then use digital signal processing to set up the new system and compensate for the vehicle acoustics. Just as the vehicle manufacturer chose to equalize the signal going to each speaker, we need to do this to maximize the performance of your new system.

What if We Don’t Undo What They Did?

Imagine a simple scenario: You have a car, perhaps a Nissan Altima. Even the base model audio system has equalization in this vehicle. Equalization is based on the speaker that is being used, location of the speaker and acoustic effects of the vehicle on the signal from that speaker relative to the listening position. If we change the speaker to one that is better, the factory equalization can – and most likely will – work against us. That nice new speaker may sound worse than the cheap factory speaker. After all, the manufacturer tried to make what was likely a low-quality speaker sound OK.

Trust Your Local OEM Integration Experts

When it comes time to upgrade the audio system in a fairly new vehicle, make sure you are working with a mobile electronics retailer that knows how to evaluate the existing audio system properly. If you are changing the source unit, then it’s no big deal. But if you are using the factory radio and/or amplifier, proper testing is crucial. Don’t be scared or discouraged. A well-trained retailer can transform any audio system into something truly amazing! Drop in and see what they can do for you.
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.

Filed Under: ARTICLES, Car Audio, Integration, RESOURCE LIBRARY

Why are Speakers Different Sizes?

Speakers Different SizesWhen you head to your local mobile electronics specialist in search of new speakers, there are a few criteria to keep in mind. Choosing a speaker size can go one of two ways: You can pick something that fits a specific mounting location or you can choose based on the characteristics of the speaker relative to its size. Are you interested in knowing why speakers come in different sizes? Good! You’ve come to the right place.

What Does a Speaker Do?

It is the job of a speaker to convert the electrical signal from your amplifier into motion. The motion of the speaker cone excites the air around it. As the cone moves forward, the air in front of the cone is pressurized. As the cone moves rearward, the air is rarefied. These pressure waves extend out from the speaker and our ears detect these minute changes in pressure as sound. Pretty simple, isn’t it?

Things to Consider in Terms of Reproducing Sound

Speakers Different SizesWhen it comes to reproducing sounds, the lower the frequency, the harder it is to produce the sound. For every doubling of frequency, the speaker cone has to move a quarter the distance to produce the same level of output. As example, if your subwoofer has to move 2 mm to produce 95 dB of output and 40 Hz, it only has to move 0.5 mm to reproduce 95 dB at 80 Hz. To reproduce 95 dB of output at 160 Hz, the cone only has to move 0.125 mm.

The size of a speaker cone affects how much sound the speaker will create for a given amount of input signal. Let’s generalize things a little (because a lot of external factors affect this statement): A 12-inch speaker cone has to move twice as far as a 15-inch speaker cone to produce the same amount of output at a given frequency. That also means the 12-inch speaker requires more power to produce the same sound as the 15-inch.

Bigger is Always Better, Right?

Based on this logic, you should simply select the biggest possible speaker for every application, right? Well, it’s not quite that easy. When we get into midrange and high frequencies, the speaker cone has to move back and forth very fast. A 1,000 Hz tone requires that the speaker move forward and backward 1,000 times a second. A 10 kHz tone requires 10,000 of these same motions per second. If we use a big speaker with a relatively heavy cone, it’s very hard to keep up with the input signal. Why? Inertia.

Speakers Different SizesLet’s use an analogy to help explain this. Imagine that you are at a parade and waving a flag. The pole is 6 feet long and the flag on the end is 3×5-foot. You wave the flag back and forth as fast as you can. Even if you are really strong, the fastest you can wave it back and forth is once, maybe twice a second. Now, look at the little kid standing beside you at the parade. He has a little paper flag that’s 2×3 inches on a 5-inch-long plastic stick. His little hands can wave that flag back and forth five or six times a second.

Speaker engineers have to balance several characteristics to achieve specific goals for a given design. Let’s compare the weight of a speaker cone for a 10-inch subwoofer to that of a 10-inch midrange used in concerts and public address systems. A typical 10-inch sub that is designed to play frequencies below 150 Hz has a cone assembly (cone, voice coil, former, half the spider and half the surround) that weighs around 150 grams. A 10-inch speaker designed to be used for midrange frequencies (150 to 1 kHz) has a cone mass assembly of around 40 grams.

Clearly, the lighter assembly can move faster and keep up with the reproduction of higher frequencies.

Is Lighter Better?

Now we face the conundrum of balancing low- vs. high-frequency output. A lighter cone will move faster and is capable of producing extended high-frequency output. A heavier cone has a lower resonant frequency and thus, can produce more low-frequency output. Combine these generalizations with electrical issues affecting voice coil inductance, and we further hinder high-frequency output. It starts to become clear that we need different-sized speakers for different applications.

Subwoofers

Speakers Different SizesMost subwoofers are sized from 8 to 18 inches. Since subwoofers are designed to play frequencies below 100 Hz in car audio applications, they need a lot of excursion capability and a low resonant frequency. This means subwoofers will have relatively heavy cones. At high excursion levels, cones are exposed to significant stresses, so the cone has to be strong, and this further contributes to their weight. Subwoofers have to handle a lot of power. This power allows us to move the cone over relatively large distances. Power handling requires bigger components in the form of large-diameter voice-coil formers and windings.

Midbass Drivers

Speakers Different SizesA dedicated midbass driver is typically designed to play from around 50 to 500 Hz. Sizes are typically 6.5 to 8 inches in size, but some people have used 10- and 12-inch drivers. The cone has to be heavier than that of a midrange, but not heavy enough to slow it down for higher frequencies.

If you look at the frequency content of a performer, you will see that many voices extend down to 100 Hz. Accuracy in speed is important in this frequency range. Resonances and non-linear behavior causes harmonic distortion. This is often perceived as “warmth” in the midbass region. We do not want anything extra in our music, so accuracy is what matters.

Midrange Speakers

Speakers Different SizesMidrange speakers become a balancing act of several different characteristics. Of course, the cone has to be relatively light, but managing linearity and distortion becomes an even higher priority. It’s easier to hear distortion at midrange frequencies. The cone has to balance mass, damping and strength to prevent deforming and cause harmonics. The suspension has to be very linear.

Managing inductance also becomes a more significant issue because it can reduce high frequency output. Midrange drivers for typical car audio applications vary in size from 6.5 inches and 6×9 inches on the large side down to as small as 2.5 inches. Many midrange drivers try to do double-duty as midbass drivers for use in two- or three-way audio systems. While this is a minor compromise, it is a necessity. We consider midrange speakers to cover the range from 100 Hz to 3,000 or 4,000 Hz.

Tweeters

Speakers Different SizesTo reproduce frequencies above 2.5 kHz, tweeters need very light cones. Tweeter cones don’t move very far, so they don’t require much excursion, but there still has to be a suspension. Resonances in the cone can wreak havoc with frequency response. Premium tweeters may make use of features like ferrofluid in the gap to improve power handling. Premium tweeters may also include a copper pole-piece cap to reduce inductance and distortion.

Directivity Considerations

Another consideration when choosing speakers is that all speakers above a certain frequency start to become directional. Directivity refers to a reduction in high-frequency output as you move off-axis to the speaker. If you choose your speakers and design your system carefully, you can minimize the effect of directivity. The only real consideration would be to have your tweeters pointed at you.

The Balancing Act

The applications for the information in this article vary, depending on your overall goal for your audio system upgrade. A simple set of coaxial replacement speakers will be chosen by the size application. If you are building a high-end audio system with multiple amplifiers, channels, digital signal processing and custom speaker mounting locations, then choosing the right speakers in terms of their quality and intended application becomes more important.

Learn More about Speakers and Their Different Sizes at Your Local Retailer

Your local mobile electronics specialist retailer can help you choose the right speakers for your application and performance goals. Drop in at a local shop today and have a listen to their demo board or demo vehicle. It’s an amazing experience!

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.

Filed Under: ARTICLES, Car Audio, RESOURCE LIBRARY

Why Buy A Bigger Amplifier?

AmplifierWhen an audio enthusiast goes shopping for an amplifier, the question of “how much power do I need?” comes up almost every time. There are a few factors to take into consideration when answering this question. This article looks at those factors and provides some technical background to help support your decision.

Why Do We Need Power?

AmplifierQuite simply, when you send more power to a speaker, it moves farther and produces more output. Two limiting factors within the speaker itself control how much power it can handle. At higher frequencies, the limit is heat. Speakers are notoriously inefficient. The best convert about 2% of the energy sent to them into sound and the rest is converted to heat. When you send 60 watts of power to a speaker, most of that energy heats up the voice coil and the components around it. Eventually, those components will reach a temperature where they will fail. The speaker will usually stop working at this point, or shortly after.

The second limiting factor is how far the speaker can move. Inexpensive midrange speakers may be able to move back and forth about half an inch without creating massive distortion. Higher-end speakers have as much as twice as much cone excursion capability. (Speakers don’t sound the same at high volumes as they do at low. Audition your speakers at the volume you will be using them.)

Power vs. Output

Power works like this: When you double the power going to a speaker, the output increases by 3 dB. That is not a large amount. In fact, it is the smallest change in amplitude that is perceivable across the audible frequency range. (1 dB is the smallest perceivable change in amplitude where our hearing is most sensitive – 1 to 2 kHz).

Perceived volume is a different beast. An increase of 10 times the power sent to a speaker produces a doubling of perceived volume.

Speaker Efficiency

Another consideration in choosing an amplifier is the efficiency of your speakers. An inexpensive conventional midrange speaker may produce an average output of 91 dB when measured 1 meter away from the speaker cone and when driven with 1 watt of power. A high-quality speaker will likely be less efficient, but capable of playing over a wide range of frequencies. A measurement of 85 dB efficiency at the same distance and power level is not uncommon.

How Loud Do We Need it?

AmplifierA typical RMS sound pressure level for an orchestra, when you’re seated three or four rows back from the musicians, is around 100 dB. If we use our analogy of the 85 dB efficient speaker, we need 31.6 watts to get that speaker to play 100 dB. The problem is that this is the average power, not the peak power. Perhaps the performance crests at 110 dB? In that case, we need a peak power level of 316 watts. Just keep in mind that the speaker components are likely to melt if you keep this effort up for any significant amount of time.

We don’t suggest buying any speaker based on its efficiency. Criteria like linearity, lack of distortion, application limitations and frequency range are far more important. If you need it loud, buy more speakers, or larger speakers.

Distortion Happens

What happens if we run out of power in an amplifier? We get distortion. This distortion creates all sorts of high-frequency harmonic content. That increased high-frequency energy is what causes tweeters to fail. We need to choose an amplifier that will allow our speakers to play loudly enough without running out of power.

You are better off buying a 100 watt per channel amplifier and only using 50 watts than you are buying a 50 watt amplifier and occasionally causing it to distort. Remember, those 50 extra watts only result in an increase in output of 3 dB – assuming the speaker can handle it.

It Takes Power to Make Power

AmplifierA consideration that many people overlook is the ability to supply an amplifier with the power it needs to produce the power you want. Modern vehicles have electrical systems with reduced power production capabilities. Smaller alternators, smaller batteries and smaller wiring save weight. Reduced weight transforms into better fuel economy for the vehicle.

As a general rule of thumb for power consumption calculations, you can assume that every 100 watts of power from an amplifier will require about 10 amps of current from your electrical system. Yes, some amplifiers are more efficient than others, but this serves as a good, quick guideline.

If you want to purchase a 650 watt amplifier to power your subwoofer, then your electrical system (battery and alternator) has to be able to provide it with about 65 amps of current. This power requirement is on top of what is required to run the vehicle. The computers, lights, ignition system, radio and heater all consume power as well. On a modern compact car, it would be no surprise if you only had 30 to 40 amps of power left over for an amplifier.

You can get away with a big amplifier – but you can’t play it indefinitely, even with the vehicle running. Once you have exceeded the power delivery capabilities of the amplifier, the battery will start to supply current. You can kill a car battery, even with the vehicle running. Once you shut the car off, you may not have enough energy in the battery to restart it.

Blowing up Amplifiers

AmplifierAmplifiers do not like to be starved for power. When you run out of power to drive your amplifier, in most cases, the amplifier rail voltage starts to drop. Power starvation causes the maximum undistorted power production of the amplifier to decrease. We are back to the same scenario: Distortion causes harmonics, and harmonics can damage fragile speakers.

If you have had an amplifier fail, and the failure was because the power supply section of the amp self-destructed, chances are you were not able to feed the amp properly.

How Much Amplifier Power Do You Need?

The solution: Buy as much as power as you can afford. Buy the biggest that will physically fit in your application. Get the highest-performance amplifier you can. Make sure your installer uses properly sized wiring to install the amplifier. Upgrade your car battery to a high-performance, high-capacity unit if you need more reserve power.

For more information, visit your local mobile electronics specialist retailer. Be honest about your needs and expectations for your audio system. They will be able to suggest a solution that sounds fantastic and will offer years of reliable performance.

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.

Filed Under: ARTICLES, Car Audio, RESOURCE LIBRARY

What Is An MP3 File?

MP3

You cannot have talked about audio and computers any time in the last 15 years and not have heard of an MP3 file. MP3 audio files and websites, like the original Napster, started a shift in where, how and when people acquired music. If you are on the older end of the spectrum, like many of us in the mobile electronics industry, then you bought your CDs, cassettes and maybe even your vinyl at a record store. Computers and the Internet changed that. You could go online after dinner and download an illegal copy of a song in a few minutes. It was wrong, but people acquired tens of millions of songs this way.

In the 1990s and early 2000s, accessing the Internet was slow. We started connecting to the Internet using phone lines and modems. Each byte of information took time to transfer to your computer, so anything that would speed up the process was a treat. Downloading (stealing) music using the Internet is where the popularity of the MP3 audio file met its calling.

A Primer on Digital Audio

MP3We could write 10 articles about digital audio – and we just might. For now, we are going to look at the basics and use the compact disc (CD) as our reference. CDs store digital audio sampled at 44.1 kHz with a resolution of 16 bits. These numbers mean each sample can have an amplitude that is a single value within a range of 65,536 different levels (2 to the power of 16). The information is sampled 44,100 times a second. Sampling at what is known as 44.1/16 allows capturing the audible range of audio (considered 20 Hz to 20 kHz) with good detail and accuracy.

To store 1 second of audio at this resolution, we need to store 1,411,200 bits of information. Anyone who has played with audio transcoding software may recognize 1,411 kbps as a standard data rate. This number is calculated by multiplying the number bits per sample (16) times the number of samples per second (44,100) times 2. The times-2 factor is because we record in stereo – which is two channels. So, a 3-minute long song is 254,016,000 bits or 31,752,000 bytes.

Let’s round it off to 31 megabytes of information. Can you imagine how long it takes to download that with a dial-up modem running at 14,400 baud? The answer is at least 3.5 minutes – without error checking, line noise and other factors that slow the real download time to about 5.5 minutes.

Data Compression

What if someone found a way to shrink the size of the audio file to speed up download time and reduce bandwidth usage? The caveat is that the audio still sounds essentially the same on most basic audio systems, such as a TV, computer speakers or a 1990s factory car radio. In 1991, a group of companies, including the Fraunhofer Institute, France Telecom, Philips, TDF and IRT, started working on a way to reduce file size while maintaining relevant information. That is the key to how file size is reduced using MP3 compression.

The MP3 file format is a “lossy compression” algorithm. Lossy compression means that information is thrown away to reduce file size. The development team worked on a compression method called perceptual encoding to decide what information to remove. Perceptual encoding is based on how we hear sounds relative to other information, and the limits of our hearing.

What MP3 Files Throw Out

We are going to analyze the information that MP3 files remove to reduce file size. One of the easiest ways to cut back on information storage is to reduce the highest frequency that will be reproduced. If we analyze a 128 kbps MP3 file, we see that the highest reproduced frequency is just below 16 kHz. If that were the only information that was removed, our new bitrate with 16-bit samples in stereo would be about 1,004,800 kbps instead of 1,411,200 kbps for 20.05 kHz.

MP3The next part of the compression process analyzes content that is common to both channels. It is common for some parts of a recording to be virtually in mono. The encoding process removes duplicated information from the file and adds code to copy the opposite channel. If the audio track were purely mono, the file size would be divided in two. Few tracks are completely mono, but we can see more space saving from this process.

Subsequent processing looks at low-level information during high-amplitude passages. Let’s use the example of a song with a lot of bass in it and some very quiet harmonic midrange information. Perceptual encoding processes like MP3 will remove this low-level information from the audio track. This process is called audio masking. There is enough audio information at other frequencies to distract you from hearing what is removed.

Can You Hear the Difference?

Dozens – nay, hundreds – of tests have compared MP3 files to full CD-quality audio tracks. Are there differences? There most certainly are. One thing became apparent during our research: How an MP3 file is created is crucial to its subjective sound quality. Different encoders work in different ways with different results.

Perhaps the best way to describe the difference between a CD-quality recording and an MP3 file is to look at the difference between the two. I wish we could share some samples here for you to listen to, but that would break copyright laws. What we can do is visually show you the difference.

We took a 3-second sample from Daft Punk’s “Give Life Back to Music.” We chose this track because of Daft Punk’s clear and conscious effort to make a high-resolution version of the album commercially available. We want to thank them for that! The sample is from 31.5 seconds to 34.5 seconds into the song.

This Spectrogram shows the frequency content of the sample. The horizontal scale is time. The vertical scale is frequency. Finally, the color intensity shows the amplitude.

MP3
This is the original sample.

You can see that there is frequency content up to 30 kHz, clearly demonstrating the high-resolution nature of this track. Each vertical color band represents a drum machine beat – more or less.

128 kbs MP3 File Analyzation

MP3
This is the sample converted to a 128 kbps MP3 file.

It is clear that audio information above 16 kHz has been removed. Infrasonic frequency content is clearly different as well. There is more information in the MP3 file below 30 Hz compared to the original. This increase in information will, however, present itself as less-dynamic range.

MP3 Vs Original File

MP3
This is the difference between the Original sample and the MP3 Sample.

We inverted the MP3 file and added it to the original sample to make the image you see here. The net result is the difference between the two tracks. You can see the high-frequency content that was removed above 16 kHz. In fact, information was removed at all frequencies, and that information follows the intensity pattern of the audio file.

The original file has a peak amplitude of -0.1 dB for both channels and an average amplitude of about -14.2 dB. The removed information has a peak level of -10.9 dB and an average amplitude of -37.01. The removed information is buried deep below the peak amplitude information.

MP3What does the removed audio sound like? We would describe the clip as the sound of a distant marching band. The audio is mostly high-frequency information. The track has a decidedly warbled texture to it as well: The drum machine beats are clear and present, but they sound like distorted cymbal hits.

Even with a high-end headphone preamp and studio grade headphones, the difference is hard to perceive when switching between the original track and the MP3 file. In a listening environment with a larger soundstage, it may be more apparent.

Conclusions about MP3 Files

Purists will tell you that you should have the highest-quality recordings available. There is no fault to this logic. Why skimp when you can have it all? High bitrate MP3 files, like those at 320 kbps, for example, are excellent in quality. Repeated testing has shown that when created with quality compression algorithms, the sound difference between a CD-quality recording and a 320 kbps MP3 file is almost impossible to detect. Lower bitrate MP3 files start to dispose of more information, and the differences become bigger.

The latest source units on the market are capable of playing WAV and FLAC audio files of great resolution and bit depth. Shortly, we will see units that will play MQA files over digital connections. Almost every source will handle MP3 and WMA files.

Drop into your local mobile electronics specialist retailer today, and bring along some music to enjoy. We think you will be impressed – no matter what format you choose.

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.

Filed Under: ARTICLES, Car Audio, RESOURCE LIBRARY

Amplifier Classes: Making Sense Out of Class AB and Class D Amps

Amplifier ClassesAmplifiers have a very tough job. They have to take a very low voltage signal and increase it in amplitude so it can drive a speaker. In this transformation, we expect the signal to remain pure – no distortion or no noise should be added. We also want significant amounts of power to drive our speakers, even though we only feed our amplifiers with a measly 12 to 14 volts of electricity. The laws of physics seem to want to work against us at every turn – but we prevail! Modern car audio amplifiers are amazing feats of engineering and design. This article looks at the two main types of amplifier classes used in the car audio industry and the benefits and drawbacks of each. Welcome to Class AB vs. Class D.

The Math behind how Amplifiers Make Power

No matter how we configure the components inside an amplifier, the goal is the same: Increase the voltage of the preamp audio signal so it can drive a speaker. Because the speakers we use are low in impedance (2 or 4 ohms for most midrange speakers), we need to be able to provide a significant amount of current to the speaker as well. This delivery of current to the speaker is the second task an amplifier has to undertake.

By way of some quick math, if a 4 ohm speaker is getting a 12V RMS signal, we can make a few calculations. To calculate the current flowing through the speaker, we divide the supplied voltage by the impedance of the speaker. In this example, we have 12 divided by 4, so 3 amps of current are flowing through the speaker wires and the voice coil. An easy way to calculate the power going to the speaker is to multiply the supplied voltage times the supplied current. The product of 12 times 3 is 36. This speaker is receiving 36 watts of power.

Let’s look at the same example as though this were a subwoofer amplifier. In this second example, we will assume we have a Dual 2 Ohm voice coil subwoofer with both coils wired in parallel to produce a 1 ohm load. If we supply this speaker with 12 Vrms of signal, then 12 amps of current flow through the speaker wire and the subwoofer. To calculate power, we multiply 12 times 12 to get 144 watts. 144 watts is a lot more power and current for the same amount of voltage.

General Amplifier Function Overview

Most amplifiers are composed of three or four key sections (or stages), depending on their design and complexity. The input stage is the portion of the amp where the low-level preamp audio signal enters the amp and receives any processing in the form of equalization or filtering.

An amplifier has a power supply. The power supply converts the supplied 12 to 14 V of direct current to positive and negative rail voltages. Let’s say, for example, a theoretical amplifier has +25 and -25V rails, relative to our ground reference. Depending on the size of the amp, there will be a driver stage. The driver stage is responsible for increasing the low-level audio signal to a higher voltage. How much the driver stage increases the voltage depends on how much power the amp will be making.

Amplifier Classes
Mosfets have shrunken in size over the years to the minuscule size now available (right).

Finally, we have the output stage. The output stage is relatively simple – it does not significantly alter the signal coming from the driver stage, but the devices (MOSFETs or transistors) used to provide the output signal with the current the load requires. The power supply and the output stage are the two portions of the amp that do the most “hard work.” That is to say, they are the stages that pass a lot of current.

In almost all amps on the market, we use dedicated devices for the positive half of the waveform and separate devices for the negative half of the waveform. To clarify , if we measure the output signal of the amplifier about the vehicle ground, we will see that it swings back and forth above and below 0V. Think back to our +25 V and -25 V power rails. Speakers don’t care about the value of the signal being sent to them; all they care about is the difference in voltage from one end of the voice coil to the other end.

Class AB Amplifiers

Amplifier ClassesFor this article, we are going to generalize Class AB amps into an Analog, Amplifier model. In our analog amplifier, we have large transistors in the output stage of the amp. When we want half of the positive rail voltage at the output, we feed half the voltage to the positive output device. When the signal goes negative, we turn off the positive device and start using the negative device only. Looked at a different way, the audio signal from the driver stage controls the resistance of the output devices and, subsequently, how much current can flow to the speaker.

In an Analog Amplifier, the output devices can be “turned on” in varying amounts about the audio signal. This means the output devices are often acting as resistors. Power is wasted as heat when we pass current through a resistor. Keep this in mind as part of our comparison later in the article.

Class D Amplifiers

Amplifier ClassesIn a Class D amp, the output devices receive a control from a Controller Integrated Circuit (IC). This controller sends out a variable duty cycle square wave. The square wave amplitude is high enough that it turns the output devices all the way on or off. The output devices spend very little time operating as resistors and act more like switches.

The logical question is, how in the world do we get music out of a square wave? If you thought that, good for you! The frequency of the square wave is much higher than the maximum frequency of our music. In fact, some modern Class D amplifiers switch the output devices at frequencies as high as 600 kHz.

Amplifier Classes
An additional benefit of class D amplifiers is the potential for extremely compact size.

To recreate music, the Class D controller sends out a signal that is Pulse Width Modulated. The amount of “on” time about the “off” time determines the output level of the signal. As a very general analogy, if the positive output devices were sent a square wave with a 50% duty cycle (on for as much time as it was off), then the average of the output would be 50% of the positive rail voltage. If the square wave is on for 75% of the time, then off for 25%, then we would get 75% of the rail voltage at the output.

As you can imagine, the signal from the Class D controller is quite complex. It has to modulate the duty cycle of the square wave going to the positive and negative devices fast enough to accurately recreate the audio signal. It also has to control both the positive and the negative output devices separately.

Benefits and Drawbacks of Analog Amplifiers

Because the audio signal in an analog amplifier is never chopped up into tiny pieces, analog amplifiers can remain faithful to the original signal. The best-sounding amplifiers in the mobile electronics industry are analog. Analog amplifiers are, historically, given a reputation for accurate high-frequency response.

The drawback of an analog amplifier is its efficiency. Efficiency describes how much energy is wasted as heat as compared to the energy sent to the speaker. Because of the output devices in an analog amplifier work as variable resistors, they get hot. Typical analog amplifiers operate in the 70-80% efficiency range regarding total efficiency, while operating at full power. That missing 20-30% is released as heat. At lower output level, the efficiency drops even more.

Benefits and Drawbacks of Digital Amplifiers

Amplifier Classes
The integrated circuit of the class D amplifier controls the switching.

Modern digital amplifiers switch at extremely high frequencies. We see amps capable of audio frequency response beyond 50 kHz, and some that exceed 70 kHz. This performance is a long way from the first Class D amps that were only for subwoofers and struggled to produce audio above 5 kHz. That said, because digital amplifiers require filter networks at the end of the output stage, they still cannot quite match the performance of a premium analog amp. With this information in mind, consider that there are some good digital amplifiers that sound better than many poorly designed analog amplifiers.

Because the output devices of a digital amplifier rarely operate in their resistive range, these amplifiers can be very efficient. A well-designed Class D amp can have an efficiency around 92%.

Another problem with Class D amplifiers is noise. Because the output devices are driven by a square wave, there is a lot of high-frequency energy in the output signal. The filter network we talked about removes much of that from the output signal, but that energy can still have detrimental effects on other systems in the vehicle. An unfortunately common trait for many Class D amps is that they cause interference with radio reception when in operation.

Choosing Between Amplifier Classes

Amplifier ClassesIt would be nice if we could formulate a set of hard-and-fast rules for choosing the right amplifier for your system. With so many variations on each kind of amp at so many different price points, that is truly impossible. We strongly suggest that the only way to pick an amp is to compare one to another under controlled conditions: Use the same music and the same speakers, and listen at the same volume. You will hear differences in frequency response and dramatic differences in imaging and staging capabilities.

Is one kind of amp better than the other? For an installation dedicated purely to sound quality, the choice is clear. For an installation where power delivery is limited or massive amounts of power are required, the choice is clear there as well. In the middle, it depends on your application and budget.

Drop in at your local mobile electronics specialist retailer to find out about the latest amplifiers on the market. They would be happy to help you choose one that meets your application and works with your 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.

Filed Under: ARTICLES, Car Audio, RESOURCE LIBRARY

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