We are at our second-to-last article in our car audio amplifier specification series and this time, we are going to talk about Class AB amplifier crossover distortion. This distortion has nothing to do with the high- or low-pass filters built into your amplifier; it’s a problem with the very concept of the Class B amplifier topology. Don’t fret — we’ll explain what causes it and how it’s minimized to a level of near inaudibility.

How Do Class AB Amplifiers Work?

Before we can explain what crossover distortion is, you’ll need a basic understanding of how the output of a Class AB amplifier works. In a Class A amplifier, a single transistor (or bank of transistors) takes care of controlling the voltage going to the speaker. When no signal is being produced, the transistor is set at a 50% output level (and gets VERY hot). As the alternating current (AC) from the signal source passes through the amp, the output voltage goes up and down until it reaches the maximum or minimum possible level associated with the transistor being off or at its maximum output level.

In a Class B amplifier, we use one transistor to handle the positive half of the waveform and a second transistor to handle the negative half. The primary advantage is that when there is no signal present, both devices are almost completely turned off and very little power is consumed.

The Class B topology is where our crossover distortion problem arises. As the audio signal swings from positive to negative, it needs to switch from one device to the other without any issues. It’s at this point where the signal crosses over from one device to the other that problems can happen — hence the name, crossover distortion.

Amplifier designers who truly care about the sonic qualities of their products know how to handle this transition and can effectively eliminate crossover distortion by biasing the positive and negative output devices into their active regions. While this does create a little more heat, it reduces distortion dramatically. This is where the Class A part of an AB amplifier comes from: The devices are turned on a little bit (like a Class A amp), but for high-level signals, act in a Class B configuration.

 

Why Does Crossover Distortion Matter?

At low output levels, even in moderate-quality Class AB amplifiers, crossover distortion is more prevalent than when an amplifier is producing very low amounts of power. More accurately, the effect of crossover distortion decreases as the output level increases.

Let’s look at the distortion characteristics of three different amplifier topologies to gain a better understanding of that statement. Up first is our truly outrageous low-quality four-channel amplifier. This workhorse of mediocrity has served us well through this series in explaining the kind of harmonic distortion versus its output level.

For a stark and dramatic contrast, we’ll switch to our high-end amplifier. This amplifier is unique in that it uses a high-bias design intended specifically to reduce crossover distortion in the transition between the positive and negative output devices. The graph below shows the distortion of 0.09% versus the output level.

Choosing the Right Amplifier for Your Audio System

If the goals for your audio system are world-class performance, and you intend to use Class AB amplifiers, you can reduce the effects of crossover distortion by matching the power requirements of your speakers with amplifiers that will operate higher in their output range. For example, if you want a 150-watts-per-channel amp for your mid-bass drivers, you may only need a 50-watt amp for your midrange speakers and a 10-watt amp for your tweeters to achieve the same acoustic output level. BestCarAudio.com published an article on speaker power handling that explains why.

While you can’t always choose high-quality amplifiers with extremely low power levels, you can minimize the effect of crossover distortion by not using a monster amp on your tweeters. Your local specialist mobile enhancement retailer would be happy to design and install an amazing car audio system upgrade — drop by and talk to one of their Product Specialists today!

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 look at the input stage of how a modern car audio amplifier works. The input stage is responsible for interfacing with your radio and provides features like the gain control, crossovers, bass boost, remote level control and signal summing that your installer will use to make your stereo system sound great and function reliably. While we don’t need to get into the specifics of the components in this part of your amp, understanding the importance of the features is crucial in picking the right solution for your applications.

RCA Preamp and Speaker Level Inputs

Your amplifier can’t amplify an audio signal if you can’t connect the amp to your radio. Almost every amplifier on the market has RCA input jacks that allow your installer to connect an aftermarket radio quickly and easily. We say “almost” because some amplifiers are designed exclusively for integration with factory-installed audio systems. These amps may have a set of wires on a pigtail harness to accept signals.

In most cases, speaker-level signals are no different than a preamp signal other than they are usually higher in voltage and, because they have already passed through an amplifier stage (in the factory-installed radio or amp), may contain a little more distortion.

Whether the amp uses preamp or speaker-level signals, it should have a feature called Balanced Differential Inputs, or a variation of those words. This simple circuit compares the signal (or noise) on the center conductor and the shield of an RCA interconnect and uses an Op-Amp (a small integrated circuit that can be configured to compare or combine two signals into a single output) to remove anything common to both conductors. If you are using twisted-pair interconnects, EMI or RFI noise should be common to both conductors and thus will be reduced or eliminated by this circuit. When shopping for an amp, Balanced Differential Inputs are one of the first things you should ask the salesperson about.

Signal Processing Features

The next important feature in any amplifier is the sensitivity or gain control. This adjustment allows your installer to configure the amp to produce maximum power for a specific voltage that matches the maximum undistorted output of your source unit, or your power requirements based on the design of your audio system. The gain control is something that should be set once using an oscilloscope and a real-time analyzer (RTA), then not touched again unless the design of the system changes.

Most modern amplifiers include some form of electronic crossover that limits the range of frequencies that pass from the inputs to the outputs. A high-pass crossover only allows frequencies higher than the crossover point to pass and would be used on midrange and high-frequency speakers. A low-pass crossover allows audio signals lower in frequency than the crossover point to pass through the amp. This type of crossover would be used for woofers and subwoofers. Depending on the design of your audio system and the features of your amplifier, your installer may choose to combine high- and low-pass crossovers to create what is known as a band-pass crossover. This crossover would be used on midrange or midbass drivers to cut out low- and high-frequency information.

Many amplifiers have a bass boost control. This control operates as a single-band equalizer that the technician installing your sound system can use to reinforce a narrow band of frequencies being sent to a subwoofer. In reality, adding a narrow-bandwidth boost to your audio signal, from a purist perspective, makes no sense. Most systems sound better with proper DSP equalization to deliver smooth and predictable response across the entire operating range from 20 Hz to the upper limit of your sub around 60 or 70 Hz. A peak at any particular point will sound unnatural.

Remote Level Controls

Most high-quality subwoofer amplifiers come with a remote level control or remote bass boost control that can be installed in the front of the vehicle. This control allows the driver to turn the bass up or down to suit the music he is listening to or his mood. When shopping for an amp, look for units that offer a level control as opposed to a bass boost. As we mentioned above, adding emphasis to a narrow band of frequencies doesn’t sound as natural as turning as adjusting the overall output of the subwoofer.

Amplifiers with Integrated DSP

A friend of ours was in charge of developing a series of high-end amplifiers for one of the well-known head-unit manufacturers. The goal was to include as much flexibility as possible in terms of crossovers and signal summing. The complexity of the amp grew to a point where it was becoming difficult to fit all the switches and adjustments on the side of the amp. To provide their clients with the features they wanted, the company decided to remove all the controls and implement a digital signal processor (DSP) to handle everything. Now, the installer could combine signals from multiple sources, set crossovers very accurately and equalize the signal to deliver a smooth and predictable response from the speakers. Sure, your technician will need to connect a computer to the amp, but the results will be fantastic!

Great Features and Great Design

As important as the features of the amplifier you choose are, the design and component layout of the input stage of a car audio amplifier can have a huge effect on the performance of the amp. Keeping these components away from noisy sections of the amp like the power supply and output stage can dramatically reduce the signal-to-noise ratio of the amp. For this reason, larger amplifiers often sound and perform better than their compact counterparts. As with any design, the electronics engineers and technicians who develop the circuits and lay out the components have to balance performance, features, cost and size.

Find the Latest Car Audio Amplifiers at Your Local Car Stereo Shop

When it’s time to shop for a new amplifier for your car, truck, motorcycle or boat, make some notes about why you need an amp and what you want it to do. Armed with that information, the product specialists at your local stereo shop can help you find a solution that will meet your requirements quickly and efficiently and ensure that your mobile audio system sounds magnificent.

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.

Whether the sound system in your car or truck is bone stock or upgraded with premium amplifiers, speakers and subwoofers, the system’s maximum volume may not directly coincide with turning the knob up all the way. The short and sweet of why this happens is something called gain overlap. This conscious and deliberate design decision ensures that any signal source can be played at the maximum volume level as determined by the system’s amplifiers. Are you confused? Fear not! As always, we’ll explain.

What Determines How Loudly a Car Stereo Can Play?

In most cases, the maximum output capability of your car stereo is determined by the power available from the amplifiers to drive the speakers. When you run out of power, the signal produced by the amp distorts heavily, and the sound becomes garbled and unpleasant.

In some systems, the limiting factor may be the quality or configuration of the speakers used in the vehicle. If the speakers aren’t operating in their optimal range, you may hear a snapping sound as the voice coil former smashes into the back plate. This is a rare occurrence but it does happen.

Audio System Gain Structure

When you have an amplifier connected to a radio, most people think that the installer (or system designer, in the case of a factory-installed sound system) should configure the amplifier so that it makes full power when you crank the volume to its highest setting. This simple 1:1 ratio works great for music that is recorded loudly.

There is a significant drawback to this configuration. What happens when you have an audio source that is quiet? A good example of this is the release of the Dire Straits’ 2005 album, Brothers in Arms. This (admittedly amazing) album is recorded with a wide dynamic range and a relatively low average volume level. Scientifically speaking, the track “Money for Nothing” has an average amplitude of -22 dBFS (decibels referenced to the maximum recording level), even if the peaks are recorded at the maximum allowable level on the CD. By comparison, the Daft Punk track “Give Life Back to Music” has an average level of -17dBFS. You need about five dB more gain in your system to make Mark Knopfler and Company appear to be performing at the same average volume level.

What Happens When Full Volume Isn’t Enough?

Audio system designers include something called gain overlap in the system to allow these quiet musical passages to be played at the same level as tracks that are recorded loudly. The drawback of this (very common) feature is that your system will distort heavily if you play loudly recorded music at full volume.

In these situations, the person operating the sound system (you) needs to know when you have maxed out the capabilities of your amplifier and stop turning the volume up. Many mobile electronics specialist retailers will tell you that a volume setting of 27 out of 30 may be as high as you can turn up the system. This limit is determined using test tones recorded at high volume levels. Following this guideline ensures that your system will never distort. With that said, if you have selected a source that isn’t as loud, whether it’s Bluetooth, files on a USB stick, a quiet compact disc or the 1/8-inch auxiliary input, having a few notches left over to turn the volume up a little higher is very welcome.

What Happens When You Exceed Maximum Volume?

As we mentioned at the beginning of this article, in most cases, audio systems are limited by the power available from the system amplifiers. When you reach this limit, the amp will “clip.” Clipping adds large quantities of harmonic distortion to the output signal and dramatically increases the energy delivered to high-frequency speakers. This extra energy can cause mids and tweeters to fail prematurely.

If your upgraded audio system is configured to include some amount of gain overlap, work with the person who tuned the system to understand its limits. You are getting all the performance possible from your audio system, even if the volume on the radio isn’t at full. Your local mobile enhancement retailer can provide specifics about the limits of your factory source unit and explain how they can improve the detail, accuracy and emotional impact of your car stereo system.

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.

We’ve talked about car audio amplifier features and specifications at great length, but up to this point, we haven’t discussed how a car audio amplifier works. In this article, we’ll provide a simple overview of how the power supply in your mobile amplifier can take the 12 volts supplied by your electrical system and convert it into a much higher voltage to drive your speakers.

Making Power for Your Speakers

To supply enough power to your speakers to produce music at realistic listening levels, we need a lot of voltage. For standard 4-ohm speakers, it takes a peak-to-peak voltage of almost 60 volts to deliver about 100 watts to your speaker. In most amplifiers, this voltage is configured as +30V and -30V, relative to the ground reference voltage of your vehicle chassis. So, how in the world do we get plus and minus 30 volts from 12? That’s the job of the power supply.

How a Transformer Works

Power supplies couldn’t be created with any reasonable amount of efficiency without a transformer. A transformer is a simple device that increases or decreases alternating current (AC) voltages using two coils of wire wrapped around a magnetically conductive iron core. If you have a 1:2 transformer and feed 12V of AC signal into the primary, you get 24 volts on the output. The big green boxes out on your curb or the cylinders on the power poles near your house are step-down transformers that convert the 16kV feed that enters your neighborhood into 120- and 240-volt feeds to your home.

How Do We Get AC Voltage in a Car?

As you may (or should) know, the power supplied by the battery and alternators in our cars and trucks is direct current (DC). Left in that state and fed into a transformer, we’d see a small voltage spike when the signal was first connected, then nothing. The steady current flowing through the primary winding would simply heat it up and not produce anything on the output side of the transformer.

Since we need an AC signal, modern amplifiers use a pulse-width modulator to create a series of square waves that turn on a bank of MOSFETs (electrically controlled high-current switches) that pulse the supply voltage on and off very quickly. Many high-quality amplifiers have power supplies that switch at more than 300kHz.

In the simplest of terms, by switching the connection from the battery to the input of the transformer on and off very quickly, we create an alternating current signal. The voltage of the pulsed signal is increased through the transformer and then fed into a set of diodes and capacitors to smooth it back out to what we call the rail voltage. The rails are the positive and negative power supply sections of the amp that are connected to the output devices.

In application, the circuit is far more complex, but this is the basic operation of the system.

Energy Storage and Power Delivery

As the output voltage of a car audio amp increases, so to does the amount of current flowing to the speaker. The pulse-width modulation controller that manages the power supply can change the relative “on” to “off” time in order to increase the current supplied by the amp. More “on” time means that more current is fed to the transformer, which results in more voltage being produced on the output. Regulated back down to the required rail voltage, this extra voltage ensures that we maintain stable rail voltages.

If you look inside an amp, you will almost always see a toroidal transformer and a bank of energy storage capacitors. These large capacitors help smooth the output of the transformer and diode rectifier stage, and store large amounts of energy so that the amp can keep up with the current demands of the audio signal.

Learn How a Car Audio Amplifier Works

You should now have a basic understanding of how your car audio amp converts the 12V feed from your alternator into much higher voltages for the rails. In the next article, we’ll talk about the front end or input stage of a car audio amplifier and discuss crossovers, gain controls and the features and functions that make it easier for your installer to configure your car audio amplifier for optimum performance in your car or truck.

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.