Our continuing look at the tools a mobile enhancement technician should have at his or her disposal brings us to the ever-important digital multimeter. In the same way that an RTA or oscilloscope is needed to measure and quantify alternating-current signals, a high-quality multimeter is the only way to accurately quantify a DC or fixed-frequency AC signal. Read on to find out how technicians use their digital multimeters (DMM) to test the wiring in your car to make the installation process more accurate.

What Is a Digital Multimeter?

In essence, a DMM is a battery-powered, handheld device with a digital display that can perform several measurements. A DMM is occasionally (but rarely) called a Volt-Ohm-Milliammeter (VOM), giving us some insight into its capabilities. In almost every case, the meter will have between two and four terminals at the bottom for the connection of the test leads and an LCD or VFD (vacuum fluorescent display) screen at the top of the unit to show the measurement reading. Some of these displays include an analog bar graph that can be used to help detect transient signals. Finally, the function selection is made by a rotary control in the middle of the unit.

Most units will measure direct current (DC) and alternating current (AC) voltages to either detect or quantify the presence of voltage on a terminal or wire. Meters separate the DC and AC measurements into different functions as some math needs to be applied to the AC measurement to create an RMS value. Premium meters known as True-RMS systems are more accurate when measuring AC signals outside of the 50-60Hz frequency region.

Multimeters can also measure resistance. They perform this task by feeding a very small amount of current through a device (resistor or circuit) and measuring the voltage drop across a fixed resistance value. DMMs are good at measuring resistance above an ohm or two and up to about 50 megohms. Below 1 ohm, the resistance of the cables and connections becomes an issue.

Finally, most meters can be used to measure AC and DC flow. The meters have an internal shunt resistor that is wired in series with the circuit. The voltage produced across this shunt resistor is converted to a current flow measurement. The test lead connects for current measurements are different than those for voltage and resistance readings.

How Technicians Use a DMM

When you are having a remote car starter, security system or even a new radio installed in your car or truck, the technician will use a multimeter to test the wires in the dash for the presence of voltage. In almost all cases, the tech needs to find a good ground, a constant power source and a switched (or accessory) source to feed and control the device.

Resistance measurements come in handy when testing speaker wires or terminals on a subwoofer enclosure to estimate the impedance of a speaker. For techs who are installing remote starters on older General Motors vehicles, they use the ohmmeter function to measure the value of the Passlock resistor in the key. It’s worth noting that the ohmmeter in a DMM can’t be used to test the suitability of a ground connection accurately. A proper grounding point should have well under 0.01 ohms of resistance.

Finally, taking current measurements is ideal for troubleshooting situations where car batteries are being drained. If you have a situation where your car battery is dying after a few days, the technician can measure the current being drawn from the battery to start the diagnosis and repair process.

Choose the Right Mobile Enhancement Retailer

As we’ve mentioned in the past, choosing the right person to work on your vehicle is the most important aspect of the upgrade shopping process. A moderate-quality upgrade that has been installed and configured correctly will undoubtedly outperform a premium product that hasn’t been installed with care. When you tour the shop as part of your retailer interview process, make sure they have the tools required to perform the work you need. At the top of that list of tools is a quality digital multimeter.

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.

As we continue our look at how car audio amplifiers work, let’s discuss how the classic Class AB amplifier output stage works and explain why it’s still the benchmark for high-end audio systems.

How Does a Class AB Output Stage Work?

We recently looked at the power supply, the input stage and how Class D output stages work to allow your car audio amplifier to reproduce music. If you go way back to the beginning of high-power car audio amplifiers, you’ll find the Class AB design.

A Class A amplifier uses a single output transistor to add current to the output signal to drive your speaker. When no music is playing, the output device is halfway on and gets very hot. Think of the output as a resistor. If you put two small resistors in series across your power supply, a lot of current would flow through the load. A Class A amp works the same way.

Class B amplifiers use a pair of output devices and split the voltage delivery duties between each device. When no music is playing, both devices are turned off and no voltage is produced at the speaker. The problem with Class B amps, as the guys at BestCarAudio.com discussed in their Class AB Amplifier Distortion article, is that there a dead zone that occurs as the voltage across one of the diodes in the transistor is biased on. The result of not compensating for this roughly 0.7-volt bias requirement is called crossover distortion.

Class AB Amplifier Output Stage

The Class AB amplifier is a Class B amplifier with a small bias voltage applied to the output devices to get them ready to play music. Different amplifier designs use different amounts of biasing to balance between efficiency and silky-smooth operation.

Amazing High-Frequency Extension

One thing that high-end Class AB amplifiers are known for is their extended high-frequency performance. Many top-end amps can play beyond 80 kHz with ruler-flat, low-distortion performance. If you are building an audio system that is compatible with high-resolution audio, this extended high-frequency response is crucial.

Low Output Impedance

As compared to Class D amplifiers that use inductors and capacitors on their outputs, Class AB amplifiers, except in configurations with variable voltage power supplies (often called Class G or Class H designs), don’t need these devices. The result is less variance in output voltage relative to load impedance and more predictable response.

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Efficiency versus Performance

At low to medium power levels, Class AB amps are not as efficient as a Class D amplifiers because the output devices act like resistive voltage dividers across the power supply rail voltages. As such, AB amps need larger heatsinks to ensure their reliable operation. As you approach the upper limits of an AB amplifier’s output capabilities, their efficiency becomes similar to that of a Class D amp. The output devices are close to being all the way on or off.

Experience the Ultimate in Car Audio Amplification

If you are determined to assemble the best possible audio system for your vehicle, visit your local specialist mobile enhancement retailer and ask about using Class AB amplifiers. Yes, you’ll need to find room for them in your vehicle, but the amazing clarity and detail they provide is well worth the effort.

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.

Our series on the tools that are important to test and configure car audio system upgrades moves to the topic of the real-time audio analyzer, also known as an RTA or spectrum analyzer. This important tool helps installers measure the acoustic response of your mobile audio system, and it can be a fundamental component in testing a complex factory sound system to ensure that the integration process goes smoothly.

What Is an RTA?

In the simplest of descriptions, an RTA tells the technician how loud specific audio bands are relative to one another. Depending on the resolution of the RTA, the bands may be divided into full octaves, half-octaves, third-octaves (the most popular) or, on some computer-based analyzer solutions, as fine as 1/48th of an octave. For an audio system to sound realistic and believable, the transition from band-to-band (depending on the resolution) needs to be smooth and free of peaks or valleys that can detract from the listening experience.

RTAs can be used with a microphone for acoustic measurements of your system, or they can be used with an adapter to measure the output of a factory radio or amplifier, so your installer will know if an equalizer or interface is required to make your system sound its best.

If you’re thinking to yourself that you’ve seen something like this before, some radios and under-dash equalizers had spectrum analyzers built into them many years ago.

Types of RTAs Found in Car Stereo Shops

When you’re shopping for a retailer to upgrade the audio system in your car or truck, one of the things we recommend is that you ask for a tour of their facility. During that tour, feel free to ask about the tools they use. When it comes to RTAs, they may use a classic stand-alone solution like the older AudioControl SA-3055 or a modern computer-based interface like the AudioControl DM-RTA or Audison bitTune.

Some computer-based solutions allow for more complex measurements than a stand-alone solution can produce, but at the very least, knowing that your technician has a way to look at the acoustic performance of your vehicle is paramount.

Tuning Digital Signal Processors

We’ll make it abundantly clear: If you’re having a digital signal processor added to your vehicle, your technician needs to have an RTA. In the same way that you can’t look at a glass of water and tell what temperature it is, an installation technician can’t listen to an audio system and know which frequencies need to be adjusted and by exactly how much. The RTA is the car audio equivalent of a thermometer for that glass of water.

Before you commit to an audio system upgrade, make sure that your local mobile enhancement retailer has the tools required to complete the task efficiently and reliably and the training needed to ensure that the proposed benefits and improvement in performance will be delivered to you, the customer.

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.

As we continue our look at how at how car audio amplifiers work, we need to discuss what has become the most popular design on the market today: amplifiers that use a Class D output stage. In audiophile realms, Class D designs often carry an unfavorable reputation. Do their drawbacks outweigh their benefits? Let’s have a look!

How Does a Class D Output Stage Work?

Once the input stage has handled any signal processing needs, the audio signal is passed to the output stage so that it increases in voltage and the MOSFETs can provide adequate amounts of current to drive our relatively low impedance speakers. In a modern Class D amp, the MOSFET output devices are fed by a driver IC that handles the conversion of the analog signal into a pulse width modulated signal.

What is a Pulse Width Modulated Signal?

Imagine, if you will, an incandescent lightbulb. We hook the bulb up to a power source and insert a computer-controlled switch in series with the circuit. If we leave the switch off, the light stays off. If we turn the switch on, the light illuminates to full brightness. However, if we turn the switch on and off very quickly, and the switch is on for as long as it’s off, the bulb will glow at half of its possible brightness. This variation of on-versus-off time is called duty cycle. When the on-vs.-off time is equal, that’s a 50% duty cycle. Using a square wave signal with different duty cycles is called pulse width modulation (or PWM for short).

The Class D driver analyzes the audio signal at extremely high speeds (some as fast as 800kHz) and creates a relatively low-voltage PWM signal that feeds the output devices. The output devices switch all the way on and all the way off very quickly. Because the devices spend very little time in a partially on state, they present very little resistance and, subsequently, consume very little energy. The best Class D amplifiers on the market offer overall efficiencies that exceed 92% at full power. This excellent efficiency is in contrast to Class AB amplifiers that turn between 35% to 50% of the energy fed into the amp into heat.

Compact discs use a version of PWM where the duty cycle of the output pulse is stored in a 16-bit digital word. This gives us 65,536 possible amplitude levels. Unlike modern Class D drivers, our audio is stored at 44,100 samples per second. This is still more than enough to reproduce the entire audio spectrum.

Class D Circuit Arrangement

Lastly, Class D amplifiers typically have their output devices arranged in a Class AB configuration, where one device drives the negative part of the waveform and the other drives the positive. As such, Class D describes how the output devices are used, rather than their electrical orientation in the circuit.

Drawbacks of Class D Amplifier Designs

If you have read the BestCarAudio.com articles on distortion, then you know that a square wave AC signal is made up of a large number of high-frequency harmonics. You’ve likely heard this phenomenon in your home if you have a dimmer on some of the lights. The filament in the lights will occasionally ring, depending on the level of the dimmer. Since we only want to pass an audio signal back to the speaker, amplifier designers add a passive filter network to the output of the MOSFETs. This network includes an inductor in series with the load as well as a capacitor and resistor in parallel and acts as a low-pass filter to remove this high-frequency switching noise.

One drawback of a Class D design is that these output filter components interact with the frequency-dependent variations in load impedance to alter the frequency response of the amplifier. While the effect is minute, it can give Class D amps a different overall tonal balance than you’d get from an AB design. Of course, a little manipulation with a digital signal processor (DSP) will get that back in check in a jiffy.

Another issue with all this high-frequency energy is the potential for electrical interference with other systems in the vehicle. Most commonly, AM or FM radio reception can be washed out or dramatically reduced. While the better amplifier manufacturers do everything possible to mitigate radio-frequency interference, problems can still occur — the best plan to keep the amplifier as far away as possible from the radio antenna.

Why Would You Want a Class D Amplifier?

The long and the short of it is that companies have invested in developing Class D amplifiers in an effort to shrink the physical size requirements of amplifiers, supposedly to make it easier for installation technicians to find mounting locations for them. The reality is, heatsinks for amplifiers are one of the more expensive single components in an amplifier. If the size of the heatsink can be reduced, so can the cost of the amplifier. The days of 40- and 50-watts-per-channel stereo amps with a footprint of more than a square foot are long gone, thanks to modern Class D designs. Now, you can get more than 1,000 watts of power from that same physical space.

Class D amplifiers are a good solution for subwoofer systems because they do offer increased efficiency over their Class AB counterparts, and almost all amplifier manufacturers have at least one series of Class D amplifier in their catalog. Your local specialist mobile enhancement retailer can help you choose a solution with the right power level, number of channels and features to make your car stereo sound great.

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.