Feature : Sound Tech 4 - Amplifiers

All mobile audio systems consist of a source, an amplifier (commonly built into the source unit) and numerous speakers. Each source is capable of providing a small amount of electrical output. However, by themselves, these electrical currents are too weak to provide the power needed to move a voice coil and diaphragms that make up the speakers. An amplifier's purpose is to take the very low voltage signals and raise their level to a degree sufficient for a speaker to perform efficiently.

A certain amount of distortion is inevitable in any electronic device, but for a power amplifier to be considered a high fidelity device, the level and nature of that distortion must be such that it is not audible. When designing an amplifying circuit, the designer must choose between several different types of designs, several different classes of amplification, and many different materials and components. The designer must also consider several important factors in the amp's usage and environment. Perhaps the most important factor to consider when designing a mobile amplifier is the fact that the amp must draw its power from the vehicle's 12 volt supply. This inescapable fact presents some serious problems.

An electrical signal contains three basic characteristics: voltage (electrical pressure), current (electrical flow), and impedance (resistance to electrical flow). The output transistors must deliver current (drawn from the vehicle's battery and stored in the amplifier's capacitors) from the power supply (these capacitors) to the speakers according to the impedance presented by the speakers and the voltage from the preceding amplifier stages. Since a mobile amp is limited to 12 volts incoming voltage, the only way to increase the output wattage is to draw tons of current, which is in limited supply, just like the voltage. Straight physics says that a head unit with built-in amplifier could not deliver more than 3 or 4 watts of good clean output power. Yet, many claim anywhere from 30 to 50 watts per channel of output. Are they lying?

Some are. There is little regulation in the mobile electronics business and there are some companies that bend the truth quite far in their marketing efforts. Yet, there are ways to increase power beyond what 12 volts would normally allow. The two most common ways are to use a Bridged Transformerless (BTL) design or to use a Switching power supply.

BTL amps are found in head units and small, inexpensive external amplifiers. BTL designs can be physically small and do not generate much heat in their use. BTL designs actually use four IC power amps to produce power for two output channels. These amps are wired together in pairs - a configuration known as bridging. Each pair is assigned one of the channels. To understand how they work, we must first look at how any amplifier works. It's already been mentioned that the amplifier is trying to increase a signal which it is receiving from a source. This signal arrives as a low voltage waveform, all wave forms have a positive (rise) side and a negative (fall) side. Increasing the waveform's power involves increasing the height of the rise and the depth of the fall. This increase is limited by the amplifiers available dynamic range.

Bridging a pair of amps works by assigning only one part of the waveform to an amp, instead of the whole. Normally, an amp has to handle both the rise and the fall of the wave, limiting the overall size of the wave to its available dynamic range. A bridged pair of amps assigns the rise of the wave to one amp, and the fall of the wave to the other. This way, each amp uses all of its dynamic range on only half of the wave form. Mobile audio power requirements are somewhat contradictory. On one hand, the vehicle environment is relatively small, enabling fairly large sound pressure levels to be built up by fairly small amps. On the other hand, the environmental (ambient) noise levels in a car are much higher than at home, requiring the music to be played at much higher levels in order to be clearly heard. This, in turn, demands much higher power.

BTL amps can be fine performers if their individual components are of good design and high quality. Regardless of construction, however, they are still limited to the overall levels of the combined components which are, at best, 4 watts each. This gives us (regardless of marketing hype) a real limit of about 16 clean watts of output per channel. With this in mind, it appears that the best BTL amps will just barely bring us into the acceptable output range. How do we get those higher wattage outputs that larger external amps claim? With Switching power supplys in the amps.

Switching power supply (SPS) amps are not limited by 12 volt problems because they don't function on 12 volts of power. They increase the available voltage by consuming huge amounts of 12 volt current. This high current 12 volt flow is switched off and on rapidly, creating a pulsing signal similar to rectified AC. Although DC current must maintain it's voltage, AC current isn't as limited. AC can be transformed - it can trade some of it's current for increased voltage or trade some of it's voltage for increased current. The device which does this is called a transformer.

This is why SPS amps pull so much current. The amp pulses the DC into a form of AC and then feeds the AC into a transformer which pulls amperage from the flow and exchanges it for additional voltage. The unit continues to pull current until the desired voltage level has been reached. The signal is now rectified back into a DC flow, one which now has much lower current levels but much higher voltage levels. This method of converting DC to AC and then back to DC is called DC to DC conversion.

The only practical limits on a DC-to-DC convertor are the available current (determined by the alternator, the battery, and the current draw) and the size and construction of the amp's switching power supply. As you have probably figured out, SPS amps can be very power hungry. This is why these external amps always require a power cable connection directly to the battery. You can not safely connect a big amp to one of the vehicle's hot circuits or fuse box connections, you should always use an extra-heavy gauge power and ground connection wire. Better connection cables will provide better performance and more safety every time.

In addition to the two basic types of amplifiers, BTL and SPS, amplifiers also come in Classes. Each class handles the actual waveform amplifying differently, and each class has its own set of advantages and disadvantages. Most amplifiers fall into one of three classes: Class A, Class B, or Class A/B.

Class A amplifiers have transistors that are biased (running) all of the time. Class A amps offer the least distorted sound, but they are inefficient and run very hot. Although they are considered sonically the best class of amp, there are very few pure Class A amps in mobile audio use.

Class B circuits are very efficient, since they use separate transistors to amplify the positive and negative parts of the signal. With one exception, switching nose, they are also fairly low in distortion. Because the transistors switch on and off rapidly in response to the changing polarity of the signal, Class B amps generate switching distortion, which is usually audible only during very soft musical passages. Since the mobile audio environment is relatively noisy, passages soft enough to have audible switching distortion present are usually lost in the general road noise anyway. Thus, there are quite a few Class B mobile amps.

Class A/B is almost as efficient as Class B since the transistors in Class A/B never quite shut off. Because of this, Class A/B does not have audible switching distortion. Most main line home audio amps and most better car amps use Class A/B amplification. Today's top A/B designs allow the amplifiers to operate during considerable amounts of the music in pure Class A, while having the overall efficiency of the higher distortion Class B.