Audio & Electronics

In our first two installments in this series, we looked at car audio systems in general and then took an in-depth tour of the head unit. In this, our third installment in the sequence, we'll get up close and personal with the car audio power amplifier.

Before we go any further, let's define our terms here. When we say "power amplifier" we are not referring to the entire amplification stage, but specifically to power amplification. You may recall that in Parts One and Two we touched upon the amplification process as a whole and broke it into two separate component groups: the power amplifier stage and the preamplifier stage.

Although both of these processes reflect opposite sides of the same coin, they actually work quite differently. The preamplifier serves more of a routing and control function, while the power amplifier deals with delivering brute force to the speakers.

Since its functions diverge from our discussion here, let's dispense with the preamplifier stage for now. We'll return to it in Part Five to take a final look at the preamplifier and discuss how its features aid and enhance the in-car entertainment experience.

So...the power amplifier. What does it do? How does it work?

The power amplifier's main job is to deliver a boosted, or "amplified," signal to the loudspeakers. The technology for this actually goes back more than a hundred years, to Thomas Edison and the light bulb, but it took a quantum leap in the '50s, when Sony introduced the transistor for commercial use, to pave the way for the power amp as we know it today.

Prior to this, most electronic devices — television sets, radios and industrial computers — used vacuum tubes. Vacuum tubes, or tubes as they're more commonly known, are still used in high-end home amplifiers, because of their excellent sonic qualities and "warmth" of tone. However, tubes have a major aversion to heat and vibration, so they're less than ideal in an automotive setting.

Think of the transistor as equivalent to the microprocessor in computers. Just as the microprocessor revolutionized the computer industry, making the PC possible, so too did the transistor transform the face of consumer electronics.

Earlier in this series we mentioned that heat is the enemy of most electric components, specifically amplifiers, which generate a lot of heat during operation. Therefore, except for small, lower-powered systems (10 to 20 watts per channel), which have the amplifier attached to the rear of the radio, most car audio amps will be located remotely from the faceplate, connected by a cable.

Since amplification is the last stop for the audio signal before it goes to the speakers, the amp merely takes whatever signal is delivered to it (after tone adjustments, balance/fade, etc., have altered it) and increases it. But that "merely" makes all the difference in the world, and it's the reason people will pay thousands of dollars for an aftermarket system that delivers awesome bass.

Before we define some amplification terms, a little more on heat. As stated above, larger amps (those above 10-20 watts per channel) are located remotely from the radio. This is not only because larger amps take up more room. A high-powered amp needs to "breathe" much as the cooling system for your engine breathes. Looking at the pictures of Pioneer and Kenwood amps we've included here, you'll note that they have "ridges" and "valleys" on top. Similar to your car's radiator, this design increases the surface area of the amp, allowing the metal to "radiate" the heat generated inside the amplifier back into the environment.

Through a series of transistors, conductors, rectifiers, resistors, inductors, capacitors and circuit boards, the inaudible signal coming from your radio passes through the amplifier and is "boosted" into the audible range. Once this is accomplished, the signal is delivered to the speaker terminals at the other end of the amp. Speaker wires connected to the terminals then carry the amplified signal to the speakers.

Amplifier output is typically measured in watts per channel, with a normal stereo system having two channels, left and right. You'll frequently see "RMS" after the wattage figure — this stands for "Root Mean Square," which is a mathematical measurement of the magnitude of the AC signal. More watts means more power output to the speakers, which in turn means louder volume.

Be aware, though, that in the aftermarket, and even at the OEM level, manufacturers will frequently massage the numbers to make their amplifiers appear to put out more power than they do.

The most common trick is to combine the per channel output figures into a "total system power" rating. In other words, a system that generates 50 watts per channel, RMS, suddenly becomes "100 watts total system power." As ludicrous as this sounds, it's a very common practice in the industry.

Another sleight-of-hand is to quote a "maximum power" rating. This is like driving a 1963 VW Beetle down a steep grade with a 50 mile per hour tailwind and then saying the car "averaged" 97 miles an hour. Not in any world you or I populate. Max power ratings are used to inflate real-world power output numbers and to trick the consumer into thinking they're getting more for their money, but they rarely reflect real-world circumstances in an audio system.

Third is the "peak power" rating. This is a snapshot of the amp in one particular moment of performance. Any decent audio amplifier will produce two, three, even four times its RMS power for a split second. However, to use this as its actual power rating would be like saying an Olympic weightlifter who cleaned and jerked 500 pounds had the "carrying capacity" of a quarter ton. Not exactly truth in advertising.

If you see "maximum power rating," or "total system power" or "peak power rating" on a box or a brochure with no other RMS figures to justify it, head for the hills. Either that, or divide the number in half for a more realistic RMS rating.

We once had a friend in the car audio business who called these numbers the "ILS" ratings — If Lightning Strikes.

The reason these power rating figures are bogus is that they give little usable information to the consumer. Almost any amplifier will produce twice its RMS power; however, this is usually accompanied by very high Total Harmonic Distortion (THD) — a crackling, jagged sound that indicates your amplifier is working overtime.

Another common misconception: horribly distorted sound is the result of bad speakers. Not true.

Even in today's improved car audio climate, most factory systems lack sufficient amplifier power to drive the loudspeakers. OEM suppliers such as Delphi, Visteon, Bose, Harmon-Kardon, Alpine and Clarion typically bid on contracts in a fierce bidding war in which the low man wins. Do you think you're going to get the best audio system in this scenario? Not on your life.

As a result, corners are cut. The first place that cut comes (since you can't have a system without speakers or a head unit) is in amplification. Therefore, most OEM systems will distort audibly when turned above about two-thirds volume.

Now, here's the interesting part. When you hear that distorted sound, you think Aha! I've got crappy speakers. No, what you've got is an amplifier that's running out of gas.

Therefore — and this is a hard and fast rule that always holds true — when upgrading a factory audio system, the first thing to replace is the amplifier. End of discussion. The only time this would not hold true is when you have damaged speakers.

Why? Because a factory system with a stronger amplifier will sound much better than that same system with replacement speakers but the same old gutless amp.

Another misconception: speakers blow because they can't handle power. Again, not true. They blow because the power sent to them is of an inferior quality.

You see, when a power amp is pushed beyond its limits, it generates a signal high in total harmonic distortion. That distortion gets reported to your ears by the speakers (but it's not their fault!). If pushed hard and long enough, the signal will become so distorted that it melts the thin copper wire around the voice coil. Voila — blown loudspeaker.

We'll get into this more in our next installment: the speaker. For now, suffice it to say that you should always replace your amplifier first.

We recommend that you go to a professional car audio shop if you're considering an upgrade. They can discuss the options with you and present different models. Step-up amplifiers generally run in the two to three hundred dollar price range, and go up from there. We also suggest that you have a qualified shop install the amp, unless you have expertise in this area.

One other term you may have heard in connection with amplifiers is the "crossover." What is a crossover?

Simply stated, a crossover is a device, sometimes built into an amplifier, which divides and directs frequencies to different loudspeakers. For instance, many car audio systems we review on our Web site have a two-way speaker system built into the front doors of the vehicle — a tweeter and a woofer. The tweeter handles the highs, the woofer the lows. Tweeters do an excellent job of reproducing upper frequencies, but they don't handle a lot of power. The crossover sends only the higher frequencies to the tweeter, directing the rest of the power of the amp to the woofers. It's like an electronic gate.

Most sophisticated aftermarket power amplifiers have built-in crossovers, as do many OEM systems in cars. Another, simpler (and cheaper) way to do this is to place a capacitor between the two loudspeakers and the amplifier. This "rolls off" the sound to the woofer, protecting the tweeter from receiving too much signal.

Well, that's about it for amps. In our next installment, we'll take an in-depth look at speakers. We've placed the links to the two previous articles in the series below.

Understanding Car Audio Systems
Understanding Car Audio Systems, Part 2: The Head Unit