CE Pro stands for "custom electronics professional"; this excellent publication covers the home automation industry.

This article about amplifiers was written for the December, 2001 issue of the magazine. Reprinted by permission.

All Amplifiers Are Not Created Equal

Most of us consider an amplifier to be one of the most basic components of an audio/video system. After all, an amplifier just takes a small audio signal from a preamplifier or source component and makes it into a bigger signal to drive loudspeakers, correct? So, all you have to figure out when choosing an amplifier is: how many channels you'll need for the application, how much it will cost, and whether it will fit into your installation space…right?

Well, not quite. There are literally hundreds of amplifiers on the market to choose from, from megabuck purist high-end designs to cost-effective, feature-packed multi-channel models. And some amplifiers definitely sound better than others. Which amplifier is "best?" It depends on your particular application, needs, and budget.

What's Your Type?

Amplifiers come in three basic types—solid-state, tube or hybrid, a combination of both. Solid-state amplifiers use transistors and integrated circuits as active elements. Tube amps use vacuum tubes for amplification, while hybrid amplifiers utilize a combination of solid-state and tube circuitry.

Solid-state amplifiers are overwhelmingly the most popular and offer many advantages. They are generally extremely reliable. They are also practical and cost-effective, although, as with any type of amplifier, you tend to get what you pay for—high-end models typically have better internal components, better mechanical construction and other advantages. Solid-state models range from basic monoblock and stereo designs to multi-channel amplifiers designed for home theater, custom installation and multi-room use.

Solid-state amplifiers typically measure extremely well and excel in two areas. One is their ability, (to a greater or lesser degree depending on the design) to deliver high current—an advantage in powering inefficient, difficult-to drive speakers and in installations using long speaker-cable runs. The second advantage is their high damping factor—a measure of the amplifier's ability to control the speaker, or how tight a "grip" the amplifier has on the speakers' driver motion and resulting sonic articulation. The higher the damping factor, the better the control.

Some solid-state amplifiers also offer ultrawide-bandwidth frequency response; that is, response far beyond the audible range (for example, from a few Hz to 100,000Hz or higher). Many designers feel that an amplifier must have wide bandwidth if it is to reproduce a musical signal accurately and with minimal distortion.

Tube amplifiers are especially prized by audiophiles for their inherent musicality. Although they are impractical for most users and applications, typically more expensive than comparable solid-state counterparts, give off more heat (though some high-powered solid-state models get quite hot), require occasional tube replacement and often don't measure as well as solid-state models (particularly with respect to distortion specs), many listeners—especially audiophiles—find their sonic advantages irresistible. (One particular type of distortion, second-harmonic distortion, may actually be heard as adding a pleasing, if inaccurate, richness to the sound.)

Tube amplifiers come in many design flavors—push-pull, single-ended, triode, pentode and others, using a variety of tube types and circuit topologies and ranging from low-wattage designs to high-power behemoths. Typically, tube amps are either monoblock or stereo and used in two-channel high-end audio systems.

Hybrid amplifiers combine the best of solid-state and vacuum tubes in designs that use both technologies, in an effort to deliver the best of both sonic worlds.

Also, integrated amplifiers, which consist of a preamplifier and power amplifier in one unit, are available in all the above types.

How Much Power Is Enough?

One of the most often-asked amplifier questions is: "How much power do I need?" That depends on several factors: room size and room acoustics, speaker efficiency and personal listening preferences.

The bigger the room, the more power required to achieve a given volume level. Also, if you listen further away from the speakers, you'll need more power to attain the same volume as close-range listening. If a room is "live," reverberant with many reflective surfaces such as large glass doors, it will absorb less sound and need less amplifier power than a "dead" room with heavy drapes, furniture and carpeting.

The loudspeakers in the system are an important factor in choosing the right amplifier. Some speakers are more efficient than others, meaning they'll play louder for a given amount of amplifier power. Efficiency is measured in decibels; the higher the dB number, the more efficient the speaker. This is a crucial point, because every 3dB increase or decrease in efficiency translates into a halving or doubling of the power required to produce the same volume level.

Personal preferences play a big role. If you want to listen to background music or chamber music through small up-close mini-monitors, you'll need a lot less power than to reproduce slam-bang action movie soundtracks at "real-life" listening levels.

All in all, 40 or 50 watts per channel should be plenty for most listening. (Background or quiet listening requires only a few watts, if that.) Power ratings are usually expressed in continuous average watts per channel into 8 ohms and 4 ohms (some speakers are closer to 8 or to 4 ohms impedance). Nevertheless, having plenty of reserve power and dynamic headroom, or the ability of an amplifier to momentarily amplify a signal exceeding its continuous power rating, is a good thing. Also, pay attention to an amplifier's rated full power bandwidth spec, the frequency range of an amplifier measured while delivering its full rated power output. Many amplifiers cannot put out anything near their rated power at the frequency extremes under this condition, while others have no problem.

Features and Flexibility

Amplifiers vary greatly in the features they offer. The most basic types of monoblock and stereo amps sport little more than input and speaker terminals, an on/off switch, a power indicator and a power cord. For many applications such as a purist high-end audio system, that's all you need. Monoblock amplifiers are the ultimate in audio separates, having completely separate power supplies, inputs, outputs, and so on in the aim of ultimate sonic performance.

On the other hand, many amplifiers offer a number of custom installation-friendly features for greater versatility in more elaborate home entertainment and whole-house applications. Features to look for include:

• Multiple channels. Amplifiers can have five or more channels, to power all the speakers in a multi-channel home theater system and/or multiple systems throughout the home. Such amplifiers are often configurable to accommodate a wide variety of installation situations, such as simultaneous use in a main room plus a second-room system with different program sources. Many amplifiers allow bridging or tying together pairs of amplifier channels to create a single, more powerful channel.

• Some amplifiers also have multiple speaker outputs per channel, which is useful for bi-wiring some speakers that allow the use of two pairs of speaker cables to connect the low- and high-frequency drivers for improved sonic performance.

• Independent level control for each channel. These allow precise adjustment of each channel's volume levels in any multichannel installation; they're also useful for matching gain levels of different brands of amplifiers in a system.

• Type of input and speaker output connectors. Most amplifiers have RCA jacks for input connections; some also have balanced XLR-type connectors. Balanced connections offer lower noise and the ability to connect longer cable runs, thanks to their lower impedance (600 ohms) and inherent noise rejection. Speaker connections range from the inexpensive, spring-loaded type that only accept bare-wire cable ends to heavy-duty multi-way terminals that accept most types of high-quality terminations.

• Remote control capabilities such as a remote power on/off feature, DC trigger inputs, and the ability to interface with integrated system remote controllers are important considerations when selecting an amplifier for use in multi-room and whole house installations.

• Power meters and status/overload indicators are useful but not essential amplifier features. Any well-designed amplifier will have protection circuitry that will shut the amp down before overheating or damage can occur.

• Parallel output jacks. These convenient "pass-through" connections enable several amplifiers to be cascaded, or linked together in complex multi-zone installations.

• THX certification. Many amplifiers carry either THX Ultra or THX Select certification; standards set forth by Lucasfilm Ltd. to assure high-quality audio performance.

• Size, weight and appearance—make sure the amplifier will fit into your installation space! And if it's going to be in view, you may want to consider how its appearance will blend with the rest of the audio/video components and the home décor. Some amplifiers are rather attractive, while others are the very definition of the term "black box."

Class Acts

Amplifiers also differ in their class of output operation—the way an amplifier's output devices handle the positive and negative halves of a musical signal. (All musical signals have a positive and a negative portion, constantly alternating between the two.) This has an impact on the amplifier's efficiency, cost-per-watt, sound quality and other factors. Amplifier classes can be summarized as follows:

• Class A—the output devices conduct signal at all times. They're always "on," a design considered by some to produce the best sound, but at the expense of less efficiency and greater heat.

• Class AB—half the output devices conduct during more than half a waveform's positive cycle, then "hand off" to the other half of the devices during the negative cycle with some overlap—more efficient and cooler-running than Class A with many of the sonic advantages.

• Class B—half the output devices conduct signal during one half of the cycle only, then cut off completely while the other devices take over. Class B is more efficient than Class A or Class AB.

• Class D—also known as a digital switching amplifier. A Class D amplifier either digitizes an incoming analog signal or receives a digital input signal and converts it into a pulse-width-modulated signal (PWM), which then drives an output stage and is re-converted to an analog signal. Although more complex to implement, Class D designs offer extremely high efficiency and power output along with compact size, so many newer so-called "digital" amplifiers are using this topology.

• Class G and H—these classes involve changing the power supply voltage from a lower to a higher level when a larger output is required. For most signals, the lower voltage is used; the higher level "kicks in" when more power is required. Class H designs "track" the audio input to provide just enough voltage for optimum operation at all times.

In addition to the above, you may encounter other types of rarely seen sub-categories, as well as "classes" which are not true technical modes of operation but merely a particular manufacturer's marketing description.

All Amplifiers Sound the Same…Right?

In fact, they don't. As noted, amplifiers come in many different designs. Tube or solid-state, Class A or Class H, built to a price point or cost-no-object—an amplifier's engineering and design can significantly affect sound quality.

Ideally, a good amplifier should "disappear" sonically and not add any sonic colorations of its own. That said, many amplifiers do impart a sonic signature to an audio system. Many tube amps are prized for their "warmth" and sweetness, while high-current, high-power, ultrawide-bandwidth models typically excel at delivering pristine-clean, detailed, dynamic sound quality.

The best amplifiers will sound clean and pure and reproduce sonic nuances with almost hair-raising detail and realism, while lesser models may sound "grainy" or veiled. Some amplifiers can handle dynamic contrasts and low-frequency reproduction effortlessly, while others may sound strained when pushed hard.

Price is not the final arbiter of quality—a well-designed, modestly priced amplifier will outperform an expensive lemon. And specs only tell part of the tale. Don't just buy an amplifier "by the numbers"—talk to installers, dealers and users in the field, read the reviews and last but not least—listen to an amplifier for yourself before you buy or spec a particular model.

The right amplifier can make the difference between merely adequate sound quality—and a home entertainment system that conveys music and movie soundtracks with spectacular realism, power and emotional impact.