Power and Integrated Amplifiers

The power amplifier—the last component in the signal chain before the loud speakers—is the workhorse of a hi-fi system. It takes the low-level signal from the preamplifier and converts it to a powerful signal to drive the loudspeakers.

It has a low-level input to receive the signal from the preamplifier, and terminals for connecting loudspeaker cables.

Because of the power amplifier’s unique function, it differs from other components in size, weight, and use. High-quality power amplifiers are usually large and heavy. Moreover, the power amplifier is the component you don’t need to touch and adjust. Power amplifiers are often placed on the floor near the loudspeakers rather than in an equipment rack.

Unlike most of the other components in your system, power amplifiers vary greatly in electrical performance. Consequently, choosing a power amplifier requires careful system matching for electrical compatibility, not just musical compatibility. While any CD player or preamplifier will function in a system (even though it may not be musically ideal), some power amplifiers just won’t work well with certain loudspeakers on a technical level.
Choosing a power amplifier is therefore a technical, as well as aesthetic, decision that requires careful attention to system matching. We’ll discuss these technical factors throughout this section.

Let’s first survey the types of power amplifiers and define some of the terms associated with them.

• Stereo Power Amplifier: A power amplifier with two audio channels (left and right) in one chassis.
• Monoblock Power Amplifier: A power amplifier with only one audio channel per chassis. Two monoblocks are required for stereo reproduction.
• Multichannel Power Amplifier: An amplifier with, typically, five amplifier channels, for use in a home-theater or multichannel-audio system.
• Watt: A unit of electrical power. Power is the ability to do work; in this case, the ability of the amplifier to make a loudspeaker’s diaphragm move.
• Power Output: The maximum amount of power the amplifier can deliver to the loud speaker, measured in watts.
• Load: In power-amplifier terminology, a load is the loudspeaker(s) the power amplifier must drive.
• Tubed: A tubed power amplifier uses vacuum tubes to amplify the audio signal.
• Solid-State: A solid-state power amplifier uses transistors to amplify the audio signal.
• Hybrid: A power amplifier combining vacuum tubes and solid-state device M, The input and driver stages are usually tubed, the output stage is usually solid-state.
• Class A: A power amplifier in which the output device or devices (tube or transistor) is operated in such a way that it amplifies the entire musical waveform.
• Push-Pull: A power amplifier in which pairs of output devices (tubes or transistors) alternately “push” and “pull” current through the loudspeaker, with one output device amplifying the positive half of the waveform, another output device amplifying the negative half of the waveform. Also called C/ass-B amplification, although not all push- pull amplifiers are Class-B. (Compare with “single-ended.”)
• Class A/B: A power amplifier that operates in Class A at low power and switches to Class-B operation at higher powers.
• Bridging: Converting a stereo power amplifier into a monoblock power amplifier. Some power amplifiers have a rear-panel bridging switch. Also called “strapping” into mono.
• Bi-amping: Driving a loudspeaker’s midrange and treble units with one amplifier, the woofer with a second amplifier.
• Class-D Power Amplifier: An amplifier that operates by switching its output transistors fully on or fully off in a series of pulses. Often erroneously called a “digital” amplifier, but correctly called a “switching” amplifier.

A Survey of Amplifier Types

As you can see from this list, high-performance power amplifiers are available in a wide range of configurations. In fact, high-end amplifiers run the gamut from a minimalist 3Wpc (watts per channel) design to massive 500W behemoth monoblocks. Power amplifiers also cover a huge range of technologies from vacuum tubes to today’s high- technology “switching” circuits. You can buy amplifiers with one, two, three, five, or seven amplification channels.

Let’s survey the amplifier types before looking at how to choose the right amplifier for your needs.

Monoblock, Stereo, Three-Channel and Multichannel Amplifiers

The simplest power amplifier is the monoblock. It houses a single amplifier channel in one chassis, with two monoblocks required for stereo reproduction. The familiar stereo amplifier provides two amplification channels, and a multichannel power amplifier offers either five or seven channels. If you plan on using your system only for two- channel music listening, you’ll choose a pair of monoblocks or a stereo amplifier.

A recent addition to this list of amplifier configurations is the three-channel amplifier. These units appeal to those who want to convert a two-channel system to multichannel, either to reproduce film soundtracks in a home theater or for multi-channel music listening. The three-channel amplifier also makes sense for those whose primary focus is two-channel music with occasional home theater watching— more of the budget can be put into the critical left and right stereo amplification channels, with a less expensive three-channel amplifier handling the center- and surround-channel duties.

The first division of power amplifiers—a stereo unit or a pair of monoblocks—will be decided by your budget. Monoblocks generally start at about $2500 per pair. At this price level, a single stereo unit may make more sense; with only one chassis, power cord, and shipping carton, the manufacturer can put more of the manufacturing cost into better parts and performance. I advise against monoblocks if your amplifier budget is less than about $4000. There may be exceptions to this figure, but it nonetheless offers a broad guideline. Many excellent stereo units, for example, cost upward of $6000. A very popular price range for high-quality power amplifiers is $800—$2000, with the $2000 models sometimes offering musical performance close to that of the most expensive amplifiers.

Monoblocks generally perform better than a single stereo unit for several reasons. First, because the two amplifier channels are housed in separate chassis, there is no chance of interaction between channels. Consequently, monoblocks typically have better soundstage performance than stereo units. Second, monoblocks have completely separate power supplies, even down to the power transformers: the left- and right-channel amplifier circuits don’t have to share their electrical current source. This gives monoblocks the ability to provide more instantaneous current to the loudspeaker, all other factors being equal. Finally, most manufacturers put their cost-no-object efforts into monoblocks, which are often the flagships of their lines. If you want all-out performance and can afford them, monoblocks are the way to go. A high-quality stereo amplifier is, however, more than sufficient for most systems.

A multichannel audio system can also be based on a single five-channel or seven channel amplifier. The five-channel amp is the more common configuration for two reasons; most multichannel loudspeaker arrays are based on five speakers plus a subwoofer, and squeezing seven amplification channels in a single chassis is a challenge. Some home-theater systems are based on seven channels plus a subwoofer (see Section 10 for a full explanation) and require seven amplifier channels. Such a system can be driven by a single seven-channel amplifier or by a five-channel unit augmented with a stereo amp, for example.

Integrated Amplifiers

At the other end of the scale from the monoblock is the integrated amplifier, in which a preamplifier and a power amplifier are combined in the same chassis. Though the power output from integrated amplifiers is generally lower than that from separate power amplifiers, integrateds are much less expensive, and ideal for budget to moderately priced systems. High-quality integrated amplifiers start at about $350, with some models running as high as $9000.

High-end integrated amplifiers have changed radically in the past few years. Once relegated to low-powered units from European manufacturers with idiosyncratic operation and non-standard connectors, integrated amplifiers have finally come into their own. Leading high-end manufacturers have realized that an integrated amplifier makes sense for many music lovers. The cost and convenience advantages of an integrated amplifier are compelling; integrateds take up less space, are easier to connect, reduce the number of cables in your system, and can even offer the performance of separate components. Now that high-end manufacturers have taken the integrated amplifier seriously, they’re putting their best technology and serious design efforts into their integrateds.

Consequently, many manufacturers have enjoyed booming sales of integrated amps in the price range of lower-cost separates that produce about 50 – 150 Wpc (watts per channel) of power. One such product is shown in Ill. 8-1.

Ill. 8-1 Today’s integrated amplifiers offer performance that competes with separate preamplifiers and power amplifiers. (from Naim)

Some manufacturers have even included a quality tuner with their integrated amplifier. Not so long ago, the term “high-end stereo receiver” was an oxymoron. Today, however, there’s no reason why a receiver designed and built with the dedication given to separate components should offer anything but high-end musical performance.

These newer integrated amplifiers have also overcome one of the limitations of earlier designs: the inability to upgrade just the power amplifier or preamplifier section. Today’s integrateds often include preamplifier-out jacks for connecting the integrated to a separate, more powerful amplifier. They also often have power-amplifier input jacks if you want to upgrade the preamplifier section.

When choosing an integrated amplifier, combine the advice in Section 7 (“Preamplifiers”) with the guidelines in the rest of this section. If your budget is under $3000 for amplification, seriously consider one of the new breed of high-end integrated amplifiers rather than separates.

Tubed Power Amplifiers

When transistor amplifiers were introduced in the 1960s, it appeared that the days of the vacuum tube were over. Transistors were smaller, lighter, and cheaper than tubes, ran cooler, and produced more output power. If that wasn’t enough, transistor amplifiers didn’t need an output transformer, a component that added considerably to the amplifier’s size, weight, and cost. All the audio manufacturers of the day scrapped their tubed amplifiers in favor of transistor units virtually overnight.

But many music lovers found the sound of these newfangled “solid-state” amplifiers unlistenable. They likened the sound to that of a pocket transistor radio, only louder. Unfortunately, those perceptive audiophiles couldn’t buy a new tubed amplifier after the transistor’s introduction.

The modern tubed amplifier was created in 1970 by William Zane Johnson of Audio Research Corporation. As did many music lovers, he found the sound of tubed amplifiers more musical. Johnson’s demonstration of a tubed amplifier at a 1970 hi-fi show prompted one show-goer to remark, “You’ve just set the audio industry back 10 years!” But instead of being a setback, Johnson’s amplifier began a renaissance in tubed equipment that is still going strong nearly 50 years later.

The perennial tubes-vs.-transistors debate arises when you’re faced with choosing a power amplifier. Tubed units can offer stunning musical performance, but they have their drawbacks. Here are the advantages and disadvantages of tubed power amplifiers. (For the moment, I’ll confine my observations to conventional “push-pull” tubed amplifiers, not the exotic single-ended triode varieties described later in this section.)

Tubed power amplifiers are more expensive than their similarly powered solid state counterparts. The cost of tubes, output transformers (output transformers are not needed in solid-state amplifiers), and more extensive power supplies all make owning a tubed power amplifier a more expensive proposition than owning a solid-state unit. Moreover, the tubes will need replacing every few years, further adding to the real cost of ownership.

In terms of bass performance, tubed power amplifiers can’t compete with good solid-state units. Tubes have less control in the bass, making the presentation less punchy, taut, and extended. Further, tubed power amplifiers often have limited current delivery into low-impedance loads, making them a poor choice for current-hungry loudspeakers. Tubes also require monthly biasing (small adjustments made with a screwdriver) to maintain top performance. Biasing is very easy, but some music lovers would prefer not to have to think about performing routine maintenance on their music-playback systems.

Power-amp tubes can fail suddenly, sometimes in smoke and (momentary) flames. Such dramatic failure is rare, however; I’ve used tubed amplifiers for most of my time as a reviewer (since 1989) and have had two tubes fail in that time, both of them uneventfully.

Finally, there is the possibility of small children or pets burning themselves on hot tubes. Many, but not all, tubed power amplifiers have exposed output tubes. If you have small children, consider a tubed amplifier that is surrounded by a ventilated metal cage.

Given these drawbacks, why would anyone want to own a tubed power amplifier? It’s simple: tubes can sound magical. When matched with an appropriate loud speaker, tubed power amplifiers offer unequaled musicality, in my experience. Even small, moderately priced tubed amplifiers have more than a taste of tube magic.

Many important aspects of music reproduction seem to come naturally to tubed amplifiers. They generally have superb presentation of instrumental timbre, smooth and unfatiguing treble, and spectacular soundstaging. The hard, brittle, edgy midrange and treble presentation of many solid-state amplifiers is contrasted with the purity of timbre and sense of ease conveyed by a good tubed amp. Music has a warmth, ease, and natural musicality when reproduced by many tubed designs. The soundstage has an expansive quality, with a sense of bloom around instrumental images. This isn’t to say there aren’t good-sounding solid-state power amplifiers, only that tubes seem to more consistently deliver the musical goods. A tubed amplifier’s softer bass is often willingly tolerated for its magical midrange, treble, and soundstaging.

As good as tubed amplifiers can sound, solid-state amplifiers have some decided sonic and technical advantages. For example, tubed units are no match for solid-state amplifiers in bass performance. Transistor power amplifiers have tighter, deeper, and much more solid bass than tubed units. The feeling of bass tautness, kick, extension, and power are all better conveyed by solid-state amplifiers, regard less of how good the tubed amplifier is. Speaking technically, solid-state amplifiers can deliver more current to low-impedance loudspeakers, making them a better choice for such loads.

No one but you can decide if a tubed power amplifier is ideal for your system. I strongly suggest, however, that you audition at least one tubed amplifier before making a purchasing decision. You may get hooked.

Single-Ended Triode Amplifiers

So far I’ve described mainstream power amplifiers that most audiophiles are likely to buy. But a number of variations on the basic design are a significant force in the amplifier marketplace. These include the single-ended triode amplifier, the single-ended solid-state amplifier, and the switching, or Class-D, power amplifier.

Single-ended triode amplifiers are an exotic species of power amplifier that has gained a considerable following in the past decade. The single-ended triode (SET) amplifier was the first audio amplifier ever developed, dating back to Lee De Forest’s patent of the triode vacuum tube in 1907 and his triode amplifier patent of 1912. SET amplifiers typically produce less than l0Wpc.

You heard right: Large numbers of audiophiles are flocking to replace their modern power amplifiers with 10-watt amplifiers based on 100-year-old technology.

Live the past 100 years of amplifier development been a complete waste of time? A surprising number of music lovers and audio designers think so.

In a single-ended triode amplifier ( Ill. 8-2), the triode (the simplest of all vacuum tubes) is operated so that it amplifies the entire audio signal. That’s what “single ended” means. Virtually all other power amplifiers are Class-B, meaning that one tube (or transistor) handles the positive half of the musical waveform and a second tube (or transistor) handles the negative half.

Ill. 8-2 The single-ended triode (SET) must be matched .with high-sensitivity loudspeakers. (Cary Audio Design)

On the test bench, SET amplifiers have laughably bad technical performance. They typically produce fewer than 10Wpc of output power, and have extremely high distortion— as much as 10% Total Harmonic Distortion (THD) at the amplifier’s rated output.

Despite these technical drawbacks, my listening experience with SET amplifiers suggests that this ancient technology has many musical merits. SET amps have a certain presence and immediacy of musical communication that’s hard to describe. It’s as though the musicians aren’t as far removed from here-and-now reality as they are with push-pull amplifiers. SET amps also have a wonderful liquidity and purity of timbre that is completely devoid of grain, hardness, and other artifacts of push-pull amplifiers. When I listen to SET amplifiers (with the right loudspeakers), it’s as though the musicians have come alive and are playing in the listening room for me. There’s a directness of musical expression that’s impossible to put into words, but is immediately understood by anyone who has listened for themselves. You must hear a SET firsthand to know what the fuss is about; no description can convey how they sound.

When auditioning an SET amplifier, it’s easy to be seduced by the midrange. That’s because SET amplifiers work best in the midband, and less welt at the frequency extremes of bass and treble. If the SET demo is being run for your benefit, be sure to listen to a wide variety of music, not just small-scale music or unaccompanied voice— these will accentuate the SET’s strengths and hide its weaknesses.

The importance of matching an SET amplifier to the right loudspeaker can not be overemphasized. With a low-sensitivity speaker, the SET will produce very little sound, have soft bass, and reproduce almost no dynamic contrast. The ideal loudspeaker for an SET amplifier has high sensitivity (higher than 96dB/1W/1m), high impedance (nominal 8 ohms or higher), and no impedance dips (a minimum impedance of 6 ohms or higher). Such a speaker will produce lots of sound for a small amount of input power, and require very little current.

Single-Ended Solid-State Amplifiers

Single-ended amplifiers aren’t confined to those using ancient vacuum-tube technology. Transistors can also be configured to amplify the entire musical waveform. These amplifiers are also called Class-A amplifiers because the mode of operation in which the output device amplifies the entire audio signal is called Class-A. A solid-state, single-ended amplifier is shown in Ill. 8-3. Note the large heatsinks required to dissipate the additional heat produced by Class-A operation.

Ill. 8-3 A single-ended solid-state power amplifier runs as hot at idle as at full power, requiring large heatsinks to keep it cool. (from Pass Labs)

Single-ended solid-state amps have better technical performance than single-ended triode (tube) amps, with lower output impedance, more power, and the ability to drive a wider range of loudspeakers. They share many of the benefits of SET amps, notably the very simple signal path, lack of crossover distortion, and greater linearity. Crossover distortion occurs on each cycle of the waveform when the transistor or tube handling the positive half of the waveform “hands off” the signal to the transistor or tube handling the negative half of the waveform. Although single-ended solid- state amplifiers produce less power than their push-pull counterparts, they generally have much more output power than single-ended tubed units. Nonetheless, it’s a mistake to equate single-ended solid-state with single-ended tube amplifiers: there are so many other design variables that single-ended solid-state and single-ended tubed amplifiers should be considered completely different animals.

Class-D “Switching” Amplifiers

If single-ended triode amplifiers represent a return to fundamental technology, the switching power amplifier may represent the future of audio amplification. Switching amplifiers, also called Class-D amplifiers, have been gaining in popularity due to their small size, low weight, high efficiency, and low cost ( Ill. 8-4). At the low-end of the audio spectrum, switching amplifiers are becoming ubiquitous in home-theater-in-a box units. A home-theater-in-a-box may need to power six loudspeakers from a DVD player-sized chassis—all for a few hundred dollars. Such a unit can output perhaps 300Wpc (500Wpc x 6), yet run cool enough to be placed in a cabinet. In this application, the advantages of a switching amplifier are undeniable. But are switching amplifiers suitable for high-end systems?

Ill. 8-4 Class-D amplifiers are small, lightweight, efficient, and produce very little heat. These amps are smaller than a brick and can be held in an outstretched hand. (from Kharma)

Before tackling that question, let’s first look at how a switching amplifier works. In a conventional amplifier (called a linear amplifier), the output transistors or tubes amplify a continuously variable analog signal—the musical waveform. In a switching amplifier, the continuously variable analog input signal is converted into a series of on and off pulses. These pulses are fed to the output transistors, which turn the transistors fully on or fully off. When the transistors are turned on, they conduct the DC supply voltage to the loudspeaker. When turned off, no voltage is connected to the loudspeaker. The audio information is contained in the durations of these on-off cycles. The train of pulses amplified by the transistors is smoothed by a filter to recover the musical waveform and remove the switching noise. Because the signal amplitude is contained in the width of the pulses, switching amplifiers are also called pulse-width modulation (PWM) amplifiers.

A switching amplifier operates at very high efficiency, thus the low heat dissipation. Because they run cool, switching amplifiers don’t need the large heat sinks found in conventional amplifiers, saving space and cost. Another benefit of this high efficiency is the reduced current demands on the power transformer, which can be smaller, lighter, and cheaper. A Class-A/B amplifier operates at about 40% efficiency (40% of the AC power pulled from the wall socket is converted into power that drives the loudspeakers). By contrast, a Class-D amplifier operates at about 90% (or more) efficiency.

Nonetheless, some successful high-end amplifiers employ switching technology. The field is relatively ne and manufacturers are finding ways to get good sound from switching amplifiers. The technology is in its infancy, suggesting that switching technology may have a future in products other than car stereo and home-theaters-in-a-box.

How to Choose a Power Amplifier

Quiz time: Which stereo system will play louder—one with a 10-watt amplifier or one with a 200-watt amplifier? The answer is that it’s impossible to determine without knowing one essential fact: the sensitivity of the loudspeaker that the amplifier is driving. A loudspeaker’s sensitivity specification indicates how much of the power driving the speaker is converted into sound and how much is wasted as heat. Everyone pays attention to an amplifier’s “watts per channel” rating but few consider a loudspeaker’s sensitivity. As we’ll see, the two specifications are equally important in determining how loudly an audio system will play.

Technically, loudspeaker sensitivity specifies how high a sound-pressure level (SPL) the loudspeaker will produce when driven by a certain power input. A typical sensitivity specification will read “88dB SPL, 1W/1m.” This means that the loudspeaker will produce an SPL of 88 decibels (dB) with one watt of input power when measured at a distance of one meter. Although 88dB is a moderate listening volume, a closer look at how power relates to listening level reveals that we need much more than one watt for music playback.

Each 3dB increase in sound-pressure level requires a doubling of amplifier output power. Thus, our loudspeaker with a sensitivity of 88dB at 1W would pro duce 91dB with 2W, 94dB with 4W, 97dB with 8W, and so on. For this loudspeaker to produce musical peaks of 109d13, we would need an amplifier with 128W of output power.

Now, say we had a loudspeaker rated at 91dB at 1W/1m—only 3dB more sensitive than the first loudspeaker. We can quickly see that we would need only half the amplifier power (64W) to produce the same volume of 109dB SPL. A loudspeaker with a sensitivity of 94dB would need just 32W to produce the same volume. The higher-sensitivity speaker simply converts more of the amplifier’s power into sound.

To recap, a 3dB increase in loudspeaker sensitivity is equal to doubling the amplifier’s output power. A 6dB increase is equal to quadrupling the amplifier’s output power, and so on.

This relationship between amplifier power output and loudspeaker sensitivity was inadvertently illustrated in an unusual demonstration nearly 60 years ago. In 1948, loudspeaker pioneer Paul Klipsch conducted a demonstration of live vs. reproduced sound with a symphony orchestra and his Klipschorn loudspeakers. His amplifier power: 5W. The Klipschorns are so sensitive (an astounding 105dB SPL, 1W/I m) that they will produce very high volumes with very little amplifier power. Klipsch was attempting to show that his loudspeakers could closely mimic the tonal quality and loudness of a full symphony orchestra.

The other end of the speaker-sensitivity spectrum was illustrated by a demonstration I attended at the Consumer Electronics Show of an exotic new loudspeaker. During the demo, the music was so quiet that I could barely hear it. I looked at the power amplifiers—300Wpc monsters with large power meters—and was astonished to see that the power meters were nearly constantly pegged at full power. This unusual speaker converted only a minuscule amount of the amplifier’s output power into sound.

The importance of loudspeaker sensitivity is also demonstrated by today’s 3Wpc single-ended triode amplifiers, which can produce moderately loud listening levels when coupled with high-sensitivity speakers. These examples of huge variations in sound-pressure level and amplifier power illustrate how loudspeaker sensitivity greatly affects how big an amplifier you need. Even a small difference in loudspeaker sensitivity—2dB, say—changes your amplifier power requirements.

In practice, loudspeakers vary in their sensitivities from about 83dB on the low side to perhaps 103dB on the high side, with most falling between 85dB and 94dB. Even the range of 85dB to 94dB represents an eight-fold difference in amplifier power to achieve the same sound-pressure level. That is, a 20W amplifier driving the 94dB- sensitive speaker will produce the same sound-pressure level as 160W driving the 85dB-sensitive speaker.

This knowledge allows us to match the amplifier’s output power to the loud speaker’s sensitivity and get the best possible performance for the money. In a system employing an amplifier with not enough output power for a particular loudspeaker, the sound will lack dynamics, distort on musical peaks, have soft and sluggish bass, and sound constricted. If the amplifier has an excess of power for a particular loudspeaker, the penalty is not sonic but financial; you are paying for watts in the amplifier you’ll never use. An amplifier’s cost is often proportional to its output power—watts equal dollars. And wasted watts represent money that could have been better spent on other components in your system.

Before we look at some real-world examples of matching amplifier power to a loudspeaker’s sensitivity, there are a few more factors to consider.

The first is room size. The bigger the room, the more amplifier power you’ll need. A rough guide suggests that quadrupling the room volume requires a doubling of amplifier power to achieve the same sound-pressure level. How acoustically reflective or absorptive your listening room is will also affect the best size of amplifier for your system. If we put the same-sensitivity loudspeakers in two rooms of the same size, one room acoustically dead (absorptive) and the other acoustically live (reflective), we would need roughly double the amplifier power to achieve the same sound-pressure level in the dead room as in the live room.

Finally, how loudly you listen to music greatly affects how much amplifier power you need. Chamber music played softly requires much less amplifier power than rock or orchestral music played loudly.

We can see that a low-sensitivity loudspeaker, driven by orchestral music in the large, acoustically dead room of someone who likes high playback levels, may require nearly one-hundred times the amplifier power needed by someone listening to chamber music at moderate listening levels through high-sensitivity loudspeakers in a small, live room. A 10 Wpc amplifier may satisfy the second listener; the first listener may need 750Wpc.

Now for some real-world examples. A loudspeaker of 88dB sensitivity in an average-sized room (say, 4000 cubic feet) featuring acoustics typical of today’s homes requires a minimum of about 50Wpc. If you enjoy orchestral music, which has a very wide dynamic range, an amplifier with a rating of 75Wpc would be more appropriate.

Choosing an appropriate amplifier power-output range for your loudspeakers, listening tastes, room, and budget is essential to getting the best sound for your money. If the amplifier is under-powered for your needs, you’ll never hear the system at its full potential. The sound will be constricted, fatiguing, lack dynamics, and the music will have a sense of strain on climaxes. Conversely, if you spend too much of your budget on a bigger amplifier than you need, you may be shortchanging other components. Choosing just the right amplifier power is of paramount importance.

The sure-fire way of determining if a particular amplifier works well with a particular loudspeaker is to audition the combination, either in your home or the dealer’s showroom. Later in this section (“What to Listen For”) we’ll learn how to deter mine by listening whether an amplifier is up to the task of driving a particular loud speaker.

Output Power Specifications: Read the Fine Print

Manufacturers use all sorts of tricks to make their amplifiers seem as powerful as possible on the specification sheet. When comparing amplifier power ratings, make sure the specified power is continuous or RMS rather than peak. Some manufacturers will claim a power output of 200W, for example, but not specify whether that power out put is available only during transient musical events such as drum beats, or if the amplifier can deliver that power continuously into a load. Also be sure the power rating is specified into an 8-ohm impedance. Rating the amplifier’s power into 4 ohms makes the amplifier seem more powerful than it really is.

Why Two “100 W” Amplifiers Can Have Different Output Powers

It is possible to buy an audio/video receiver today with a rated power of 100Wpc into 8 ohms across all five of its amplifier channels for less than $300. You can also spend $4000 for a stereo amplifier that is identically rated at 100Wpc into 8 ohms. Do these amplifiers have the same output power?

Yes and no. If both amplifiers are driving a continuous signal such as a test tone into 8 ohms, as occurs on a test bench, they both deliver 100W. But music is not a Continuous signal, and loudspeakers rarely have an impedance of 8 ohms at all frequencies. Most loudspeakers have dips in their impedance; a speaker rated at 8 ohms could present a 4-ohm impedance to the amplifier over most of the audio-frequency band. Some speakers even have impedance dips as low as 2 ohms. All solid-state amplifiers can increase their output powers into a 4-ohm speaker relative to an 8-ohm speak er, but they vary in the amount of power increase. For example, the audio/video receiver may be able to deliver perhaps 140W into 4 ohms, but the high-end stereo amplifier can deliver 200W into 4 ohms. This difference has real-world consequences. The ability to increase output power into low impedances indicates how much current the amplifier can deliver to the loudspeaker. It is current flow through the loudspeakers’ voice coils (in dynamic loudspeakers) that creates the electromagnetic force that causes the cones and domes to move, producing sound. If current flow through the voice coil is constrained, so is the music.

In addition, higher-quality amplifiers have greater dynamic headroom. This term describes an amplifier’s ability to deliver instantaneous bursts of short-term power well above the amplifier’s rated continuous power. For example, snare drum beats that require 180W for a few milliseconds would be reproduced cleanly by a 100-watt amplifier with good dynamic headroom, but sound compressed, lifeless, and even distorted by a lesser-quality 100-watt amplifier with very little dynamic headroom, even if the amplifiers’ continuous power ratings were identical.

The result is that two “100W” amplifiers can have very different power-delivery characteristics—and musical performances—when driving loudspeakers under real- world conditions.

A hallmark of a high-end amplifier is the ability to deliver performance beyond the specification sheet. The high-end amplifier is designed to reproduce music as faithfully as possible under real-world conditions, not to look good “on paper.” An amplifier’s ability to increase its output power into a low impedance, and deliver good dynamic headroom, is related to two aspects of its design: the power supp4i and output stage. The power supply is the circuit in the amplifier that converts the AC power from your wall into direct current that feeds the amplifier’s circuits. Most of a high-end amplifier’s weight is in the power supply. The amplifier’s output stage is a bank of transistors (or tubes) that do the actual work of pushing and puffing electrical current through the voice coils of your loudspeakers. To achieve the exemplary performance I’ve described, the power supply needs to be large and heavy, and the output stage must employ multiple heavy-duty transistors cooled by large heat sinks (the cooling fins visible on many amplifiers). Power supplies, output transistors, and heat sinks are by far the most expensive components in a power amplifier; skimping on these saves the manufacturer money at the expense of real-world musical performance. High-end amplifiers are easily distinguishable from mid-fl amplifiers of comparable power rating by their large power supplies and ample heat sinks. These are just two aspects of why high-end designs sound better than mass-market products, but there are countless other design elements throughout the high-end amplifier that make it a more faithful device for reproducing music.

Other Power-Amplifier Considerations

Balanced Inputs

If your preamp has balanced outputs, you may want to consider a power amplifier with balanced inputs. Most power amps with balanced inputs also provide unbalanced inputs, allowing you to compare these two connection methods before deciding which one to leave in your system. Some preamplifier/power-amplifier combinations sound better via balanced connection, others via the unbalanced jacks. The best way to discover which method is better is by listening to both. (See Section 7 for a discussion of why some preamplifiers may sound better from their unbalanced outputs.) Borrow a pair of interconnects from your dealer so you can make this comparison.

Bridging

Some stereo power amplifiers can be “bridged” to function as monoblocks. Bridging configures a stereo amplifier to function as a more powerful single-channel amplifier. The amplifier will have a switch on the rear panel to convert the amplifier to bridged operation. Note that two bridged amplifiers are needed for stereo. If you have a stereo amplifier that can be bridged and you want more power, simply buy a second, identical amplifier and bridge the two for more total power.

Bridging changes the amplifier’s internal connections so that one channel amplifies the positive half of the waveform and the other channel amplifies the negative half. The loudspeaker is connected as the “bridge” between the two amplifier channels instead of between one channel’s output and ground.

What to Listen For

How can you tell if the power amplifier you’re considering will work well with your loudspeakers? Simple: Borrow the amplifier from your dealer for the weekend and lis ten to it. This is the best way of not only assessing its musical qualities, but determin ing how well it drives your loudspeakers. In addition, listening to the power amplifier at home will let you hear if the product’s sonic signature complements the rest of your system.

The next best choice is if the dealer sells the same loudspeakers you own and allows you to audition the combination in the store. If neither of these options is practical, consider bringing your loudspeakers into the store for a final audition.

All the sonic and musical characteristics described in Section 4 apply to power amplifiers. However, some sonic characteristics are more influenced by the power amplifier than by other components.

The first thing to listen for is whether the amplifier is driving the loudspeakers adequately. The most obvious indicator is bass performance. If the bass is soggy and slow or lacks punch, the amplifier probably isn’t up to the job of driving your loud speakers. Weak bass is a sure indication that the amplifier is underpowered for a particular pair of speakers because the woofer puts the greatest current demands on the amplifier. Other telltale signs that the amplifier is running out of power include loss of dynamics, a sense of strain on musical peaks, hardening of timbre, reduced sense of pace and rhythm, and soundstage collapse or congestion. Let’s look at each of these individually.

First, play the system at a moderate volume. Select music with a wide dynamic range—either full orchestral music with a loud climax accompanied by bass drum, or music with a powerful rhythmic drive from bass guitar and kickdrum working together. Audiophile recordings typically have much wider dynamic ranges than general-release discs, making them a better source for evaluation. Music that has been highly corn pressed to play over the radio on a 3” car speaker will tell you less about what the sys tern is doing dynamically.

After you’ve become used to the sound at a moderate level, increase the volume—you want to push the amplifier to find its limits. Does the bass seem to give out when you turn it up, or does the amplifier keep on delivering? Listen to the dynamic impact of kickdrum on a recording with lots of bottom-end punch. It should maintain

its tightness, punch, quickness, and depth at high volume. If it starts to sound soggy, s1o or loses its power, you’ve gone beyond the amplifier’s comfortable operating point. After a while, you can get a feel for when the amplifier gets into trouble. Is the sound strained on peaks, or effortless?

NOTE: When performing this experiment, be sure not to overdrive your loud speakers. Turn down the volume at the first sign of loudspeaker overload (distortion or a popping sound).

Compare the amplifier’s sound at high and low volumes. Listen for brass instruments becoming hard and edgy at high volumes. See if the soundstage degenerates into a confused mess during climaxes. Does the bass drum lose its power and impact? An excellent power amplifier operating near its maximum power capability will preserve the senses of space, depth, and focus, while maintaining liquid instrumental timbre. Moreover, adequate power will produce a sense of ease; lack of power often creates listener fatigue. Music is much more enjoyable through a power amplifier with plenty of reserve power.

All the problems I’ve just described are largely the result of the amplifier run ning out of current. Just where this happens is a function of the amplifier’s output power, its ability to deliver current into the loudspeaker, the loudspeaker’s sensitivity and impedance, the room size, and how loudly you expect your hi-fi system to play. Even when not pushed to its maximum output, a more powerful amplifier will often have a greater sense of ease, grace, and dynamics than a less powerful amplifier.