Published On: June 16, 2014

How to Pick the Right Amp for Your Speakers (or Vice Versa)

Published On: June 16, 2014
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How to Pick the Right Amp for Your Speakers (or Vice Versa)

Amplifiers and loudspeakers. Two devices that go together "like a horse and carriage," as the Sultan of Swoon would have said. After all, you can't have one without the other, assuming you actually want to listen to your music and...

How to Pick the Right Amp for Your Speakers (or Vice Versa)

By Author: Dennis Burger

Dennis Burger is a native Alabamian whose passion for AV began sometime before the age of seven, when he dismantled his parents' brand new 25-inch solid-state Zenith console TV and exclaimed--to the amusement of no one except the delivery guy--that it was missing all of its vacuum tubes. He has since contributed to Home Theater Magazine, Wirecutter, Cineluxe, Electronic House, and more. His specialties include high-end audio, home theater receivers, advanced home automation, and video codecs.

MArk.jpgAmplifiers and loudspeakers. Two devices that go together “like a horse and carriage,” as the Sultan of Swoon would have said. After all, you can’t have one without the other, assuming you actually want to listen to your music and movies in an actual room. But if speakers and amps were a Facebook couple, there’s no doubt that their relationship status would be “It’s Complicated.”

Despite being quite dissimilar devices (one being an electrical device designed to increase the amplitude of a signal from a preamplifier or preamp stage; the other being a mechanical device designed to convert that electrical signal into acoustical energy), the performance characteristics and capabilities of both loudspeakers and power amplifiers are described using many of the same words: primarily, watts (both RMS and peak) and impedance (usually denoted as Ω, or ohms). As such, you might think it would be reasonably simple to match a speaker with an appropriate amplifier: make sure the numbers match, and you’re in good shape, right? Well, sorta yes and sorta no. To understand why, we need to take a look at what those terms mean.

Additional Resources

But first, a caveat: this article is intended as a quick primer for beginners. As such, it contains a number of simplifications, many of them bordering on grossness. The goal here isn’t to write the definitive treatise on the subject, but rather to give budding audio enthusiasts a foundation of knowledge on which to build. To keep things simple, we’ll mostly be focusing on the characteristics of your typical solid-state amplifier and ignoring things like OTL (output transformerless) tube amps. Still, even a simple discussion of the relationship between speakers and amps can get a bit mathy; so, if you’re merely looking for a cheat sheet to help you find a new amp for your favorite speakers (or vice versa), you can skip straight to the conclusion.

With that out of the way, let’s discuss some of the common terms listed in the specifications for most amps and loudspeakers, and what those specs mean when shopping for components that play well together. Since wattage is the spec that most people look at first (and the only spec that some shoppers consider), we’ll start there.

Watts It All About?
Pioneer-TS-A1375R-5.25-inch-Three-way-Speakers-Back.jpgWattage is a measure of electrical power described most simply as voltage (volts) times current (amps). But what does that mean when it comes to speakers? Despite conventional wisdom, it really isn’t a good indicator of how loud a speaker will play. Instead, it’s an indicator of how much power a speaker can physically take without distorting or physically breaking (whether that be from fried voice coils, blown speakers, or cooked crossovers).

An amplifier’s rated wattage, by contrast, is calculated a little differently: by throwing numbers into a hat and pulling them out until one sounds impressive.

I’m kidding, of course. Mostly. The power output of amplifiers is usually listed as RMS and Peak. The former (which would be more simply described as continuous power) is calculated by playing a sine wave signal and measuring the average (root mean square) voltage and current that the amp can deliver without clipping (which is usually measured in terms of Total Harmonic Distortion, but can be seen on an oscilloscope as a flattening out of the curved tops and bottoms of a sine wave).

A good amplifier can, of course, deliver more powerful, albeit brief, bursts of clean energy than its continuous power output rating without clipping, and good speakers can likewise handle such brief bursts. How much more? It really depends on the speaker and the amplifier. I recently spoke with Larry Reagan, vice president of sales for Pro Audio Technology, about his company’s LFC-24sm subwoofer, whose power-handling capabilities are rated as 1,000 watts (AES) / 2,000 watts continuous. When I asked him what that meant, exactly, he had this to say: “The AES test is 1,000 watts of constant pink noise for two hours straight. It’s the Audio Engineering Society’s torture-test rating. With real musical or movie content, the thing can handle up to 2,000 watts constant easily, but you could actually throw 10,000 watts at one of these things in one of those momentary bursts, and it wouldn’t flinch.”

Needless to say, what’s true for subwoofers isn’t necessarily true for all speakers, but what is undeniably true is that most everyone seems to have an opinion on exactly how much amplification you should feed a speaker given its rated power handling. And almost none of them agree. Some recommend an amp with a 10 percent higher RMS power rating than the speaker’s continuous power-handling capabilities. Other suggest 1.5 times. Others still recommend doubling or even quadrupling the speakers’ RMS power-handling rating.

Why such high recommendations? I think that notion comes from the mistaken belief that what blows up drivers is a lack of power, not an excess of power. It’s easy to see why this canard is so commonly accepted as gospel, though. What’s actually happening when an underpowered amp blows up a speaker is that the amp is being asked to deliver more voltage or current than it’s physically capable of doing safely and cleanly, which sends the amp into clipping. And when an amp clips, it subjects your speakers to a not-insignificant amount of direct current. So, technically, if you’ve ever fried your speakers with too small an amplifier, the murder weapon was too much DC power, not too little AC power.

triton_detail_overview_image_new.jpgPerhaps that’s why a number of knowledgeable speaker manufacturers have started to eschew RMS and peak power ratings in favor of a broader “recommended amplification” rating. GoldenEar Technology is one such company, so I called head honcho, Sandy Gross, to ask why his speakers are rated this way and how, exactly, the “recommended amplification” rating relates to RMS and peak power handling. “The truth is,” he told me, “RMS and peak power specifications don’t really relate to anything in terms of real-world listening material. You can blow up speakers with virtually any size amp, or you can use them safely with virtually any size amp, depending on how you drive them.” To make his point, Sandy told me that he’s currently driving his pair of GoldenEar Triton Two towers (which carry a recommended amplification rating of 20 to 500 watts) with a 22-watt-per-channel tube amp from Line Magnetic. “I had David Chesky over recently to listen to the setup, and he was blown away by how good it sounded,” he told me. “I’ve even driven the Triton Two with amps as low as six watts per channel, and it did a pretty good job. You’d be surprised.”

Click on over to Page 2 to find out why Resistance Is Not Futile, Sensitive Speakers and Wrapping Up . . .

29af0585-2cd7-30b5-89e2-56a6c6e359b5.jpegResistance Is Far From Futile
Of course, in all of this talk about wattage, we’re overlooking another spec that can have a huge impact on the relationship between speaker and amp: impedance, which is usually denoted by the symbol Ω (ohm). Merriam-Webster defines impedance as: “the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current.” Which is why practically everyone who has tried to explain impedance in a comprehensible way has relied on the tried-and-true analogy of water flowing through a pipe. If you imagine voltage as analogous to water pressure and current as analogous to the flow rate, you can think of impedance as the size of the pipe: the lower the impedance, the larger the pipe.

What the heck does that have to do with speakers, though? Well, the impedance of a speaker determines how much electrical resistance it has against the flow of current from your amplifier. Another way of putting it is that the lower a speaker’s impedance, the more current it’s going to pull from your amp given the same voltage. (That’s one expression of Ohm’s Law: I=V/R, or Current is equal to Voltage divided by Resistance.) Most of the time, impedance isn’t a major concern, since most consumer speakers these days are rated at eight ohms or, at the very least, six ohms, and most amplifiers (even the ones built into mass-market AV receivers) can deliver sufficient clean current to drive them.

It should be noted that, although we’re speaking about impedance in terms of single numbers, that’s never actually the case. A speaker whose rated (nominal) impedance is eight ohms may well drop as low as four ohms (despite the fact that the International Electrotechnical Commission’s standards dictated that a speaker’s minimal impedance should be 80 percent of the nominal impedance or higher) and climb well into the double digits at different points across its frequency range. That’s why many speaker manufacturers list nominal and minimal impedance. It’s also why other manufacturers (like GoldenEar) list a more vague “compatible with” impedance spec. I asked Sandy why his speakers are specified as “compatible with eight ohms,” and he told me: “Most components on the market are designed for eight-ohm speakers; so, although one of our speakers might have impedance as high as 10 or 12 ohms, or as low as four or five ohms, depending on the frequency, I say that they’re compatible with eight ohms just to assure consumers that they’ll work with most consumer gear.”

sx8300r_ng.jpgThat isn’t true of all speakers, though. If your tastes start to veer off the beaten track, you’ll definitely want to look at the rated impedance of your speakers and shop for amps accordingly. RBH’s excellent SX-8300/R tower speaker (right), for example, is rated at four ohms. Although it’s safe to assume that anyone in the market for such a speaker is likely also shopping for a good dedicated amp, it’s worth stating for the record that I wouldn’t attempt to drive a set of SX-8300/Rs with most AV receivers for fear of overheating or outright frying their amps.

There are a few exceptions, of course: Pioneer’s Elite SC receivers can handle a four-ohm load just fine, and I’ve auditioned a few higher-end Onkyo receivers that will allow you to drive four ohms by limiting the current. AudioControl also offers a high-current version of its Concert AVR specifically for use with four-ohm speakers.

Still, your best bet with low-impedance speakers is always going to be a separate amplifier – one that specifically lists how much power it’s capable of delivering into four-ohm loads, and ideally one that gives its power-output rating with all channels driven, not merely one or two. If you’ve been paying attention to the equations above, it won’t come as a surprise to you that an amplifier will deliver more power to a four-ohm speaker than it will an eight-ohm speaker. (Current is equal to voltage divided by resistance, and wattage is equal to voltage times current.) So, while the Anthem A5 amp in my home theater system is rated to deliver 180 watts of continuous power into an eight-ohm load (with less than 0.1 percent Total Harmonic Distortion, all channels driven), it’s rated to deliver 265 watts of continuous power (ditto the specifics) into a four-ohm load.

That would certainly make the A5 a great match for the RBH SX-8300/R. Is it the right match (in terms of power) for my own Paradigm Studio 100 towers, though? Before we get into that, we need to explore one more speaker specification that matters when it comes to questions of amplification.

Some Speakers Are Just So Sensitive
VSi60_lens_1(0).pngUnlike most of the topics we’ve covered thus far, there aren’t a lot of “yeah-buts” when it comes to sensitivity – a straightforward measure of how much acoustical energy a speaker delivers into the room in relation to electrical power delivered to it. It’s also ultimately one of the most important specifications that determines how much amplification you really need, the other being how loudly you want to listen.

Sensitivity is calculated by measuring the sound pressure level of a speaker from one meter away, fed with 2.83 volts. You might also see some speaker manufacturers list the sensitivity rating as “one watt at one meter,” since 2.83 volts into an eight-ohm speaker gives you one watt, and 2.83 volts into a four-ohm speaker gives you two watts, which seems like cheating. Does that mean that higher-sensitivity speakers are necessarily better? No. The GoldenEar Triton Seven loudspeakers currently in my two-channel listening room are rated at 89dB sensitivity, which is generally considered average, and they sound incredible. The Paradigm Studio 100 towers in my home theater are rated at 92dB sensitivity in-room, which is a far cry from the 99dB sensitivity of something like Klipsch’s P-39F floorstanding speaker. That doesn’t necessarily make the Klipsch better (or worse). It simply means that it requires less amplification to reach the same sound pressure level in the same room. For every three dB of increased sensitivity, you need half the amplification to reach the same SPL.

Wrapping It All Up (Or: A Cheat Sheet for Those Who Skipped the Math Lessons)
In truth, I’ve probably made amplifier selection sound infinitely more complicated than it actually is if you stick to reputable brands. However, given that a lot of consumer-focused manufacturers fudge their specs, sometimes it’s unfortunately a more complicated process than it needs to be. When choosing an amp for your speakers (or vice versa), here are the most important things to consider:

Do your speakers have low nominal impedance?
If their impedance is listed as six or eight ohms nominal or “compatible with eight ohms,” you don’t really need to be as choosy when selecting an amp. If their nominal impedance is four ohms (or if their minimum impedance drops below three ohms), make sure you shop for a high-current amp, or one that lists separate power-output ratings for four- and eight-ohm loads.

What is the sensitivity of your speakers?
If it’s in the lower range of typical speakers these days (89 dB or below), understand that you’ll need more amplification to reach higher listening levels.

What’s the size of your room, and how loud do you like to listen?
6985095703_3443c5b643_b.jpgI worked out the math for my own home theater system to demonstrate exactly how much of my Anthem A5’s (right) output I’m using when listening to music and watching movies. In fact-checking myself online, I discovered that Crown has created a calculator for exactly those purposes. It should give you a great idea of how much amplification you need for your room, your speakers, and your listening tastes. You can find it here.

Most of the entries in the calculator are easy to understand. Listener Distance from Source? In my case, that’s right at two meters. Loudspeaker Sensitivity Rating (1W/1M)? For my Paradigm Studio 100s, that’s 92 dB. Headroom? Oy, that’s another article in and of itself. Leave it at three dB.

As for the Desired Level at Listener Distance, that’s different for every listener. My dad has an unmitigated conniption if the sound ever climbs above 90 dB or so, which is why I always turn on the dynamic range compression (or Night Mode) of his receivers when setting up or updating his home theater system. Given that my dad sits about three meters away from his speakers, that means he’s generally using a grand total of 11 watts of amplification in his home theater.

Me personally? I hate dynamic range compression and more often than not watch movies at reference level, which means it’s not out of ordinary for me to experience brief peaks of around 105 dB in my own home theater, depending on the movie, even if the dialogue is down around the 75dB range.

Plug those numbers into Crown’s calculator, and I get a recommended amplifier power of 176 watts per channel. Given that those brief 105dB bursts are highly unlikely to be coming from every speaker at once, and given that the A5 is capable of delivering 180 clean watts per channel with all five channels driven, I’d say it delivers pretty much the perfect amount of amplification for my room, with my listening tastes.

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