Resistance 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."
That 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
Unlike 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?
I 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.