Saw the post about what speakers work with low wattage systems, but I was wondering what specific attributes one would look for when picking out speakers to go with a low wattage amp.
One thing to be aware of is the fact that there are two paradigms in amplifier design: Amps that deliver approximately constant voltage (and therefore increased wattage into a lower impedance, and decreased wattage into a high impedance); and amplifiers that deliver approximately constant power (wattage doesn't change dramatically with impedance of speaker, within reason). Amp designer Ralph Karsten calls these the "voltage paradigm" and the "power paradigm", and has written a well-worth-reading paper on the subject:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpHere is why all of this matters: If a speaker was designed for "voltage paradigm" amplifiers, chances are the frequency response curve will be off when driven by "power paradigm" amplifiers. And vice versa. For example, suppose an "8 ohm" speaker has twin 16-ohm peaks in the bass region, a dip down to 4 ohms at 150 Hz, and then a 32-ohm peak in the crossover region, 2.5 kHz (this is probably an exaggeration, but the math is easier for me to do my head). Suppose this speaker is designed to be used with voltage paradigm (typical solid state) amps. At 2.83 volts output, such an amp will be putting out 1 watt into 8 ohms, but only 1/2 watt into those twin bass impedance peaks, 2 watts into the 150 Hz impedance dip, and 1/4 watt into the impedance peak in the crossover region. If "voiced" for this type of amp, the speaker will be correctly balanced despite the fact that the wattage is changing with the impedance curve.
Now suppose we read all about the magic of tubes, get excited, and buy a (power paradigm) tube amp to drive this speaker with. Is the magic going to happen? Well, we get twice as much power as the designer intended into those twin bass impedance peaks (1 watt instead of 1/2 watt), so we have a 3 dB rise in the bass region, and the bass sounds fat and sluggish. At 150 hz we're only getting half as much power as intended (1 watt instead of 2 watts), so the upper bass/lower midrange sounds recessed and lacking in body. Then at 2.5 kHz we're getting four times as much power as intended (1 watt instead of 1/4 watt), so we get a screaming 6 dB peak at 2.5 kHz (not a good place for a screaming peak). We logically conclude that tube amps suck (we changed the amp and the system went from good to suck), and we dump that piece-of-junk tube amp on the first unsuspecting sucker we can find, and march off with a sure conviction that only deaf idiots listen to tubes.
Well, what really happened was, we had the wrong kind of speakers for a tube amp (low wattage or otherwise). And if we'd have matched up power paradigm speakers with a voltage-paradigm amp, we'd have likewise have gotten poor results (actually the inverse of what I described above, but that would still suck).
So the moral of the story is, speaker/amplifier matching is important, but really not that hard once you know what you're looking for.
So what kind of speakers work well with low-power amps? High efficiency speakers intended to be used with that
type of amp. If you want speakers that work well with both types of amps, then the impedance curve needs to be very smooth. We're used to worrying about the effects of impedance dips, but as you can see from the above example, impedance peaks can cause problems too.
And by the way if you want to get a good deal on a used tube amp, make friends with the guy who just bought one to try with speakers having roller-coaster impedance curves that worked great with his solid state amps.
