Drew Eckert wrote:
"Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.
"For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.
"Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.
"With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way...."
In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??
It is because the audio world is accustomed to the way transistor amps behave, and most speakers are designed to work well with transistor amps. The designer designs the speaker to sound right when driven by an amplifier that puts out 1 watt into the 8-ohm portion of its curve, 2 watts into the 4-ohm portion of its curve, and 1/2 watt into the 16-ohm portion of its curve, all at the same time (2.83 volts).
Now, what if the designer's goal was a speaker that works great with an amplifier that puts out approximately constant wattage, regardless of the impedance curve (within reason)? Well that can be done just as easily, but there are fewer amplifiers designed that way out there, so his potential market is smaller.
These two approaches to amplifier and speaker design have a name: Voltage paradigm, and power paradigm. You can read more about the subject here:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
Okay, what kind of amplifier sounds best? Boy that's a long debate for another day, but a lot of people familiar with many types of amps prefer the sound of a good OTL or SET amp, assuming a good speaker pairing, and that includes yours truly. Some speaker manufacturers give priority to building speakers that will work best with power paradigm amps because they believe that combination sounds best, and just accept that they are fishing in a smaller pond.
Is it possible to build a speaker that works well with both types of amps?
Yes, by keeping the impedance curve as smooth as possible, the speaker will work well with both types of amps. And with such a speaker, you can really make an apples-to-apples comparison of the different amplifier types, rather than actually evaluating whether a (typically roller-coaster impedance) speaker synergizes best with a voltage paradigm amp or a power paradigm amp.
Drew's numbers above illustrate an argument in favor of using a high impedance speaker with a low-output-impedance amp: The amp's output is approximately constant-power when the speaker's impedance is varying between 16 and 64 ohms, but the amp's power output changes significantly when the speaker's impedance varies between 4 and 16 ohms (same 4-to-1 variance in both cases). Also note, in both cases the amp's power output change vs speaker impedance is in the opposite direction of what happens with a voltage-paradigm (solid state) amp, and this is probably the main reason why simply dropping a specialty tube amp into your current system is a roll of the dice (and the odds are against you) unless you already know your current speaker is a good match.
Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality). Also, most woofers are 8 ohms, so if the designer wants to use two such woofers, he has to choose between series connection (16 ohms) and parallel connection (4 ohms). Most choose parallel connection because 4 ohm speakers outsell 16 ohm speakers. I believe that most amps - tube or solid state - sound better into a 16 ohm load, so my home audio two-woofer designs are 16 ohm loads (whereas my prosound two-woofer designs are 4 ohm loads, because there we're trying to maximize available SPL with solid state amps). What do I do about the typical halving of maxium power from solid state amps when driving a 16 ohm load? I start out with speakers that are about 3 dB more efficient, and of course pay a corresponding price in box size vs bass extension. However there is one "free lunch" to using power paradign amps: They generate equal or even increased, rather than reduced, power into the speaker's virtually inevitable bass impedance peaks, and if we keep this in mind and design our box accordingly, we get back most of the bass extension we otherwise would have lost when we traded off in the direction of higher efficiency.
So I take the position that, all else being equal, 16 ohm speakers sound better than 4 or 8 ohm speakers, and that power-paradigm amps are well worth seeking out matching speakers for.
Duke
dealer/manufacturer/power paradigm groupie
"Some audiophile amplifiers have silly high output impedances which interact with the speaker's varying impedance to change the frequency response and this is exacerbated with low load impedances. Output Transformer Less Tube amps are especially bad although single ended triodes without global feedback can also have problems.
"For instance an Atmasphere M-60 Mk.II.2 has a 4.1 Ohm output impedance.
"Driving a 3-way speaker with impedance varying from 16 to 64 Ohms this would cause a 1.4dB output difference between the minimum and maximum impedances.
"With 4 to 16 Ohm impedance the difference would be 4dB. This is not atypical for a 3-way...."
In reply, I'd like to point out that, with the same 4 to 16 ohm impedance difference described here, the power that a transistor amp puts out varies by 6 dB, because it is putting out constant voltage rather than constant wattage. Why does the audio world accept this without a blink, and yet think there's a problem when a tube amp exhibits less variance in power output into the same load??
It is because the audio world is accustomed to the way transistor amps behave, and most speakers are designed to work well with transistor amps. The designer designs the speaker to sound right when driven by an amplifier that puts out 1 watt into the 8-ohm portion of its curve, 2 watts into the 4-ohm portion of its curve, and 1/2 watt into the 16-ohm portion of its curve, all at the same time (2.83 volts).
Now, what if the designer's goal was a speaker that works great with an amplifier that puts out approximately constant wattage, regardless of the impedance curve (within reason)? Well that can be done just as easily, but there are fewer amplifiers designed that way out there, so his potential market is smaller.
These two approaches to amplifier and speaker design have a name: Voltage paradigm, and power paradigm. You can read more about the subject here:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
Okay, what kind of amplifier sounds best? Boy that's a long debate for another day, but a lot of people familiar with many types of amps prefer the sound of a good OTL or SET amp, assuming a good speaker pairing, and that includes yours truly. Some speaker manufacturers give priority to building speakers that will work best with power paradigm amps because they believe that combination sounds best, and just accept that they are fishing in a smaller pond.
Is it possible to build a speaker that works well with both types of amps?
Yes, by keeping the impedance curve as smooth as possible, the speaker will work well with both types of amps. And with such a speaker, you can really make an apples-to-apples comparison of the different amplifier types, rather than actually evaluating whether a (typically roller-coaster impedance) speaker synergizes best with a voltage paradigm amp or a power paradigm amp.
Drew's numbers above illustrate an argument in favor of using a high impedance speaker with a low-output-impedance amp: The amp's output is approximately constant-power when the speaker's impedance is varying between 16 and 64 ohms, but the amp's power output changes significantly when the speaker's impedance varies between 4 and 16 ohms (same 4-to-1 variance in both cases). Also note, in both cases the amp's power output change vs speaker impedance is in the opposite direction of what happens with a voltage-paradigm (solid state) amp, and this is probably the main reason why simply dropping a specialty tube amp into your current system is a roll of the dice (and the odds are against you) unless you already know your current speaker is a good match.
Back to the original question, what's with 4 ohm speakers if they're harder to drive, well in general they can play louder with a solid state amp, and most people have solid state amps, so they get more sound per dollar with 4 ohm speakers (quantity outsells quality). Also, most woofers are 8 ohms, so if the designer wants to use two such woofers, he has to choose between series connection (16 ohms) and parallel connection (4 ohms). Most choose parallel connection because 4 ohm speakers outsell 16 ohm speakers. I believe that most amps - tube or solid state - sound better into a 16 ohm load, so my home audio two-woofer designs are 16 ohm loads (whereas my prosound two-woofer designs are 4 ohm loads, because there we're trying to maximize available SPL with solid state amps). What do I do about the typical halving of maxium power from solid state amps when driving a 16 ohm load? I start out with speakers that are about 3 dB more efficient, and of course pay a corresponding price in box size vs bass extension. However there is one "free lunch" to using power paradign amps: They generate equal or even increased, rather than reduced, power into the speaker's virtually inevitable bass impedance peaks, and if we keep this in mind and design our box accordingly, we get back most of the bass extension we otherwise would have lost when we traded off in the direction of higher efficiency.
So I take the position that, all else being equal, 16 ohm speakers sound better than 4 or 8 ohm speakers, and that power-paradigm amps are well worth seeking out matching speakers for.
Duke
dealer/manufacturer/power paradigm groupie