speaker impeadance question
You don't exactly get constant wattage, or constant voltage. What happens is that with high impedance amps and low impedance speakers is you can loose output power (i.e. dynamic range). By having a 4 Ohm tap, your 75W tubes still put out 75W. Otherwise, they might only put out 60 @ 4Ohms using the 8 Ohm tap. This example is exceedingly soft. :) I suggest you find some Stereophile reviews to examine this more specifically. |
Hi Keith, First, it’s probably worth defining these terms for the benefit of others who may read this thread. An amplifier which provides "constant voltage," aka one that acts as a "voltage source," is one that for a given input voltage provides an output voltage that does not vary significantly as a function of speaker impedance, assuming the amp is operated within its capabilities. Hence for a given input voltage it will deliver significantly more power at frequencies for which speaker impedance is low than at frequencies for which speaker impedance is high. (Power delivered into a resistive load equals voltage squared divided by resistance). Most solid state amplifiers fall into this category. An amplifier having "constant power" aka "constant wattage" characteristics will, for a given input voltage, tend to deliver less output voltage at frequencies for which speaker impedance is low, and more output voltage at frequencies for which speaker impedance is high. That will result in loosely approximating delivery of constant power into those varying impedances. Most tube amps fall into that category, to a loose approximation. How loose that approximation is will depend on both the output impedance of the particular amplifier (which varies widely among different tube amps), and on how the impedance of the particular speaker varies over the frequency range. Which characteristic is better depends mainly on the particular speaker. The majority of speakers these days are designed with the expectation that they will be driven with voltage source amplifiers. But of course many are designed to work well with tube amps of various kinds. And some are suitable for use with either type. (The Daedalus speakers I use are an example of a speaker that is equally happy with either type, and that versatility is made possible by the fact that Daedalus speakers have a very flat impedance curve, i.e., their impedance does not vary very much over the frequency range). If you haven’t seen it, Ralph (Atmasphere) has a good paper on this subject at his website: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php Best regards, -- Al |
I don't really like to use the phrase "constant voltage" here. I think that the term "voltage source." is more clear. Meaning, the voltage is a function of the input voltage, not the speaker impedance. For what we call a power amplifier, this is usually around 20x. That is, 1 V in = 20 V out. 0.1V in = 2 Volts out, so long as the speaker’s impedance is at or above the rated speaker impedance of the amplifier. |
