Voltage source vs. current source amplifiers
Only certain ML models have powered woofers - my Aerius doesn’t for example. But I agree kijanski has it right. The amp will need to double the power when the load is halved in order to maintain a constant voltage. In contrast, the tube amps almost always either maintain the power or in some cases even increase the power when the load is reduced. What prompted me to read up on this was my current set up where I have my McIntosh MC2200 - 200 wpc @ 8,4,2 ohms, driving my Aerius MLs and I have a Vincent - 150 wpc @ 8 and 300 wpc @ 4 ohms, driving my Def. Tech. BP10s. The "science" seems to suggest that I have to swap amps/speakers but the MLs sound better with the MC2200. I’m sure there are other factors in play but it was worth the experiment. Mainly wondering if others have found a correlation between the amp design and the type of speakers, etc. |
Regarding the McIntosh solid state amps that have identical maximum power ratings into 8, 4, and 2 ohms, paradoxically they are best considered to be voltage source amps. They are a special case among solid state amps due to their use of autoformers at their outputs. Their max power ratings reflect the assumption that an 8 ohm load will be connected to the 8 ohm tap of the autoformer, and a 4 ohm load will be connected to the 4 ohm tap, and a 2 ohm load will be connected to the 2 ohm tap. However, for a given tap the current and power delivered will increase essentially in proportion to a decrease in load impedance, with voltage being held essentially constant, as long as the current drawn and the power delivered do not exceed the maximum capability of that tap into that impedance. Although the amp’s **maximum** current and **maximum** power capability into a 4 ohm load that is connected to the 8 ohm tap will presumably be less than if that 4 ohm load were connected to the 4 ohm tap. Likewise for a 2 ohm load connected to the 4 ohm tap vs. the 2 ohm tap. All of that can be seen based on the amp’s specified damping factor, which is ">40" for most or all of the solid state McIntosh designs which utilize output autoformers. A damping factor of 40 corresponds to an effective output impedance of approximately 8/40 = 0.2 ohms for the 8 ohm tap, and 4/40 = 0.1 ohms for the 4 ohm tap, and 2/40 = 0.05 ohms for the 2 ohm tap. Those numbers are low enough to essentially result in voltage source behavior. (A theoretically ideal voltage source has an output impedance of zero). While most tube amps have damping factors that are in single digits, and not much more than that in most other cases, corresponding to effective output impedances that are measured in ohms, not fractions of ohms. Regarding Martin-Logan speakers, Ralph (Atmasphere) has stated in past threads that many of them have been designed with solid state amplification (having low effective output impedance) specifically in mind. Most other electrostatics, certainly including (among many others) the classic Quad ESL-57 that was designed before solid state amplification existed, are considered by most (although not all) audiophiles to be best suited to tube amplification, that will come closer to providing at least a rough approximation of constant power delivery into the speaker’s varying load impedance than to providing constant voltage. See his paper on the subject here. Regards, -- Al |
