speaker impeadance question

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

Hi Bruce (Bifwynne),

McIntosh uses autoformers providing multiple output taps on many of their solid state amplifiers, and as far as I am aware they are unique in that respect. The rationale is presumably to make life easier on the output stage when low impedances are driven, by reducing the amount of current the output stage and the power supply must provide, and reducing the heat they will generate. One benefit of that presumably being a reduction in distortion. Their literature also talks about the autoformers providing benefits relating to speaker protection, including protection against DC being applied to the speakers in the event of a malfunction in the amp. My understanding is that an autoformer (as opposed to a transformer) can’t block DC, but perhaps the near zero impedance it would present to the output stage at zero Hz (i.e., at the "frequency" of DC) would trigger some other protection mechanism in the amp if that were to occur, and cause the amp to shut down.

A consequence of the autoformer approach is that those amps will provide the same rated maximum power capability when a 4 ohm load is connected to the 4 ohm tap, and when a 2 ohm load is connected to the 2 ohm tap, as when an 8 ohm load is connected to the 8 ohm tap. As is typical of many tube amps, and in contrast to just about all other solid state amps which of course will increase their power capability significantly as load impedances decrease, until some limit is reached.

And while this amp (each monoblock) weighs 93 pounds, I’d imagine that a solid state amp not using an autoformer, and that is capable of providing 600 watts into 8 ohms and far more into lower impedances, and that is comparably designed in other ways including robustness, would weigh a lot more due to the higher currents and additional heat sinking that would be required.

Just curious about the use of autoformers in this application and why the OP perceives that there is a sonic difference between the 4 ohm and 8 ohm taps.

The best answer I can provide is probably what I said in the last paragraph of my first post above, beginning with "So I would expect..."

I think Ralph may have posted several years ago that a DF of over 8 or maybe 10 isn’t all that important.

More specifically, what Ralph has said in some past threads is that "there is no speaker made that requires more than 20:1 for a damping factor."

In the OP’s case, I am surprised that his DF isn’t higher since his amp is solid state.

Just a guess, but I wouldn’t be surprised if the resistance of the autoformer winding is the main reason the DF is less than in most other solid state amps.

Best,
-- Al

No, you won’t hurt anything by using the 8 ohm taps.

In some other situations, involving amps that are much less powerful and/or speakers that are much less sensitive, the reduction in the maximum power capability of the amp that would presumably result from using a tap that does not correspond to the impedance of the speaker at most frequencies (especially in the bass and mid-bass regions, where lots of energy is typically required) might be conducive to driving the amp into clipping. Which in turn can be harmful to tweeters.

However, given the "conservatively rated" 600 watt capability of your amp (when load impedance corresponds to the designation of the tap that is used), and given the 88.3 db/2.83 volt/1 meter measured sensitivity of your speakers, and given the "Power Guard" function provided by your amp (which reduces its gain if and when necessary to keep the difference between output and input from exceeding 0.3%), that risk would appear to have zero chance of arising in your particular situation.

Regards,
-- Al

:-)

Best,
-- Al

(For others who may not be aware, Duke's statement just above was made famous by "Roseanne Rosannadanna")

Hi Duke,

I'm afraid I must offer another correction, this being a matter of considerable significance :-)

It was actually a different character, "Emily Litella," who concluded her commentary on Saturday Night Live with the words "never mind."  As I'm sure you realize, Emily Litella and Roseanne Rosannadanna were both characters portrayed by the amazingly talented Gilda Radner, who very sadly is no longer with us.

Best,
-- Al
    

Lots of excellent and informative responses above, except that I believe some of them have been made without the realization that the OP’s monoblock amps are solid state. And the multiple output taps are created via autoformers, the result being that the taps have higher damping factors and lower output impedances than pretty much all tube amps.

Specifically, the MC601 has a specified damping factor of "greater than 40," which if accurate would at least in theory mean an output impedance from the 8 ohm tap of <0.2 ohms, and <0.1 ohms from the 4 ohm tap, and <0.05 ohms from the 2 ohm tap. All of those numbers will result in the amp essentially behaving as a voltage source with this and most other speakers.

So I would expect in this case that interactions between amplifier output impedance and variations of speaker impedance over the frequency range are very minimal contributors to the sonic differences the OP hears in going from one tap to another. The major contributors would seem likely to involve how the sonics of the amp itself (and perhaps also the amp’s maximum power capability) are affected by differences in loading of its output stage that depend on which tap is selected.

Regards,
-- Al

P.S: Bruce (Bifwynne), great to see you posting again!