speaker impeadance question

@ieales

"I note that the OP accomplished the selection in a day or two. In decades of tube amp ownership, I've never found it to be so cut and dried, often taking many weeks / months to come to a determination"  

Funny you should say that. Sometimes you have to digress a little. Im no different. 

Here is my observation so far. 

Moving the Olympica iii's to the 8 ohm taps definitely makes the system sound more lively, but at a small cost. I use subs in my system and initially I listened with the subs on. With the subs off its apparent that on the 8 ohm connections without the subs playing as someone earlier stated the bass response is not quit as tight. Not that big a deal to me because obviously the subs make up for that. 

I have also noticed a slight decay in the sound stage however. This could just be an adjustment with speaker placement, however that is no small task. It has taken me months of dialing these speakers in to where I think they sound perfect so for this fact alone that could be a deal breaker for me. Now with that said the system does seem to have a little more airiness which I like! 

Along those lines definitely a little brighter sound, but not to the point that its annoying so far. No sibilance that I can detect on recordings that I think would reveal that. My room is rather flat so the brighter sound really is not enough to make me switch back if I were just measuring that only. With that said however I have to admit the speaker is not quite as natural sounding as it was for sure. One of the the things SF is good at is they're natural sound quality in this line of speaker anyway, and I have to admit that on the 8 ohm taps they sound a little top heavy, but not really in a bad way if that make sense. 

For me at this point its about trade offs. While the systems sounds a bit more lively, and airy which I like, but loosing some of the lower octave quality, and sound stage quality.  Could I get used to it and keep things the way they are probably , but being honest sound staging, and a natural sounding speaker is of high priority in my case. I think I would have to go back through speaker placement tweaking to make a good determination, and for me I don't think I want get in a cycle of speaker placement doing compare and contrast between the two taps. I did not intend on this being a laborious exercise. 

I will report back and let you know what I decide in a few weeks. Excuse my fickleness for lack of a better word, but I guess that's how this works in this hobby. 

-Keith

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.

@almarg  Just as a bit of clarification on this bit: any tube amp can be made to act as a voltage source if you can add enough voltage feedback. If the amp is zero feedback it will behave more like a power source. Further muddying the waters is the simple fact that just because an amp doesn't double power as impedance is halved does not mean that it can't behave with a constant voltage characteristic. Many tube amps do exactly that.


In the 1950s before the idea of voltage driven speakers caught on, it was seen as a good thing to make sure that the amp had low distortion as well as the correct damping factor for a given speaker. Now some open baffle speakers need a damping factor of only 1:10 and you did read that correctly: where the amp has 10x the output impedance as opposed to the loudspeaker. OTOH some speakers do need 20:1 to sound right.


To allow this to occur, in the 1950s a number of amps were made with a 'Damping Control' which balanced voltage feedback against current feedback. So at one extreme the amp behaved with constant voltage; at the other with constant current, and in the middle where the voltage and current feedback were in equilibrium, constant power. BTW its worthy of note that this idea has nothing to do with solid state vs tubes.


IME/IMO, while the Voltage Paradigm has taken over decades ago, the simple fact is that there are speakers out there that don't and **can't** work under the voltage rules- specifically ESLs, a number of magnetic planars, a good number of horns, 'full range' drivers and certain box speakers that designers noticed that they sound better with tubes than solid state...  I think the damping control isn't a bad idea at all; if you want plug and play, the damping control is more likely to do the job for you.

Thanks Ralph. I agree.

My amp has 14db of NF and "low'ish" output impedance: JA measured low-end output impedance of 1 ohm off the 8 ohm tap and .55 ohms off the 4 ohm taps.  Converted into output voltage db variation as a function of FR, JA measured +/- .8db off the 8 ohm tap and about half that amount off the 4 ohm tap.  See https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements 

One poster said that the sonic flavor added by my amp's output impedance is akin to a tone control.  Crude analogy, but I see where he is coming from.

However, how many speakers, even if driven by voltage source SS amps, do not have ruler flat FR plots.  Add room characteristics, more coloration.  So, IMO, we go back to the old sage advice.  Go with what sounds good to your ears. 

One more thought.  I get your comments above that any amp that uses NF inserts a measure of high order harmonic distortion into the signal path, which sounds harsh and bright to the human ears.  Not sure if the 14 db used by my amp generates a lot of high order harmonic distortion, but to my old untrained ears, I still like the sound.