speaker impeadance question

barnettk, I own an ARC Ref 150SE tube amp.  Same issues have come up for me over the years.  I find myself switching back and forth all the time.  Right now, I am using the 8 ohm taps. 

Both of us have power paradigm amps.  As a result, the voltage output will change as a function of the speaker's impedance curves.  IMO, your speaker's have pretty moderate impedance and phase angle plots.  I surmise that your amp has little trouble driving the speakers.  

What would be interesting to know is what the output impedance of your amp is off the 4 and 8 ohm taps.  That would give you a clue as to how much the amp's output voltage with vary as a function of changes in the speaker impedance.  

For example, the output impedance of my Ref 150SE is pretty low.  As a consequence, the output voltage varies by +/- .8 db off the 8 ohm tap and about half that off the 4 ohm tap.  See John Atkinson measurements here:  https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

Point being that your speaker output will be flatter off the 4 ohm tap and a little brighter off the 8 ohm tap because your amp will response to speaker impedance bumps with more output voltage.  The reverse is the case with respect to speaker impedance dips.

So the bottom line is that what you hear will sound flatter and bass will be slightly tighter off the 4 ohm tap and a little more lively off the 8 ohm tap.  Btw, I mention that bass will be tighter off the 4 ohm tap because I surmise that your amp's output impedance is lower off the 4 ohm tap.  As a result, the amp's damping factor will be higher, thereby controlling the woofers a little better.  

But the real bottom line is that you use whatever taps sound best to YOU!!

Hi Al, 

I've been checking in from time to time.  Actually, I've been waiting for you to post something because I always learn something from your responses.

The OP's post caught my interest because he tapped [pun intended] into a topic that you, Ralph and I posted about several years ago.  Perhaps you could expound on why the OP's solid state amp would be using autoformers at all.  In the usual case, solid state amps have very low output impedances.  Hence, their damping factor should be high. 

I think Ralph may have posted several years ago that a DF of over 8 or maybe 10 isn't all that important. In the OP's case, I am surprised that his DF isn't higher since his amp is solid state.  Is there some design feature relating to the output transistors that makes the use of autoformers necessary?    

Further, in the OP's case, you posted that his amp is rated at 600 watts and his speakers have a reasonable 88 db sensitivity.  So what is the issue?  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.  I am a little surprised. 

BIF
  

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.