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speaker impeadance question

Hi everyone, 

Out of curiosity I was playing around with the 4 and 8 ohm taps on my Mcintosh MC601 mono blocks last night. I currently use Sonus Faber Olympica iii speakers which have a nominal impedance of 4 ohms. I have run them on 4 ohm from day one (non bi-wired), but I was quite surprised to hear that there is a very distinct difference in SQ (positive) moving from the 4 ohm taps to the 8 ohm. So after doing a little digging I was able to find the impedance plot for the speaker and according to the plot I can see why they are rated at 4 ohm but what I don't understand is why they sound so much better at 8 ohm. What I am concerned about is the huge spike at 3khz ..see link below:

https://www.soundstagenetwork.com/index.php?option=com_content&view=article&id=1285:nrc-meas...

So I thought I would throw this out on the forum to see what you guys think in regards to using the 8 ohm taps based on this plot. 

Thanks in advance

-Keith
barnettk
audiokinesis
2,755 posts
 
" What I am concerned about is the huge [impedance] spike at 3khz..." 

Notice that this impedance spike (which is closer to 2.5 kHz) corresponds with a frequency response dip, when driven by whatever voltage-paradigm amp the NRC uses. So an amp like your McIntosh, which is more of a power-paradigm amp (outputs constant wattage rather than constant voltage) fills in that frequency response dip somewhat. This is a really nice synergy.

Duke
erik_squires
16,592 posts
 
What huge spike??

That's very typical for a speaker with a 2 way crossover. What I'm impressed with is how flat the rest of the speaker impedance curve is.
barnettk OP
410 posts
 
Thanks for the response Duke and Erik. Right I am aware that speakers change impedance at different frequencies. I was only concerned because I changed from the 4 ohm to 8 ohm outputs. Which is why I looked for the data in the first place before changing it permanently.  I too was quite surprised to see how flat the impedance curve was. the amps run very cool, but I guess that’s not unusual for McIntosh Amps, but I’m sure the rather consistent impedance helps also.

Anyway. Listened most of the day today to various samples of music and so far I do like these speakers on the 8 ohm taps. I think that is where they will stay for a while at least.

Thanks again

-Keith
bifwynne
2,206 posts
 
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!!

audiokinesis
2,755 posts
 

Erik wrote:

"What huge spike [in the impedance curve]??

"That’s very typical for a speaker with a 2 way crossover. What I’m impressed with is how flat the rest of the speaker impedance curve is."

The impedance curve can matter a great deal when it comes to matching amplifiers and speakers. Most amps approximate a constant voltage source (and most speakers are designed for that) but many tube amps (including McIntosh) approximate a constant power source; that is, the wattage they put out doesn’t change much as the speaker’s impedance changes.

Over most of its range that speaker’s impedance looks like about 5 ohms, but then between say 1.5 kHz and 3 kHz the average is more like about 12 ohms.

If the amp approximates a constant voltage source, then assuming 1 watt into 5 ohms, it will only be putting about .42 watts into 12 ohms.

But if the amp approximates a constant power source, when it’s putting 1 watt into 5 ohms it is also putting approximately 1 watt into 12 ohms.

The SPL difference between .42 watts into 12 ohms, and 1 watt into 12 ohms, is about 3.8 decibels. This is enough to spoil the frequency response OR - in this case - enough to fix it!

You see, the measurements show the Olympia III to have a roughly 3 dB dip centered between 1.5 kHz and 3 kHz.

(In the real world, the actual SPL difference between the two amplifier types would probably be less than the math in this post predicts, because in the real world amps don’t quite behave as true voltage sources nor true power sources. BUT the trend described above is valid in my experience, and explains why sometimes tube amps get blamed for bad sound when the real culprit is poor amp/speaker matching.)

Duke

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