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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

Showing 3 responses by erik_squires

erik_squires
16,591 posts
 
I don't really like to use the phrase "constant voltage" here. I think that the term "voltage source." is more clear.

Meaning, the voltage is a function of the input voltage, not the speaker impedance.
For what we call a power amplifier, this is usually around 20x. That is, 1 V in = 20 V out. 0.1V in = 2 Volts out, so long as the speaker’s impedance is at or above the rated speaker impedance of the amplifier.

erik_squires
16,591 posts
 
You don't exactly get constant wattage, or constant voltage.

What happens is that with high impedance amps and low impedance speakers is you can loose output power (i.e. dynamic range).

By having a 4 Ohm tap, your 75W tubes still put out 75W.  Otherwise, they might only put out 60 @ 4Ohms using the 8 Ohm tap.

This example is exceedingly soft. :) I suggest you find some Stereophile reviews to examine this more specifically.
erik_squires
16,591 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.
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