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Output Impedance and Speaker Impedance

I'm a bit of a novice so please excuse this if it is a stupid question.  I have a Mac MA 7900 (200 watts) that I use to power Aerial Acoustics 5T's and a MC275 (75 watts) powering Triton 2+'s . The preamp in the 7900 controls both amps. 

In an attempt to find ultimate sonic balance, I switched the amps - 7900 powering the Tritons and the 275 powering the Aerials. Not as good of a set up and would guess it has to do with the sensitivity/ efficiency of the speakers and the corresponding power provided by the amps. Tritons are 91dB / 8 ohm and the Aerials 87dB / 4 ohm (nominal, 3 ohm minimum) . In this configuration the Triton's drowned out the Aerial's. When balanced, these speakers compliment each very well IMHO..... Plus the sonic benefits of tubes and SS. 

While switching the speakers around with the amps, I connected the 5T's back to the 7900, but connected them to the 8 ohm outputs. It really opened up the lower frequencies and I didn't notice any loss in the mid or high frequencies. Finally, my question... Is there any inherent danger powering 4 ohm speakers through the 8 ohm outputs on the amplifier. Recommended power for the Aerials is 25- 200 watts. 

To me, it sounds better and volume output is nicely balanced. However, I do not want to damage the amp or the speakers. Thank you in advance for any guidance you can provide.       
ubbcbus

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,099 posts
 
@atmasphere - do you think there is a chance to damage the drivers by overdamping them ? They're paper (papyrus if you're so inclined :) The bass when connected to the 4 ohm posts is less pronounced.... maybe another question for Aerial
No. You can't damage the speaker just by changing taps on the amp!

That the bass is less pronounced on the 4 ohm taps makes sense as the amp would make slightly less power into that speaker when using the 4 ohm taps. The speaker would be more accurately rated at 8 ohms than 4.
atmasphere's avatar
atmasphere
12,099 posts
 
the cone excursion is not halted, but over travels slightly on the out and the inward movement, in other-words it’s not as tightly control as Aerial would like it to be.
This is only partially true. A loudspeaker cone can be overdamped; looking at the impedance curve (https://www.stereophile.com/content/aerial-acoustics-5t-loudspeaker-measurements) we see that the impedance never dips to 4 ohms and spends most of the bandwidth at much greater impedances. This is why the 8 ohm tap sounds better.

Normally running an 4 ohm load on the 8 ohm tap will cause the power tubes to make more distortion and run hotter (ultimately failing sooner) since the load impedance is transformed by the transformer to a load impedance that would be half of that for which the circuit is designed.


But in this case this isn't happening so no worries!
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