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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
bdp24's avatar
bdp24
9,697 posts
 

The output impedance of the power amp (more commonly referred to as damping factor)/loudspeaker "modulus" of impedance relationship is one of the most consequential in all of hi-fi (along with loudspeaker/room interaction, pre-amp output impedance to power amp input impedance, and cartridge compliance to tonearm effective mass).

Music Reference's Roger Modjeski encourages owners of his amps to try "light loading" (running the loudspeaker on the impedance tap half the speaker's nominal impedance), but advises against doing the opposite (an 8 ohm speaker on the 4 ohm tap good, a 4 ohm speaker on the 8 ohm tap bad).

onhwy61's avatar
onhwy61
5,186 posts
 
Are you playing both set of loudspeakers at the same time in the same room?
pragmasi
212 posts
 
Looking at the spec for the MA7900 I'd guess you're probably okay. I don't know the mechanism they use to achieve 200W into all different impedances.
The difference between the speakers' sensitivity is 4dB which means the less sensitive speakers will need 1.6x the voltage to achieve the same SPL.
For the amplifier to output 200W into 4Ω then the RMS voltage needs to be 28.3V drawing about 7A. For 8Ω the figures will be 40V drawing 5A current. So assuming the amplifier controls the output voltage to match the output levels then we might expect that the 8Ω output puts out 40/28.3 or 1.4 times the voltage of the 4Ω output. 1.4 x the voltage equates to around 3dB. So this would explain the better level matching.
But now when you run at full volume you're putting 40V into 4Ω which will draw a current of 10A. In a lot of cases that would be a problem and you could end up with overheating but in this case the amplifier is capable of running with a 2Ω load so the chances are you are okay. If you put a 2Ω load into the 8Ω output you'd draw 20A which would likely put the amplifier into protection mode but is not worth the risk of trying.
Another way of achieving the same (or better) result would be to put a simple voltage divider (two resistors) across the inputs of the higher sensitivity speaker to reduce it's sensitivity by 4dB. You may be able to buy a 4dB attenuator to do the job for you.

Hope that makes sense.

atmasphere's avatar
atmasphere
11,753 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!
georgehifi's avatar
georgehifi
10,312 posts
 

Before you read the rest ubbcbus , the Aerial Acoustics 5T is not an overdamped design, it’s normal.

A loudspeaker can’t be overdamped electrically, this is why some of the best amps for bass are from class-D, that have output impedances in the milli-ohms, damping factors of 10,000 or more and why they are great for subwoofers.

However you can have too much damping if the electrical damping is combined with too much mechanical damping in "over damped" enclosure design.

A typical example was the Linn Isobaric speaker which had very lean almost no bass with big Krells of the era, who would have thought a Krell with no bass!!!!
I actually heard a Naim amp with very mediocre damping factor sound better than the Krell into the bass of the Linn Isobarics.

BTW: this overdamped speaker with underdamped amp, was a great marketing strategy thought up by Linn/Naim at the time, you had to get both their amps and speakers, one did sound right without the other in A/B’s at shops.

Cheers George
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