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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
millercarbon's avatar
millercarbon
12,531 posts
 
No danger. Seems to be a general thing as a lot of guys have the same experience when trying this with all kinds of different amps and speakers.

But wait a minute- you are running two pairs of speakers at the same time??? What is this?? Home Theater?
georgehifi's avatar
georgehifi
10,312 posts
 
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.
What’s happening using the 8ohm tap is that in the bass there is little "damping factor" from the amp to control the speakers movement, so 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.
Stereophile Aerial 5T 6 ohms in just two regions—between 29 and 54Hz and between 110 and 310Hz—and a minimum value of 4.28 ohms at 160Hz. The electrical phase angle (fig.1, dotted trace) is occasionally extreme, and the combination of 6 ohms and –38° at 108 ohms means a good 4 ohm–rated amplifier would be the best match for this speaker.

But hey! you "could" have an "over damped" room for bass, or a "lean" source in the bass, so an under-damped speakers bass could be ok for you, not ideal for detail retrieval but fine for tonality..

Cheers George
bdp24's avatar
bdp24
9,698 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.

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