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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
9,788 posts
 

@ubbcbus, the MR amps are all tube designs, and they, like almost all tube amps, produce different amounts of power at different impedances (and to the amp's different binding posts). McIntosh amps, having autoformers, are the rare exception.

Almost all tube amps produce less power into lower impedances than into higher ones, the opposite of solid state amps. The only exception of which I am aware is the Music Reference RM-200, which produces slightly more power at 4 ohms than at 8.

ubbcbus OP
11 posts
 
@bdp24 Thanks for the info. SS and tube power is a perplexing, abstract concept. I'm still trying to wrap my head around impedance, volts and wattage. But what I do understand, traditional tube amps do better with higher ohm rated, more efficient speakers. I took a look at the RM-200 hybrid online. Again, in my simple mind, bending the laws of physics. When it comes to tubes it makes me wonder if true tube sound is adulterated by autoformers, hybridization of the power supply and other technologies that have been developed to improve variables in tube amps.... Probably start a small war posting this question on the forum. Sound is like bourbon and artwork, price doesn't necessarily make it better. You like what you like. 

Regards, 

Uncle BB   
bdp24's avatar
bdp24
9,788 posts
 
Roger Modjeski is an outside-the-box designer, his RM-200 an example of his originality. He bristles a little at the amp being called a hybrid, but yes, it does not have a tube input stage (the driver and output stages are pure tube, however). But it isn’t a normal solid state input stage, either. It’s comprised of a pair of transistors and some resistors, in a zero-gain circuit. In the RM-200 he manages to get a hundred watts out of a pair of KT-88’s (or 6550's) by employing low screen voltages and higher-than-normal plate voltages on the tubes. A clever fella who knows as much about tubes and tube amp design as anyone living.
atmasphere's avatar
atmasphere
11,784 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.
pragmasi
212 posts
 
You can really take this as far as you want to...
1) You say you know what you like (which is great) so if you like the sound you're getting stick with that... everyone seems to agree you're not going to damage anything.
2) You can make up a lead to attenuate the Tritons, the simplest way to do this would be to go back to your starting configuration:
Mac MA 7900 (200 watts) that I use to power Aerial Acoustics 5T's and a MC275 (75 watts) powering Triton 2+'s .
And adapt (I couldn't find an off the shelf solution) a pair of interconnects to connect between the MA 7900 and the MC275. To do this you'd need a handful of components, some basic tools and rudimentary soldering skills. The advantage of this is that you can experiment with different levels of attenuation. If you're interested in going that route I can elaborate.
3) This is probably heresy here but you could use DSP... that would let you completely control the output of the two sets of speakers by controlling both the signal level and the frequency ranges sent to each speaker. So you could roll off the top end of the Tritons and the bottom end of the Aerials. It's easy to sniff at DSP but Siegfried Linkwitz (he's got quite a good CV) used it in the designs for his reference speakers.

If you're happy with the sound you've got then definitely go for option 1.
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