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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
ubbcbus OP
11 posts
 
Thank you ALL for your responses. I'm relatively new to AG and lurk daily, but do not have much to add to the knowledge base so I don't post frequently. Really appreciate the knowledgeable veterans replying to my inquiry. Regarding the responses:

@millercarbon - not a home theater, only for music. I started out with the 7900 and the Aerials, but was pining for more bass (48Hz wasn't cutting it). Listened to a few sub-woofers but was not impressed with what I heard. I listened to the Triton's and was impressed with the bass response (especially considering the price). I know these get poo-pooed on the forum and that's OK, I know what I like. After reading up on AG about tube sound and sound staging, I decided to invest in the 275 knowing that it could be piggybacked on the 7900 to power the Tritons. Granted, it's an odd approach but it's synergistic in a sense. SS + tubes, good midrange and highs (especially voice) from the Aerials and good lower frequencies and sound stage from the Tritons... I've never been too conventional.


George- overdamped room for sure. Been considering taking up the carpeting and putting down cork floors.


 @bdp24 - M.R. amps may have different wattages for different impedance ? The 7900 & the 275 claim same wattage for all impedances as @pragmasi pointed out. I'm going to check in with Aerial to see what they have to say. 


@ pragmasi - regarding the resistors, the Aerials have 4 connection posts. +'s and -'s upper and lower. The upper and lower posts have  bridge connections  i.e. + to + / - to -. Right below the upper posts is labeled "midrange and treble" . Could these potentially be resistors or a bridge that functions in the manner you speak of ?   


@onhwy61 - Yes, both sets of speakers at the same time. I have a decent sized listening room 18x30. Its has 45 degree pitched walls, two dormers and a funky diamonded shaped cut out for the pitch of the adjacent, perpendicular roof. With 4 speakers, it's an acoustic circus but pretty enveloping sound. At medium to higher volume, if you close your eyes you cannot tell where the speakers are placed. 


@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

Thanks again to ALL of you,

Uncle BB 
ubbcbus OP
11 posts
 
Thanks again George. As I get more in to this hobby, seems I'll need to do some reading on classes of amplifiers and speaker pairing. There's a lot to learn. I have a Unit Atom and when the firmware went awry with my 7900, I hooked up the Aerials to the Atom (40 watt amp) so I could listen to music. It sounded a lot better than expected. And conversely, when I hooked them up to the 275 they sounded very thin and anemic. Your point is well taken. 

Best regards,

Uncle BB     
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.
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