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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
georgehifi's avatar
georgehifi
10,312 posts
 
ubbcbus OP
do you think there is a chance to damage the drivers by overdamping them ?
Not if overdamped by an amp, you can if they are very underdamped, as it’s travel is increased uncontrollably and the speaker cone "could" hit it’s end stops both inwards and outwards, especially with vinyl at warp frequency. Too much of this can damage the speaker.
Ever seen an underdamped bass driver? it wobbles around like jelly when doing bass
Cheers George
ubbcbus OP
11 posts
 
@ atmasphere and George, that was kind of what I was thinking. I have a vintage pair of 601 IV's that I had since college and I played them at "11" (Spinal Tap reference) all the time. It's really the only way they sounded decent. I had the drivers re-foamed as low frequencies made them shake like a Georgia Bluetick s#*%ting peach pits. I've boxed them up and closed that chapter in my life. Maybe kenjit will buy them. I did test a with some "bassy" Al Di Meola music. Same (high) volume level #1- speakers on the 4 ohm taps,
 #2 - speakers on the 8 ohm taps. There was no noticeable (visual) difference in the extension of the driver. The 5T's are ported so maybe there's a little gain there ? With the autoformer balancing the wattage and my ultra-scientific test  I'm no longer overly concerned. Appreciate your edification and thank you for responding to my post.

Salute,

Uncle BB      
ubbcbus OP
11 posts
 
@pragmasi - Even though I'm past the warranty I would not feel comfortable rigging up an attenuator. No offence to your suggestion, I just don't trust myself. I wish the 275 had balance inputs but no such luck. However, with the 5T's connected to the 8 ohm posts, they're pretty well balanced with the Tritons. 

All the best,

Uncle BB 
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