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What does an impedance-matching issue "sound" like?

I recently added a 2nd Luxman MQ-88uSE to my 2 channel system, bridged to mono so I am running a Luxman to each of my 6ohm Volti Rivals.

The SE version of this Luxman amp only has single speaker taps which output 25Wpc to 6 ohms, and 20Wpc to 4 ohms and 8 ohms (from the manual). I used a jumper between each of the "positive" taps so I am not sure what the bridged output is to the 6 ohm Rivals.

Now what I am hearing is definitely more power and presence in the sound, better bass, as well as improved separation. BUT occasionally I am hearing what sounds like a bit of breakup deep in the mix of some songs. Like the whole channel isn’t breaking up, but suddenly a rhythm guitar part sounds noisy like it’s being played through an AM radio or something. On some songs, the entire mix may sound reduced in scale and less dynamic.

I am not sure if my ears are playing tricks on me, or if I’m crazy, or if there could be something happening in the pairing of these amps and speakers that is bad for the equipment and sound. I realize this is a complicated issue because one amp is brand new and the other is broken in, and they are both tube amps., So chasing down the culprit could get messy.

Is it possible there are impedance issues at play here? What would that sound like?



128x128jsqt
georgehifi's avatar
georgehifi
10,312 posts
 
jsqt OP

Bridging amps in solid state,
Doubles the output impedance
Halves the damping factor
Increases the distortion
Has have the current ability.
Makes the amp less stable into lower impedance’s to what it was in stereo.
So everything takes a hit, except you gain higher watts. I believe the same happens in tube amps also.

https://www.stereophile.com/images/617Voltifig1.jpg
If so where you may have had a amp that could "just" sort of drive the Volti Rivals 3ohm load at 100hz with 50 degrees of -phase shift in the bass. Now you have no chance bridged.

Along will come the suggestion of autoformers, they are just a band-fix don’t get sucked into that.

Solution go back to Stereo.
You could vertically bi-amp the two stereo amps (one channel bass, the other mids and highs. (same for the other side)
Change speakers or change amps.

Cheers George

almarg's avatar
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almarg
9,670 posts
 
@jsqt

I don’t envision any possibility of damage. But whenever the impedance of a speaker is significantly mismatched to the load impedance a tap is designed for there may be compromises to both maximum power capability and distortion performance.

As I indicated, paralleling two channels each of which has a tap that is optimized for a 6 ohm load in effect creates a 3 ohm tap. Applying a 3 ohm resistive load to such a tap would typically result in the amp being able to supply approximately twice as much power as it could supply into 6 ohms when running in stereo mode. With the impedance of your speakers being much higher than 3 ohms at most frequencies I’m pretty certain that the amp is falling significantly short of realizing that 2x increase. And although I’m just speculating perhaps the mismatch is contributing to the adverse sonic effects you described.

Regards,
-- Al

almarg's avatar
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almarg
9,670 posts
 
@georgehifi 

As I've indicated multiple times, the amp is not being bridged!

Regards,
-- Al

georgehifi's avatar
georgehifi
10,312 posts
 
@almarg
Ooops sorry, just saw what he said in the first sentence.
added a 2nd Luxman MQ-88uSE to my 2 channel system, bridged to mono so I am running a Luxman to each of my 6ohm Volti Rivals

In any case that phase/impedance load around 100hz is nothing that tube amps want to be seeing.
https://www.stereophile.com/images/617Voltifig1.jpg
Change the amps if you want to keep the speakers. or visa versa


Cheers George
jsqt's avatar
jsqt OP
117 posts
 
Thanks guys, this is all a little confusing to me as both the speaker mfg and the dealer who sold me on running 2 Luxmans said there would be no issues. 95% of the time this sounds amazing, and I do hear the benefits of a more powerful sound and better separation between channels. But that fuzzy sound in some of the midrange on some material is what brought all this up. Hate to think that I'm stuck with these amps now if all I've done is decrease the quality of my sound. 

Also regarding tube amps and the Rivals, they are voiced with Border Patrol SET amps which are used to demo the speakers at all the shows, etc. So I don't think tubes are the issue, but it's the 3 ohms into a 6 ohm speaker part that has me concerned. 
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