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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
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. 
georgehifi's avatar
georgehifi
10,312 posts
 
So I don’t think tubes are the issue, but it’s the 3 ohms
That 3ohms at 110hz with -50’ phase angle at 75hz to me is an issue, especially with SE tubes, this is where all the amps power is needed for bass repoduction.

I find it hard to believe a speaker manufacturer would voice a speaker with this kind of load graph with "feeble" SE Border Patrol amps, that are probably good amps with the right speaker, easier loads than this..

Cheers George
jsqt's avatar
jsqt OP
117 posts
 
@georgehifi this Luxman is a push pull triode amp ... and to be more accurate I don’t want to make assumptions for the manufacturer so I cannot say they are voiced with the BP amps but they are definitely known for being paired together at shows.
almarg's avatar
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almarg
9,670 posts
 
@georgehifi

George, I understand what you are saying about the speaker’s impedance being challenging for a tube amp in parts of the bass region. But note that the OP has indicated that the addition of the second amp, with the two channels in each amp paralleled, has improved the bass (which is to be expected given that paralleling the channels in effect creates a 3 ohm tap), and he has not indicated that there are any problems in that region.

Note also that he has said that "fuzzy sound in some of the midrange on some material is what brought all this up." And as I indicated in my first post in this thread the speaker’s impedance in parts of the upper mid-range and lower treble is in the vicinity of 20 ohms or more! Resulting in more of a mismatch to the tap on the amp than when a single amp had been operated in stereo mode. Which leads me back to the first of the two hypotheses I proposed in that first post, with the second hypothesis also being a possibility especially given that the amp has an output impedance that is very low for a tube amp (0.6 ohms in stereo mode, as I mentioned earlier, which would mean 0.3 ohms in paralleled mono mode):

With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

Best regards,
-- Al


jsqt's avatar
jsqt OP
117 posts
 
With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

This is more than likely contributing to the issue since one amp has about 300 hours on it (and the tubes) and the other new one has about 15. 

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