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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
almarg's avatar
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almarg
9,670 posts
 
More than likely the impedance of the speaker is too low for the amps in this configuration.

That would probably be the case if the amp were in fact bridged, as was initially indicated, since as you probably realize bridging causes an amp to "see" the speaker’s impedance divided by two.

But note that as indicated in the subsequent posts the amp’s two channels are being paralleled, not bridged. Which has the effect of converting its single 6 ohm output tap into a 3 ohm output tap. 3 ohms being less than the speaker’s impedance at all frequencies, and much less at most frequencies. So in this configuration the amp is being lightly loaded, perhaps even too lightly to be optimal.

Regards,
-- Al


jsqt's avatar
jsqt OP
117 posts
 
Al,

So if I am running the amps in this configuration to the 6 ohm speaker, you are saying based on JA's measurements and the information about the Luxman amp that it could be having a negative effect on the performance? Is it possible there is any damage being done to the amps or speakers? I was told by both the dealer that sold me the 2 Luxmans and the speaker manufacturer that this would not cause any issues with the equipment. 

Thanks!
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

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almarg
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@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
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@georgehifi 

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

Regards,
-- Al

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