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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
 
Erik, I've seen some cases in which paralleled operation of the two channels of a stereo tube amp has been recommended by the manufacturer.  Examples include the McIntosh MC275 and the old Dynaco ST-70, despite the fact that like the OP's amp those amps have output impedances that are particularly low for a tube amp.  (JA measured the OP's amp as having an output impedance of about 0.6 ohms; the ST-70 and the current version of the MC275 have specified damping factors of 15 and >22 respectively, which in both cases is suggestive of output impedances of well under 1 ohm).

A concern I've always had about doing that, however, is the possibility that if a small signal tube serving one channel were to fail, and music is then played, one channel would be attempting to output a full amplitude signal while the other would be attempting to force the output to zero volts.  Which doesn't seem likely to be healthy for the amp, especially if the listener isn't nearby or for whatever reason doesn't promptly realize there is a problem.

Best regards,
-- Al
 
mijostyn's avatar
mijostyn
8,570 posts
 
More than likely the impedance of the speaker is too low for the amps in this configuration. Get yourself a high quality active cross over and bi amp the Rivals. They are set up for it! 
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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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