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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?



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erik_squires
16,021 posts
 
Interesting diagram, and that means I'm out of my experience level. I've never seen an amp "bridgeable" like this before, but the high output impedance of the transformer probably makes it less frightening than a SS amp with feedback coupled output stages. 
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
 
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mijostyn
8,540 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! 
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


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