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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
 
@jsqt, no, what you’ve described in the second paragraph of your post above is a horizontal biamp configuration.

In a vertical biamp configuration you would dedicate one amp to powering the left speaker and the other amp to powering the right speaker. One channel of each amp would power the low frequencies of the corresponding speaker, and the other channel of that amp would power the mids and highs of the same speaker. When identical amps are used vertical biamping is considered to be preferable to horizontal biamping.

There are several potential advantages of a vertical biamp configuration, in addition to the overall increase in power capability compared to stereo operation of a single amp:

1) In contrast to a horizontal biamp configuration, in a vertical configuration both channels of the amp are processing the same signal, and therefore interchannel crosstalk that may occur within the amp is essentially eliminated.

2) The amps can be located close to the speakers, reducing the length and potentially the sonic effects of the speaker cables.

3) Quoting a statement made a while back by GeorgeHiFi in a thread about biamping:

... the biggest advantage of vertical bi-amping is that the whole power supply joule storage of one amp, is dedicated to just one bass driver and not shared, so the bass/upper bass should be better on big dynamic transients. (unless the amps have true dual mono power supplies) which are rare.

Regards,
-- Al

jsqt's avatar
jsqt OP
117 posts
 
Thanks Al, so this vertical biamping actually sound preferable to running the 2 amps in parallel, if I am reading correctly.

So the connection from the preamp is the same as what I am using described in my post above (both L outputs to one amp both R outputs to the other amp) only no jumpers - I use both sets of speaker taps to go to the mid/hi inputs on one speaker and the bass input on the same speaker, then do the same on the other amp.

So this also effectively doubles the power going to each speaker - how does it affect impedance and is it safer for the amp and speaker?
almarg's avatar
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almarg
9,670 posts
 
So this also effectively doubles the power going to each speaker - how does it affect impedance and is it safer for the amp and speaker?
Regarding safety, earlier I described a scenario involving failure of a small signal tube that could conceivably result in damage to an amp that has been monostrapped. (Thanks, Ralph, for citing that term). However that scenario obviously has a relatively low likelihood of occurring, and so IMO it would be reasonable to consider both approaches as being safe.

Regarding impedances and sonics, IMO it's probably one of those situations where you have to try it both ways to decide which is preferable. Since paralleling/monostrapping would in effect provide the amp with an output tap optimized for a 3 ohm load, as I had indicated, that would presumably result in the best match in parts of the bass region where both the impedance of the speaker and the content of a lot of music tend to be especially challenging.   But it would result in a worse match at mid and high frequencies, compared to both stereo operation and vertical biamping.

Regards,
-- Al
 
ieales's avatar
ieales
1,201 posts
 
For BiAmping, you really should have crossover, otherwise the amps see an infinite impedance out of band.

A simple 6db HP crossover can be effected with a single capacitor for the upper and a LP for the low with a choke.

It would be better to use slopes similar to the in-built crossover.

For a quick cheap & cheerful XO, pick up a dbx 223 or similar. It's 4th order or 24db/octave. 
jsqt's avatar
jsqt OP
117 posts
 
@almarg - thanks, I have now experimented with the vertical bi-amping. It’s probably too early to have a firm opinion but as you noted, I noticed a little weaker bass, if not a smoother mid/hi top end. Also strangely enough it sounds like the stereo field is a little smaller than when I was mono strapping. Funny thing, the song I noticed the "breakup" from my OP was the arpeggiated rhythm guitar part in Tears for Fears’ "Woman in Chains," which now in the bi-amp configuration is NOT mostly placed in the left stereo sound stage but is now spread out across the entire sound field. Very weird as I didn’t change the output config of the preamp at all.

@ieales - the Rival has an external passive 3 way crossover. I’m sending one channel now into the mid-high input and the other channel into the bass input. I’m not sure how I would implement an active crossover into that configuration as the external crossover is built specifically for the Rival and from what I understand must be used (you cannot connect a direct signal on the speaker inputs).

Here is a photo of the bi-amp connection into the Rival passive crossover

https://jsqt.smugmug.com/Luxman-MQ-88uSE-Diagrams-Measurements/n-cgz5rM/i-dFFnJ5v/A

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