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Who knows something about Tri-channel 3-channel stereo for music (not HT)?

I happen to have 3 Martin Logan 'Source' e-stats, and want to overcome one of my main complaints with these speakers... narrow sweet-spot.

Having heard of 3-channel stereo, had the idea that instead of using a stereo pair, I might incorporate all three to solve several issues (narrow sweet-spot and low impedance), while improving 3-dimensional holographic imaging for added benefit.

I know there was an era (before my time and before surround sound), when this was (considered by some) the pinnacle of hifi. I found this article on Magnepan site that details some history on the subject https://www.magnepan.com/betterthanstereo

I have tried connecting the 3rd speaker located in the center in series with the stereo pair - jumpered to positive speaker pole of one (say left) and negative to the other (say right) speaker. In this configuration, I'm not sure what signal is going to the 3rd (center) speaker, but it doesn't follow either left or right, and switching pos/neg connection to the other two doesn't cause a change. So as long as the 3rd speaker is connected pos-pos/neg-neg, it seems to be in-phase and not make either left or right channel dominant.

The effect does increase the sound stage width and impedance (this is a good thing for these low-impedance speakers); however, imaging is no more holographic than is typical for these (and most) electrostats.

So, I wonder what products like the Maggie/Bryston SP-3 and (likely) other similar products of the era offer, that would be an improvement over what I'm doing with series wiring? What about the benefits/pitfalls of various circuits L+R, L-R... etc., and what works best for imaging?

Happen to have some equipment (unused Denon AVR, miniDSP 2x4HD, Emotiva A800) that could potentially be used to try recommended schemes (play) and REW with calibrated mic (to measure results). Currently reconfiguring my 2-channel listening room with two single-stack racks to allow space for the center-channel floor-stander ML Source at center for progression of this endeavour.

Any history lessons on 3-channel stereo (from the good old days) or ideas to try are sincerely appreciated.
128x128mwatsme
9 responses Add your response
pragmasi
212 posts
 
I have tried connecting the 3rd speaker located in the center in series with the stereo pair - jumpered to positive speaker pole of one (say left) and negative to the other (say right) speaker
I wouldn't go about it this way, if I understand you correctly the left channel has two 5Ω loads in parallel which means the combined load for that channel is 2.5Ω, depending on your amplifier this might not be a good thing. Also depending on the design of the output stage you're either just sinking the current from the 3rd speaker to ground via the right channel, or worse (if the output is bridged) then you're 'pushing' the drivers with one channel and 'pulling' from the other which is unlikely to sound good. 
I'm afraid I've not come across 3 channel stereo before but I do remember reading that crosstalk between the left and right channel can in some cases appear to widen the sound stage. To emulate that you'd only need a low level signal going to the centre channel. You could also adjust the toe-in of the stereo speakers to widen the sweet spot.
I suggest that you use the minidsp before your main amplifier and use a spare amplifier to power the 3rd speaker. In the software you'll need to mix the left and right channels down to mono for the 3rd output (the input to the mono channel should have the left and right set to -3dB to prevent clipping). This means you can adjust the phase, level and frequency response to both the stereo pair and the 3rd channel.
Hope that's helpful.

mwatsme's avatar
mwatsme OP
187 posts
 
Pragmasi,

Thank you for the response! 

The way I've tried already,  puts the center in series with L&R (increasing impedance and causing no left or right bias). It also increases dispersion for wider sweet spot. It fails to increase holographic image.

I like the idea of using miniDSP as you described - think there is merit there! 

I have an 8-channel Emotiva, so all equipment is in place to make it happen. 

It will be fun to play with, and see if I can achieve something close to the LXmini sound I heard last night for the first time - WOW!
pragmasi
212 posts
 
puts the center in series with L&R
I think I'd need to see a diagram... I assumed you meant left and right were wired normally with the centre taking a tap off one positive and the other negative. If that was the case then follow the positive wire that's connected to two speakers, each speaker returns to ground putting them in parallel. The only way I can think you'd have them in series is by daisy chaining them from a single channel. Anyway... the miniDSP is the way to go for experimenting.

It will be fun to play with, and see if I can achieve something close to the LXmini sound I heard last night for the first time - WOW!
I've been meaning to build LXminis for a few years but other projects keep getting in the way. The full range speaker in the LXmini is designed to have a cardoid dispersion so I guess it's all sweet spot in front of them. Do you know if the ones you heard used miniDSP or Nelson Pass' active crossovers?
pragmasi
212 posts
 
 I found this article on Magnepan site that details some history on the subject https://www.magnepan.com/betterthanstereo
I only just read this... my bet is that the centre is out of phase with the left and right to some degree. That would create the illusion of a wider sound stage, but possible a hole down the middle. I don't think that would affect the sweet spot though.
mwatsme's avatar
mwatsme OP
187 posts
 
I’ve been meaning to build LXminis for a few years but other projects keep getting in the way. The full range speaker in the LXmini is designed to have a cardoid dispersion so I guess it’s all sweet spot in front of them. Do you know if the ones you heard used miniDSP or Nelson Pass’ active crossovers?
I’m using the miniDSP 2x4HD, and think you meant Nelson-designed passive crossover. There is something very special about the LXmini sound, and it carries throughout the room (except above or behind them). It’s amazing how accurate the full-range drivers sound, which was completely unexpected since they are 3.25" (measured) surround diameter - most tweeters being 1" or less. They rival the highs of B&W FPM4’s, which have metal (think titanium) tweeters. And the LXmini’s do it without the sibilance I’m sensitive to (heard with B&W, all I’ve heard anyway - can’t include Nautilus).

They have a very similar sound to my truck stereo (when the windows are down and the drivers can breath) - made of Morel 3-way separates (older version of ’Elate Titanium 3-way’) driven by a high-current amp. I know that sounds like a lambaste of LXmini, but I assure you it is a comparison of high praise. The Morel tweets and mids are amazing, and the midbass are 8.75"dia (3" voice coils), all drivers have hex-wire coils. The Morel drivers are so good, I considered using them to build bookshelves, but think I’m done with ’box’ speakers. Actually it was my experience with the truck stereo with windows down that inspired me playing with open-baffle concepts - like Linkwitz designs.

Next build might include Manger full-range driver in the LXmini form-factor - they look very interesting. I wouldn’t be the first, so am not taking credit for the idea. https://mangeraudio.com/en/discover/about/the-manger-sound-transducer

Everything ultimately gets compared to B&W (highs) and GoldenEar Tritons (all-round) as reference in my home - but the Tritons require moving heavy things up/down stairs.
mijostyn's avatar
mijostyn
8,570 posts
 
Right. Klipsch started doing this for the simple reason that K horns had to go in corners and in many rooms this put them too far apart leaving a "hole" in the middle. So they developed the La Scala as a center channel speaker to run in the middle. The idea was that you would mono the signal to the center channel and turn up the volume just until the "hole" disappeared. It worked ok and made the system seem even more powerful. Nobody else did this because if you had a "hole" in the middle you just moved your speakers a little closer together which was a whole lot cheaper than buying a third channel.
In regards to your system a third middle channel will do nothing to increase the "sweet" spot. Draw it out on paper and you will see the dispersion stays exactly the same and you have shrunk the image dramatically. 
If you want a wider sweet spot buy speakers with wider dispersion. 
pragmasi
212 posts
 
@mijostyn - Okay that explains why Klipsch did it... the link was to Magnepan.
In regards to your system a third middle channel will do nothing to increase the "sweet" spot.
I took 'sweet' spot to mean good off axis frequency response... theoretically a third speaker could help to achieve that.
mwatsme's avatar
mwatsme OP
187 posts
 
Next build might include Manger full-range driver in the LXmini form-factor - they look very interesting. I wouldn’t be the first, so am not taking credit for the idea. https://mangeraudio.com/en/discover/about/the-manger-sound-transducer

Well, Manger transducers are out - they don't sell them for DIY anymore, and were $1000 USD/each - when they were available.
mwatsme's avatar
mwatsme OP
187 posts
 
My thought on dispersion was to be able to widen placement of L&R and remove toe-in for wider soundstage by adding the third one in the center - all facing straight forward. I don't know if that will be completely achieved.

Finding the LXmini compliment Source e-stats very well. Both have similar fast response, while the LXmini's fill-in the rolled-off highs of the e-stats, the 'hybrid' ML Source rounds-out weak'ish low bass of LXmini and add warmth to the midrange - awesome combo! I might have arrived at the end speaker system here, once a couple subs are added.

LXmini's really help with sweet-spot too - highs come from everywhere. Really liking this combo, and looking forward to getting them spread-out, position dialed, and third e-stat included. Tomorrow's focus will be finishing the gear racks.
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