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Multi-Way Conventional loudspeakers with crossovers

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Old 2nd June 2006, 01:53 PM   #1
DKAudio is offline DKAudio  United States
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Default DIY Center Channel

I have been thinking about building this...
http://www.partsexpress.com/projects...ure/index.html
I am sure many of you have seen it. I also thought about just buying one but the common WTW I learned was very bad for horizontal dispertion (off axis performance). The only thing is that $200+shipping+wood is a little more than I wanted to spend. Any ideas/opinions on this design? How does it seem like it would sound? Any other good DIY center channels that you know of? Thanks
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Old 2nd June 2006, 02:06 PM   #2
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At the end of the day, that design is still a MTM and will suffer from the same problem with polar radiation as every other MTM. However, if your listening position is always right on axis with the centre speaker, then the narrow beam of sound is not really any issue.

In my own designs, I always favour few but good quality drive units rather than several cheaper ones.
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Old 2nd June 2006, 02:33 PM   #3
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Good point. I thought that maybe the way he crossed them over (the three mids on each side had different x-over points) would help. Any links to good ones? Thanks

Also, I meant MTM in my first post, not WTW, sorry.
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Old 2nd June 2006, 04:47 PM   #4
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If you a means of turning the speaker vertically, there are no problems. If not then I agree with richie00boy that you can reduce the number of drivers by getting better ones

What are the left and right speakers you have now and how sensitive are they?
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Old 2nd June 2006, 05:41 PM   #5
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They are Cerwin Vega LS-12s. They have a sensitivity of 98db. I don't think that is truely acurate though. I would keep the center channel laying down on top of my TV or on top of my entertainment center.
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Old 2nd June 2006, 06:21 PM   #6
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Are the Cerwin Vega speakers MT or MTM? People may disagree with me but to get the best surround effect the center channel speaker should be as identical as possible to the L&R speakers -- even including the crossover.

Surround sound imaging depends on a lot of subtle phase shifts. Do the CWs use a 2nd order xover? Do they have the tweeter wired in reverse phase to the woofers, as is common with 2nd order systems? If so, your center channel should also use a 2nd oder xover and probably connect the tweeter in reverse phase.

Without phase and timbre matching of all 3 up front speakers directional cues will depend primarily on amplitude as those subtle phase shifts will be washed away by disimilar speakers and xovers. The net effect could be a disappointment. Read about how Dolby Pro Logic works and you will better understand what I am saying.....

My $0.02
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Old 2nd June 2006, 06:49 PM   #7
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Hmm, this is all the information I can get on them



LS-12 Description: 12" 3-Way Floor Standing Model

Frequency Response: 28 Hz - 20 kHz

Power Handling: 300 watts

Sensitivity: 98 dB

Impedance: 8 ohm compatible

Crossover Frequencies: 500 Hz/3 kHz

Protection: Yes; Fuse (system)

Video Shielding: No

Dimensions: 33" x 14 3/4" x 13 1/4"
(H x W x D) 83.8 x 37.5 x 33.7 (cm)


I have no idea about the crossover or tweeter polarity. The Vega's are common 3 way WMT floor models. I am new to crossovers and am not very good with them. I did find this one too, a WMTW design
http://www.partsexpress.com/projects...nicc/index.htm

What is the deal with this reverse polarity tweeter configuration? How do you know when to do it? I have read about it a lot but never an explanation as to why. Thanks
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Old 2nd June 2006, 07:22 PM   #8
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A second order xover uses two reactive elements (a coil and a cap), each of which introduces a 6 db/octave reduction in amplitude and each element introduces a phase shift with a 90 degree rotation. Therefore to keep both the W and T close to each other in phase throughout the xover range the tweeter is hooked up 180 degrees out of phase with the woofer. This is basic xover theory.

A 3-way system usually has its midrange driver hooked up in reverse phase from the tweeter and woofer. The project you listed looks to be a good possibility. Note that it has both its mid and tweeter in reverse phase to the woofers. But, this is the way the xover's design worked out. The question I have is what are the xover points of this design? How closely do they match the ones of your CWs?

Phase shift also occurs as the driver's response drops off at each end of its passband. A well designed xover compensates both for the phase shift of its reactive elements and the driver's drop off induced phase shift. An amateur without significant knowledge, measuring and simulation software, will not get a crossover put together that is optimum. Too many variables.

Does CW make a center channel that matches the L&R you have? If not, you may want to consider selling them and making 3 identical speakers from kits. Go to the Parts Express discussion board and ask a few questions and look at some of the projects posted there in the Showcase.
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Old 2nd June 2006, 09:11 PM   #9
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Hi, CW does make a center channel but it is discontinued. I was going to bid on one on Ebay durring lunch but my works internet connection went down (figures). The auction is now over. It was a MTM design. I understand about the reverse polarity now, it is because caps and inductors present a phase shift in the current.

Maybe I will just wait then. I eventually want to make all the speakers (matching). I am looking at a Yamaha HTR-5850 and HTR-5840 on ebay that I was going to bid on, maybe I will wait on those too?

Thanks
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Old 2nd June 2006, 09:46 PM   #10
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Oh, I also forgot to mention. I also have some KEF Q90s. I am not using them right now, I thought maybe they would make good rears eventually? They are not effecient but they sound nice.
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