Multi-driver, horizontal center speaker with TB-871?

[morbo]

Hi Everyone;

I've been reading here for some time, and am very happy to see the healthy amount of discussion of the Tangband 871 fullrange drivers. I've lived with 2 of these (in the form of the CSS ELF 1.0) for over a year now, and have been frankly quite amazed at the value, and in particular, the imaging when used as nearfield monitors. In my admittedly limited experience, there are few speakers as addictive as these for small scale, 'intimate' music.

Currently I am using one of these as a center channel speaker in my small home theater. At low volumes, it sounds great for dialogue. Once the action picks up a bit though, things get very congested, even crossed at 100hz. What I would like to do is build a new center using one or more of these drivers, as the quality, cost and simplicity of the design are 1st rate. The only thing I'd like to improve on is the output capability.

I am aware of the CSS Spires center, which is similar to what I describe here, but I have a number of concerns about this kit including putting the crossover at 1k and having 5 drivers in a line horizontally, which I understand may cause poor horizontal dispersion.

Any ideas or suggestions?

[BillFitzmaurice]

Even an MTM will have poor horizontal dispersion horizontally placed; more drivers will make things worse. Either switch to a driver that has more output capacity or find a way to keep the array vertical. One method is two small arrays to either side of the screen to maintain the center of screen imaging, while placing the two sources far enough away from each other to minimize horizontal plane phase glitching.

[morbo]

Hmm, that is an interesting suggestion Bill... how far apart would the 2 arrays have to be? I could actually accomodate this type of arrangement in my setup fairly easily, but I'm not sure I'd have the necessary power from my receiver, plus the cost would be much higher.

What about using 2-3 drivers only? I was under the impression that it takes 4 drivers for 'line array' behaviour to occur, though I can certainly be wrong as my knowledge of the subject is extremely limited.

[planet10]

Originally posted by morbo
crossed at 100hz

With an Fs in the 130 Hz range an XO at least twice that is needed if you want them to start opening up at higher levels.

dave

[navin]

one option that requires good woodworking skills is to use 3 drivers (1 x 871 and maybe 2 of the W4s) and build them like Vandersteen does his center channel as shown below.
http://www.vandersteen.com/pages/VCC-5lit.html

[BillFitzmaurice]

Hmm, that is an interesting suggestion Bill... how far apart would the 2 arrays have to be? I could actually accomodate this type of arrangement in my setup fairly easily, but I'm not sure I'd have the necessary power from my receiver, plus the cost would be much higher.

Once you're over a half wavelength apart phase glitching becomes non-problematic; three feet apart is a half wavelength at 180 Hz, and below that room modes become strong enough to overshadow phase difficulties anyway.

Line array behavior begins as soon as you're source is higher than a true point in space; what changes with height is the frequency that line characteristics extend (downward) to.

Amp power is no problem. With every doubling of cone area you pick up 3dB of increased radiation efficiency. Just make sure you keep the impedance right. I think 2 drivers per side wired series in each box, with the two boxes wired parallel. This would give you 6dB more sensitivity with a level impedance load.

[morbo]

Dave;

I know the 871 really wants a higher crossover... but surprisingly enough, when using the reciever's 100hz crossover, it really doesnt start to distort at all until the high 80db range. Only the peaks really present 'congestion' problems. Does sound a little 'thin' though, especially on deep male voices.

Navin;

That looks like an impressive center channel! Unfortunately my woodworking skills are rudimentary at best, I'd likely botch it horribly. Also I'm not sure whether the design is contingent on using the external fullrange driver and inward firing mid/basses or not. I'd like to avoid crossovers if possible, if not at least complex ones.

Bill;

thanks for the elaboration. So you think 2 per side would be suitable? In that case, perhaps I should just look at 2 of the superelf that CSS has, which uses the 871 as a mid/tweeter and another tb 3" driver as the bass. Or is the idea to use 2 of the same per side, wired as you described?

[Timn8ter]

Originally posted by navin
one option that requires good woodworking skills is to use 3 drivers (1 x 871 and maybe 2 of the W4s) and build them like Vandersteen does his center channel as shown below.
http://www.vandersteen.com/pages/VCC-5lit.html

I kinda like this idea although I don't think you'd need to make it as complicated as the Vandersteen. Maybe a pair of W4s in a vented chamber and a W3 in a sealed.
Hmmm......

[morbo]

Maybe a pair of W4s in a vented chamber and a W3 in a sealed.

W4s firing inwards as in the vandersteen design? And how would one cross such a thing?

[BillFitzmaurice]

thanks for the elaboration. So you think 2 per side would be suitable? In that case, perhaps I should just look at 2 of the superelf that CSS has, which uses the 871 as a mid/tweeter and another tb 3" driver as the bass. Or is the idea to use 2 of the same per side, wired as you described?

What you do isn't as important as having both sides identical. I'd shy away from anything that uses a 3 for a woofer; if you're going to try to get worthwhile response below 100 Hz you really need a 5 to get it. If you want to go 2 way I'd use an 871 paired with a W5-876 on each side, closed box, xover at 500 Hz. This would, however, give you two 8 ohm boxes, so your receiver would have to handle 4 ohm loads to parallel two boxes. If you think that your current F3 is OK and just need more sensitivity using a pair of 871 per side makes things very simple.