DIY center with ribbon, any tips?

[4real]

I decided to make a DIY center. Since it will be a little experimental, I don't realy care if they don't match my fronts. This gives me a little more design freedom

Still, I have a few fixed demands:
- I want to experiment with a ribbon tweeter
- Horizontal MTM is taboo, Verical MTM is oke!
- 2-way system, with a 13 or 17 cm woofer(s) (5" of 6.5")
- Needs to be as fullrange af possibe
- Butget: 300 euro's max for the ribbon + woofers, encl. filter
- Needs to be shielded

So, this is what I came up with:

A Vertical MTM system with

Tweeter: Aurum Cantus G2si aka Celeritas RB60. Shieled ribbon for a nice price.
Woofers: Monacor SPH-135KEP 133 mm kevlar woofer with phase plug, with a wide frequence range (almost upto 10 Khz). Magnetic shielding is possible.

Bass extension is not that good, but for a center it will do (11 liters Bass-reflex):



Now for the hard part: To make a good D'appolito setup, the units need to be lined up correctly. The woofers need to be less then 1x wavelength of the crossover frequency away from the tweeter, or even better less then 2/3 (at least, that seems to be the theory). The problem is the mimimul crossover frequency of the ribbon. It is 2.5 Khz. The 2/3 wavelength won't fit (unit's are to big). If I put the units as close as possible together (so unit against unit), I get a maximum crossover frequency of about 2.8 Khz. That will just fit . If I use a third order filter, the ribbon should be safe. The speaker should be directed to the audience, because of the horizontal dispersion of the ribbon, but that is no problem.

Here is a rendering of what it should look like:


[ klick for a high res version ]

My question is? Are my calculations correct? Maybe some tips for other woofers that might be usabel. I looked at Peerless CSX, but that one is a little larger, and I can't cross that low. Any general tips are welcome of cource

[4real]

Anyone?

And another question: Is it wise to put the ribbon in a seperate compartment? How big should this be?

[paulspencer]

4real, this won't help with your questions, but here's an idea that might interest you. Two TM's, one above and one below the tv - this creates a "phantom centre" in the middle of the tv so the voices are matched to where they appear to be coming from. As they are small, they are effectively oriented vertically, but don't look quite so vertical.

I have considered this myself, as well as a 3 way which is essential a vertically mounted TM with woofers either side.

Are you sure you can live with a veritcal MTM for a centre channel? where will you put it? It seems to me that it would be hard to place it so that it will look and sound right. (unless it is behind a screen)

Otherwise ... great looking rendering!

[4real]

Cool idea! It would be great, except that I do not have a TV . I just have a PC monitor (21") on my desk. I also watch TV and DVD's on it . So another speaker below is not such a good idee, and would me way to expesive.

So, It needs to be on top of the monitor. Luckyly for me, there is a shelve just above my monitor, where I can put my center . a horizontal center (what you call vertical , so flat, and not upright ), is no option. I want good sound , and I think it will even look cool like this

[paulspencer]

Very confused!!!

If you put an MTM on its so the tweeter is in the middle and the woofers are on either side then I call that horizontal. If you stand it up so the woofers are above and below the tweeter, then that's vertical. Now I'm not sure which you are intending to do!

Center speaker for your computer?!!!!!!
OK, you obviously want to build this thing whether you need it or not! (I can relate)

cheers,
Paul

[4real]

Yes, exacly as you mean, horizontal is:

MTM

vertical is:

M
T
M

I want to build the last one (as in the rendering).

And no, it's not a center speaker for my computer, but for my "HT" (well, at least partially). That I don't have a TV does not meen that I can't watch DVD's, or listen music . I have a fairly decent Receiver (Marantz SR-5200), and fine DIY front's. If you know what kind of a (non DIY) center I have now, you'll see that I definently need a new one (a tip: the "fitler" of my current center consists of a resistor and a bipolar cap )

Furthermore it is more or less an experiment. I also want to test this thing out as a front or surround speaker. If they perform better that my current DIY's, I might consider building new ones

So.. any help with my questions

[Lusso5]

One of the benefits of the D'Appolito-MTM is its horizontal focusing of the tweeter/limiting it's vertical dispersion.

The ribbon will already have these dispersion characteristics, individually.

Have you considered doing a TMM 2.5-way?

Just FYI - a speaker system is not a true D'Appolito, unless it uses odd order slopes; otherwise, it's just a MTM.

Nice CADD work BTW, it's going to be pretty...

[4real]

I didn't realy consicer a 2.5-way TMM. That I would rather do a TM design. TMM looks strage for a center I think . For a TM design I would use a 6.5" woofer ( Monacor SPH-165KEP) , but then the problem of a low crossover rises again (because of the beaming of the 6.5" at over 3 Khz.

So you say, a true D'appolito design (I was planing on using 3rd order crossovers, because the ribbon needs that anyway at that frequency) is not realy nessesery when using a ribbon?

So what will happen when I cross higher (3 to 3.5 Khz) then? I'll still have lobbing, won't I? I just looked at another topic about the Seas Thor by D'Appolito: Joe D'Appolito's THOR (Seas) . If I look at that design, the crossover is 2.5 khz. According to this own theory (in my opinion anyway), the maximum distance from tweeter to midbass is 13,76 cm (5.417"). The actual distance in the design is 15,2 cm (5.9843"). So what's behind this? Also the crossover does not seem to me D'Appolito "compatible"... I don't know what to think anymore

[4real]

Doesn't anyone know something about it?

It seems to mee that this is a fairly well gaurdes secret or something... bit's and pieces can be found, but not the whole picture

I think I'll just go for it and order the unit's. I'll simulate a filter and oder it, too. Tweaking can be done after measurements

[4real]

Ah, finaly an answer, in another topic, but still:

Originally posted by gary f



I think that the woofer-tweeter distance in a MTM and lobbing effects are given too much importance. Let's be clear: Keep the distance between woofer and tweeter to a minimum, that can't be bad. But crossing a tweeter too low is much worst IMO. In many successful MTM design (Ariel or Thor), crossover point is around 2500 Hz, a little higher than theory would allow.

In a real room, I think that lobbing effects are reduced because of the presence of a strong reverberant sound field. I hear many 2 ways with crossover point over 4000Hz and some sounded great. Some full-range design also use a supertweeter above 10KHz. The lobbing effects do not seem to be such a problem...

F

Thanks Gary

Well, I found some strange working tool to convert bitmaps of SPL and imdepande data to real SPL and impedance files to import in lspCAD

And came up with the folowing filter (don't look at the phase!):



both high- and lowpass filters are 3rd order. crossover is somwhere around 3200 Hz. I guess this way, I know for sure the ribbon is safe. And with the comment above, I'm not so strickt anymore about the lobbing crap