Driver selection help

[jimbones]

I am looking into building an MTM or MMT with some Dynaudio 17W75's. Any recommendations on a tweeter that will work well with these mid woofers? Also do you recommend MTM or MMT? Thanks

[AllenB]

If you are planning to measure and accommodate the acoustic phase of the drivers, then the MTM may be the better choice. In that case you'd want a tweeter that can go low. Otherwise the MMT may be more likely to give good results.

[jimbones]

Ok Allen, I am resurrecting this thread. I am using 2 dynaudio's 17W75 and a Vifa XT25TG30 tweeter in am MMT arrangement.
I did the crossover calculations for an mmt 2nd order @2500hz and come up with 0.36mh and 11uf cap on dynaudio's with a zoebel if 15uf and a 3.2 ohm resistor in parallel.

for the tweeter I have a 5.6uf cap and 0.7mh coil with 3 db of padding. does this sound like a good starting point?

Should I do baffle step calculations? make it a 2.5 way?

Thanks,
Jim

[AllenB]

Hi jimbones. I'll trust you on the calculations for the time being. An MMT is a good opportunity to go 2.5 way. The fundamental process is relatively straightforward.

When designing the crossover, assume you have one woofer, your tweeter, and no baffle complications. For the other woofer you first need to calculate the centre frequency of the baffle step, which you can estimate based on the width of the cabinet. Calculate impedance compensation (zobel) for it and add an inductor. The size should be that which would cross it at the baffle step frequency.

[jimbones]

OK so there will be some value changes. I'll see if I can figure this out.
Thanks!!

[jimbones]

OK here it is. the baffle is 10 5/8 inch wide:
Lvc = 0.23 mh
Re = 5.1 ohm
Wb = 10.625 inch

F3 = 4560/Wb = 429 hz

LBSC = Rparallel /(2π f3) = 1.8 mh


R parallel = Re x (10 db/20 -1) = 5 ohm

R zobel = 1.25x Re = 6.375 ohm

C zobel = Lvc/(1.25 x Re)2 = 0.23/40.64 = 5.6 uf

Looks ok?

[AllenB]

One benefit of the 2.5 way is you don't need the shelf, ie. you don't need the resistor in parallel with the inductor. Just cross it over first order at the baffle step and that's all it does.

[jimbones]

Oh ok so the 5 ohm is deleted.

[jimbones]

After I build these I have another variation on this but that will come later. We can always throw the design around and build when I have time. Thanks.

[AllenB]

To treat the woofers separately, give them each their own zobel. Referring back to your first post today...double the resistance and halve the capacitance so they work with an 8 ohm driver, then make up four of them.

For the filter, double the inductance and halve the capacitance from what you posted so you can use it with the top woofer only.

The tweeter may need to come down a few more dB (and would even if you were making this into a standard two-way). An L-pad would be advisable at this point.

The lower woofer may want an inductor of around 3mH.

(I'm assuming by the way that you are wanting to move forward without making further measurements)