Enlisting Help

[mischmat]

XO's are not my forte for building and I was hoping to enlist somebodies help with a centre channel MTM. Box will be .75 cu/ft with external dimensions of 19x18x13 sealed using 3/4" mdf. If anybody is willing to help below is a listing of parts I have. Was hoping not to buy more components but if needed I could. If the total project could be 4 ohms or higher it would be nice. Centre would be placed in a TV stand if that helps. Open back stand so open to suggestions to porting.

Parts are:

Woofers
2 x RS180s-4

Tweeter
1 X 27TDFC

Inductors
2 x .39mH 14awg, 0.12ohms DCR, 16mm H x 64mm D
2 x 1.5mH 18awg, 0.60ohms DCR, 14mm H x 57mm D

Capacitors
8 x 24mfd 250v, 0.0005ohms DF, 30mm D x 43mm L
2 x 30mfd 250v, 0.0006ohms DF, 31mm D x 48mm L
2 x 8.2mfd 250v, 0.0003ohms DF, 21mm D x 33mm L

Resistors
2 x 2 ohms 10w
2 x 8 ohms 10w
2 x 10 ohms 10w
2 x 5 ohms 10w

Anybody willing to help would be greatly appreciated or any input of course is appreciated also. ;x(

[JonMarsh]

That's a bit tricky without having measured data for the specific enclosure. Connecting the two RS1804's in series would be a starting point, then a crossover from the Modula MT project using the Seas 27TDFC might get you most of the way. But whether the existing crossover parts you have would be work, you'd need to compare against the BOM.

[mischmat]

Thanks for the poke in the right direction. I had a look around the Completed Guide and it appears most MTM's seem to be the 180-8 in parallel. Never noticed any designs for the 4 ohm version in series unless I missed something. I'll give your suggestion a go and see what happens. If I knew how to at least model something to see if it'll be half decent I'd be light years ahead of the game but I guess playing with components and the ear will do for now.

[JonMarsh]

Thanks for the poke in the right direction. I had a look around the Completed Guide and it appears most MTM's seem to be the 180-8 in parallel. Never noticed any designs for the 4 ohm version in series unless I missed something. I'll give your suggestion a go and see what happens. If I knew how to at least model something to see if it'll be half decent I'd be light years ahead of the game but I guess playing with components and the ear will do for now.

With dual 4 ohm drivers you don't. Have much of a choice but to wire them in series. But, this will give you a kinder load for the amp. The Modula MT is designed for the 8 ohm version of the RS180- two 4's in series should have 2-3 dB higher sensitivity, so you should be able to reduce the value of the series resistor.

Take a look at the NatalieP design, versus the original Modula MT; it's basically an MTM version using two RS180's, with the crossover adjusted as needed.

[mischmat]

Take a look at the NatalieP design, versus the original Modula MT; it's basically an MTM version using two RS180's, with the crossover adjusted as needed.

Interesting you say this. I was actually thinking of asking the question about adjusting this XO for series, but reading past posts came to the conclusion it's alot more involved than a small tweak to make it work. I guess I can order the parts for a single Natp and order a bunch of different resistors and adjust R8?

[JonMarsh]

Interesting you say this. I was actually thinking of asking the question about adjusting this XO for series, but reading past posts came to the conclusion it's alot more involved than a small tweak to make it work. I guess I can order the parts for a single Natp and order a bunch of different resistors and adjust R8?

I believe I have data on hand for the RS180-4- I could do a sim for you, if you can wait a week or two- things are a real b*tch at work right now! I'm doing something similar for Bill for a tweeter upgrade.

[mischmat]

I believe I have data on hand for the RS180-4- I could do a sim for you, if you can wait a week or two- things are a real b*tch at work right now! I'm doing something similar for Bill for a tweeter upgrade.

I hear you on the work front. I'm about to go solo for 3 weeks on my site. I can wait a week or two. I'll just pick away at the box till then. Would adjusting the Natp be easier and/or better or would series on the Modula MT be a better choice?