Design assistance - HiVi D6.8 & Morel MDT33?

[randytyler]

I'd like to build a new MTM system definitely using the HiVi D6.8 woofer and possibly the Morel MDT33 tweeter. I've had these drivers on hand for some time now, and am hoping someone can lend me a hand on an enclosure and crossover design. If the MDT 33 is not a good fit in an MTM with the D6.8, I also have a pair of Dayton DC28F-8's and a pair of Dynaudio D21/2's on hand. If none of those will work, I'm open to suggestions.

I'd like to build enclosures with the front sides rounded over like you see in the picture below. The speakers will be used mainly for music, but may eventually be used in an HT system with a sub added. No real preference for sealed or vented enclosures; whichever works best.

So, I'm looking for someone to help me out with a basic design drawing for the enclosure and a schematic for the crossover. I can build the stuff, I just can't design for stink!

I'd be happy to trade the unused drivers from above, or other drivers I have on hand for design assistance.

Please post here, or contact me if you can help me out.

Thanks,

Randy

[zach_t]

Re: Design assistance - HiVi D6.8 & Morel MDT33?

Try a design yourself using free Excel tools.

http://www.rjbaudio.com/Audiofiles/FRDtools.html

Model the D6.8 in Unibox, find a ported or sealed volume and post here to get feedback.

Trace some mfg. curves. Do a Baffle diffraction study, combine the curves and start playing with Passive Crossover Designer.

Ask plenty of questions along the way! Best way to learn.

[critofur]

Re: Design assistance - HiVi D6.8 & Morel MDT33?

Just for fun...

Here is a simulation WITHOUT baffle, or box modeled.

It's easy to use WinISD to come up w/a box, I can do that maybe tomorrow, not sure, if I have time. Busy with some errands...

[Chris Roemer]

Re: Design assistance - HiVi D6.8 & Morel MDT33?

Just for fun...

Here is a simulation WITHOUT baffle, or box modeled.

It's easy to use WinISD to come up w/a box, I can do that maybe tomorrow, not sure, if I have time. Busy with some errands...

I like having fun too! Before I looked at crit's work, I whipped up an XO for you. It's not the same as crit's, but it's not terribly different either.

High Pass (all this is compared to crit's XO):
I also used a 10uF series cap, but my shunt coil is a 0.50mH instead of a 0.40.
My L-pad is also a bit stronger, dropping the mdt-33 by about 2dB, which is accountable to baffle step compensation. I used a 4 ohm SR, and an 8 ohm PR. In MY sim, I connected the tweeter up with reverse polarity to align the phase better. If you're unsure which way to go, run them both ways before sealing up the box (all you have to do is swap the tweeter terms at the driver, nothing more) and use whatever sounds best to you.

Low Pass:
I used a 1.2mH series coil (instead of 1.0), then a 16uF shunt cap (instead of the 33uF). Beyond that, I just used a Zobel with a 4 ohm resistor and a 33uF cap (no coil at all in this shunt leg).

I also whipped up a box model. I'd run that pair in 1.4 cu. ft. with a 3"dia vent, out the back would be my preference. I'd try to get the morel up to seated ear level (at least 30" off the floor, about 39.5" high, OD), and probably go with about an 8" wide baffle (which would give you an outside depth of a little over 11.25"). These should reach down into the mid 30's, which is pretty dang good, especially for just music.

You could use an 8" long vent out the back (1/3 of the way up the box from the floor would be good) if you don't want to bend it. If it were mine, I'd put an elbow on the vent and make it more like 11" total length. This will give you a shallower rolloff, closer to what a closed box would have, but still make it into the mid 30's. The shorter vent will be more prone to boominess, especially if you have a room mode around 40 Hz (which most rooms seemto have), but they'll play a bit louder with slightly less tendency to bottom out.

Chris

[critofur]

Re: Design assistance - HiVi D6.8 & Morel MDT33?

Sorry the polarity of the tweeter is wrong in my post - that was from when I reversed it to show the reverse null.

The baffle will introduce dips and bumps in addition to the overall boost that BSC is used for.

Once you build an actual box, we should measure the speaker and refine the crossover - unless you were to simulate everything the way Jay does.

I found box modeling to be quite obtuse and un-intuitive in LspCAD, but the simulations in Soundeasy don't look quite so bad.

Next idea would be to model up a TMM 2.5. If you do want to do an MTM, we may want to see if we can push the Morel tweeter a little lower? I'm not sure how low the MDT33 will go comfortably. I do however, think that most guys on these forums are somewhat conservative in regards to how low they use tweeters (this comment is partially based on experimenting with 3/4" tweeters crossed over below 2Khz and partially based on the studies of the audibility of distortion that I've read).

3" vent looks good; I think I'd be happy enough with a 2 1/2" or 2" vent, or even a slotted vent.

One thing I don't know how to do in LspCAD is to show the combined output of just the two woofers in the graph? I assume it must be a fairly simple thing to do, I just haven't seen how yet...

One thing we may want to add to the crossover is a notch filter at the tweeter's FS, particuarly if we were to crossover lower, I will take a look at that (unless my wife yells "dinner's ready" before I get to it ;) )