Morel mdt 33 for HT use

[Hezz]

Everyone,

I am designing a new high end three way HT front speaker for my HT system. I have a bunch of drivers around from trading work for projects that I want to use. This will be a medium\small floor standing speaker.

It is a two cabinet design with 2 cubic foot lower bass section. These are the drivers that I have:

Morel MDT33 tweeter.

Silverflute 5.25 inch shielded mid\bass.

Vifa w20 8 inch woofer. Not sure off the top of my head the exact model number.

The issue that I am wrestling with is the stray magnetic field of the MDT33. I really want to use this tweeter because of its sound quality.

I have some lower quality shielded tweeter that I could use but I would like to try and build the system with a first order crossover for higher impulse response and the MDT33 has much better power handling and a lower Fs.

The big issue is that there is still a CRT TV in the system for using to watch old VCR tapes. And full screen dvd. I am using a homemade LCD projector to watch widescreen dvd.

Does anyone know how bad the stray magnetic field of the MDT33 is. I have tested the Vifa woofer and it does not seem bad.

Also, has anyone tried to use something glued to the inner cabinet wall of the speaker box to provide some shielding.

Any thoughts.

[Juergen Knoop]

Hi,
my MDT33 are less than 1m away from the crt-tvset. There are no notecible artefacts on the screen. But this may be different on your tv. Just give it a try.
Regards
Juergen

[Hezz]

I just did a test of the MDT33's near the TV and compared them to some Vifa D27*** non shielded tweeter.

WOW, those mdt33's have a strong stray magnetic field. It is at lest ten times stronger (subjective evaluation) than the Vifa d27 I tested.

Still the effect is minimal at around 20 inches away from the TV were the speaker location will be. But I may have to use the D27's to be safe.

I am still considering some kind of thin shield on one side of the inner cabinet. Anyone tried to do this and what material would be best. Copper or aluminum. I would still like to use the MDT33 since the CRT tv may be replaced with a widescreen HD LCD with in a year or so.

[joe carrow]

Actually, iron or steel would be best- something with a high magnetic permeability.

I used the MDT20 in a project once, and I really did like the sound of it. I realize now that it may not have had the highest accuracy, but it had a fairly nice sound.

A 1/8" steel sheet on the inside wall of the cabinet ought to be a big help for shielding. If you can pick one up from your local hardware store, you can do a simple experiment without cutting it, and return it if the results aren't satisfactory.

[GM]

Greets!

FWIW, I used 12 ga 'CS' grade cold rolled steel (CRS), not the much more common hot rolled (HRS). Large enough scraps 'on the cheap' can usually be found at a local metal fabrication shop. I could set a 1.7 T prosound 15" woofer on a sheet big enough to lay on top and overlap my old 25" TV's CRT without it causing any problems.

GM

[sdclc126]

How about just adding bucking magnets to the tweeters?

[joe carrow]

How does that usually work with tweeters? I know that it can change some of the characteristics of the driver, but I'm not sure how much of that is normally accounted for in the design of a shielded version of a tweeter.

[GM]

Greets!

Depends on the tweeter and the size of the bucking magnet, but a worst case is the tweeter will have a rising response that will require flattening.

GM

[Hezz]

Guys,

Thanks for all of your responses. I have studied up a little on shielding as I really didn't know much on that topic.

It turns out copper and aluminum aren't much better than nothing at all. Steel is 200 times more effective and some special EMI films up to 20,000 times more effective. One of the films is rather inexpensive so I think I will line the tweeter cavity with that material and then use a piece of sheet metal on the inside of the cabinet near the TV.

I guess the materials need to be highly permeable as Joe has stated. Here is a link I found some some materials and films.

I'll post a picture of the CAD workup. I'm excited about this speaker as I think it has the potential to be really first rate.

http://www.lessemf.com/mag-shld.html#274

I think what I am going to do is create a small cavity for the tweeter housing that is isolated from the rest of the internal acoustic energy to reduce housing induced vibrations from the mid\bass. Then I will line that cavity with two layers of this film. Then put a 1/16 inch steel plate on the inside of the cabinet.

[Hezz]

I wanted to keep the budget very low on this set of speakers by utilizing existing drivers. However, if I want to keep the crossover simple with a first order design I'm going to need the ideal mid\bass driver.

The Silverflute W17RC38-S-08 6.5 inch shielded driver seems to have a frequency response made in heaven for a first order crossover. Plus the driver are really inexpensive at 30 USD each.

This driver has the smoothest response that I have seen in this size of a cone driver and falls off very smoothly at about 5.2 kHz. So I can use a single high quality capacitor on the MDT33 at around 5.2 kHz. This keeps me three octaves above the tweeter Fs. So I'm hoping that should be adequate. I want to run the mid\bass driver with no crossover components in series for high sound quality and dampening and impulse response. The Vifa P21WO woofer is very smooth and good sounding in the lower midrange so a simple inductor at a very low frequency to supplement the low bass should work just fine. If not I'll put a second order high pass on the woofer and wire the mid out of phase with the woofer.

This system is looking to be very good and simple.