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Old 14th June 2015, 05:32 PM   #1
Marwhal is offline Marwhal  Sweden
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Default Building an active MTM 2-way

Hi,

I'm planning to build a pair of active floorstanders this summer.

My first thought was to build the Troels gravessen CNO-25. But I have wanted to try active filtering for a long time so I decided that now is the time to try it out.
If it don't turn out well I can always build the passive X-over later as a separate box.

But then I got the idea that maybe I could mount the drivers in a MTM-array and make it a 2-way instead of 2.5-way.

The plan is to use miniDSP as active crossover and a 6 or 7-channel amp so each driver gets it's own dedicated channel.
RoomEQ will be made by Audyssey XT32 or even PRO in the future.


So, is it crazy to make an MTM as my first build? What could go wrong?
Should I just stick to the original CNO-25 and just make it active?

I'd like as much input as possible about what to think about before i start this project?

Attached is a CAD-drawing of this project.


Best regards,
Martin
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Old 14th June 2015, 06:33 PM   #2
Marwhal is offline Marwhal  Sweden
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Another view of the Cad-model attached below.

I kept the internal volume of the troels design, but added a couple of braces and made it a little bit taller. The plan is to fill the space above the plinth with sand.


Best regards,
Martin
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Old 15th June 2015, 12:24 AM   #3
xrk971 is online now xrk971  United States
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If you have miniDSP it's not crazy to make an MTM as first build. Just make sure drivers have smooth response a few octaves past your intended XO point if going with less than LR4. Try to model the response before buying and building. Get as close as you can with a model first then build. You will find that the process forces you to think about a lot of issues that make the difference between a good speaker and a great speaker.
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Old 15th June 2015, 01:51 PM   #4
Marwhal is offline Marwhal  Sweden
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Quote:
Originally Posted by xrk971 View Post
If you have miniDSP it's not crazy to make an MTM as first build. Just make sure drivers have smooth response a few octaves past your intended XO point if going with less than LR4. Try to model the response before buying and building. Get as close as you can with a model first then build. You will find that the process forces you to think about a lot of issues that make the difference between a good speaker and a great speaker.
I was hoping for that answer. Thanks so much!
My thought was to use the same crossover frequenzy as Troels uses in his design, which is 3300 hz. But with the miniDSP I can experiment a little with other frequenzies.

The drivers are 2 x Seas W18NX001 and Seas T29CF002 Crescendo.

Which program is recommended for simulation?


Best regards,
Martin
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Old 15th June 2015, 03:00 PM   #5
xrk971 is online now xrk971  United States
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Two great programs available for free by master XO designers:

Xsim by Bill Waslow (Bwaslow)

And

PCD by Jeff Bagby

I use Akabak once the XO is known and you want to add speaker cabinet response modeling vs measured FRD files.
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Old 17th June 2015, 03:06 PM   #6
Marwhal is offline Marwhal  Sweden
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Quote:
Originally Posted by xrk971 View Post
Two great programs available for free by master XO designers:

Xsim by Bill Waslow (Bwaslow)

And

PCD by Jeff Bagby

I use Akabak once the XO is known and you want to add speaker cabinet response modeling vs measured FRD files.

Thank you, that's awesome!

Does the miniDSP adjust the relative volume between LF and HF in the digital domain? (Losing bits in the process?)

If so, maybe it would be smarter to get a power amp with gain controls for each channel?


Best regards,
Martin
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Old 17th June 2015, 10:44 PM   #7
xrk971 is online now xrk971  United States
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MiniDSP can adjust relative gain but math is done internally at higher than input sample rate so you aren't losing bits. It can be useful to have a class D amp with a volume pot though. I prefer to use the miniDSP to set relative gain.
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Old 18th June 2015, 04:30 AM   #8
Marwhal is offline Marwhal  Sweden
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Alright!

I'm keeping an eye out for Nuforce mch-3c7. But may as well get a Marantz power amp since I'm probably gonna get one of their processors. Good to know that gain can be set within the miniDSP without losing bits.


Best regards,
Martin
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Old 5th July 2015, 12:40 PM   #9
Marwhal is offline Marwhal  Sweden
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I guess I need to have some kind of filter on the tweeter to protect it from turn on pops and stuff like that so I don't accidentally burn it.

How do I calculate this and what do I need?

Is a low slope HP-filter at 1000 hz fine or isn't that enough protection from low frequenzies?



Best regards,
Martin
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Old 5th July 2015, 12:48 PM   #10
Marwhal is offline Marwhal  Sweden
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I also want to know if the braces inside the box could affect the sound badly if they are placed to close to drivers?
Do i need to think about reflections etc. inside the box or is more bracing always better?



Best regards,
Martin
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