Modified MB0W1 - does this look like a reasonable plan? - diyAudio
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Old 9th December 2003, 11:19 PM   #1
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Default Modified MB0W1 - does this look like a reasonable plan?

Greetings all. I've been interested for some time in building a pair of small, reasonably flat 2-ways with shielded drivers for use in the near field at my computer desk. They will be driven by an old Adcom 535 amp and supplemented by a small subwoofer.

I'm attracted to Dennis Murphy's well-regarded MBOW1 design, which uses a GR Research M-130 and a Hiquphon OW1 with reasonably simple crossovers.

Since I need shielded drivers, and I've been having a terrible time finding suitable designs that use them, I'd like to attempt substituting a (shielded version) Vifa XT25 with slight crossover adjustments.

Here is the original crossover as designed by Dennis Murphy; it is designed to result in a 2nd order L-W acoustic response at about 2600 hz, and thus needs a tweeter at least as sturdy as the Hiquphon:

Click the image to open in full size.
Click the image to open in full size.

I propose to change the values of the tweeter's Zobel network (in pink) to values suggested at the bottom of this page in order to flatten the XT25's substantial measured impedance peak, and adjust the parallel padding resistor (in blue) to compensate for the slight sensitivity difference.

I'll probably just purchase a pre-made box (readily available) since my access to tools is limited right now.

I realize that modifying perfectly good designs is not generally what one does, but at this relatively modest level I'm willing to take some risks. And the drivers are certainly useful to have around if it works out badly.

Does anybody see any gaping flaws that my general n00bishness did not uncover? Comments of any type would be most welcome.
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Old 9th December 2003, 11:44 PM   #2
sreten is offline sreten  United Kingdom
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Near field design is not the same as far field design.

So I'd say the MBOW1 is not good for your application.

Near fields do not require the "baffle step compensation"
that is needed in the far field.

DM has several other designs on his site that are the MBOW1
but with different tweeters.

But they are all 'far field' designs.

/sreten.
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Old 9th December 2003, 11:53 PM   #3
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Thanks for your insight, sreten.

It is possible that I am using the term "nearfield" incorrectly - my situation will place the speakers about a meter from my head, with the nearest wall a bit more than 2 meters behind the speakers.

In this case, should I still be looking for a design without baffle step compensation?
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Old 10th December 2003, 12:09 AM   #4
sreten is offline sreten  United Kingdom
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I'm sure that will be nearfield though I don't know much about
nearfield design - perhaps you need a "studio" forum.

Certainly the studio brands do make "nearfield" monitors.

/sreten.
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Old 10th December 2003, 12:17 AM   #5
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Thanks again.
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Old 10th December 2003, 06:41 AM   #6
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Can anybody enlighten me as to how the need (or lack of need) for baffle step compensation correlates with listening distance?

That is to say, as one moves from a "farfield" position - where a speaker's response suffers from diffraction loss - closer and closer to approach a "nearfield" listening position, I can't imagine there is some arbitrary distance where the need for compensation is magically eliminated. It seems to me that there must be a gradual change, if there is in fact a change at all.

Does anybody have any information in this area?
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Old 10th December 2003, 07:11 AM   #7
usekgb is offline usekgb  United States
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Your intended application would be considered nearfield. Generally a loose definition of nearfield would be, a listening position close enough for the direct sound from the speakers to mask all or most of the reflected sound from the room. Often, a 3dB pad is used on the tweeters because they can sound very bright at closer distances. Making near field monitors that are "flat" can be a very difficult challenge. You have to consider things like early reflections from your desktop and monitor, boundry gain in the LF from the speakers sitting on the desk, etc. I would say just go for a nice simple x-over that is relatively flat, and you should be pretty happy. Maybe even add an adjustable L-Pad to tame the highs if they are too bright at your listening position. Give it a shot and, more importantly, have fun!

Cheers,
Zach
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