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Old 6th September 2010, 12:44 AM   #1
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Default Not Your Daddy's Dipoles

Just finished an OB design that incorpoartes some interesting concepts. I spent the weekend listening and I am favorably impressed.

Type: OB dipole up to approx 5.5k Hz. Biamped helper woofer, actively crossed and roll-off compensated. Coaxially mounted pro audio mid w/traditional 1" fabric dome tweeter. Acoustically dampened and isolated panels w/closed cell suspension. Visco-elastic feet.

Helper Woofer: Peerless 10" Nomex XXLS woofer. 250Hz lowpass, 24dB/octave.

Mid: Eminence Acoustinator 8" poly mid range, coax 1 1/8" 18TPI threaded, allowed to roll-off naturally.

Tweeter: Morel MDT-20 mounted coaxially, 1.8mf single cap, 6dB per octave.

Initial crossover points surprisingly good. Will spend some time tweaking. The drivers produce some respectable SPL's w/o strain. These were initially designed with many genres in mind. The acoustinator mid is a hair dark, but still makes for a realistic presentation. The voicing is very pleasing to me. More to come.
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Old 6th September 2010, 08:45 AM   #2
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Hello, I assume you use them for PC speakers? Great work !

How far do you place them from the rear wall?
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Old 6th September 2010, 01:23 PM   #3
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Quote:
Originally Posted by gainphile View Post
Hello, I assume you use them for PC speakers? Great work !

How far do you place them from the rear wall?
They seem to be somewhat less sensative to rear wall interaction. I have tried them anywhere from 1.5 to 3.5 ft. w/o any smearing of the image, to my ears. Currently I have them at 2', which is more for aesthetic reasons than anything else. They are also at least a few feet away from the nearest corner boundary.

Interestingly I also tried them outdoors to listen w/o any boundary effect. They sound surprisingly full and warm, possibly due to the high excursion capabilities of the XXLS (13mm) and a higher than typical 250Hz lowpass filter. The XXLS remains clean much higher than that, but the 8" Emi is no slouch either, so a higher xo point is not desired. The low to upper mids are actually quite satisfying, which make them just as apt to reproduce electronic music... something I have found to be lacking in many dipole designs. This was a primary design goal for me... thus the title of the thread.
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Old 6th September 2010, 01:30 PM   #4
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Some details of the dome mounting please? Didn't think that was at all possible.
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Old 6th September 2010, 02:49 PM   #5
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Default Coaxially mounting a hi-fi dome tweeter.

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Originally Posted by mayhem13 View Post
Some details of the dome mounting please? Didn't think that was at all possible.
I've been doing it for years and it's actually quite easy. Use a standard 18tpi to 2 bolt adapter. Selenium makes a metal one that is quite good and inexpensive.

Selenium ADM25-25 Horn Adapter | Parts-Express.com

It's easiest when the drivers flange diameter is 4" or greater. Align the tweeters center with the 1" throat and make a reference. I drill three holes through the horn adapter first, making sure they pass between the tweeter's outer flange diameter and body, away from the terminals. Then I clamp the tweeter back in position and use the adapter's pilot holes as a template, drilling straight therough the tweeter flange. Finish with flat head screws, locking washers and nuts, with the nuts facing outward. 10/32" screws work well.

Acoustically, the tweeter's dome should wind up in the same relative position as the element of a HF horn driver on a similar mid. If matched correctly, the single point source geometry is quite effective, using the mid driver's cone geometry as a wave guide. Driver selection is key, and with the proper x/o dispersion and off axis response aren't bad at all.
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Old 6th September 2010, 03:08 PM   #6
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I just finished my first pair of OBs and from all the reading and talking I did, a wide baffle and having the driver off center is needed to ensure the lower freqs are not cancled. How does that work with such a small baffle on yours?
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Old 7th September 2010, 11:48 AM   #7
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Originally Posted by djn View Post
I just finished my first pair of OBs and from all the reading and talking I did, a wide baffle and having the driver off center is needed to ensure the lower freqs are not cancled. How does that work with such a small baffle on yours?
On the contrary, many successful dipole designs using 10" woofers have a baffle width of only 12-13 inches (i.e., SL Orion). Remember this particular design uses some EQ to address dipole cancellation so a wider baffle is not necessary.

Offsetting the driver speaks more to retaining a proper stereo image than cancellation. In my test baffle I tested on center, 1.5" and 2.5" off center. With my single point source combination there was almost no distinguishable difference. I actually have 3 front panels cut with varying driver offsets. They are interchangable and can be swapped by removing 4 bolts.

Let me assure you that the quality and quantity of bass is not lacking. Unlike many dipoles I have heard, they excel in the 150Hz to 500Hz range; a solid presence w/o sounding boomy. In my opinion this is key to enjoying OB speakers with more modern music.
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Old 7th September 2010, 08:41 PM   #8
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That makes sense. I am new to OB and used the EDGE program to figure out my baffle size. I opted not to use the EQ the EDGE provides and went with big baffles. My wife and I are both artist and we are going to take one baffle board each and use them for canvases. Should be interesting. Here is a pic of my speakers. I'd never heard OBs before I built them and I am hooked.
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Old 7th September 2010, 09:02 PM   #9
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djn, i am looking forward to seeing the art from you and your wife on those speakers.

matevana... i drink you every once in a while and my kids enjoy your hint of chocolaty flavor.

These two examples of open baffle speakers are a perfect example of how there are different ways to achieve similar goals.

Zilla
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Old 8th September 2010, 01:47 AM   #10
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Quote:
Originally Posted by djn View Post
That makes sense. I am new to OB and used the EDGE program to figure out my baffle size. I opted not to use the EQ the EDGE provides and went with big baffles. My wife and I are both artist and we are going to take one baffle board each and use them for canvases. Should be interesting. Here is a pic of my speakers. I'd never heard OBs before I built them and I am hooked.
Great idea on the graphics! Please post again when they are finished. If it were me I would paint a picture of my living room so the speakers just disappear It is true that once you hear the 'correctness' of dipole bass, it's difficult to listen to monkey coffins.
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