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Old 23rd August 2013, 07:52 AM   #1
4real is offline 4real  Netherlands
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Exclamation Narrow OB experiment

Hi there,

I have been meaning to build a OB for quite some time, but never found the guts and time to do it. Various designs were in my head, and I finally took the step to actually build one of them as a prototype. So beware, this thing was never intended to be crafted perfectly, it just had to resemble the design enough for me to: (1) See how my wife likes it and (2) see if the design would actually sound good.

Well, first about the units used. I've had them for years already, just waiting for this project:

- tweeter: B&G Neo3 PDR. I think this one won't need any introduction... It's great, and I even have to spares
- mids: Peerless HDS 180 Exclusive, two per side. These are fine units form Peerless, with pearl colored basket and phase plug. They behave nicely and distortion figures are quite good.
- Woofer: the elusive 15" Peerless SLS 400. I'm probably one of the few to have a pair of these, which is in itself a problem, since I'd rather have two pairs, but sadly, you can't get them anymore and back then I only had the budget for two. They do have quite a low Qts for use in an OB, but nothing a little boost won't be able to fix.

Filtering will be done using a miniDSP 4x10 HD. Amplification is done with eight L25D IRS2092 Class-D amps, powered by a 1KW SMPS. Currently I only have two channels wired up, one powering my old speaker, the other the SLS 400. Mid's and tweeter are currently hooked up to a small T-amp. It works for now, until I can build a nice casing for the L25D amps. With four amps per side I can be flexible, I could bridge two channels to power the big SLS 400, or power each mid with one amp. Currently the mids are in parallel tough, so I only use three channels.

Anyway, enough about that now. Let's talk about the design! Most OB's are fairly wide designs with the woofers flat on a big slab material, for obvious reasons. Well, I did lot's of drawings of these, and found I didn't really like the looks that much. I will be building one of these however, just to find out the difference in sound quality. But that's for later on. For now, I wanted to build something more elegant an narrow, but still retain at least some of the low end performance. So I came op with the idea to put the SLS 400 in a kind of ripol/N-dipole/slot loaded OB. This way, I could have a sleek design and still retain good performance. The only thing I still needed to to is, get rid of the ugly box on the back side. I solved by making it rounded on the back. I think It looks pretty cool like this:

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

The rounded backside is made up of slices of material (MDF). The experimental unit I build doesn't have this, I used a much simpler (and less MDF consuming) method. If I choose to go with this design, I will have it made this way however. Also, in the Sketchup design I cut some bits off of the NEO faceplate. I did not do that with the experimental unit. So the mids are a but further apart. I build the thing last weekend, and hooked it up yesterday. Currently it looks like this:

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

While building I immediately noticed some shortcommings: Both the baffle and the outer sides of the rear end are probably to narrow. Hooking the thing up confirmed this. I'd probably need to double them up to lessen vibration.

Currently I've set up some a half baked filtering scheme, with some dipole correction, and filters at 120 and 2200 Hz. I didn't measure the system yet, but the first music tracks already showed some promise Bass performance is actually quite good. There are some resonances however, but that is expected, and measurements will tell me if it can be fixed or not. mid's and high's are already very nice, but it's clear that it still needs a lot of tuning. I'll probably also brace the sides of the backend with some MDF strips to make them stiffer.

I hope to have time today to do some measurements and tweak the miniDSP. If so, I'll post some of the graphs.
Attached Images
File Type: jpg front_small.jpg (41.1 KB, 729 views)
File Type: jpg back_small.jpg (48.2 KB, 716 views)
File Type: jpg pic_3.jpg (168.4 KB, 720 views)
File Type: jpg pic_2.jpg (174.2 KB, 720 views)
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Old 23rd August 2013, 08:19 AM   #2
Rudolf is offline Rudolf  Germany
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Nice project!
Quote:
Originally Posted by 4real View Post
While building I immediately noticed some shortcommings: Both the baffle and the outer sides of the rear end are probably to narrow. Hooking the thing up confirmed this. I'd probably need to double them up to lessen vibration.
I wouldn't double up the baffle. How about a "spine" in the back which fixes the magnets of the HDS drivers.
Could you explain your rationale behind keeping the mounting frame of the Neo3? Wouldn't it be better to discard it and get the HDS drivers closer together?

Rudolf
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Old 23rd August 2013, 08:33 AM   #3
4real is offline 4real  Netherlands
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Hi Rudolf,

Yes, fixating the magnets is another option, I'm not sure how to get that right esthetically though

The mounding frame of the Neo3 is currently more of a convenience. I'm not sure I would keep it. The wife doesn't like it anyway
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Old 23rd August 2013, 08:35 AM   #4
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Nice looking design!

Be aware that the Neo3 front plate also works as some kind of waveguide, that improves the frequency response and dispersion somewhat. Taking it off will change the response. But in the other hand, by taking it off you will get more symmetry in front/rear radiation....

To isolate the mids and tweeter from the woofer vibration you could build the mid/tweeter frame as a separate unit that is decoupled from the woofer frame. I did this on one of my designs, and it worked well.
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Old 23rd August 2013, 09:02 AM   #5
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I would agree with adding a back spline to the HDS's

What is the CTC distance? Looks to far apart to be set @ 2.2kHz, which is ideal for these HDS drivers.
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Old 23rd August 2013, 09:34 AM   #6
bvan is offline bvan  Denmark
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I'm curious the advantage of having a wider baffle around the tweeter, in theory and in listening? I ask cause I'm about to build something like this myself. Linkwitz has gone super skinny on his latest which makes me think its probably a good idea? (though perhaps something less circular/symmetrical?)

Also 4real, I'd be curious to see where you measure your ripole resonance at. I'm going ripole with 12s and really hoping to get up to 200hz 4th/8th order without problems.....

Cheers
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Old 23rd August 2013, 11:33 AM   #7
4real is offline 4real  Netherlands
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Quote:
Originally Posted by StigErik View Post
Be aware that the Neo3 front plate also works as some kind of waveguide, that improves the frequency response and dispersion somewhat. Taking it off will change the response. But in the other hand, by taking it off you will get more symmetry in front/rear radiation....
The final design will probably have a symetrical front and back side, both having a bit of a wave guide (the baffle is 18 or 22 mm in thinkness). Different response can be fixed easyly.

Quote:
To isolate the mids and tweeter from the woofer vibration you could build the mid/tweeter frame as a separate unit that is decoupled from the woofer frame. I did this on one of my designs, and it worked well.
Yes, I've seen this done all over the place. I guess it could even be done without killing the design.

Quote:
What is the CTC distance? Looks to far apart to be set @ 2.2kHz, which is ideal for these HDS drivers.
Phaseplug-to-phaseplug distance is currently 310 mm (90+130+90 mm). Ideal crossover point is probably somewhere near 1.8 to 2 Khz. I guess I can drive the NEO3 that low with a high enough order filter. I might even go as low as 1.5Khz..

Quote:
I'm curious the advantage of having a wider baffle around the tweeter, in theory and in listening? I ask cause I'm about to build something like this myself. Linkwitz has gone super skinny on his latest which makes me think its probably a good idea? (though perhaps something less circular/symmetrical?)
The basic advantage in my case is not have the upper woofer break the MDF, and off I have seen lots of variants, from a naked NEO, to vert small sides, or even the use of damping material. From what I've gathered it looks like a small baffle or the use of damping on the sides yields best results. I't didn't quite fit my visual design however, so I didn't go with it. I can probably make the tweeter section a bit narrower, specially if would stiffen the baffle by fixing the magnets.
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Old 23rd August 2013, 12:20 PM   #8
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If there is any way to reduce this distance the better. Driving the tweeter that low will induce a significant amount of distortion. Trim those bezels if necessary. Optimum would be 2.2kHz wave distance to the tweeter center.

Really like your setup
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Old 23rd August 2013, 02:32 PM   #9
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Originally Posted by Greebster View Post
If there is any way to reduce this distance the better. Driving the tweeter that low will induce a significant amount of distortion. Trim those bezels if necessary. Optimum would be 2.2kHz wave distance to the tweeter center.

Really like your setup
BG crosses that tweeter at 1500
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Old 23rd August 2013, 06:26 PM   #10
4real is offline 4real  Netherlands
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Today, I made some measurements and threw together a fast and simple filter that at least gives me something resembling a flat response.

First off the woofer:
Click the image to open in full size.

Dipole peek is at about 170Hz, with a second resonance right after that. The somewhat corrected woofer response looks something like this.
Click the image to open in full size.

Next up, the mids:
Click the image to open in full size.

And the corrected version:
Click the image to open in full size.

Tweeter looks like this, corrected (somewhat:
Click the image to open in full size.
There are still quite some ups and downs here, probably related to the rear end

Lastly, the full response with some more tweaking done:
Click the image to open in full size.

As you can see, It still needs tweaking, but I doesn't sound half bad at all. Bass resonances are mostly gone, and it really sounds quite nice

So, now, I'll lay back and enjoy the current settings for tonight
Attached Images
File Type: png woofer_raw.png (40.2 KB, 629 views)
File Type: png woofer_corrected.png (31.0 KB, 623 views)
File Type: png mid_raw.png (36.1 KB, 623 views)
File Type: png mid_corrected.png (33.2 KB, 622 views)
File Type: png tweeter_corrected.png (29.0 KB, 618 views)
File Type: png full_response.png (31.0 KB, 622 views)

Last edited by 4real; 23rd August 2013 at 06:33 PM.
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