JX92 + Alpha 15 OB - Design & construction

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Hi Everybody,

For the past few weeks I have playing with MJK's OB worksheets, trying to design a good OB. Here are some thoughts about this.

The kind of music I listen is mostly Pop/Rock both Indian and English. I also listen to some classical as well.

1. Considerations on Bass
JX92 is not very sensitive driver, so I am trying to get by with just one Eminence Alpha 15 driver/channel. First I tried the usual simple OB and kept getting a slight bump in mid bass. Tried several different configurations, but was not too happy. MJK has shown in his studies that H frame had the deepest flattest bass response, while simple OB had higher SPL. Then it occured to me that, I just have to omit using a piece of wood on the bottom for the ob to stand on. Instead, if I use 2 squares serving as feet perpendicular to the OB, I have a H frame.

That was it. H Frame. 2 square pieces of plywood 17.5" X 17.5". One straight groove / dado in the middle of the squares into which the OB measuring 17.5" X 40" slides into to be glued. Bass driver mounted on the OB centered at 8.75" X 8.75 at the bottom.

Simulations show me a pretty flat response down < 20, crossover filter at 125 Hz.

2. Considerations for the extended driver.
This I am still not sure. No matter what I have tried there is always some ups and downs in the response. The width of the baffle did not have too much of an impact as compared to the how much off-centered the driver is placed. Infact it seems, the closer to the edge the driver is placed, the better. Am I right or am I messing something up?

Right now I am considering the OB to be simply 17.5" X 40" and mounting the driver's edge just an inch from the baffle edge.

Crossover point, perhaps 350 Hz.

3. Other considerations
All edges will be rounded over to reduce diffraction.
I
have found a 13 ply Russisn Birch plywood in a speciality plywood store locally and have found a wood working shop as well, just so my challenges with a router need not jeopardize this project.

Finishing will be nothing fancy. I will sand it fine down to 600 grit. Then a natural stain and polyurathane. Do I need to do anything else?

The drivers I have are the older JX92 not JX92S.

What do you guys think of this. Please comment.

Regards,
Dinesh
 
Dinesh,

I think your idea is a very good one, a Jordan with an Alpha 15A in an H frame sounds like a winning combination. But there are a couple things that I think need to be clarified.

1. The H frame design needs to be closed at the top, the floor could be used as the bottom but it does not sound like you plan to use a horizontal piece to enclose the top of the H frame. The H frame is really just a pair of very short TLs added to the front and back of an OB.

2. Your crossover point of 350 Hz for the H frame sounds a little high. You have to watch out for the first axial standing wave in the H frame which I found to be just above 200 Hz when I used a 7.5" deep H frame. I think a lower crossover point between 100 and 150 Hz would be more appropriate.

3. I would probbaly also mount the Jordan near the center of the OB part of the speaker, maybe shifted an inch or two so that it is not totally centered and symmetric. I can't provide any real facts as to why this is better then a location near the edge, it just feels better to me.

Hope that helps,
 
Martin,

1. Thanks a lot for pointing out the mistake. I was not planning on closing the top. I will place 2 additional rectangular pieces one on front and one on back.

2. Please help me with the crossover point selection. I was planning on an LP filter with a corner freq of 125 Hz driving the Woofer. and a HP filter at 350 Hz, driving the extended range. Are you suggesting that I move the HP corner freq down from 350 to between 100-150?

3. I will move the driver back to an inch off the center.

Thanks a lot setting me straight.
Regards,
Dinesh
 
Hi Dinesh
I've been listening to a freind's JX92s's on and off recently. I too have been considering an H frame with a 15A. The cost of a decent quality passive crossover at low frequencies can get quite high and I would say that the flexibility of a modern digital crossover (a behringer ultradrive perhaps?) would more than repay it's initial investment. Additional amplification is required, but this can be quite modestly priced in practice. An additional advantage is that my friend's jordans really benefitted from some additional energy above 10Khz, something that can be reasonably done with a digital crossover.
I've built a number of Martin's OB's now and really like the way they interact with the room.
Good luck with your build. Let us know how you get on...

Rgds
Mark
 
Hi Dinesh,

dviswa said:
Martin,

2. Please help me with the crossover point selection. I was planning on an LP filter with a corner freq of 125 Hz driving the Woofer. and a HP filter at 350 Hz, driving the extended range. Are you suggesting that I move the HP corner freq down from 350 to between 100-150?


OK, I understand. The 125 Hz low pass really produces more like an effective 300 Hz low pass for the H frame. So your plans for a 350Hz high pass on the Jordan makes perfect sense. My mistake was not understanding what you were trying to do with the crossover, I would chose exactly the same crossover frequencies.

dviswa said:


3. I will move the driver back to an inch off the center.


I think that makes a lot of sense. I can't wait to hear how it turns out. Send a picture for my gallery.
 
marec said:
Hi Dinesh
I've been listening to a freind's JX92s's on and off recently. I too have been considering an H frame with a 15A. The cost of a decent quality passive crossover at low frequencies can get quite high and I would say that the flexibility of a modern digital crossover (a behringer ultradrive perhaps?) would more than repay it's initial investment. Additional amplification is required, but this can be quite modestly priced in practice. An additional advantage is that my friend's jordans really benefitted from some additional energy above 10Khz, something that can be reasonably done with a digital crossover.
I've built a number of Martin's OB's now and really like the way they interact with the room.
Good luck with your build. Let us know how you get on...

Rgds
Mark

Mark,
Thanks, I will keep you all informed.

I have been toying with the idea of investing in a Behringer. Recently did some googling about digital crossovers and read some of the threads here in modding Behringer. What makes me think twice is all the reviews about how mundane the output stages and the power supplies are. Further I am surprised many of these devices do not have a S/PDIF or AES inputs, they insist on having only analog input, digitizing it, then processing and converting it back. With an S/PDIF input one can go straight to processing it, improving the S/N and SQ quite a bit. Many of the mods I have seen cost more than the unit itself, which makes me think twice.

Maybe I should make a new thread to discuss if Behringer is the only game in town or if people have had good luck with other similarly priced products. DEQX pricewise is out of my league.

If I am go with a digital crossover, I will need another amp to bi-amp the OB. While, I agree whole heartedly with you, using a digital cross over significantly makes things easier, there are some budget questions that need answering. :xeye: Your argument about the extra energy helping the Jordans, might just push me over :D

Regards,
Dinesh
 
MJK said:
OK, I understand. The 125 Hz low pass really produces more like an effective 300 Hz low pass for the H frame. So your plans for a 350Hz high pass on the Jordan makes perfect sense. My mistake was not understanding what you were trying to do with the crossover, I would chose exactly the same crossover frequencies.
Thanks for clarifying.


MJK said:
Send a picture for my gallery.

Yes, I will, not sure if it will do much good to your gallery, finishing is not one of my strengths :D
 
MJK said:
1. The H frame design needs to be closed at the top, the floor could be used as the bottom but it does not sound like you plan to use a horizontal piece to enclose the top of the H frame. The H frame is really just a pair of very short TLs added to the front and back of an OB.
Martin,
I had clearly not understood the meaning of H Frame. Definitely did not think of it as 2 small TLs. From that perspective, perhaps H Frame is a bit of a misnomer. Hope somebody suggests a better name for it.

britbug said:
The JX92s is really so good. Let us know how it sounds, I have the jordans, and might well build them too.

britbug,
Join in the fun man. :D More the merrier. BTW, I have older JX92 not the JX92S. Difference is absence of shielding and a little less on the high end.
 
dviswa said:


What makes me think twice is all the reviews about how mundane the output stages and the power supplies are.


Hi Dinesh,
I guess it's a matter of what you hear when you listen...
I can sometimes tell that amplifiers sound different, but can rarely and consistently say that one is better. I often run one having convinced myself it's better, only to reverse the process a little later. To me, these differences are apparently insignificant.
The differences created by using a highly adjustable digital crossover are not insignificant! Perhaps you have an aberration of -6db over a couple of octaves caused by your loudspeaker -room interaction. Remove it, and the differences are not only obvious, but the system sounds better!
IMHO, if you are building loudspeakers, nothing for £150 will make anything like as much improvement...

Rgds
Mark
 
dviswa said:


Further I am surprised many of these devices do not have a S/PDIF or AES inputs, they insist on having only analog input, digitizing it, then processing and converting it back.



Hi Dinesh

I quote verbatim from my DCX2496 manual, without understanding or opinion...
"Balanced XLR input connectors A, B and C are used for connecting input signals. Input A can also be used for digital AES/EBU input signals"

Others may know what this implies. I regret I do not!

Rgds
Mark
 
marec said:
Input A can also be used for digital AES/EBU input signals"


Mark,
I am surprised by this. I do not understand why Behringer or its dealers do not advertise this better. :xeye: I kept seeing no digital inputs listed in the features and assumed it did not exist. Thanks for pointing this out. Man, you are now costing me money:D ;) I have to go a build a new amp and buy myself a DCX2496. Not to mention I also have to figure out what it takes to interface consumer co-axial to balanced AES. That's a whole new project I guess.

Also please tell me if the unit has a master volume control that is accessible from the front panel? The reason why I ask is that my cable box and many of the DVD players now on the market ignores volume setting on their digital outputs, forcing units further down the stream to do the job of volume control. For a bi-amped situation, power amp is not the place for this function. The Cross over will have to do this job.

Regards,
Dinesh
 
Dinesh; Great idea man, I have been working on an OB using the CCS WR125 and the Alpha, and wondering about reconciling the differences in sensitivities---never considered the "H" baffle. Interestingly enough, the unit could be constructed as an OB and the "H" added with wooden tubes attached to front and rear of the OB. This would make a nice way to compare the two formats and to try different drivers. Thanks for sharing your thoughts, and Martin, thanks for all you contribute to the DIY community.

Pete
 
marec said:
Others may know what this implies. I regret I do not!

Mark,

Not all D/A convertors are made equal, as you can see from D/A convertors so cheap that they are just thrown in for free on one end, while on the other end a state-of-art DAC will run you up for several thousand dollars. The same is true for A/D conversion. So, if I feed analog signal to the unit, it has already suffered one D/A conversion. The unit then does a A/D conversion before it can digitally process the sound and apply the crossover filters and then perform another D/A conversion. It is this final signal you feed to your amp. All these conversions result in lowered signal quality and increased noise. Hope, you don't mind explanation of the basics. If I can feed digital signal (S/PDIF or Coaxial for instace), then I will have avoided one D/A conversion, loss of signal quality by transporting it in the analog over the interconnect, and one A/D conversion.

Shorter the analog signal path, the better. From the little googling, I did now, it appears like I can take a coaxial output, make a cable that terminates in an XLR connector and feed it to the DCX. So it might not be a project after all.

Regards,
Dinesh
 
Hi Dinesh;
Sure, I understand the concept. i've just never used anything but SPDIF for digital communication. I can't imagine it's difficult to wire up a co-ax digital to XLR, but I've never done it!

Volume controls: There's a nominal overall gain control that gives about (IIRC) 15db of cut (and maybe boost). I don't regard this as a volume control and have never used it as such. You might recognise this from the fact that I've never implemented a digital input? :)

Rgds
Mark
 
Peter Menting said:
Dinesh; Great idea man, I have been working on an OB using the CCS WR125 and the Alpha, and wondering about reconciling the differences in sensitivities---never considered the "H" baffle. Interestingly enough, the unit could be constructed as an OB and the "H" added with wooden tubes attached to front and rear of the OB. This would make a nice way to compare the two formats and to try different drivers. Thanks for sharing your thoughts, and Martin, thanks for all you contribute to the DIY community.

Pete

Thanks Peter, Good luck with your efforts. Keep us posted on how your OBs turn out.

Now that I know better what H frame stands for, I am figuring out the easiest way to construct a H Frame. I will try and post some sketches in a few days.

marec said:
Sure, I understand the concept. i've just never used anything but SPDIF for digital communication. I can't imagine it's difficult to wire up a co-ax digital to XLR, but I've never done it!
More googling turned out a bit more info. You also need a small pulse transformer to convert 75 Ohm Coax to 110 Ohm AES XLR. It still is no biggie.

marec said:
Volume controls: There's a nominal overall gain control that gives about (IIRC) 15db of cut (and maybe boost). I don't regard this as a volume control and have never used it as such. You might recognise this from the fact that I've never implemented a digital input? :)

What can I say. :xeye: :rolleyes: I am Openly baffeled by the lack of such a basic feature. They can implement all kinds of complex signal processing, but no Volume control? Seeing those DIY Volume Control projects on the web made me suspect this.

I hope Behringer will take note and implement this feature.

Regards,
Dinesh
 
dviswa said:


What can I say. :xeye: :rolleyes: I am Openly baffeled by the lack of such a basic feature. They can implement all kinds of complex signal processing, but no Volume control?


In fairness to Behringer (and they do give us good value products) I suppose this is a bit of Pro gear with a specific purpose, not a GP pre-amp with digital processing tagged on....

Rgds
Mark
 
Just an update. For a few weeks I was not able to put in any time on this for personal reasons. Today I went to the wood working shop only to find that the business had gone belly up. This is a blow to the project, till another wood working shop can be found who can cut the plywood and route it. Can anybody suggest such a place anywhere here in Metro DC?

Thanks,
Dinesh
 
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