JX92 + Alpha 15 OB - Design & construction

[dviswa]

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

[MJK]

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,

[dviswa]

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

[marec]

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

[MJK]

Hi Dinesh,

Originally posted by dviswa
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.

Originally posted by dviswa


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.

[dviswa]

Originally posted by marec
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. Your argument about the extra energy helping the Jordans, might just push me over

Regards,
Dinesh

[dviswa]

Originally posted by MJK
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.

Originally posted by MJK
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

[britbug]

I am sure they will sound great. The JX92s is really so good. Let us know how it sounds, I have the jordans, and might well build them too.

[dviswa]

Originally posted by MJK
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.

Originally posted by britbug
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. More the merrier. BTW, I have older JX92 not the JX92S. Difference is absence of shielding and a little less on the high end.

[marec]

Originally posted by dviswa


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