Neebie question.

Isophon · « **on:** 15 Jun 2010, 02:47 am »

Hi All,

First time posting here.

I recently went to a DIY show in Montreal, Canada and loved the OB sound..I was hooked so I had to build one.

The set-up I'm working on, sorry about the long post but I believe the more you know the better the answers will be :

After much reading on this site and the usual OB web sites I decided to go with an H frame with 2 x Alpha 15A per side. These would be one on top of the other, overall with about 18-19 inches and 16.5 inches deep.

The full range sitting of the Hframe are the Fostex FE206E. All the drivers are already bought. (a gift from my wife)

I will go active (bi-amplification) with a SE 6BQ5 (RH84) for the Fostex and a Tripath TA3020 for the Alpha 15A's.

My X-over is the Behringer CX2310 (for now) that I intend to set around 125Hz based on MJK recommendation of a H frame with the Alpha 15A. I have no possibility to change the upper frequency for the Fostex so it would be a high pass filter at also 125Hz, 24 db/oct just like the LPF.

Since there is no real possible modification to the H frame, what about the baffle shape and size for the Full range. I'm a bit confuse as far as the baffle step question for the Fostex. What would you recommend so there is a smooth transition in the frequency response between the woofers and the Fostex ?

Also: Any comment/recommendation on the set-up are welcome.

Thanks a lot for your help.

Regards,
Isophon

Rudolf · « **Reply #1 on:** 16 Jun 2010, 12:40 pm »

Quote from: Isophon on 15 Jun 2010, 02:47 am

My X-over is the Behringer CX2310 (for now) that I intend to set around 125Hz based on MJK recommendation of a H frame with the Alpha 15A. I have no possibility to change the upper frequency for the Fostex so it would be a high pass filter at also 125Hz, 24 db/oct just like the LPF.

This will NOT work. You definitely need different crossover frequencies and filter orders for the LP and HP. Even for the low pass on the H frame alone the fixed 24 dB LR slope of the CX2310 is less than optimal.

gvimhoof · « **Reply #2 on:** 16 Jun 2010, 11:41 pm »

All the drivers are already bought. (a gift from my wife)

Congrat's on that! You've got some good drivers, so you're more than halfway home. I use a single 15A in an H-Frame under an FE103E OB and I still find myself smiling a lot. Use the minds of those here to help and make sure you check out the Open Baffle Theory Articles by MJK. Helped me tremendously! Open Baffle = Open Sound

D OB G · « **Reply #3 on:** 17 Jun 2010, 12:22 am »

Hi Isophon,

I have used the FE206E in a different configuration (as a mid- it's really designed for a back loaded horn I believe).

It has a low Q, meaning the response will drop off below a relatively high frequency.

Even Fostex's own graph, which I presume might be taken on an IEC baffle (1650 mm x 1350 mm) shows it droping off below 300 Hz. A sim in Speaker Box Design shows it dropping off below 400 Hz, even in an infinite baffle.

i.e. it will need equalization to make it down to 125 Hz, but then again MJK may have a solution (as Rudolf has said, maybe with some crossover manipulation).

Regards,

David

chlorofille · « **Reply #4 on:** 17 Jun 2010, 07:42 am »

Hi Isophon,

You could follow the Visaton noBox BB OB speaker dimensions.
http://www.visaton.com/en/bauvorschlaege/breitband/noboxbb/bauanleitung.html

Isophon · « **Reply #5 on:** 17 Jun 2010, 10:58 pm »

Thanks to all for the replies.

Since I already bought the x-over, maybe I could modify it so I can choose between 12db/oct or 24db/oct and have 1 frequency for the LPF and 1 for the HPF. I'm hoping to get the schematics by this Friday so I'll see if this is possible.

Since this system will be bi-amp I do not really care about the efficiency of the 206E so what about adding a serie resistor to the 206E to increase the Re and therefore increasing Qts, could this work so that the 206E could reach a lower frequency on a reasonnable baffle size (18W x 20H) ?

As a last resort, I do have a analog EQ so this could also help in boosting the 125-500Hz dip.

Thanks for all your help, this is a great learning project.

Regards,
Isophon

D OB G · « **Reply #6 on:** 18 Jun 2010, 05:01 am »

Hi Isophon,

I had a go at doing a sim in Basta.

IF !!! I have done it correctly, using your baffle size, and with 20 ohms in series, you can get to about 5 dB down at a 125 Hz crossover (using all values I couldn't get better than this), and before allowing for the 3 or 6 dB down for the crossover. This would give you an efficiency of about 82 dB/W (about the same as the BBC minimonitor). You would then have a hump centred at 650 Hz, which would need a notch filter.

So, a fair bit of work to not exactly achieve your aim I'm afraid (IF I have done the sim properly).

Maybe the analog EQ, with or without a resistor, is the best bet (would help to have test gear of course- even maybe a cheap SPL meter).

Regards,

David

Isophon · « **Reply #7 on:** 21 Jun 2010, 11:21 pm »

Hi,

This Friday I will be returning my CX2310 (credit) and purchase a DCX2496.

This should simplify my life and improve the overall sound.

Still even when considering the new equipment, how big should the baffle for the Fostex 206E be (approx.) to have a smooth transition with the H baffle with the eminence Alpha 15a's. The H baffle will be cut at 125Hz, 12db/oct L-R. (Based on MJK earlier tests with the Alpha.

Thanks for your help.

Regards,
Isophon

scorpion · « **Reply #8 on:** 22 Jun 2010, 12:20 pm »

Here is a MJK-model simulation of Alpha15 and FE206E in MJK's Goldwood/Jordan baffles with speakers placed exactly as he has them. Crossover is active at 125 Hz 12 db/oct Linkwitz-Riley for the Alphas and at 500 Hz 12 dB/oct L-R for the Fostex. This seem to give the best match like MJK already has discovered.

With the DCX you can use channel 1 and 4 for bass and channel 3 and 6 for the Fostex to achieve the crossovers. Channels 2 and 5 is used just to dial in the right frequencies. Then you can use the DCX to pad down the Fostex a bit and probably apply some filtering for the Fostex units also. Good Luck !

/Erling

Isophon · « **Reply #9 on:** 22 Jun 2010, 02:08 pm »

Thanks a lot for the sims Scorpion.

I was wondering, the efficiency looks kind of low at 87db for the Alpha but I know this is what MJK have also found in his paper.

In my set-up I have 2 x Alpha 15A per side, can I just assume that the lower portion will be raised by 6db ? Will it then match with the 206E..

Thanks a lot for your help.

p.s. Kinda weird to use the 206E which is an 8 inches full range (Fs of 39Hz) and use it with a HPF at 500Hz, sort of like a waste...

Regards,
Eric

scorpion · « **Reply #10 on:** 22 Jun 2010, 04:55 pm »

I should have read better from the beginning. Two Alphas change things. This is what it looks like:

Alphas in an H with 32" (H) x 16" (W) x 15" (D) inner measures excluding baffle, on top of that a baffle sized 18" (W) x 12" (H) Fostex placed 8" up and 13" left of the left corner, like this:

Crossovers now 125 and 325 Hz 12 dB/oct Linkwitz-Riley. Level matching OK, some filter might still be needed on the Fostex depending on how you like their FR.

/Erling

Isophon · « **Reply #11 on:** 22 Jun 2010, 06:52 pm »

This is AWESOME, thanks a lot !!

Can I ask you if the drivers were acoustically aligned during the simulation ? I was thinking of mounting the 206E in front of the H baffle (instead of at 7.5 inches for Depth) but then I would have to correct the acoustic alignment with a certain delay with the DCX, correct ? Maybe the ripple are caused by the sound bouncing on top of the H baffle ?

I haven't seen many H baffle with 2 woofers (I'm new here...), I will soon go cut the wood so I need some advise; For the H baffle do you need a piece of wood between the 2 woofers, sort of like 2 single H frame mounted one on top of the other or can you live without it and not change the performance.

Thank you very much for your help.

Lots to learn but all so interesting.

Best regards,
Isophon

scorpion · « **Reply #12 on:** 22 Jun 2010, 07:55 pm »

I think the MJK-model assumes the top baffle to be aligned with the H-front as MJK's pictures show. I don't think some inches difference is so important with wavelenghts of about 1 meter or more for these crossover frequensis. Dipoles can only be physically aligned to perfection as John Kreskovsky and Rudolf have shown. Ripples are both baffle effects and floor bounces.

You don't need to separate the H with a shelf or something, but one or two crossbraces coiuld not be wrong. Letting both speakers facing forward one can maybe add 1" to backchamber depth to compensate for the volume that the speakers occupy.

/Erling

Isophon · « **Reply #13 on:** 22 Jun 2010, 09:27 pm »

Thanks a lot for such a quick reply

I looked at your gallery and I really like the look of your Build 5 with the slanted 45 degres angle, what is the purpose of this angle and do you think it would be possible for my project. I kinda like the idea instead of locating the 206E all the way in the corner.

Thanks in advance.

p.s. Thanks for the tip about the backchamber depth being slightly longer than the front one. Would the depth then be 16 inches.

I have a long weekend coming up so I think there will be some MDF in the air...

Regards,
Isophon

FullRangeMan · « **Reply #14 on:** 22 Jun 2010, 09:59 pm »

Also being a newbie in OB diy I ask permisson to the thread owner to question about this OB panel image.
What about this OB panel?? How it works?? What is the sound effect it made?? It have better sound than the regular ''C'' panel?? Etc...
The drivers projected are two Alpha15A and one B200.
I thanks in advance any tip, Gustavo

Isophon · « **Reply #15 on:** 22 Jun 2010, 10:34 pm »

Hi Gustavo,

This is actuallt the Quasar II from B.D Design, is now uses the FE206E instaed of the AER but the woofers are the BD15.

From what I read just be careful with cavity resonance since the panel is 4 inches thick so make sure all your drivers can breath and they are not in too tight.

Lots of info can be found on the B.D. Forum.

Take care,
Isophon

FullRangeMan · « **Reply #16 on:** 23 Jun 2010, 03:00 am »

Thanks Isophon for your help. I copy this image on the BD site, this OB design always call my attention due the lack of side walls, maybe the self cancelling effect is too much, or not, I do not know.
Regards, Gustavo

D OB G · « **Reply #17 on:** 23 Jun 2010, 03:05 am »

THANKS Erling.

Isophon · « **Reply #18 on:** 25 Jun 2010, 10:20 pm »

I just got my DCX2496 today

Could someone please help me with this;

I looked at Scorpion gallery and I really like the look of your Build 5 with the slanted 45 degres angle, what is the purpose of this angle and do you think it would be possible for my project. Is there any advantage ? I kinda like the idea instead of locating the 206E all the way in the corner. I tried to do some sims with Edge but I did not see any difference.

Thanks in advance.

Regards,
Isophon

Isophon · « **Reply #19 on:** 28 Jun 2010, 12:23 am »

I now have a better understanding of 'Edge' so I have found my answer

The amplitude of the dip on the slanted baffle is slightly less compared with the square baffle. A difference of 1db so I'll see which one I choose,there are alomost identical.

Isophon