OB H frame Emin Alpha 15A & CRH-70 - Help needed

[andb]

I've read over MJK's Jordan + Goldwood and many threads here and now am interested in building an open baffle speaker. I'm asking for some help from more experienced members to make sure I get this right. I'm hoping to build using an Eminence Alpha 15A with a Mark Audio CHR-70. I have the MA's, which is the main reason for this project.

I'd like to use passive crossovers and push these speakers with an audiosector LM3875 chipamp and a soon to be built Triode electronics st-70 clone. I have a small listening room, about 4mx5m, with the listening area being half of the shorter axis. Living in a flat in the city prevents me from listening as loud as I might want to, so I don't need to go for really high volume levels.

I see these issues that I need to resolve:
1. Will this driver combination work well?
Frequency response seems to be well matched, but the sensitivity of the 15A seems to be 10db higher. What is the best way to deal with this? Can these two drivers play nicely together with just a passive crossover?

If the CHR-70 isn't a good driver for the project, Ill can build a lotus box for them, give them to a friend, and choose another driver.

2. Proper dimensions for the H frame bass baffle (Alpha 15A)
In MLJ's research, he used a internal cross section area of 16” wide and 16” tall. The depth of the cavity was defined as 7.5”. Based on pictures, is it correct that the cavity depths in front and behind the driver are symmetrical?

MJK wrote:
The efficiency of a U or H frame could be increased by using a larger cross-sectional area. The depth of U frame or either side of H frame enclosures should be set to place the quarter wavelength resonance above the selected low pass crossover frequency.

Based on this, is it correct that the cross-sectional area should be tuned based on the efficiency of the other drivers in the system, in this case the CHR-70? Is the 16" square appropriate in relation to this FR driver? It seems that it will be ok since the sub is already much more efficient then the full range driver.

3. Proper dimensions for the fullrange baffle (CHR-70)
Im pretty new to this so tips on baffle size and driver location would really be appreciated. I'm also very interested how to derive these values.

4. Crossover wiring
Will this combination be work with just a passive crossover? I want to understand whats going on so I've ordered Weems' "Designing, Building, and Testing Your Own Speaker System with Projects". Hopefully I'll get a better understanding of passive crossovers from this. It would be great to be told what crossover components to start with for these 2 drivers as a starting point. Fortunately this is something Ill be able to play with over time, but input is very welcome. Suggestions about good sources for audio quality electrical parts in the EU are welcome.


I'm hoping I'll be able to build something that makes real music with this, so thanks in advance for help!
Andrew

p.s. Im also preparing a Fostex FE126eN A126 build, Dave is working on definitive plans, then I'll get cutting! I expect it will be interesting to compare these two speakers when they are done.

[MJK]

Run the simulations to answer your questions. In my opinion that is the best and only way to do the design work.

[holdent]

I ran the simulation assuming an active filter (I know you specified passive) and it looks very good. The H-frame for the Alpha 15A is 15" long and internal dimensions are 17" x 17". I tried a baffle with the CHR-70 mounted on top of the H-Frame that is 18.5" wide by 19" tall with the CHR-70 mounted 7" in from the edge and 14" up. The active x-over was set to be a 110 Hz low pass and 400 Hz high pass with 2nd-order Butterworth filters.

[andb]

Thanks, holdent. Good to know that the pair makes sense, so I'll continue to look into the design.

After thinking about it, I'll have both the chipamp and the tube amp, so maybe using something like a Behringer CX2310 to dual amp might make things easier.

[ra7]

Yup... those two drivers should work well together in a MJK style OB.

[kees52]

Hi There

I have also the sheets, but are still green behind my ears what concern the use.

My idea was two eminence bass speakers each channel, but how to fill that in, do I have now change the H frame to 17 x 34 inch? or still use single H frame for each speaker .

I have the eminence alpha for this.

thanks.

[Rudolf]

Originally Posted by kees52

My idea was two eminence bass speakers each channel, but how to fill that in, do I have now change the H frame to 17 x 34 inch? or still use single H frame for each speaker .

MJK talks about two drivers in one baffle in this small PDF, which is a bit hidden on his web page. Basicly you use the double Sd, but put it in a frame with the dimensions for two stacked drivers (17 x 34 inch). It looks funny in the graphics, but should work alright.

Rudolf

[kees52]

Thanks

with playing it go now a little,my is 17 x 36, with two eminence beta 15a I get to 25 hz 85dB but have a bump of 95 dB at 40 hz, simulate stuffing on the walls do make a very nice graph without the bump so maybe there are some refections, I think the hframe needs to not see each other but that cannot be simulatied.

See that I have to put in some changes so I can always fill in single driver but mathcad see two drivers in the to driver sheet I make.

Oke, but I have to learn more first so I put in duoble vas etc.

[kees52]

I have did this (see pdf) after first learning some, I am still green behind the ears with Martins sheets, but get slow the ping feeling.

Anyway, I have use two visatons wsp26 s speakers in a single H frame and a fostex full bandwith speaker.

like to get some ideas reactions, because I have a H frame without a baffle between the upper and lower woofer, normal stacking has there a baffle but the sheet don,t work with that anyway.

[Rudolf]

Originally Posted by kees52

I have use two visatons wsp26 s speakers in a single H frame and a fostex full bandwith speaker.

Try to work with better precision. You got the divided Re of the Visatons wrong.
Why didn’t you enter the proposed “theoretical values” as Actual Component Values in the “Crossover Definition”?
You might lower the low pass filter frequency or raise the high pass filter frequency to avoid the response bump at 250 Hz.

like to get some ideas reactions, because I have a H frame without a baffle between the upper and lower woofer, normal stacking has there a baffle ...

If people put a brace between the upper and lower driver, it is only for mechanical reasons (and recommended). There is no acoustical necessity for it. And leaving it off doesn't change anything acoustically either.

Rudolf