Jamo R909 Passive Open Baffle Project

[josephpo]

I just received lots of speakers in the mail. A pair scanspeak revelators, a pair seas excel drivers and 2 pairs eminence alpha's 15". I'm going to build a something similar to the Jamo R909 using passive crossovers. I think the Jamo design is ingenious, it matches 2 high effeciency 15" woofers with a low effeciency mid and tweeter using a large inductor to correct for open baffle rolloff.

Anyone interested in providing me with guidance (I'll need lots of it) who is willing to conbribute and learn from my mistakes is welcome to join me in this project.

[dogbert]

If you go to John Kreskovsky site he has developed a spreadsheet that has the tools you need to design a dipole. You will just need to build some baffles and take some measurements of your drivers in them. If you don't already have a measurement system you can always download Speaker Workshop.

http://www.musicanddesign.com/A_B_C_Dipole.html

[el`Ol]

Here are guidelines for passive OB speakers:
http://www.audiocircle.com/circles/i...?topic=41297.0
The basic idea: a 6dB lowpass (series inductance) with very low cutoff (high inductance) that shelves the 6dB rolloff, the real cutoff frequency is defined by the baffle width. The drivers must have a good FR in the higher regions because of the 6dB cutoff. The guy at audiocircle used low Qts because his air coils have high resistance and therefore influence the Qts quite a lot. With standard iron coils I would recommend the Hemp 15" OB driver.
The Alphas both have too high Qts and too bad top end for this design.


Greets, Oliver

[grantnsw]

...just a very interested onlooker! best wishes to you Joseph...grant

[Ang]

apparently MJK used the alphas w/ good results http://www.quarter-wave.com/Project07/Project07.html

[el`Ol]

MJK uses huge baffles for the Alphas.
The Hawthorne augie works more or less in narrow baffles with conventional x-over.
http://www.audiocircle.com/circles/i...?topic=44258.0

[Shaun]

Hi josephpo

Do also post your request over at avforums.co.za. There are some members there experienced in dipole design.

[josephpo]

I have been very busy playing with different crossover designs, modeling them in Speaker Workshop and building them. I currently busy with design no 5. After every design I had to fix inherent flaws but every time with better results. A lesson I learned the hard way is that the choice of the low range drivers is critical to ensure a reasonable efficient speaker, it is possible to trade in a higher bottom end (F0) for efficiency. Admittedly the Alpha's might not be the best choice for this speaker but it was locally available. Unfortunately it is very expensive to ship these large woofers plus not every web retailer is willing to ship to South Africa. I will thus have to work with the limitations of the Alpha's. I'll be posting some pictures and designs once I've got reasonable results.

[sreten]

Hi,

Consider using the Alpha's in a 0.5 way configuration, i.e. the lower
driver has a larger series inductor - may ease crossover design.

You should also add trap filters for the 2.5Khz peak IMO.

/sreten.

[el`Ol]

Here is MJK`s version with the Alphas and 12dB passive x-over:
http://www.quarter-wave.com/General/OB_Design.pdf