[panomaniac]

Before JBSpeakerman get his 2 cents in, I'll just remind you guys that the CX38 series uses the big 1.4" compression driver, the CXA series the smaller 1" exit driver.  My guess is that the 1.4" CD is the BMD740 and the 1" the BMD440.  (the CD drivers on the coaxes may be slightly different)  That's got to be part of the cost issue.

The big BMD750 certainly works well for the 18" coax because it can be crossed so low. And that big 18" cone/waveguide lets you do so.  As to whether it's an advantage to have that 1.4" CD on a 12" coax, I can't say.  For power handling it will be, but maybe not for hi-fi.  You won't be able to cross it as low on the 12" as you can on the 18 anyway, so some of the big CD's advantage is null, in that case.

I've almost never heard a 1" exit compression driver mate well with a 15" woofer. 1.4" exit CD, no problem.  But with a 12, you should be OK with a 1" exit CD.

[JBspeakerman]

Hello all....

Just composed a nice post about the viability of the BM-18CX38 as an OB driver and after some distractions it timed out and I lost it all ....  Hate it when that happens... A Monday thing I guess.

Anyway... will try to redo now....

The main catalyst for me choosing to work with this driver is rooted in the my original design goals...

To build a passive... medium/high efficiency OB that could be as dynamic as possible... Musical.... practical for most residential environments.   It needed to be simple and something you could hook up to just about any competent amplifier... heck receiver and really enjoy what you hear.   Liked the coaxial designs horizontal alignment advantage and knew to get the dynamics and bass extension I felt were needed ... a single driver just would not get me there.   It had to be a two way at minimum.  And to make it compatible with as many sources as possible... it needed to be ALL passive.

What makes for some of the magic is how the woofer sections (the pair I have anyway) measure.   Raw free air QTS is .45... mount it on a 1,000 or so sq. inch baffle will shallow wings and you get to .52.  Add in the needed and substantial inductance to tilt the midrange down the appropriate amount for passive OB use.. you get to .70 or so.    Then there is a FS in the mid 30's.  And large area (1,250 sq. mm.) cone and wave guide.  Xmax is 4.5 but BL is pretty good out to 6 mm Peak.   This is enough woofer to play loud and flat to below 40 Hz.  Loud being 105-107 db.   The wave guide loading that big 16" diameter cone provides will load the HF CD driver to about 750 Hz.   It goes on and on.   

Like any design there are compromises and draw backs... including but not limited to a large - substantial baffle, lack of bass below the mid 30's... 6 db or so of the woofers natural efficiency being traded away in the crossover design...  and so on.  Even with the 6 db loss... you are still in the 94-95 dbw area. 

As for the HF CD... it has an FS in the low 500's and is very smooth.  As smooth as any Altec... Eminence.... JBL .. Radian or other CD I have ever heard.   It is particularly smooth and musical on the lower end.   And it really does reach up to above 18K on axis.  The key to this thing coming to life is ... as with most speakers... cross over implementation - design.   

As for the results... and I have to admit this is still a work in progress.... but this driver really is dynamic... articulate... musical and just plain fun to listen to.  I have been listening (and from time to time) designing OB since early high school (like 45+ years) and I have heard few designs of any kind that can compete... again given the design goals.   Are these the last word in OB performance... of coarse not... but when you factor in their simplicity, cost and flexibility (no sub... no special electronics.... just a pair of speaker wires to fool with) they are IMHO remarkable.   How do they measure... don't know.   How do they sound... Like music...LIVE music.   Better than they have a right to... but there it is.   Are they for every one... heck no.  Have they been fun to build ... YEP!!

I will be happy to answer any questions I can... still learning here myself.  Hope this has not been too long.

JB Speakerman

[ecir38]

So Jb, how close are you to getting the crossover where you like it?

BR

[opnly bafld]

So Jb, how close are you to getting the crossover where you like it?

BR

 

Likes it or is happy with it?

He likes it now. 
He will never be happy with it. 

Lin 

[ecir38]

Well said lin. I think you got that right being you are one that has heard his ob and met JB  .

BR

[JBspeakerman]

Hello Gents....

Some one had to ask the $64,000 question.....

Funny... had a discussion earlier today with Russell Dawkins about the 18's and the subject also came up.... the conclusion being that it is difficult to know when you are as "there" as you are going to get.

But there are some performance areas that need some more work... no doubt about it.  Have my Manzanita references out and may also get my Altec 605 OB's back up and running .... again as references.... the 605's with a helper 18 and sub I took to LSAF in Dallas this year are very good and will keep me on track.

So... you guys pegged it... "like" for now... but a lot of this is about romance... may never be able to love and be completely satisfied... after all it is a moving target! 

Will keep all advised of any major changes.   Also working on a dual 10" passive OB with the Scan Speak D3800 on top.  Again this is a KISS project... rumaging around the parts bins and see what I come up with.  Just curious about the D3800... the TRUCK of tweeters.   It can really play down to 1 Khz.  The real background on this new project is to see ... or help diagnose a bit of edge the Manzantas have.. I suspect the Seas 27TDFC is stressing at the low end of its range. (1,350 Hz).. the D3800 can work
down there with ease.... so checking the reference with another system.  Sounds a bit silly... but hey... any reason to play with another design.

JBspeakerman

[Russell Dawkins]

Funny... had a discussion earlier today with Russell Dawkins about the 18's and the subject also came up.... the conclusion being that it is difficult to know when you are as "there" as you are going to get.

That reminds me of an apocryphal story:

There was a very old man working at Steinway whose job it was to put the final sheen on the piano finish as the last stage of manufacture. He would patiently work away for hour on hour with his French polishing.
One day a visitor to the factory who had spent some time watching him patiently polishing, making circle after circle on the finish, came up to him with a look of puzzlement.

"However do you know when you are finished?" he inquired.
A look of surprise came across the old man's face, as if he had never pondered that before.
"When they come and take it away!" he said.

[panomaniac]

Yes Russell - I think that nails it!

[JoshK]

Magnetar,

Have you compared the Warrior 15" to the War18"?  Do you like the 15" better? 

[AK]

also, Magnetar, I noticed you have experience with PYLE PDW21250 woofer.
On photos it looks like very well built woofer. What do you think of it for dipole use? How high in frequency it can play?
I'll eventually buy a pair. These woofers appear to have best price/performance ratio.

[otto88]

All

I'm rick57 - I tried to update my email address but the activation didn’t work  eventually I created a new account . .


Hi JB

> Just composed a nice post . .  . timed out and I lost it all ....

A suggestion: anything other than a brief post or email, I compose offline in something else (I use
Word), save regularly, then when ready, paste into the forum.


> QTS ... mount it on a 1,000 or so sq. inch baffle will shallow wings and you get to .52. ... 
Add in the needed and substantial inductance to tilt the midrange down the appropriate amount for
passive OB use.. you get to .70 or so

The substantial inductance to tilt the midrange down – that’s to offset the 6 dB/ octave OB rise?
How do you calculate the values?

Cheers

[JohnR]

All

I'm rick57 - I tried to update my email address but the activation didn’t work  eventually I created a new account . .

We have a "login help" forum where you can ask for help to solve these kinds of problems.

I've disabled this new account and re-activated your old one. Thanks

[rick57]

Thanks John, I feel my old self again 

[JBspeakerman]

Rick 57..

Yes the inductor is to mainly offset the 6 db/octave theoretical roll off... but with most large OB woofers once you are a couple of octaves or so below f equal... you usually see more like 9 db per octave drop.  High Q drivers tend to go lower before you see a 6 db to 9 db drop transition.

I do not have a formula to determine the amount of inductance.... mainly seat of the pants kept real by listening to the results & some crude outdoor half space measurements.... helps a bunch that I have been at it quite awhile... and honestly... there are so many variables.... such as driver Q factors... driver impedance...baffle size & diameter.... wing depth... how high the woofer can or in some cases, needs to play to properly mate with what ever midrange is at hand and so on.  You can get some rough Idea where to start by modeling a sealed box woofer using the native woofers QTS (including baffle gain as well as adding a healthy RG ... say an ohm or so) and use the amount of inductance required to get the woofer to yield a flat response assuming a QTC of 0.5.  (at it's low end)  With a 0.5 QTC sim you see the woofer start it's shallow roll off up pretty high.  I assume a box size to be essentially a infinite baffle.  Also assume a first order solution to flatten the 0.5 QTC simulation up to the beginning of the roll off point... no shunt cap.  After that.... as above.... back to the seat of the pants.  Then it gets tricky and combinations of Zobels and shunts (usually asymetrical) to fine ture the overall response to suit the application.   Hope this make sense..

JB

[rick57]

Thanks JB. That mostly makes sense.

I'll look at this more clsoely when they're on the actual baffle 

Cheers

[chakija]

I think this one could be pretty good candidate for best value OB woofer. I think that Lukas Lampizator Fikus is using them.

[rick57]

I’ve picked my drivers, but had a look:
Sd of 1392 and Qts of 1.04, but Xmax only 4 mm. .
Only in Europe? How much?