Anyone used Beyma or B&C coax 8" drivers? - Page 2 - diyAudio
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Old 6th March 2005, 05:20 PM   #11
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Quote:
Originally posted by Pallas
My understanding of Dr. Geddes' position is that coaxes are kind of dumb, anyway. (I disagree with him, though I've never heard his OS horns.) His comments about B&C were about their compression drivers not their coaxes, and the BMS coaxial driver to which we was comparing them was this one: http://www.bmspro.com/products/4590.html. It has a 1" ring radiator coincident with a 2" ring radiator.

My take on the discussion matches the above - they were discussing compression drivers, not conventional coax drivers.

Pallas, when considering Geddes position on coax drivers, you have to understand that his whole focus is on pattern control, and achieving constant directivity as far as possible. Typical coax driver arrangements make this pretty much impossible as they impose geometrical constraints that are at odds with pattern control. It'll also probably generate more high-order-modes, but I'm still a bit fuzzy on the math on that one.

For the 'best' approach to conventional coax drivers, I suspect this can't be beat:
http://www.eaw.com/products/AX/CSA_technology.html


Quote:

Maybe I need to hear more horns, and especially oblate spheroid ones, but I was astounded by the performance of the 4590 on 90x40cd horns.
The 4590 is pretty well regarded, so this doens't surprise me. However, once again keep in mind that Earl is after constant directivity, and the BMS coax arrangement doesn't lend itself to this, since the radiation pattern at the throat will be non-uniform, and hence won't mate too well with his OS waveguides.

So, just because Earl doesn't like/use it doesn't mean it won't perform well. His OS waveguides do look theoretically attractive, but to a degree they're a 'finishing touch' - about getting that last 5-10% of the performance out of the system.

Quote:

I also think onethis 15" cone + compression driver BMS coax looks on paper really neat for a big home monitor for use with a subwoofer, though I haven't heard it.
Yeah, I think those are interesting as well. I'm not sure how well they'll work in the nearfield, though. I suspect the cone relies to a degree on diffraction around the horn to fill-in the center of the soundfield, and it might need a bit of space to develop. Still, with a 60degree conical horn, it should mate reasonably well with the directivity of the 15 at the 1-1.2k xover, so in a larger-ish room it might work very well.


My personal track has put my Unity-concept experiments aside for the moment (I think the Unity is truly the ultimate coax-arrangement, but it's tough to get right) and am looking at something like this EAW setup: http://www.eaw.com/technology/proprietary/va4.html I'm looking at using a waveguide-loaded conventional tweeter though, to minimize the cross-section impact on the midhorn. Preliminary experiments are very promising - with my DCX2496, I got something that sounded way better than my Unity experiments after a couple hours of fiddling.
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Old 7th March 2005, 02:46 AM   #12
Pallas is offline Pallas  Pakistan
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I always thought that pattern control was one of the reasons why I almost always prefer coaxes over separate drivers. (I've not heard many GOOD systems with low-cutoff horns crossed over low to high-quality pro-audio woofers, as Dr. Geddes sells; the only one I really liked was the aforementioned 90x40 BMS 4590 with a 15" Beyma woofer and a huge Cabasse subwoofer. I would love to hear the Gedlee speakers....)

I like big coaxes much more than small ones, but even when I was shopping for inexpensive bookshelf speakers in the late 1990s the only one that sounded like it had decent pattern control (no horns in the power response) was the KEF Q15, and accordingly that's what I bought. I've also heard the same from Gradients, which use a Seas variant of the KEF Uni-Q.

Also, the Tannoys that Gordon W. introduced me to at Audio Atlanta and the DIY2003 gathering (the Tannoy D500's were the star there for me) sound like they have very good pattern control, with a consistent soundfield as you move around the room that differs in treble extension but not basic character.

Those EAW speakers look very interesting. Check your e-mail
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Old 7th March 2005, 09:31 PM   #13
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I think a coax can be engineered to have fairly uniform/smooth power response, but remember that Geddes is an academic/theoritician, and so 'good' isn't necessarily good enough.

Done properly, it should be possible to put the xover in a coax at the point where the directivity of the woofer matches the tweeter. If we follow the Geddes model, this would be ~1k for a 15" and maybe 1.5k for a 12".

I think the practical problem with this is that in many coax drivers the tweeter horn isn't large enough to provide true directivity down to the low end of the spectrum. On my Tannoy System 600s for example the tweeter 'waveguide' is only a couple inches long at most, and only maybe an inch in diameter. Contrast this to Earls waveguide in his speaker, which is 15" in diameter. This is one reason why those BMS drivers look interesting - the horn might actually be big enough to actually work.
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Old 10th March 2005, 02:47 AM   #14
GordonW is offline GordonW  United States
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WRT: the Tannoy tweeter being relatively low power handling- I can attest to that. I actually managed to kill one element, during "abuse testing" of the speaker I did. Mind you, it was at levels that pretty much NO ONE would find tolerable for more than a few minutes in a home setting (unless you, like some people around here... *cough... GM... cough* happen to have had Danley servo-subs in your home ).

For the application I've been designing for, the Tannoys seem to be pretty much up to the task, power-handling wise. I work for a dealer that has carried Tannoy DC-drivered speakers for the last 15 years, and I can count on one hand, the total number of replacement tweeter elements we've ever had to order for the Saturn, Definition, Dimension, Sensys, Eyris, Churchill, Prestige or any other dual-concentric. I'm talking hundreds of pairs that we've sold, in use, with only 3-4 total tweeter failures... and some of these speakers are owned by DRUMMERS, for heaven's sake! (insert obilgatory apologies to any drummers reading this who don't fit into the normal musician's "drummer jokes" category, of course... )

Hence, my thoughts are to possibly take something like a Beyma or B&C driver, and modify it to use a directly-coupled dome element into the back of the bass driver magnet assembly (possibly including maching out the waveguide, to reduce the compression ratio, and such). There are some pretty efficient dome drivers out there, that behave reasonably well (Fostex, Vifa, etc)... and with the fact that the waveguide/horn load provided by the flare inside the coax, INCREASES their low-frequency output substantially (this is what allows Tannoy to use a 1.4KHz acoustic crossover point... it's about a 4kHz electrical network)...

Specific questions come to mind, about the B&C coax- is it a curvilinear cone, or straight-sided like the Beyma? I'd much prefer to stick to a curvalinear... as it seems to GREATLY improve the tweeter on-vs-off-axis energy dispersion, and makes the frequency response of the tweeter more "linear" (in the sense, that a crossover can be designed with minimal components, to achieve flat response... not many "fixes" are required to compensate for response discontinuities and drop-offs).

I'm also tempted, to see if Eminence might be able to build the Beta 8 CX into some sort of cast basket, one with low-profile basket legs. This would greatly help me decide the Eminence is a good idea- currently, with the stamped steel basket, there is far too much flat metal in the basket legs, reflecting sound back at the woofer cone...

I'm also not ruling out the possibility, of building/reconing/modifying a driver to do what I want. I've reconed enough speakers to be confident in my ability to assemble driver moving assemblies, pretty reliably... and I will only get better, with more practice, I would expect. So, even if I wound up with just the frame of something like a Beyma or B&C driver, and made up a custom cone assembly for it, that could possibly be a viable alternative...

Basically, my point here, is to try to find a driver where I'm not at the mercy of "spare parts" from a manufacturer NOT in the business of building raw drivers (but instead, finished systems). In actuality, I got TWO of the THREE spare-part Tannoy TDC 8" dual-concentric drivers in the western hemisphere, apparently... that kind of makes any sort of repeat build, rather difficult...

Regards,
Gordon.
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Old 10th March 2005, 02:56 AM   #15
GordonW is offline GordonW  United States
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Quote:
Originally posted by dwk123


I think the practical problem with this is that in many coax drivers the tweeter horn isn't large enough to provide true directivity down to the low end of the spectrum. On my Tannoy System 600s for example the tweeter 'waveguide' is only a couple inches long at most, and only maybe an inch in diameter. Contrast this to Earls waveguide in his speaker, which is 15" in diameter. This is one reason why those BMS drivers look interesting - the horn might actually be big enough to actually work.
Well, if you really get into the Tannoy DC drivers, the tweeter does hold up pretty well, right down to 1.4KHz, where it "hits the wall", at least on the bigger (8" and larger) DC models. This is, as far as I can tell, because the tweeter actually uses the neck and body of the cone, beyond the waveguide (the cone flare does almost exactly match and continue the flare of the outside of the waveguide), and bolsters the bottom end response and pattern control. In actuality, the 8" DC driver acts as if it's got an 8" round conical horn attached to the tweeter... which easily can hold pattern control down to a pretty decently low frequency. It's still got a "mouth radius" of a half-wavelength, at 1.4KHz. Especially if, like in the Tannoy Prestige models, the driver itself is mounted into a "flare exension" which further extends the "flare"... this further extends the lowest point where pattern control can be upheld...
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Old 10th March 2005, 06:29 PM   #16
Pallas is offline Pallas  Pakistan
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Quote:
Originally posted by GordonW
Hence, my thoughts are to possibly take something like a Beyma or B&C driver, and modify it to use a directly-coupled dome element into the back of the bass driver magnet assembly (possibly including maching out the waveguide, to reduce the compression ratio, and such). There are some pretty efficient dome drivers out there, that behave reasonably well (Fostex, Vifa, etc)... and with the fact that the waveguide/horn load provided by the flare inside the coax, INCREASES their low-frequency output substantially
FYI, RCF Acoustics has an 8" coax designed to accept a conventional dome tweeter. It's called the PAC 81 CX, and has a cast frame with a carbon fiber cone that looks to have some curvature to it. IIRC, Solen carries RCF, though I don't know if they carry this specific driver or how much it costs.

As you mentioned, the waveguide substantially increases the low end of the dome. According to the Hobby HiFi review of the RCF piece (2/2002, pg. 62-3), the included dome tweeter (of which they weren't complimentary) rises ~12dB from 5kHz to ~1.4kHz, below which it falls off like a rock.

As for your other question, I'm 99% sure that the B&C 8CX21/Hi-Level RX200 has a straight-sided cone.
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Old 12th March 2005, 05:04 AM   #17
GordonW is offline GordonW  United States
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Huh, that RCF sounds interesting. Especially since RCF now is, for all intents and purposes, an "independent" speaker manufacturer again (once they split away from Mackie). Probably much more likely to keep something like this in the line for a while...

I'll have to check it out... maybe my pro-audio contacts here in Smyrna GA might know where to find RCF more near-to-home.

Regards,
Gordon.
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