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Beyma 8CX300Nd/N crossover re-design

17K views 27 replies 9 participants last post by  beastaudio 
#1 ·
Blame beastaudio for this one! :D



The Beyma 8CX300Nd/N is a nice 8" coaxial unit with an external horn for the HF driver rather then using the woofer cone, while this does eliminate some problems with the transition from internal HF flare to woofer cone but there are diffraction problems associated with the "floating" waveguide and the small size of the horn causes it loose pattern control starting around 3k and it is complexly lost by 2.2k.


Now the stock crossover does an ok job, the drivers themselves don't sound too bad and they do use padding on the HF driver so that it is not screaming at you but the level is still rather high with a peak about 5dB hot on axis at 5.5khz. I did not measure the stock crossover point because the crossover was located inside the cabinet and disconnecting one of the driver wires one at a time would have involved more driver un-mounting and mounting then I felt like doing at the time. But if I had to guess I would say it was around 1.8k. This is one of the shortcomings with the stock crossover unit, that crossover point is well below the pattern loss frequency of the HF horn. The off axis response graph I took with the stock crossover shows the extreme effect this has on the polar response (1/6th octave smoothing). I have also attached a graph showing the stock response at 20 degrees (1/24th octave smoothing) which is what I chose to be my design axis for the new crossover, you can see at that one reference point it does not look terribly bad, maybe the HF is a little elevated. Both graphs are taken indoors 4pi with gating applied so the LF rolls off a little early.


Lets see what can be done...
 

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#4 ·
I would suspect that if the stock crossover is at 1.8K then the designer chose that in order to minimize the upper end breakup modes in the woofer. These types of drivers often have issues with not enough overlap between the woofer and tweeter.
 
#7 ·
Thanks for the woofer graph. There's an edge resonance at 1.4K (5db dip) then the first breakup peak at 2.2K - not an easy combination to work with. I would love to take a crack at it for the challenge.
 
#9 ·
Here is what I came up with, attached are the new schematic, parts list (xover BOM), off axis plots, 20 degree plot showing individual woofer/tweeter response & combined. and comparison of the crossovers at 20 and 60 degrees.
 

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#12 ·
It sounds good, flattening it out above 1k really helped tame things, those on axis HF peaks are still present but they are far less annoying now. The little bit of BSC also helped bring out the lower midrange detail, before it was really lacking there. Overall much improved, I think you will like it.
 
#14 ·
You have EQ right :D


It should not be too bad. I only used a little bit of BSC knowing these are going to be wall mounted so placing it in 2pi should not cause any problems with to much or bloated bass. I am unsure how much things will change on the upper end of the woofers response due to the larger baffle size but that is where EQ comes in.
 
#15 ·
Just thinking out for other designs. I know I have one plan for a close the wall mounted flush design, among some other slanted designs as well as what others might be interested seeing done with these. Love being able to build little boxes and get multiples out of one sheet of ply. Big change from building 24" subwoofer boxes that need 3 sheets+ per cabinet. Quite nice :)
 
#19 ·
Yep, agreed. FYI - those measurements are in a 1 cubic ft backer box.

Do you think we could EQ that 7k dip? I don't have any experience with CD's

I don't think its a huge deal, given it is for rears/surround, plus they're cheap and its a screw in a box proposition instead of stuffing around with x-overs
 
#22 ·
The dip is pretty wide across that area but is high enough that you could handle it I think. Problem there is the response from there is still rising and I would prefer a slightly sloping down response. Could still be handled, but.... AFA the 1 cuft box, I am guessing they still did that with a very wide baffle design that helps the low end a shade. enough to make a difference, probably, but I know the beyma can get as low as that graph does in a 1 cuft box, sealed. The test box I sent to matt was only .6 and he had the gating on as well in those graphs!
 
#26 ·
Still plugging away at surround duty :) Some of the guys got to hear them in stereo mode at my last g2g when we pulled them and brought them upstairs for a bit. I really like how they perform, but am looking at some other ideas right now with a little larger dispersion pattern that might be more optimal for surround/atmos duty. These perform fantastically as rears, and how enough power to really reach all the way to my front row which is over 15 feet away.
 
#27 ·
what's the lowest those beyma drivers resonably play?

For instance - could you use them with a 10" sub on an a large active design bookshelf speaker?

Say I wanted to play with that Beyma driver powered by 1 channel of an iNuke DSP 3000, and a 10" sub powered by the other side with the built in iNuke DSP providing all EQ for both channels?
 
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