Beyma 8CX300Nd/N crossover re-design - AVS Forum
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post #1 of 22 Old 07-28-2014, 07:30 PM - Thread Starter
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Beyma 8CX300Nd/N crossover re-design

Blame beastaudio for this one!



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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post #2 of 22 Old 07-28-2014, 07:34 PM
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post #3 of 22 Old 07-29-2014, 01:31 AM
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Excellent, I look forward to the results. FYI, I have an off the shelf plan B - here

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post #4 of 22 Old 07-29-2014, 07:04 PM
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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.

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post #5 of 22 Old 07-29-2014, 09:02 PM - Thread Starter
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It does have cone breakup between 4-7khz as seen in the graph below which shows the native woofer response (no xover) at 0-40 degrees in 10 degree increments.
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post #6 of 22 Old 07-30-2014, 10:31 AM
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Sweet!!! So, still think it will be a good option with the custom XO, or are there better options available. You can shoot me straight Seems the stock XO still does a pretty decent job if I do say though...

(European models do not accept banana plugs.)

 

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post #7 of 22 Old 07-30-2014, 12:47 PM
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Quote:
Originally Posted by mtg90 View Post
It does have cone breakup between 4-7khz as seen in the graph below which shows the native woofer response (no xover) at 0-40 degrees in 10 degree increments.

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.

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post #8 of 22 Old 07-30-2014, 12:56 PM
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Rick, you should have told me! I could have shipped you my other one as I disconnected both rear surrounds after I sent one off the Matt! Lemme know and I could still ship you one, just the driver itself, or the box and driver combo if you wanted

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post #9 of 22 Old 07-30-2014, 01:06 PM - Thread Starter
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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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post #10 of 22 Old 07-30-2014, 01:21 PM
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Wow what an improvement between the two crossovers.
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post #11 of 22 Old 07-30-2014, 01:24 PM
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So listening impressions? Have some fun with it

(European models do not accept banana plugs.)

 

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post #12 of 22 Old 07-30-2014, 01:34 PM - Thread Starter
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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.
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post #13 of 22 Old 07-30-2014, 01:38 PM
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Awesome! How bad would it be to pop this design into a box with a much larger front baffle, or even to flush with the wall?

(European models do not accept banana plugs.)

 

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post #14 of 22 Old 07-30-2014, 01:47 PM - Thread Starter
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You have EQ right


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.
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post #15 of 22 Old 07-30-2014, 02:24 PM
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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

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post #16 of 22 Old 07-30-2014, 04:48 PM
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Quote:
Originally Posted by beastaudio View Post
Rick, you should have told me! I could have shipped you my other one as I disconnected both rear surrounds after I sent one off the Matt! Lemme know and I could still ship you one, just the driver itself, or the box and driver combo if you wanted
Sure - feel free to send me a driver and box.

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post #17 of 22 Old 07-30-2014, 05:29 PM
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My enthusiasm for this driver has waned a little after I found this "in ceiling" model from JBL


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post #18 of 22 Old 07-30-2014, 07:13 PM
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The 6-8khz area has me slightly concerned but other than that, response looks pretty darn good. Keep in mind this design is "in-box" as opposed to in ceiling, and the response was gated so the low end should improve to around 100hz or so as I measured without any major gating.

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post #19 of 22 Old 07-30-2014, 07:19 PM
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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

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post #20 of 22 Old 07-30-2014, 07:28 PM
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Nice job Matt. I like what you did with the big tweeter resistor to allow for small caps. Nice trick.

No woofer break up? I was wondering about that myself. Being an 8", that lower frequency stuff might be more diffraction off the horn than break up. Hard to tell with FR measurements.
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post #21 of 22 Old 07-30-2014, 07:34 PM
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Oh yeah...subscribed.

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post #22 of 22 Old 07-30-2014, 08:09 PM
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Quote:
Originally Posted by Elill View Post
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
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!

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