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Cabinet for Coral beta 8
Cabinet for Coral beta 8
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Old 21st February 2018, 12:22 PM   #1
marekocska is offline marekocska  Hungary
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Default Cabinet for Coral beta 8

Hi everybody.

I'm new to this forum, so maybe it was asked earlier, but I did not find a thread.
So I have acquired a pair of coral beta 8 drivers. I'm now looking for cabinet for them. My goal is to build a not too wide, not too deep cabinet for them. Could be a floor standing box design. Anybody tried such design (bass reflex?) with good results? I don't really want a horn, since I have other horn designs built earlier, but for other drivers (fostex and lowther), and want to try some simple design.
Regards: Mark
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Old 21st February 2018, 05:30 PM   #2
DrBoar is offline DrBoar  Sweden
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Voigt pipe (sometimes called Voigt horn) ?
Or bass reflex along Heco Direct Einklang. Fullrange driver, wide baffle speaker and floor loaded bass reflex ports
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Old 22nd February 2018, 07:37 AM   #3
marekocska is offline marekocska  Hungary
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Hello!
Thank you for your reply. TI like this "Heco Direct Einklang" very much. Are there any drawings or dimensions of the box optimised for coral beta 8? Or maybe somebody has built such enclosure for beta 8?
I'm very interested aout the sound of such setup.
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Old 23rd February 2018, 03:19 AM   #4
GM is offline GM  United States
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Oh Corals! - diyparadise

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Old 23rd February 2018, 10:02 AM   #5
DBMandrake is offline DBMandrake  Scotland
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Not sure how close the T/S parameters of the Beta 8 are to the Flat 8II, but I have my Flat 8 II's in 45 litre bass reflex cabinets with two 75mm diameter ports tuning the box to 41Hz.

A nearfield measurement shows an "early gradual rolloff" alignment vaguely like a QB3 which is -3dB at 50Hz and -6dB at 40Hz, however in room they have good solid, well balanced bass down to about 40Hz in a typical British size living room.

In fact I have -6dB 1/3rd octave PEQ notches at 41Hz, 71Hz and 108Hz to compensate for room modes and there is still plenty of bass down to 40Hz. (By coincidence the lowest room mode is at the same frequency as the port tuning)

I did model the alignment in WinISD Pro years ago but unfortunately I've lost the T/S measurements I took of the drivers and can't be bothered measuring them again as I don't want to pull the speakers apart!
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Last edited by DBMandrake; 23rd February 2018 at 10:10 AM.
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Old 23rd February 2018, 10:43 AM   #6
marekocska is offline marekocska  Hungary
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As I know the T/S parameters are not so close. Regarding factory cabinet drawings, in a bass reflex cabinet, flat 8 needs 38 liter capacity, but Beta 8 needs 94 liters. It is more than twice the volume! Beta 8 needs even bigger box than flat 10 (57l vs. 94l)! I'm thinking about building a cabinet of the factory box size, but with modified proportions to be a floorstanding cabinet. I have a pair of Flat 10 drivers too, and building a "factory" Bass Reflex box for them. How do you like the sound of your speakers? What amp do you use?
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Old 23rd February 2018, 12:15 PM   #7
DBMandrake is offline DBMandrake  Scotland
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Something to keep in mind is the surround stiffness and therefore T/S of these drivers vary a lot from one pair to another, and partly due to age.

I've had my Flat 8 II since 2003 and the surrounds are still in excellent condition, nice and supple and have plenty of xmax (about +/- 3mm) and produce bass well.

I have another similar pair of Flat 8A and the surrounds on them are so stiff and hard that they basically can't produce any bass below 100Hz - they need the surrounds replacing if they ever have any hope to produce bass.

Because of these wide variations in T/S parameters the original coral recommended cabinets may not be applicable.

I would not use a cabinet smaller than 45 litres for the Flat 8II - this is on the lower acceptable size limit and this already gives an overdamped early gradual rolloff alignment. (Although I like this)

From what I remember of the WinISD simulations I did years ago the bass response can be extended considerably by increasing the cabinet volume to about 60 litres and re-tuning the port - this will give a more traditional butterworth alignment with a 3dB point of around 35-40Hz.

I designed a brand new floor standing cabinet of this size (the current cabinets sit on small stands to get correct ear level height) but never got around to building them.

If you do a search on the forum for me talking about coral's you'll see lots of measurements and also some driver modifications I've done.

I improved the cone damping with a pattern of self adhesive strips on the outer part of the rear cone - this gives a lot smoother response in the 2Khz to 5Khz region.

I like the way these drivers sound a lot, I've used them in many configurations over the years since 2003, including using them as a wide range midrange driver in a large 3 way system, as a full range driver by itself, (in the current cabinet) and as a midbass driver in a 2 way system also in the same cabinet.

I'm currently using them as a midbass driver crossed over at 3Khz with an Aurum Cantus G2 ribbon using a passive crossover. Here is a not very good picture taken recently while I was taking measurements for the crossover design:

Cabinet for Coral beta 8-img_0296-jpg

Two 75mm ports are on the rear at the bottom and top. Previously the bottom port was on the front panel but when I re-veneered the outside I decided to move the front port to the rear panel as front mounted ports on 2 way systems put out a lot of spurious midrange. (There is a large port resonance at about 700Hz which was easily audible and measurable when it was on the front)

The crossover (designed in Vituixcad) ended up fairly complex but I'm extremely happy with the results and while the crossovers are still outside the cabinets and having minor tweaking done over a long period of time they are otherwise finished and in daily use in the living room.
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Last edited by DBMandrake; 23rd February 2018 at 12:25 PM.
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Old 24th February 2018, 06:31 AM   #8
marekocska is offline marekocska  Hungary
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Simon, what amp are you using? In my experience with SET amps, any kind of crossover or inductive/capacitive filters just kill the presence in music. That's why I don't want to use multiple speakers, only one. Now I use Lowther DX4 drivers in accousta horns, but after playing around with them (I had at least 4 horn projects) I don't really like horns. I'll check out your modifications of coral flats, I highly appreciate this kind of innovative thinking
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Old 24th February 2018, 06:54 AM   #9
DBMandrake is offline DBMandrake  Scotland
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My current amplifier is a Luxman A-371 which is a 55w per channel solid state amp with low output impedance. The crossover is designed with the low impedance in mind.

I don't agree with the "crossovers kill the presence in the music" that a long of full range driver advocates ascribe to. You just have to get the crossover design right. (and it isn't easy)

The fine tuning process of the crossover to get it just right can be quite time consuming and tricky. I started with the simulated design I did in Vituixcad based on raw driver measurements, and the initial results right out of the gate were surprisingly good but weren't quite right.

I found there were a couple of components that were too far out due to tolerances, fixed those, but it still wasn't quite right.

I ended up increasing the baffle step coil values a bit as the design was based on free standing measurements and I have the speakers a bit closer to the wall than that. Presence still wasn't quite right and I ended up increasing the first cap value on the tweeter slightly (4th order L/R) from 3.4uF to 3.6uF which has lifted the presence region about 0.4dB - now it sounds pretty much spot on.

Very small changes like this can be needed to get subjective impressions of imagine right and I'm not sure that a lot of people go to the extra trouble to do this.
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Old 24th February 2018, 06:20 PM   #10
marekocska is offline marekocska  Hungary
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I don't know the Luxmans, but I've never heard a solid state amp that did not kill the presence. I don't like the most these two things: crossovers and solid states
I'll keep you posted how the Flat 10 in factory box sounds...only I have to wait for the box to be finished.
I'm using an Unson Research S2K amp with NOS tubes, but have a 2A3 DIY amp with custom amourphus core output transformers, and a WE91 clone with NOS 6C6 tubes and KR 300BXLS output tubes, custom nanocrystalline core OPT's, full mundorf and Black Gates, Tamura amorphous chokes. I'll have at least four sets of full range speakers to test: Enabled fostex 125WK in a fonken cabinet, Lowther DX4 in acousta horn, Coral Flat 10 in factory BR cabinet and Beta 8 in redesigned BR cabinet. Yum-yum Maybe I'll also order from my carpenter a set of "factory" back loaded horns for the beta 8
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