Prelim T/S Parameters of Coral 8CX-50

[red76]

This was measured last week using a digital freq. meter, oscilloscope, cd player w/ a test tone cd (burned in cd-r using a shareware test tone generator) that acts as a substitute for an audio freq. oscillator, and an amp with volume control.

We used Elliot Sound Products free downloadable .xls calculator as you see in the screenshot. The screenshot was captured using free trial software.


I have not included Vb, Vc, Fb, and Vas values yet in the screenshot, but will do soon.

My initial plan is to mount these in an open baffle (marine ply) with these dimensions:

Height = 1050mm, Width = 650mm, Driver mounting height = 500m, Driver offset from inner edge = 247mm



If I like the results I may use higher quality wood for the OB. I will also try it mounted in a floorstanding enclosure for comparison - Bill Woods’ TL/Backhorn hybrid using a Karslon type waveguide for the front mouth opening near the driver.

Driver pics were directed to me by setup1 from a Japanese website.. Just to give you an idea of what they look like:
http://www.h3.dion.ne.jp/~roktal/audio- ... x-50_1.jpg
http://www.h3.dion.ne.jp/~roktal/audio- ... 0_back.jpg

[BenC]

Hi Fred,

How'd you arrive at the dimensions of your OB? Looks very interesting! Kelan mo bubuin yan? Padinig pagnatapos ha?

[ichabod]

Ben that was also my question. Maybe Fred can give me its dimensions again in inches lang, but this is oppose or different from the OB dimensions familiar here. I kinda like the smaller size. The OBs we got are similar or close to the Quad 57s, while these are like the 63s.

I have an extra pair of drivers (8s") I would like to experiment on with these dimensions.

[BenC]

Hi James,

Have you tried elevating the OB's? I experimented with heights of 30" and 13". The soundstage at 13" is better from my listening position. At 30" plus the angle of the OB's, the soundstage is unnaturally high ... It's fun doing experiments like this. Give it a try. You might like it too!

BenC

[red76]

Hi Ben and James,

I ended up with these dimensions via the sim I played around with. I tried changing the dimensions by increments and even tried 'farout' larger dimensions but the response at the most audible range just becomes non-linear. BTW, the baffle dimensions somehow also follow the golden ratio... One imporant factor I found out in this sim is that the height of the driver itself, change it a bit higher or lower and the response anomalies are more apparent than by changing baffle dimensions in increments. I will not try any rearward tilt on the baffle first - since my drivers come with diffraction horn tweeters at the center of the cone.

Just to make things clear, the baffle dimensions above happens to suit my Coral drivers, for other drivers other dimensions maybe "better". JE Labs baffle is always a good starting point for experimentation to hear the potential of your driver for OB (universal testing baffle for drivers used by MJ zine) .

The free downloadable .xls spreadsheet/sim could be found here:

http://www.t-linespeakers.org/downloads.html
(spreadsheet written by T. Loesch, he has the same dimensions for his modified Supravox but at a different driver height plus he also tilts his baffle at an angle unknown to me)

The sim has addressed certain boundary reinforcment issues etc. and is a good predictive tool, but of course it still has its limitations like most sims (read link above)... Actual listening and real world conditions is "where it's at" IMHO.

Common length conversions can be done here (eg, mm to inch):

http://www.onlineconversion.com/length_common.htm

[ichabod]

Info much appreciated Fred.

I have a pair of R & A drivers lying idle at the moment for my second OB. The dimensions I saw in your previous post for an OB made me curious. Ben a while back was also talking about experimenting I assume on the dimensions for an OB. So the drawing you posted struck me while I was trying to make up my mind on the "finish" on my next OB. Well straightaway I'd be happy to hear from you by the time you will have had one pair going.

Ben, I would appreciate to hear from you as well along these lines. Regards.

James

PS

Sorry Ben. I didn't see your post after mine, but yes I will try elevating them like you said and see what happens. My listening position being very close, closer than nearfield might seem to favor raising the speakers.

As they are though, I'm happy with their sound. John Keung in HK likes the Tannoy red on his leak amps, but he also says the altec 755As are very good drivers, and he will soon make a comparo between the altecs and the LS 3/5a. Most all of us into OBs are ardent LS 3/5a users as well.

How about a Quad 57 pair? I sure will hunt for these speakers later these year. Like Sir W. Churchill who was voted a long time ago as "Man of the Century" by Time Magazine was it, the 57s are said to be "the" Best Speakers of the Century!

[ichabod]

I'm happy to report guys after a few hours of listen and perhaps breaking in of the R & A drivers on OBs, they sound deep and luscious. I would keep both altecs and Allans along with my LS. As they say, variety is the spice of life. Can't enjoy them if you're strap to one. This is not like marriage folks!

[BenC]

Hi James,

Yes, indeed it's fun not being strapped to just one ... set of speakers! Are the R&A's big drivers?

Cheers,
BenC

[ichabod]

Hi Ben. These are still 8s. Nicely made. Thanks to the Brit's peskiness when it comes to quality. THat goes for sound too! Keep well. James

[T-Rex]

Fred,

Been more than a month since your last post, whats the latest deve?

Rex

[red76]

Fred,

Been more than a month since your last post, whats the latest deve?

Rex

It's been only 18 days since I started this thread.... Look at tube/diy thread on Handyman's projects... Next week baka ma-try ko na itong OB.

[T-Rex]

Oo nga no, wala pa palang one month . Akala ko Mar-03 (eh Mar-30 pala)

Basta si Bon ang gumawa ay maganda. Sige balitaan mo ako. Pero kung gusto me imbitahin mo na rin ako.

[red76]

These prototypes have been here for a week... just uploaded pics now.




Notice that brass hinge at the bottom...


...Bon did some cool innovative stuff here by making the backwards tilt adjustable via nut&bolt plus a hinge... Although, I stuck with 83.4deg.



These are prototype OB's. I most probably will re-cycle some of the wood here and change tha main baffle with better quality ply (acrylic?). The final version would have 2 sturdy feet but not as high w/ shorter length, a fixed tilt angle, and a small wood sub-baffle at the back around the driver down to the floor for better support and damping. OB dimensions would remain the same.

They do have that ESL-like quality but sprinkled with dynamics... These would easily integrate with a good sealed active subwoofer (ideally a plate amp with LT Eq crct. and high level inputs) in an 'acoustically treated' room, but that's diff. subject altoghether. Room acoustics ko nga la kwenta pa....