Three way OB with Isophon p203c

[zmyrna]

I have been dreaming about building my own OB speakers for 3 years.
My inspirations are Visaton No Box, OB plan on Supravox website and Lampizator.
I have looked at Supravox and Tone Tubby drivers for the wide ranger that will be at the heart of my project.
However when I saw these,

eBay - The UK's Online Marketplace

and could not resist and purchased a pair for £30 plus shipping.
They are new old stock Isophon p203c 8" alnico drivers, circa 1969.
And to my surprise, they arrived in sealed original boxes!!!!
It was a great experience to open the boxes and handle them for the first time in 40 odd years.
And here is the info I could find about them so far: 4,5 ohm, Fs=65, 6 watt, and probably Qts of 0.5 to 0.8 because they are made for open back boxes.
Vintage Saba and Isophon drivers are Lmpizator's favourite because of their high efficiency (95-99 db), light paper cone and alnico magnet.
It looked too good an offer to miss at this price and condition.
I have now listened to them a few times connected directly to my amp (no crossover, no baffle) and they sound beautiful. Excellent open soundstage and dynamics esp with acoustic music and vocals. I could live with them as they are if I was to listen only to that type of music. I was not expecting any bass but instruments do have a nice body including even the acoustic bass.
Now my plan is to give them a helping hand in bass and treble in a 3 way system. I plan to put the Isophon and the tweeter on a narrow baffle of about 100x28cm. And there will be 2 woofers on a separate 70x50cm baffle which will be attached perpendicularly to the bottom half of the front baffle.
The bass baffle will be a shallow H baffle with the front baffle forming one of the side walls (all 4 side walls will complete a frame 28cm wide). I am also hoping to incorporate some sand filling into the H frame for dampening.
Imagine Lampizator's P13

Project13 speakers

but the bass section H baffled instead of a vented box and two 12" woofers arranged vertically instead of a single 15 or 18". I hope you can visualize.
Here is what I am considering for additional drivers
Tweeter: Beyma CP25 horn, 104db, 8ohm
Woofers: Beyma SM212, Fs=40Hz, Qts:0.38, 98db, cone 44g, 8ohm, Xmax=7mm (2 on each side)
Since I don't have any measuring equipment and can't afford to buy any, I will need your help for the way forward (please).
First your thoughts on the baffle design and drivers (and on crossover later).
For example I am tempted to place the two woofers firing in opposite directions. What do you think?
My current speakers are Oscar Heil Syrinx with some beautiful dipole AMT tweeters. It would be nice to nick those tweeters but they might not be efficient enough. And also I plan to sell them once the OBs are complete.
Looking forward to hear from you all.

[zmyrna]

Another option is to use a slightly wider 35cm front baffle and fit a side firing upright W baffle fixed behind it.

[zmyrna]

Okay, I made some research and it looks like side firing open baffle bass is a no go.
So my options for a narrow profile OB speaker are limited to wideranger and tweeter sitting isolated on a dual 12" woofer:
1. M-frame.
something like this
http://bilder.vgb.no/1296/img_437325ac0308d.jpg
or this
http://www.gr-research.com/mis/v2nude.jpg
2. W-frame
something like this
http://www.bastanis.de/showroom/tom_03/DSC00297A.jpg

[Godzilla]

After years of listening and building i can't get enough of OB. Maybe it's the simple build? Maybe it's the sound? I did have a long listening session with a nice pair of back horns (BIBs) the other night but i miss OB when my head begins to pound. For me listening sessions are longer with OB... too long sometimes.

I think you are on the right track and recommend anything you like sitting on top of H-frames. If you do two 12" drivers in an H-frame or something like in the pictures you attached i think you will have something really nice.

>>> Woofers: Beyma SM212, Fs=40Hz, Qts:0.38, 98db, cone 44g, 8ohm, Xmax=7mm (2 on each side)

Not sure when you double them up if having a higher Qts is beneficial. In my experience, higher Qts woofers provide deeper bass on OB. Based on the spec above, a Qts of .38 may not give you the bass you want unless equalized.

Good luck! I think you purchased a nice driver there.

[zmyrna]

Thank you for your reply Godzilla.
Yes, there is always going to be a conflict between;
A- fast and tight bass (narrow open baffle, low Qts, small driver, light cone, low Xmax)
B- deep and less articulate bass (box, H/U/W/M frame, high Qts/Xmax, big driver, heavy cone)
That is why I am torn between Lampizator's P12 and P25 designs

opoen baffle alnico speakers project 12

Greencone_Saba_Maximus_Lampizator

P12 is always going to be the more elegant solution.
My current living room has a fireplace in the middle with a recess on each side and there are 80cm high, 1m wide, 30cm deep cupboards built into these recesses.
I cannot do the bookshelf design on P12, but I can turn those cupboards into onken type boxes with a 15" driver on each side.
I was thinking of Beyma SM115/N for a plan like that (Qts=0.31, Fs=29, Xmax=5.5, paper cone=77gr)

http://profesional.beyma.com/pdf/SM-115%20NE.pdf

I have top listed Beyma CP25 for its reasonable price (£40).

http://profesional.beyma.com/ingles/pdf/CP25.pdf

But I am scared of metal diaphragms.
Further research brought BMS 4524 into my attention (around £50 each)

Overview

I like the idea of using this driver with the Flo T1040 wooden horn from Rainer Flock.

http://www.flo-horns.de/FloT1040/FloT1040.htm

Tractrix | Tube amps DIY

However his does not seem to be an active business anymore (no reply to my email yet).
Cheaper option is the aluminium horn by Faital Pro STH 100.

http://www.faitalpro.com/products/sc...p?id=903010100

BTW, I plan to cross the CD at around 5000Hz, second order.

[Godzilla]

Those wooden horns look wonderful! Keep working on your project. I think you will have something really amazing when you are done even if you must use the cheaper horn on the tweeter. As for the woofers, just choose an option and go with it. Either way you will have good bass. After selecting your drivers and getting them the fun begins in figuring out the hook up. Enjoy!

[zmyrna]

I did get a response from Mr Flock.
But because I have no experience with horn drivers, I will probably go for the Faital horn initially. I think the horn is less critical at over 5k Hz, plus the Faital horn is one of the better PA horns.
My girlfriend is not very keen on letting the cupboard storage space go. I will have to settle with the M baffle with the two 12" woofers. It should be okay because we have a very small living room anyway.
I plan to build the M frame out of plywood or mdf, and the isophon+horn baffle out of acryllic. The acryllic baffle will sit on the M frame, isolated by sorbothane feet or cones.