For a while I had been thinking of building a pair of OB speakers but couldn't find anything I liked that had a certain WAF and appeal to me until I came across the PureAudioProjects web site and their Trio 15 range of open baffle commercial speakers, http://www.pureaudioproject.com/trio...eaudioproject/, and then MaxN posting up on Audio-Talk forum http://www.audio-talk.co.uk/phpBB3/v...php?f=4&t=6916 his own version of these using the MarkAudio Alpair 12P full ranger and a pair of Eminence 15" woofers inspired me to start checking out suitable alternative drivers for my own project.

I wanted however to do something a little different to the normal 2-way baffles using a full-range driver with bass helper(s) and came across a new range of professional 2-way coaxial PA drivers from Monacor as part of their 'Stage Line' range of offerings. These come in 12", 10" and 8" versions. I asked Scott (Scottmoose) to check these out for me and he suggested, based on the published datasheets, the 10" version would be best suited for our purposes. This has a 96dB bass unit with a coaxially arranged horn tweeter with a 1" voice coil and 105dB efficiency. Scott and I have found in the past the published data of the Monacor drivers has been pretty accurate to be relied on so this gave some degree of confidence.



Before progressing a design further I decided to purchase a pair the coax drivers for testing purposes and Scott came up with a test baffle design and a suitable 2-way XO to get me started.





These are the baffles I took along to the last 'Yorkshire HiFi Club' meet at David's (MCRU) premises in Huddersfield since Nick Gorham had expressed an interest in hearing the driver as a possible replacement for his Fostex FE167/FT17H drivers on his own OB speakers that he has not been 100% happy with. It was at this meet that Nick commented he liked what it was doing, but noted that beyond a volume level something was breaking up badly, I would guess it was around the 2k area, can't say if it was the cone or the horn, but it was a clear something is far from happy noise. I should say David's listening room is quite large and not any easy room to drive speakers and perhaps we had the baffles placed too close to the rear boundary wall but the volumes we were having to use in that room are far in excess what one would be listening in a home environment. However, there is a ragged rise in response in the driver FR published plot from 2.5K - 3.5K that likely is the causes of this issue. This led to a revised filter for the 2-way XO and best I can tell has resolved the issue.

With the results of the coax confirming this is a very nice driver, 2-pair of 15" Monacor woofers were purchased and I started to draft up plans based on the said 'PureAudioProjects' Trio15 OB's. Here the PAP web site came in very handy as they provide dimensions for the individual baffles and plenty of photos of the frames, base plate and the finished speakers. With some judicious interpolating this led me to CAD up the baffles and frames to something akin to the originals.

So I set about making up some routing templates from 6mm birch ply,, to make up duplicate copies of the side frames and bases and then making up a set of baffles from 25mm chipboard.





The side frames I made from 9mm birch ply and the bases from 15mm birch ply. These were painted matt black and the six individual baffles covered in some rather nice Tamo Ash veneer and given 3-coats of satin acrylic varnish.



The speaker build is now finished but currently only the coax drivers are wired to the revised 2-way XO whilst awaiting the full 3-way filter make up. The coax drivers are still driven full range and as is sound rather good. There is something that these coax drivers with a horn tweeter do that is rather special. In the final XO design they will however only be doing midrange and HF duties. Here are some photos of the finished speakers.











Though I did investigate the use of miniDSP's for an active filter set up I took the decision to stay with a passive filter for this project for the following reasons:

1. The initial cost of the DSP's for a 3-way speaker.
2. The thought of having to use 6 quality amplifers
3. My LDA P6 amps are making some wonderful sounds with the coax drivers I doubt I can find 6 smaller chip amps that will give me the same sound quality.
4. There has been some criticism of the sound quality of the DSP modules.
5. I have faith in Scott's ability to design a passive filter that will be at least as good as any active setup and I can use the P6 amps to drive the speakers.

These speakers can very easily be a DIY project though it is recommended one have a router to cut the holes for the drivers and a straight cutting router bit with ball race template follower for cutting the side frames and bases from the 6mm ply templates. The templates are best cut with a jig saw and finished by hand to final shape. The frames are simply bolted to the base using M6 furniture joint bolts & Cap nuts as are the baffles similarly bolted to the vertical frames. The speakers sit on M6 x 12mm adjustable tilt levelling feet.

These are a personal build project and I'm not sure these will be added to the Reiver Acoustics range as really the DIY ply bases and side frames really need to be professionally CNC laser cut from sheet aluminium and then professionally powder coated. My initial investigations suggest these will cost in the region of £600 - £700 for the machining and then there is the added cost of the powder coating. Still depending on interest..............?

As said earlier these are not that difficult a build and are suited to DIY build and so I thought I'd share the design with you folk if anyone has fancied building a pair of OB's but have not got WAF appeal. These can be made to look more domestically acceptable than a lot of other DIY baffle projects. Anyone interested in making a pair of these baffle speakers can get in touch with me and I will provide a copy my draft CAD plan for the panels and the frames/base.