Latest Open Baffle Build [Archive] - Audio & Video Forums


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11-23-2011, 12:56 PM

11-23-2011, 01:02 PM
These are bi-amped "Wild Burro Audio Lab" Betsy full rangers over Eminence Alpha 15A bass drivers. The backs are covered with Ikea cork placemats to dampen the 3/4 inch plywood frames.

11-23-2011, 01:11 PM

11-23-2011, 01:11 PM
Nice! Still enjoying my open baffle Betsy speakers. Have a great Thanksgiving.


11-23-2011, 01:16 PM
Thanks Mark and the same to you. I still have my folding wing Betsy baffles but these new ones are so much easier to move around and they have ample bass.

11-23-2011, 01:42 PM

11-23-2011, 01:56 PM
Nice use of the MJK style baffle on the Betsys. Do they sound better now?

Have you given any thought to doing the exact MJK build with Fostex 4" drivers and Eminence bass? It would be interesting to hear your thoughts on how it compares to your tangband hframe combo....

11-23-2011, 02:00 PM
These surgical interventions ( bass implants and tweeterectomy ) took Betsy to another level. She's now got ballsy bass whereas before she was an alto with baritone aspirations.

11-23-2011, 02:07 PM

11-23-2011, 02:26 PM
I think MJK used the 4.5 Fostex in his OB build because he had them on hand. The Betsy's and Tang Bands are purpose built for Open Baffles whereas the small Fostexs are designed for box speakers and horn enclosures. The Betsy's aren't as refined and smooth as the Tang Band W8-1808s but they destroy most conventional speakers with their killer mids and newly acquired bass weight.

11-23-2011, 03:57 PM
I think MJK used the 4.5 Fostex in his OB build because he had them on hand. The Betsy's and Tang Bands are purpose built for Open Baffles whereas the small Fostexs are designed for box speakers and horn enclosures. The Betsy's aren't as refined and smooth as the Tang Band W8-1808s but they destroy most conventional speakers with their killer mids and newly acquired bass weight.

Actually, he uses the Fostex FE108EZ 4" (which is his current favourite of several 3" and 4" drivers he has tried so far).

Also, I think you are probably short-changing the design... It's clear he put a lot of thought into it + measurements... So it's way beyond just the idea of him using some inexpensive drivers he had lying around the house...

If I'm not mistaken part of the reason he opts for 3" or 4" drivers is to avoid the need for a tweeter/supertweeter... If you already have a 15" driver handling bass, then you don't need the fullrange driver to go low (so you can opt for an ideal midrange-high freq driver)...

Project 9 : Eminence Alpha 15A Experimental Open Baffle Design (

11-23-2011, 07:36 PM
Don't get me wrong the Sigma is one fantastic little full ranger but Fostex only recommends it for a back loaded horn design. It has a Qts. of .3 while the Betsy has a Qts. of .78. A Qts. under .5 is considered borderline for OB. There is a definite preference for the larger drivers in the DIY OB community these days. Hawthorne OB's are 10-15 inchers and Hammer OB's are even larger. I don't need a super tweeter for the Betsy's or Tang Bands but I love to play with tweets. I even built a shelf on the Betsy's to try a rear firing design.

My Fostex 4.5 inch Tektons sitting atop the H-Frames can't equal the huge soundstage the Betsy's/Alpha's throw out.

11-23-2011, 09:30 PM
Is Qts about more than bass extension in an OB versus box? If not, then Qts shouldn't matter that much if you use an Alpha 15 to handle the bass...

11-24-2011, 09:45 AM
I tried a Fostex FE126En in an OB center channel paired with surround Frugal Horns and it was very thin sounding. I finally put it into the bass reflex cab that Fostex recommends and got a much fuller sound. I don't think you can achieve the mid bass with a 4 inch cone compared to what you can get with an 8 inch.

11-24-2011, 01:37 PM
I don't think you can achieve the mid bass with a 4 inch cone compared to what you can get with an 8 inch.

Agreed... However, I think the counter argument is that you can't achieve the highs of a 4" with an 8" cone... I suspect the real issue is the competence of the final design: So a properly implemented 15" cone should make up for any mid bass deficiencies of the 4" cone...

I really would love to hear your TB/Alpha/SuperTweeter design (essentially a 3 way design - using the term loosely of course) versus the MJK Fostex/Alpha (a 2 way design).

11-24-2011, 06:04 PM
The Tang Band W8-1808 drivers now cost $500 a pair while the Betsy's are $100 a pair but the TB's don't sound five times better. Actually they are not that far apart and I could live comfortably with either for many years to come. IMO they far surpass the the Aerial Acoustics and Zu's I once owned.

I recently received a pm from a member who sold his expensive Omega's after building the Betsy Folding Baffles ( pictured above in post #6 ).

11-27-2011, 09:51 PM
Those speakers look great and it sound like you are enjoying making them as much as listening. Mucho respect for the DIY inspiration.

Does the Open Baffle configuration only work with fullrange drivers? I ask because I am interested in making some but I only have an assortment of regular drivers from different speakers. I heard it might have something to do with the speakers QT, but I am not sure what "QT" is?

Great speakers!!! I appreciate the DIY aspect because if you have enough money anyone can buy gear, but when you tell people you made and it sounds great, the grin must be huge on a persons face.


11-28-2011, 05:21 AM
Thanks for the kind words. Open baffle speakers really need full range drivers with Qts. around .5 and higher. A project like this is very inexpensive and could sound really good with even cheaper drivers like the $13 GSR 8 from Parts Express:

GRS 8FR-8 Full-Range 8" Speaker Pioneer Type B20FU20-51FW 292-430 (

With these 8 inch drivers plus two Alpha's at $65 each and two 80 hz crossovers at $17 each one could build this OB system for around $200 plus a sheet of plywood or MDF. If you buy the wood from Home Depot they'll make the cuts for free and the driver holes can be done with a simple jig saw.

11-28-2011, 05:33 AM
This link might be helpful regarding driver specs:

Interviews (

11-30-2011, 05:25 PM

11-30-2011, 05:30 PM

11-30-2011, 05:42 PM
I used the link below to recap and mod these super horn tweeters. I deadened the enclosure with lots of duct tape and applied rope caulk to the magnet, horn throat and plate seal. It's now got great high end sparkle and sounds as good as the $$$ Fostex super tweets.

Radio Shack 40-1310 Super Tweeter (

11-30-2011, 06:17 PM
I'm driving the upper full range Betsy's and super tweeters with a Miniwatt SET amp and the bass Alpha's with a Denon AVR-1905 in stereo mode. The analog output from a Rotel RCD-855 is fed to the Miniwatt while the digital coax signal is sent to the Denon. The bass Alpha's use a Parts Express low pass crossover ( part # 266-442 ) to filter out frequencies above 80hz.

If you decide to duplicate this build and have questions please ask them here or send me a PM. One word of warning, once you enter the realm of OB there's no going back.

12-04-2011, 04:10 PM
How do you like the Miniwatt? I have been thinking of going the low watt Set direction. I was looking at the Decware kit to do. Then, and related to this post, I would like to make some OB's or backloaded single driver horns (maybe add a ribbon tweeter).

How do you figure the placement of your drivers?

I think that combo looks great and should make me happy one day. I read the super tweeter mod and looks like it makes sense and easy to do.

12-05-2011, 06:02 AM
I bought the Miniwatt in the spring of 2009 and at $240 it was quite the deal then. I had a friend do the diode mod which opened it up even more. I owe a lot to the MW as it showed me the superiority of the SET vs the better solid states ( killed my Classe CAP-150 ) in terms of non-fatiguing holographic detail and warmth.

While I really enjoy the MW with these OB's It's not quite on the same level as the Bottlehead Paramour 2a3 mono-blocks. The para-feed Paramours are a kit amp I bought used recently and they do everything the MW does but with greater finesse and more meat on the bone.

In the original 6moons review of the MW it's sound was described as "crystalline, fast as a whip like a lit up Decware". I've never heard a Decware but they are held in the highest esteem by the full range speaker crowd. The entry level Decware makes 1.5 wpc compared to the MW's 2.5 while the Paramours have gobs of headroom with 93 db HE speakers with only 3.5 wpc. But it's all about that first watt!

I've built four BL Frugal Horn Mk3 with Fostex drivers which I use in a bedroom 5.1 surround system. While the sound is very good they don't have the huge sound stage of these much simpler to build OB's. The Frugals may image with greater detail but lack the bass weight that comes with the 15 inch Alpha's. The OB sound is highly addictive and more "live" sounding than you get from conventional speakers. With box speakers music sounds like it's coming from a box but with OBs you're inside the box with the music and the instruments.

The $250 Musical Paradise MP-301 from Canada via China seems to be the latest SET sensation and it betters the MW in both power and number of inputs ( 2 vs 1 ). I've not heard it but the reviews are excellent and we all know there's no such thing as a bad sounding SET.

The 2 inch off set of the mid driver is based on Martin J King's OB design parameters. I've linked his site in other posts.

I haven't seen any OBs with ribbon tweets but it sounds do able. The tweet's efficiency should be at least 6 dbs higher that the driver which limits the field. I hope to amp the tweets separately in a future build. I really like the sit-on-top Fostex super tweets with their built in crossover filter but they're not cheap. They are very popular with BL horn owners.

I'd suggest you build a simple flat sheet plywood OB like mine just to see how stupid good they can be.

12-05-2011, 08:56 AM

Great looking speakers!


12-05-2011, 02:08 PM
thanks cubdog

12-10-2011, 11:41 AM

If I wanted to make an OB design out of the drivers I have now, I would have to box the mid driver because it is not fullrange and the QT's are off.

These are the drivers I was thinking of using:

2 x 1" Titanium Vifa Tweeters

2 x 6.5" poly mid drivers (company not known; they came from a pair of Dahlquist DQ-18)

2 x 10" CT Woofers that came from a pair of Rectilinear 3 High Boys.

Would be possible to build an OB with the woofer and tweeter open and the midrange closed? or is that self defeating and I should just buy a pair of fullrange 6" drivers?

12-10-2011, 03:37 PM
Hi Poultrygeist,
Can you give me any plans and or suggestions on building a pair of these?

12-11-2011, 05:11 AM
Baffle size 20" wide X 38" tall ( Home Depot will make these cuts when you buy their 3/4" plywood ) Mine are 19X38

Woofer ( Alpha -15A ) centered 10" above bottom edge ( hole size 13.77" )

Full range driver centered 32" above bottom edge and shifted 2" off center ( Betsy hole size 7 3/4" ) $100 per pair @ Wild Burro Audio

Pair constructed mirror image of each other

Parts: two Alpha 15A ( Parts Express # 290-407 ) @ $64.99 each
two 80Hz XO ( Parts Express # 266-442 ) @ $17.69 each
3/4 " plywood from HD ( half sheet may be enough )
4 large L bracket for feet if you want a simple build
16 guage hook up wire and blue female solderless
Pan head screws and adhesive backed foam window
stripping for driver gaskets
Lowe's speaker binding posts ( discard plastic keystone )

Other great full range driver options : Dayton PS220-8 Point Source driver ( Parts Express # 295-346 @ $109.49 each ). The Tang Band W8-1808 is my top choice but they are $250 each at Parts Express

Martin J King's helpful link:

12-11-2011, 05:47 AM

Low Qts. driver ( under .5 ) usually indicates a stiff cone suspension
High Qts. driver ( above .5 ) usually indicates a loser cone suspension

A stiff cone suspension while perfect for a box speaker will not sound good in open air it's too rigid to move enough air. I think your mid drivers have low Qts. and are meant for enclosures. It's possible the woofers could be used if their Qts. is above .5. I'm playing with some 16 inch woofers from an old Sansui XP because they have F.range printed on the magnet and their suspensions seem flexible. Your tweeters may work but they need to be 8 ohms and have a sens. rating in the high 90's.

04-12-2014, 12:56 PM
What's the purpose of the little shelf at the top of the speaker? Just to add a super tweeter or something else?

04-12-2014, 04:52 PM
What's the purpose of the little shelf at the top of the speaker? Just to add a super tweeter or something else?

I am not sure if PG still posts on this forum.