Wild Burro 'Betsy' and Eminence Alpha 15a - open baffle build

[pcgab]

I decided to build an open baffle speaker for my dad based on a pair of betsys and 4 alpha 15a's.

Inspiration came from:

https://zimmer64.wordpress.com/2015/09/13/betsy-eminence-alpha-15-a-mk-ii/

http://www.diyaudio.com/forums/wild-burro-audio/279757-my-next-iteration-betsy-ea15.html

They're made from baltic birch, 12mm and 18mm. Baffles are 12mm and 18mm, and the yet to be made 'supra-baffles' will be 18mm. I plan to slot load them as the build in the inspiration links did.

I made some progress this weekend, here are a few pics.

Baffles:

Gluing the first baffle together:

Rear brace is 12mm in the middle, flanked by 2 18mm pieces on either side:

The baffles and rear braces will mount to a base made from 2 pieces of 18mm laminated together. That's the plan for now at least.

I'd like to build a passive xo, but for now I'll use a mini-dsp and a couple chip amps.

 

[pcgab]

Cleaned up the first baffle and first rear brace a bit today. Marked mounting locations for the alpha's and did a little prelim fitting of the rear brace.

The rear brace isn't trimmed to mate to the baffle yet. I left a good bit of material there so I can true it up nicely one the baffle and base are ready for final fitment.

Hopefully I'll get some time tomorrow to glue up the second baffle, and start on the rear brace for that one as well.

 

[guangui]

Coming along very nicely!

 

[pcgab]

I got a bit more done yesterday evening, didn't have as much time as I would have liked, but progress is progress!!

Got the chamfers marked and cut for the Betsy driver on the first baffle:

 

[adason]

looks very good, I would even install a better driver...tb?

 

[pcgab]

looks very good, I would even install a better driver...tb?

I considered that option, and tbh I may if I make a pair for myself. As I already had the Betsy's, I decided to use them for this project.

Hmm, that gets my mind thinking. I could make a temporary baffle for the betsy's cut-out and try some Fostex, or other <~8" full ranger..

 

[pcgab]

Made a bit of progress today:

Got the supra-baffles cut, lightly sanded and prepped for glue-up:

and started glue-up:

I'm hoping to get enough time over the next few days to get all the supra baffles glued. Once that's done I'll cut and laminate the base and move on to making the last rear brace. One step closer!

 

[norman bates]

will this be active dsp ?

with that narrow of a baffle, you may be needing 12db of boost per octave below 500hz.

I can pull up jamo r909 and explain.

Or the Visaton no box.
that basically took a 15" and ran a 40hz low pass crossover on it to get it to go flat up to 300hz or so.

Don't get me wrong, I'd like to try something like this also someday.
And my wife would approve of an open baffle 19" wide x 50" tall (jamo R909).

 

[pcgab]

will this be active dsp ?

with that narrow of a baffle, you may be needing 12db of boost per octave below 500hz.

I can pull up jamo r909 and explain.

Or the Visaton no box.
that basically took a 15" and ran a 40hz low pass crossover on it to get it to go flat up to 300hz or so.

Don't get me wrong, I'd like to try something like this also someday.
And my wife would approve of an open baffle 19" wide x 50" tall (jamo R909).

I am planning on using DSP, MiniDSP 2x4 to be specific.

I'm sure I'll need some boost here and there. I really like the profile of the slim baffle.

 

[guangui]

Sims

Not 100% accurate, but very close. Enclosed you will find simulations for both woofers, as if they were fully open, and as if slotted. I did some basic math, and by using the slot you decrease the woofer effective diameter by 82%. So, for the simulation of your speaker I decreased the woofer diameter by 82% (diameter X 0.18). With the slot the woofer seems to be more controlled, but, IMO, not much of a difference.

 

[pcgab]

That's awesome, thank you very much guangui. I plan to run some sims this week, if time allows.

 

[norman bates]

here is the freq response of the jamo R909 (from stereophile).

Notice the dip at 1khz.
That (green) is a 5.5" driver's response on a 19" wide baffle.
I figure driver to edge baffle is 7".
14" front to back, and the driver shorts out, wave is 180 degrees out of phase).
1129/(14"/12") = 967.
967hz, close to the 1khz dip in the measurement.


And a peak a little less than an octave below that, where the front and back waveforms are 90 degrees out of phase, thusly adding.


The visaton no box has a similar response dip around 600hz (24" wide with wings). It is passive. They use 12db crossover under 100hz.

The VISATON NoBox BB Loudspeaker Kit Review By Dick Olsher

"To illustrate the point, one of the VISATON BGS-40 15-inch woofers, mounted on the NoBox baffle, was measured near field (to eliminate room effects), driven first by a conventional 300B SET amplifier, and then by the constant-current F2. In both cases, a second order low-pass network consisting of a series 13 mH steel-laminate coil and a 200 uF shunt capacitor was used to roll off the high frequencies. The measured frequency response is shown below."

But that is passive crossovers.

I look forward to your results.

 

[adason]

dip is less audible then peak

 

[guangui]

That's awesome, thank you very much guangui. I plan to run some sims this week, if time allows.

BTW, the T after Alpha 15A means Top, and the B means Bottom, referring to the woofer position on the baffle.

 

[pcgab]

I got a little bit of time yesterday to make some small progress.

The 'supra-baffles' are glued and mostly trimmed on the first baffle:

Rear View:

Still need to do some final clean up in the cutouts/cavities for the Alpha 15A's, and a bit of trimming on the lower portion of the supra-baffles.

Mounted the Alpha 15A's to check for alignment and to check the gap. I think I'll need some grey foam weather stripping to seal the driver's face gaskets to the baffle. The slight hump of the tee-nuts may be in the way, just a tiny bit.

New MiniDSP 2x4 arrived yesterday as well, now to sort the amps out.

Unfortunately I may not have much time over the next week or so to get any appreciable progress. Work and other obligations are likely to consume all my time for a bit. We'll see, maybe I can sneak out here and there and chip away at it.


As far as testing, I have a Umik from MiniDSP, mac book pro retina(and several linux computers). I've not done any real driver/speaker measuring before. If anyone has some recommendations as far as software and setup, I'd love to hear about it. Maybe REW? I would like to measure/test them, especially to setup the DSP, and whatever needed EQ, etc.

Thanks y'all~

 

[guangui]

Always satisfying to see progress of a project.

 

[Pirmin]

Like this!

 

[pcgab]

I finally got a bit of time to get back on these. I made some quick and dirty frames so I could start some of the testing, etc. Also, as these are for my dad, it worked out well that I was able to play them for him today.

I'll post more details of the crossover work, and when I get to testing them in the next week or so.

A few pics for now.....

Hopefully I'll have more to post later this week.

 

[jwjarch]

Pcgab, interested to know how this build ended up. I'm contemplating something very similar.

 

[pcgab]

Pcgab, interested to know how this build ended up. I'm contemplating something very similar.

I never took it as far as I initially wanted to. Using a minidsp and various amps, I did tweak on it a bit, and by the end had a really nice sounding speaker.

However, the baffles really are just too narrow for LF. I might add some wings, and finish out the rear supports at some point. I think that would make the LF much easier to get to the right spot. They needed a lot of eq boost to get what I felt like was enough low end support.

IIRC, the 15a's were low passed @12db lr2, ~200Hz, and the betsy's high passed @12db lr2, ~300Hz.

at one point I tested them with a B&G Neo3 added, and that was much more to my liking, adding a nice bit up high.