2-Way OB Betsy-K / Alpha 15a with MiniDSP Active Cross Build-log and Measure-log - diyAudio
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Old 4th February 2013, 11:15 AM   #1
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Default 2-Way OB Betsy-K / Alpha 15a with MiniDSP Active Cross Build-log and Measure-log

I am in the process of building a pair of 2-way actively crossed-over bi-amped open baffle speakers, which utilize the following for drivers:

Betsy-k by Wild Burro Audio Labs -- $110/pr

Eminence Alpha-15a 15" Driver -- $129/pr

Right now my current amplification scheme is a battery-powered 15 watt/channel Class D amp from Jerry's Electronics (with upgraded caps). This will power the Betsy-k speakers.

The Alpha 15a speakers are powered with a Rotel RB-1050 high current amp.

The electronic crossover is a Mini-DSP and my DAC is currently an Apogee Duet firewire interface from my laptop.

I'm to the point where I have my draft baffles built from pine project boards. The final baffles will be a bit more substantial and built from 13-ply birch. I will update this thread with pics from the build and measurements as soon as I have them. I am open to suggestions and comments throughout the process.

Last edited by danvprod; 4th February 2013 at 11:27 AM.
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Old 4th February 2013, 11:30 AM   #2
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Here are some pics of the current build.
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File Type: jpg 2013-02-04_005.jpg (348.5 KB, 524 views)
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Old 4th February 2013, 11:36 AM   #3
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And quick measurement of the response with about 15 mins of minidsp tweaking. (Left channel, 1 m from drivers halfway between LF and HF driver.

Decided on 250 Hz X-over @ 48 dB/Octave. Time-alignment of HF with a slight delay and 6 dB 20 Hz boost (like the First Watt B5 x-over, with 15 Hz subsonic filter). I notched out a few things as well in the response (mainly the 1800 Hz Betsy-k 3 dB boost). It's not perfect, but does show promise, especially when I get them into bigger, more substantial baffles.

I should add that I started this from the MJK 2-way design -- with his suggestion for passive x-over. I was quite unhappy with the spectral balance of the Betsy-K and the Alpha. I was also unwilling to add series resistance to the Betsy-K. The active x-over has much more flexibility.
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Old 4th February 2013, 11:41 AM   #4
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My ideas for the final baffles include an iteration of one of the following:

1) Big 30 x 60 1.5" thick OB made from 2 pieces of ply laminated together with a slight curve on the top -- like the Lowthers here.
2) Baffle with base and center support like this one here.

I am very open to ideas...
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Old 4th February 2013, 12:43 PM   #5
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Location: Hamilton, was Ottawa (Canada)
Lookin' good!

Did you use the dimensions and positioning of Martin's Paper?

MJK ob a15 f103.png

Why not stand on the shoulders of giants! The design integrates a 15 and full range nicely around 200Hz.


The wings will create a resonance. Could you hear it?

I've been having some open baffle/DSP fun myself!

open baffle party.jpg

Great speaker tool: REAL-TIME DSP crossover adjustment!

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Old 4th February 2013, 12:51 PM   #6
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Hey, Jeff

I did use the measurements from the MJK paper -- rounded to the closest I could find in that pine board at lowe's. MJK calls for 20" x 38" baffles, mine are 20" x 36".

Yes, I can hear the resonance. I am planning on lining the inside cavities (if you could call them that) with cotton batt insulation. I may stick with those dimensions and do a better job flush-mounting the drivers (I recently purchased a router and jasper circle jig). You can see my hack-job with the holes in the pictures in the current build.

I may also do the single center brace to hold up the baffle vs. the two wings.
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Old 4th February 2013, 01:44 PM   #7
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Well, 2" is 2"!

Snicker, ... that's what she said! Snicker!

I don't think lining will help as is won't attenuate enough (higher freq only). Stuffing however will ... but then you're into quasi-cardioid territory and ... there be dragons!

NaO U-frame

Another thing you could do with the wings is taper them. Have them start like they are from the base for support and then angle is so that they're done by the time they pass the top of the driver. As far as I understand this a more spread-out and lower-q res.

Or yes, a single wing for support and it could be large (longer distace from front to back = lower rolloff) but ?no? resonances (no 'cavity'). Hmmm, I'm not sure how to model that one ?!? ...


Last edited by AudioLapDance; 4th February 2013 at 01:47 PM.
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Old 4th February 2013, 01:53 PM   #8
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Location: Hamilton, was Ottawa (Canada)
Or you can use that resonance!

MJK ob hframe res.png

To extend the high freq of your 125Hz filter to end up with a 300Hz filter:

MJK ob hframe res w 125hz LR2nd.png

MJK is great! 16x16x15 H Frame:

MJK ob hframe 16x16x15.png


Last edited by AudioLapDance; 4th February 2013 at 01:57 PM.
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Old 6th February 2013, 07:12 AM   #9
hajj is offline hajj  Lebanon
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Concerning the wings, as long as it doesn't have a top, the resonance should be minimal since the nature of the "mouth" will be mostly resistive.

A waterfall plot should help in figuring out if any resonance is taking place.

You could also try to cross lower (say 130hz), as the Alpha 15s won't play well above 200hz (which might be why you hearing "resonance").

I've been through the process of building and designing an OB before, and if there's one advice I can give you, first and foremost, try to measure woofer and FR separately, with and without crossover, only then would you be able to find the right crossover point. Martin's worksheets are great tools to get you in the ballpark, but final tweaking should be done through measurement and listening.

I personally went for high slope crossovers first (because they look good on paper), only to find out that they can sound pretty nasty.

Good luck, and most importantly, enjoy the build and the tweaking.


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Old 6th February 2013, 12:38 PM   #10
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Hey Nick -

Thank you very much. The technique that you have described is in the paper by Nelson Pass on the FirstWatt B5 -- he recommends taking an impulse response of the combined drivers and the LF and HF individually and making sure that the combined response is not less at the XO point than the individual (to indicate that the woofer is subtracting energy due to phase cancellation).

I think I will do that soon -- and also reduce the x-over point to 100-130 Hz as you have suggested.

Also agreed on the high slope x-over -- may be better to dial back to 12 dB or 6 dB an octave too and hear how that sounds.

I really like that the minidsp lets me tweak like this.
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