2-Way OB Betsy-K / Alpha 15a with MiniDSP Active Cross Build-log and Measure-log

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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.
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Here are some pics of the current build.


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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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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.
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 ?!? ...

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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.


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.
If your measurement above is a good representation of what you're hearing, I'd try pulling everything above 300Hz down 1-2dB, or maybe just 300-6000Hz.

edit: whoops, just realized where your crossover is, so if that's where you ended up by ear, then just dropping the Betsy 1-2dB from 6kHz on down is what I mean. Also, reverse polarity on the Betsy and mess with delay to make sure you're getting a decent on-axis null at your crossover point (otherwise you're in danger of chasing your tail because bad summing in that frequency range will really throw off your ear).
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Made some good progress on the rebuild today. Designed my final baffles and built them from 3/4" birch ply (which looks more like cherry to me because it is so dark and pretty).

I've attached the plans. Basically a copy of the lowther baffles -- 24" x 48" with 16" deep sloping wings. The wings have a 2" set-back for time alignment of the drivers. 3" flat top and bottom and angled back towards the bottoms.

Also, finally got some proper tools -- our new Bosch router, Jasper jig, dewalt circular saw and rip cutting jig. This made the build MUCH easier. Gave a nice 3/8" radius around the edges and the wings. Wings are held on with glue and also 1.5" chair cleats in bronze metal.

Everything is nicely flush-mounted and is looking good. I did not do a detailed measurement by rather set the minidsp by Nelson Pass' B5 crossover settings -- LF to the alpha @ 80 Hz, 20 Hz +6dB Q2 peak filter. 15 Hz Subsonic filter. 12 dB for the LP, 48 dB for the 15 Hz HP. Betsy K has a 12 dB/octave filter at 130 Hz. -3 dB Q 0.5 @ 1800 Hz to take away the betsyk honk. No BSQ filter, but I will work on this tomorrow.

First impression is these baffles are a step above the draft baffles. Very rich sound, deep bass and good imaging. Will take some more measurements tomorrow and tweak a bit more.


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Yes @AudioLapDance -- I need a better equipment stand for sure! My plan is to build the battery, the amp and the mini DSP into a single chassis and hardwire everything. I can power the MiniDSP and the HF amp with the SLA battery, and then can wire up chassis-mount Neutrik RCAs and a USB port.

Similar to what Paul did in his build here on the miniDSP forum.
Here are some measurements of the system with my updated crossover settings.

I'm not really getting the bass extension I expected out of the betsyK speakers -- even with no low-pass, it seems to be rolling off about 18 dB/octave at 250 Hz. So that is what I'm working with.

I'm crossing the BetsyK speakers at 250 Hz, with a 3 dB high shelf at 1 kHz to tame the rising response.

The Alpha15as are crossed over at 250 Hz as well with a 6 dB boost at 20 Hz, Q2. I am also cutting 4 dB at 125 Hz, because there was a big bump there.

I need to listen to my settings a bit more tonight. I'm getting pretty decent bass response down to 35/40 Hz, which I am happy with.

Would love any thoughts...


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Here are some updates. Lots of changes in my audio system as well as these speakers. I went ahead and finished the cabinets over the course of the past week. 4 coats of Danish oil and now they have a rich cherry finish that matches my audio rack. I need to let them dry for a week or so and then I will add a few coats of poly or shellac to finish them up.

Ended up adding the rack that you see in the picture, which holds my turntable (Dual 1249 currently), my preamp (Antique Sound Labs, Radii), my amps and my power transformer.

Minidsp is still on the same settings, after the speakers really break in, I'll measure again, for now I am pretty happy with the sound. Thinking of maybe adding a ribbon tweeter in the future, crossed over passively to the betsys at 10-12k. Maybe.


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