Boson, Designing An Affordable Dipole

[JasonP]

So I believe I've learned what I can about design and DIY from the Wavefront at this point, including a harsh lesson on using drivers correctly. Now onto something else entirely!

Here is what I'm planning at the moment and I'm wondering what people think and if they can give me any ideas.

The Boson is going to be a tall, not to wide, Open Baffle Dipole with affordable drivers. The goals are moderately high output, and good distortion/sound quality. I've started the design already, and here is what I'm thinking.

It's a three way with an MTM on top and <something> on the bottom to fill in the low end. I'd like the speaker to be usable down to 40, but for myself I'm looking at it as the top end of a subwoofer assisted offering.

I picked the Vifa TC9D drivers for the M, and the Vifa OX20 for the T. This means the MTM drivers are 87$ in total. I placed them on the dipole baffle in PCD as so, and went to work for an XO just to see what I could shake loose:



Working in PCD for just minutes (and wow do I mean just minutes) I got this with a targeted 4th order LR at about 3k and 2nd order BW at 350 or so:



I put a DA270 for the low-end support in this mockup version, since I've already used it in the Wavefront for the same. However that won't offer anything below 100 Hz, so its violating my goal of offering a full-range speaker here. I need better options for the bottom end, if anyone can chime in.

Off-hand, I'm impressed how well the MTM came together with a simple 15 element filter. I didn't work up the cost on that yet, but if it can be worked down to something simpler I will.

I'm also open to looking at other tweeters if they are <20$

[piazzafatb]

Re: Boson, Designing An Affordable Dipole

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[JasonP]

Re: Boson, Designing An Affordable Dipole

I'm kind of looking at these for the low end right now 2 per speaker: http://www.parts-express.com/pe/psho...number=292-410 I mean, I'd probably order one to start and measure it just to see what it looks like. Its got the high Qts I need for the open baffle though, and a lot of radiating area with two, and the price is... hard to accept.

[fdieck]

Re: Boson, Designing An Affordable Dipole

I'm kind of looking at these for the low end right now 2 per speaker: http://www.parts-express.com/pe/psho...number=292-410 I mean, I'd probably order one to start and measure it just to see what it looks like. Its got the high Qts I need for the open baffle though, and a lot of radiating area with two, and the price is... hard to accept.

I wonder if you have taken in to account the dipole peak from an opben baffle more than about twice the width of the cones radiating diameter.

"How open baffles work" In the download section at:

http://www.dipolplus.de/frameset.htm

Baffle simulation tool for multiple drivers:

http://www.tolvan.com/index.php?page=/edge/edge.php

[JasonP]

Re: Boson, Designing An Affordable Dipole

I wonder if you have taken in to account the dipole peak from an opben baffle more than about twice the width of the cones radiating diameter.

I put the drivers through Jeff B's Baffle and Diffraction Simulator, which has support for Dipole baffles, and merged that into their given responses with Response Modeler. There are peaks in the responses before the filter, I didn't show those, since I'm just playing right now and will measure before I do the actual XO anyway. I wonder how Edge will compare with what I have now, I'll check it out.

[JasonP]

Re: Boson, Designing An Affordable Dipole

Ok, things are looking up. I traded out the OX20 for the DX25 since I had to move the crossover point on the MTM down to just above 2khz because of some nasty diffraction effects on the TC9 M's. I think the sims are looking pretty nice, and because I placed the XO points near the baffle peaks, no real notch filters needed in the XO. Instead I just have some creative filter slopes to compensate.


I think that is looking pretty impressive, with a nice power response and reverse null on the tweeter. Note that this is still be modeled with the DA270 Midbass only, with output rolloff at just under 100Hz. This is likely the version I'll be building for me, since I'm going to have a sub per channel anyway. A 2x GRS 10" version will be worked up, and likely built for my second room as a full-range speaker.


The impedance of the streamlined XO seems pretty decent, and the weird response stuff on the bottom of the bandpass is because I omitted the expensive 3MH coil shunt on the high-pass, since I know the DA270 version isn't going to get any real power below 90 Hz or so anyway.

Progress!

[JasonP]

Re: Boson, Designing An Affordable Dipole

Ok, so I think I've settled on the drivers for the MTM top of the Boson. Here is what I a have settled on, given no one can talk me into a cheaper tweeter in the next couple days!

(per Speaker)
2x Vifa TC9FD-18-08, 3.5" poly driver, to do duty from 350 to 2400 hz
1x Tang Band 28-537SH, 28mm soft dome, to work 2400 and up

Cost: 104$ on PE right now for the drivers needed for two speakers.

I was looking at the OX20 and the DX25 earlier, thinking at first to go all Tymphany on top, but neither driver seemed to like the XO point and needed some additional parts to make them happy. In essence, the 30$ TB is about the same price in the end and likely will sound better. I was looking at some cheaper seas, but even those start at 40 and that is starting to push this design outside "affordable" IMO. If anyone wants to talk tweeters and push some cheaper model, feel free!

[JasonP]

Re: Boson, Designing An Affordable Dipole

Drivers are on the way, now to design and build the baffle so I can take measurements and model a final XO. I did settle on just using the DA270 again for the midbass duties, as there is just no cheap/affordable way to do serious low end response in this design. I fear the GRS will sputter out at any serious excursion and everything else would blow my affordable budget point of 200 or so for drivers. I had thought about a ripole using two DA270's per side and pushing the cost up just a bit, but my instinct tells me the ripole peak will be low enough to not allow them to easily mate to the small Vifas in the MTM. Choices, choices...

[dcgold2000]

Re: Boson, Designing An Affordable Dipole

These buyouts are a bit cheaper.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=299-283

[JasonP]

Re: Boson, Designing An Affordable Dipole

Woah, I know you can't trust the listed specs, but man: Qts: 2.49. I wonder what that peak would look like :eek: