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Old 15th April 2002, 08:35 PM   #1
Bill F. is offline Bill F.  United States
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Default DIY transducers--the final frontier

At some urging from others on this forum, I'm starting this thread about DIY transducers, the woefully underexplored final frontier of DIY audio.

A couple weekends ago, I spent a weekend dreaming about transducers (as usual) and thought I'd show you what I came up with, just for fun. It's way off the beaten path, but that's where I like to spend my time.

(See image link below)

What got me started was looking at all the bass drivers I have sitting around in boxes and wondering if I could cannibalize a motor from one as a foundation for something new, maybe a wide-ranger.

These bass drivers have 3" and 4" voice coils, not usually the cat's meow for wide-range use, given the large, resonant dustcap and massive inductance-boosting pole of typical bass drivers. However, I have recently begun a love affair with faraday rings, which have the seeming magical ability to cancel the pole's effect on inductance. I began wondering if a 4" voice coil motor could work for a wide-ranger.

As I see it, there could be several advantages to large-diameter VCs. One could begin with a ready-made bass motor--giant mags driving lots of flux through a thickish top plate. The wide gap spacing of the bass motor would allow room for inner and outer faraday rings without modification. Since the VC is 4" instead of the more usual 1", It would have a 4x advantage in terms of force it could derive from a given field density. Given a thick top plate, say 1/2 to 3/4", and the fact that we'd be winding a short coil of light-gauge wire, an underhung voice coil becomes possible. All else being equal, an underhung coil between faraday rings is the finest recipe I know for extreme linearity.

Furthermore, having played with an online inductance calculator, I believe this short, large-diameter coil should exhibit less self inductance than smaller-diameter, taller coils that are closer to the dimensions of an ideal inductor.

So, to review, we've taken a large bass driver motor and added faraday rings and a light-gauge underhung coil.

Now, the typical way to patch the hole in the cone left by a 4" voice coil is to just slap a dustcap on it. For wide-range, the resonances this would create are unacceptable. So, for this exercise, I decided to try something different. I decided to use a big ol' ugly phase plug--perhaps a wood turning. This phase plug performs a number of valuable services: It replaces the dustcap, progressively horn loads the cone, and serves as a waveguide.

For the cone, I decided on an 8" diameter. With the giant 4" cutout in the center, the effective area of the cone is 16pi-4pi=37 square inches, equivalent to a normal 7" cone. However, instead of the normal cone's 3" of cantilever out from the edge of a 1" former, this cone has a 2" cantilever (from a 4" former). This gives the cone a geometric stiffness profile closer to that of a 4" driver, a significant advantage that will raise the rolloff frequency.

The expanding radius of the phase plug progressively horn loads the inner edge of the cone where the higher frequencies are generated. As frequency climbs and the effective radiating area shrinks back toward the inner edge of the cone, the radius of the phase plug also guides the output away from the center, dispersing the high frequencies and improving the polar response. Of course, the profile of the cone and phase plug could be adjusted for different loading/dispersion characteristics.

To sum up this design exercise, I think this driver could offer the following benefits: use of a production magnetic circuit, high linearity, efficiency, wide-range response, and good high-freq. dispersion.

Allright everybody, let's punch through the glass ceiling and DIY our own drivers!

Bill



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Old 15th April 2002, 09:04 PM   #2
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Bill that looks very interesting.

I think a phase plug made of wood that large would cause problems (I saw the "perhaps"), eh?

Why not use carbon fiber, maybe reinforced with balsa wood?

Digi
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Old 15th April 2002, 09:18 PM   #3
Bill F. is offline Bill F.  United States
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All you need the phase plug to be is stiff and dead. Like maybe a machining made from a dead cat...although that might not be stiff enough.

I just spec wood because (if you have your own lathe) it's a snap to turn whatever shape/profile you want.

Bill

PS. By the way, how do you make the darn pic appear in the dag nabit post?
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Old 15th April 2002, 10:07 PM   #4
Bill F. is offline Bill F.  United States
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Click the image to open in full size.

Here's the side view, just fer fun. If there's any interest, I'll post some novel variations I came up with.

Bill
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Old 16th April 2002, 12:55 PM   #5
maik is offline maik  Germany
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Default RE: DIY transducers--the final frontier

Bill,

check out the "Virtuoso" design at http://www.beauhorn.com. They use a
very huge "Phase Plug" made from wood to load a
Lowther driver with a front horn. It might give
you some ideas to play around with different phase
plugs.

Regards,

Maik
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Old 16th April 2002, 04:30 PM   #6
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It always seemed to me that electrostatics were fertile ground for DIY experimentation. Agree? Disagree?
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Old 16th April 2002, 05:49 PM   #7
Bill F. is offline Bill F.  United States
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>check out the "Virtuoso" design at http://www.beauhorn.com. They use a
very huge "Phase Plug" made from wood to load a
Lowther driver with a front horn.

Yes, that speaker was one of my inspirations.

>It always seemed to me that electrostatics were fertile ground for DIY experimentation. Agree? Disagree?

Strongly agree! I've seen some interesting examples, though I'm exhibiting my fixation on dynamic drivers.

(Rant follows)

Actually, the whole reason I started this thread is because I believe ALL transducers are fertile ground for DIY experimentation.

For all the hairsplitting DIYers lavish on everything upstream of the transducer, I'm shocked at the dearth of attention transducers get. Aside from the T/S params, which get plenty of attention, the factory transducer offerings available seem to be taken for granted as sealed systems of devine engineering. It's pure audio gnosticism, and I'm trying to punch a hole in it.

Speaking in general and allowing for exceptions, driver cosmetics have far more to do with what reaches production than distortion and dynamics. As publicly-held companies serving predominately ignorant consumers, the big names have certainly learned to avoid the diminished returns of lengthy, meticulous engineering.

However, you and I do not answer to beancounters. We can (and frequently do) voluntarily give up evenings, weekends, and holidays to raise the level of refinement and generally push the envelope of our sound reproduction gear. I'm suggesting that our quasi-religious dedication to tweaking can and should be extended into the realm of the transducer. It's time to split the audio atom. If you doubt the importance of such a crusade, just place the THD numbers of you source/pre-amp/amp beside the THD of your output. The difference is your drivers/cabs.

Are you willing to take the linearity of your drivers' magnetic circuits for granted, or even consider it a luxury? Or are you going to get your hands dirty and make it so?

Have I whipped you all into a seething, chanting fervor yet?

Bill
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Old 16th April 2002, 05:58 PM   #8
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Quote:
Originally posted by J Epstein
It always seemed to me that electrostatics were fertile ground for DIY experimentation. Agree? Disagree?
Sure, check out:

http://audiocircuit.com/9041-esl-cir...041IMAI-DI.htm

for several DIY efforts.
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Old 16th April 2002, 10:46 PM   #9
Bill F. is offline Bill F.  United States
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<img src="http://www.villagephotos.com/pubimage.asp?id_=215538" width=632 height=901 >

This is a little twist I came up with. How about running the spider inside the VC former? You could damp the resonance of the spider by tuning its air load by stuffing the pole vent that forms sort of a transmission line. Since the spider is isolated in the former, it's noise is less likely to radiate through the cone. Also, the cone's backwave path is opened up. In the drawing, I included some felt on the top plate to further prevent reflections back through the cone.

And here's the side view. Isn't that nice and sanno?
<img src="http://www.villagephotos.com/pubimage.asp?id_=215537" width=632 height=659 >

Bill
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Old 18th April 2002, 04:01 PM   #10
Bill F. is offline Bill F.  United States
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Sorry about the unavailable photos yesterday. I guess I ran into a daily bandwidth limitation at villagephotos.com. Anyway, they seem to be back today.

Does anybody know of a free pic host with more than 8 megs of daily bandwidth?

Bill
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