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Old 2nd July 2012, 11:27 PM   #31
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Thats some beautiful machining on the magnet structure. Cones look very functional too. I think I would stick with the conical single seam cone. The multisection cone looks like it will be hard to make with strength. You can experiment with different cone stock, from heavily calendared manila card stock to blotter paper. If I remember the old Lowther units used a thin manila card stock. I've seen them with square ruling on the back as if they used a type of graph paper.

Good work. We look forward to the results.

David
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Old 3rd July 2012, 09:01 AM   #32
Hentai is offline Hentai  Romania
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Hello,


Radugazon, those are good questions. Inductance of voice coil is one of the problems I'm fighting. You noticed that the higher diameter of voicecoil, the thinner wire and the underhung motor structure, all will lead to a higher inductance. I have given it a lot of thought and I have a model that i will present here as soon as i will take some pictures and do some measurements to verify my calculations.
In my model tho what i wanted to see is how much of an influence will a shorting ring on a FC motor will have on impedance curve, how much of an influence will the low impedance field coil itself have on the impedance (if any), influence of eddy currents.
I chose 2" voicecoil because i wasnt sure how rigid i could make the cone and i was afraid a smaller diameter will stress it too much. The thin wire at first although allows for greater lenght in the gap increases the number of turns and so increases inductance but there is a way to wind it to get around that... i think

Once i will determine which type of cone works best i might try a pressed cone. I already discussed different mixtures for cone material. It will be pretty costly tho so this will be an option if i will continue to build the cones.

David, i also have a couple of stock cones from recone kits i will try and compare.
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Old 3rd July 2012, 09:44 AM   #33
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[QUOTE=Hentai;3079940]Hello,


The thin wire at first although allows for greater lenght in the gap increases the number of turns and so increases inductance but there is a way to wind it to get around that... i think

QUOTE]

....cross winding? Time consuming or even possible?

Beautiful work!

Peter
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Old 3rd July 2012, 08:41 PM   #34
Hentai is offline Hentai  Romania
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Im not really sure what you mean by cross winding applied to voice coil.

What i am considering is 2 coils in parallel. One on the inside and one on the outside of the former.
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Old 14th July 2012, 07:24 AM   #35
Allar88 is offline Allar88  Estonia
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Have you thought about the spider design and materials also? It would be intressting to see what kind of DIY solutsion would match the coals for a project like that.
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Old 15th July 2012, 11:30 AM   #36
Hentai is offline Hentai  Romania
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Moving forward,

This weekend i was able to perform a few tests. Placed the field coil inside and did a few measurements of field strength. FC measures 7.1 ohms and 124mH. The higher DC resistance will require a higher supply voltage than my initial goal.
Because of the enamel coating the wire thickness is smaller, for a 5.3 ohms FC the motor structure will need to be a tad bigger.

Click the image to open in full size.
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In order to determine the field strength i had to measure BL, after that knowing L i could determine B value. BL can be computed by measuring the impedance of a complete assembly so i used a factory cone to quickly put together the loudspeaker.

Click the image to open in full size.
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I used an 16V/7.5A IBM power supply at first and that gave a current of about 2.2A through the FC. I made another power supply with a multitap transformer a 35A bridge and 2 x12000uF/56V to test other supply voltages.

Results are the following:

A---------- V---------- Rdc-------- Fs--------- Qes-------- SPL-------- Mms--------- BL--------- B
2.38------- 17.98------ 13.47------ 33.65------- 0.258------ 96.47------ 38.94------- 20.72------ 1.48
2.52------- 19.34------ 13.57------ 33.65------- 0.255------ 96.52------ 38.94------- 20.93------ 1.50
2.65------- 20.3------- 13.59------ 33.65------- 0.2496----- 96.62------ 38.94------- 21.17------ 1.51
4.33------- 33.6------- 13.74------ 33.65------- 0.21------- 97.34------ 38.94------- 23.13------ 1.65

It seems that FEMM simulations predicted a good value and I am right on target with a B value of 1.5T at 2.5A. The factory cone is heavy so the SPL isn't spectacular.
at 2.5A heat is not an issue, at 4.33A the motor structure was warming up but still not a big problem, you can also observe the rise in Rdc because of that.

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Next i will get a temperature probe and a variable power supply with high current capability (build one maybe?) so i can plot Temp, BL, B and Rdc at various voltages/currents.

Here is a short video showing remanence in pole piece.

Youtube Link

Allar, so far i have prepared a number of factory spiders, right now im not so hyped about wood spiders and that kind of thing but i might try something like that. The mechanical - acoustical transducer is the most challenging part i guess.

Well more will follow.
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Last edited by Hentai; 15th July 2012 at 11:34 AM.
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Old 15th July 2012, 11:41 AM   #37
Zen Mod is offline Zen Mod  Serbia
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drool !
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Old 15th July 2012, 11:56 AM   #38
Hentai is offline Hentai  Romania
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"Heavy is good, heavy is reliable" Boris the Blade

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Old 15th July 2012, 12:45 PM   #39
chops is offline chops  United States
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I wish I had the knowledge, tooling, and knowledge to do such work. Great job!













And yes, I realize I said knowledge twice.
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Old 15th July 2012, 01:15 PM   #40
Allar88 is offline Allar88  Estonia
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Nice, it is intressting to see some results with diffrent FC steering Cant wait when the right cone is beeing fitted and the measuring is beeing repeated.

About the power supply: maybe something from CCCP times, that shouldent be so hard to find, will do for testing? '


Impressive work indeed!
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