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Old 14th April 2007, 09:34 PM   #1
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Default Building a midrange driver

I have experiance in building drivers but im not a pro as such.I have a very light coil and neo magnet from an audax tweeter and i want to turn this into a midrange driver that will be very detailed and sound great.I want to see what peoples thoughts and ideas are for building such a unit.I know that not tomany people build drivers but i want these to be something i can be proud of.I already use my diy ribbon tweeters and cant get enough of them.I dont seem too good at getting good midrange with a light paper cone as it seems to resonate bad at certain frequencys.Any ideas?
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Old 15th April 2007, 12:31 AM   #2
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Default need some info...

what size coil are you planning to use? How low down do you want the driver to run?
You might consider a short coil in a long gap with an extended pole. Vent the pole piece and make sure that you put a good sized chamfer or bevel on the inside of the pole piece. Use copper shorting rings on the the top of the pole piece as well as on the inside of the top plate and bottom of the pole piece. Use high iron content parts and plate them with zink or paint with an epoxy paint to protect them from corrosion. Vent the inside cavity via the back plate. You might consider using (if your cone size is not too large) felted paper cones from Piezo tweeters. These are cheap and are a true high tech product (I refer to Motorola units). You could even consider laminating two such cones together. A light sealer coat of spray laquer on all sides then use a wet coat to bond the two cones together would provide a very stiff well damped yet light composite cone. Mount the cone onto the voice coil so that the coil extends past the edge of the cone by some small distance as this will result in a more ridgid joint. Open cell foam about 1/16 inch thick can make an excellent suspension. You will need to consider treating the foam to prevent rot over time perhaps a high rated sunscreen and latex diluted would get the job done. You can probably expect a life span of 10 to 15 years as long as you can seal the foam. You can experiment with convex and concave arrangements for the cone to see which you like. You might even experiment with mounting two single layer cones face to face to balance the assembly. In this arrangement you could even play with prestressing the two cones via some form of attachment between the two apexes. Using a thin walled tube connecting the two cones.
You might also want to consider using a ferro fluid in the gap to help keep things cool and stable. Make sure that the adhesive on the coice coil is immune to the ferro fluid and if not seal the coil with an epoxy paint. Don't know if this is the direction you were thinking of but this should be food for thought. Regards.
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Old 15th April 2007, 01:35 PM   #3
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The coil is 20mm in diameter and about 7ohm.The thing that i cant get is building the correct diaphram for midrange.I was going to just buy some visaton TI100 drivers as i really like them but the price is quite off putting.Do you think a standard paper cone out of A4 paper can sound natural and pleasent to listen to?
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Old 15th April 2007, 06:59 PM   #4
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Default you have a tweeter voice coil...

the piezo cones that Motorola made have a diametre of 1 5/16 inch plus a flat flange of 3/16 inch width all the way around. As I said these were the result of a very serious R&D program and they are very well designed felted paper cones. Some years and a lot of money went into these cones. I am not familiar with A4 paper so I cannot make a comment. It is however not very likely that you will find a commly available paper stock with similar properties to that used in these cones.
Do you have access to a supply of the voice coils that you want to use? Do you have access to measurement equipment? If you don't I would say that your project is a non starter. I assume that you have the machine equipment to make all the necessary parts? You have still not said how low you would want the driver to play. Remember that a good goal is to have two full octaves of response on either side of the crossover point. I would guess that you could shoot for about 300 Hz low end resonance (perhaps lower) with the Motorola cones. That would mean that you would want to start using the driver at 1200 Hz, a driver Fs of 150 Hz would let you start as low as 600 Hz.
There are several one inch tweeters available with Fs as low as 600 Hz (Seas) which would permit crossover at 2-2.4KHz. A good midbass driver will be pistonic up to this range and two ways of this type sound excellent.
I get the impression that you might be better off spending the money to buy an off the shelf driver. You can often find used drivers for a fraction of the money that they go for new. ATC have a 3 inch dome midrange unit that is impressive and is built like a battleship.
If you don't have access to a machine shop and test equipment I would not recommend building a unit of your own as it will cost far more in time and money than buying a new off the shelf unit. Certainly you can knock up a unit on your own from available parts but it will not compeat with available commercial units.
From a low tech point of view your least expensive and best performance project that would be competitive would be an electrostatic panel. You can buy all the necessary parts premade and it will be easy to assemble on the kitchen table so no real shop requirements. Have you considered this approach? This type of panel will have the lowest distortion and the flattest response possible. By building the panel so its height matches that of the midbass driver you can match the two drivers horizontal dispersion at crossover which is a nice touch. Give it some thought. Regards.
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