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Old 26th March 2005, 08:26 AM   #51
Zvon is offline Zvon  Australia
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and the other image

Regards
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Old 26th March 2005, 03:40 PM   #52
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So now I guess the question is, will it hold up for years with 5 kVDC bias applied more or less continuously?


Thanks for the input!

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Old 27th March 2005, 03:42 AM   #53
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The Licron diaphragm is installed and working fine. I read 15-30 ns conductivity between a couple washers laid on the diaphragm a couple inches apart.

We'll see if it continues to work. It seems like one can of the Licron should be good to build a lot of drivers, maybe a couple hundred of the Quad sized drivers.

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Old 27th March 2005, 04:34 AM   #54
Zvon is offline Zvon  Australia
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I_F

Glad that the speaker is working.

Based on your surface resistance measurement of 50Mohm/sq I guess the Licron coat(s) was(ere) heavy, which may have caused forming of the droplets.

If there is no perceivable (or better measurable) difference in the speaker sensitivity your are OK .

My speakers have been working with Licron two years, I'll drop you an e-mail when the membrane coating fail

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Old 27th March 2005, 05:12 PM   #55
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Default Re: Coating both sides of the diaphragm

Quote:
Originally posted by Brian Beck


This is the same argument, also overlooked at times, that teaches that you should not coat the stators with anything approaching a “perfect” insulator – the speaker won’t play! Strickland and Janszen both wrote about how using Teflon or polyethylene coating on stators (stator wires in particular) would rob the air gap of its electric field by simple voltage division between a small resistor and a larger one. They recommended PVC coating, since PVC is about 1000 time more conductive than PE.
My recollection of what Strickland said is that it worked fine in Florida where the relativel humidity is high, but did not work when they moved to Arizona, where the humidity was low...



The Licron is the Tin-Oxide based anti-static stuff??

What about a spray'd Nylon? Anyone make that?

What sort of surface does the Licron provide when dried on the Mylar, and did you say it doesn't want to come off??



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Old 27th March 2005, 06:46 PM   #56
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Can anyone explain to me why coating the stators with an insulator is supposed to kill the output from a driver? I have heard this said in a few different places and it just doesn't seem to match up with reality. The air between the stators and the diaphragm is an insulator, so how can adding another layer of insulation effect the performance of the driver? Adding additional insulation will increase the capacitance of the driver, and will reduce the maximum excursion of the diaphragm, but that's about all I can see happening.

Also, a measuring resistance is not the same thing as measuring sheet resistance. Sheet resistance is accurately measured by using a 4 point probe. A known current is passed between two of the points and the voltage is measured across the other two points. Then the whole thing is rotated 90 degrees and another set of measurements are made. The resistances (measured voltage/applied current) are plugged into the Van der Pauw equation which can then be solved for sheet resistance:

exp(-pi*Ra/Rs)+exp(-pi*Rb/Rs)=1

In the case where the resistivity is isotropic, the formula becomes

2*exp(pi*R/Rs)=1 -> Rs= -(pi*R)/ln 2

R, Ra, Rb are the calculated resistances, and Rs is the sheet resistance.

In practical terms, the coating won't be uniform, and not many people have a four point probe, or a calibrated high voltage low current source, so a resistance measurement is what we have to live with and it is adequate. Just don't call it sheet resistance!

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Old 1st April 2005, 05:20 PM   #57
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Taking a complete guess at the answer to the question about coating the stators...

It doesn't kill the output. Obviously Martin Logans are coated and JanZen and Acoustat use PVC insulated wire, etc...

IF and only IF the insulation is *very good* - as it was in Stickland's case (Acoustat) using Teflon (r)(tm DuPont) insulated wires - then I am hypothesizing that the voltage gradient outside the insulation was *reduced* sufficiently in a dry air environment.

It seems that the idea is that a voltage gradient would be set up, wherein the air + insulation becomes a capacitive dielectric, getting charged. The "leaky" quality of the PVC insulation becomes a minor benefit as it permits sufficient charge (voltage & current) to leak so as to keep the air dielectric's field charged...

That's my seat-o'-da-pants hypothesis.

Anyone else?

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Old 1st April 2005, 06:10 PM   #58
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That's exactly right. Good explanation. Martin Logan's powder coating is probably loaded with carbon to make it sufficiently leaky so as to keep the air gap's electric field high. But the resistivity is still high enough to reduce the energy of any arcs and for safety’s sake.
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Old 4th April 2005, 05:54 AM   #59
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Default Guess

Hi,

the problem with high resistance stator coatings is that in case of an overload the diaphragm touches the stator and will stay attached for a while and that after the membrane disconnects itself from the stator the output will be reduced for a while . Coatings with a lower surface resistance will recover more quickly.
For a music signal the resistance of the coating is of minor value. The problem of high insulative coatings like Teflon or PE is rather a very low dielectric constant. The higher this constant the less is the loss of signal over the coating thickness (capacitive voltage divider with the air). While Teflon is very low (2-3) PVC and Nylons are higher (4-12). So PVC and Nylon permit thicker insulations which is good in safety tems without giving up too much on efficiency.

yap
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