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Old 21st May 2009, 02:25 PM   #101
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Hi Arend-Jan,

Thanks for your explanation of the surface resistance. However, I do not understand how the low resistance can cause higher distortion.

If you look at the ESL like it is a capacitor, the lower the resistance would equal to the faster the charging time for the capacitor. Wouldn't that make the ESL's bias voltage more stable?

It's just my thought.

Wachara C.
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Old 21st May 2009, 02:55 PM   #102
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What effect would small tears in diaphragm have?
Actually I have some Stax SR Lambda's with small pinhole.
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Old 21st May 2009, 04:15 PM   #103
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Hi Wachara,

with a high conductive diaphragm the voltage remains constant (because of the power supply), but as a result the charge must vary as the capacitance formed by the diaphragm - stator changes with diaphragm position!

V = voltage
Q = charge
C = capacitance

Let's assume constant voltage operation:
Q = C*V => Q / C = V = constant, so Q must change if C changes. The power supply must supply (and discharge) charge to and from the diaphragm to hold the voltage constant.

Now with a high resistance diaphragm the charge can't move, so Q = constant (and V changes with C, but the power supply does not notice this because the resistance is so high).

So why does this matter?

Let's look at the force on the diaphragm. Simply put we can use F = q * E. So we can see that F is linear with E if q is constant. Hence we must keep the charge constant to get a linear motor.

I hope this clears things up.

P.S. Again I have been simplifying things to make them understandable. But the reasoning remains valid.
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Old 22nd May 2009, 03:52 AM   #104
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Hi Arend-Jan,

Thanks so much for your explanation. I will need to read a few times and try to digest it a few days to really understand it.

Anyway, to summarize, high surface resistance is good. A good range should be between 10E9 to 10E12 ohms.

Given so high resistance, I figure that a normal multimeter will not be able to measure it. Do you have an easy way to measure the resistance? Does it matter so much if the surface resistances are not the same on 2 panels?

Wachara C.
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Old 22nd May 2009, 06:37 AM   #105
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Hi Wachara,

if you can get the S/R around 1E7 or up this is fine. ( I see that I wrote 10E.. in my previous post but I meant to write 1E.. )

There are meters for high resistance also called insulation meters. A well known brand is Megger. Such a device can measure resistance up to 1E11 at high voltages like 500, 1000, 5000 Volts.

I'm not sure if it really matters if S/R is not the same on two panels (within limits of course), but I would try to get them as close as possible. With a good coating this should not be a problem.


@coloradosound

If you don't hear any mechanical noises from air going trough the pinhole (e.g. farting) then I would leave it well enough alone. No problem with a small pinhole that I can see.
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Old 22nd May 2009, 06:57 AM   #106
ddd is offline ddd  Bulgaria
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Quote:
Originally posted by arend-jan
...
if you can get the S/R around 1E7 or up this is fine...
Assuming 100pF membr/2plates capacity, it'll give 1mS - the half period of 500Hz.
Isn't it very bad?
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Old 23rd May 2009, 04:03 AM   #107
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Last night I had a chance to try 3M 77 adhesive spray for gluing the 1.4 microns mylar to the PCB spacer. It worked out very well. The trick is to spray the adhesive very lightly and evenly. One pass of this stuff is more than enough to glue the mylar down.

Wachara C.
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Old 26th May 2009, 10:07 AM   #108
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I have good news for everybody. I've found a self doable coating material which is very easy to apply on the diaphragm and it gives very high surface resistance in the range of around 1E12. The test result is from a high voltage insulation tester using 5000V.

What you need are:

1. PVA glue.
2. Water.
3. Graphite powder.

You add the mixture of 1 unit of PVA glue with 2 units of water and 1/10 unit of graphite powder. Take your time to stir the mixture well. It will become dark grey in color. Using a soft sponge, apply the material on the diaphragm. Only very light coating is needed. Once it's dry, it's good to go.

The only thing I don't know yet is its stability over a period of time. So far I have only use it for 3 days. But so far so good. Let's see.

Wachara C.
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Old 26th May 2009, 12:53 PM   #109
Phil47 is offline Phil47  France
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Quote:
Originally posted by thinkbad
Phil47

I do not intend to hijack this tread, but could you tell us about the linesource ESL-speaker which I suppose was a follow-up to the headphone design mentioned here, the article appeared in L`audiophile.

Regards JB
Hi thinkbad,

Here is the thread of Wachara, about DIY electrostatic headphones but, indeed, my ESL speaker also uses "piano wires" in trapezoid electrodes plates.

Best regards

Philippe

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Old 26th May 2009, 01:09 PM   #110
Phil47 is offline Phil47  France
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Quote:
Originally posted by chinsettawong
Hi Philippe,

Nice work. It is just beautiful.

I wonder how you make the wires to stay that way.

Do you think if I use my CNC to machine a PCB similar to your pattern, will it be good?

Thanks for sharing.

Wachara C.
Hi Wachara,

Having a CNC to machine PCB is a good opportunity. Manual work of spacers, drilling electrodes, is tedious.

Philippe
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