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Old 5th July 2006, 11:02 PM   #1
Few is offline Few  United States
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Default ESL diaphragm contact approaches

Are most of you ESL builders using a single-point electrical contact to apply the bias voltage to the diaphragm or some method that yields more contact area? The sensitivity (SPL out for a given applied voltage) of my aging ESLs seems more dependent on the ambient humidity than they used to be, but the effect has always been there. I used a rubbed graphite coating on the diaphragm and about a 1 square inch electrical contact between some copper tape and the graphite coating.

For my new and improved ESLs I've wondered whether finding a way to apply the bias voltage all the way around the perimeter of the diaphragm, instead of at a single point, would reduce the loss of diaphragm charge under humid conditions. I vaguely recall reading the Martin Logan uses such an approach--or am I imaging that? Has anyone tried both methods? Any suggestions for a simple way to do the "contact all the way around the perimeter" method?

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Few
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Old 6th July 2006, 01:34 AM   #2
BillH is offline BillH  United States
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Hi, Few.

I'm using a variation of the 'charge ring' approach. The charge is carried to the diaphragm with copper foil tape made for stained glass work. I bought 3/16" wide tape from a seller on Ebay and cut it in half the long way before sticking it to the stator. The narrow tape could be coaxed around corners without having to connect pieces together.

I don't have a picture of it installed at the moment, but I'll get one for you when my other ESL panel is ready.
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Old 6th July 2006, 06:52 AM   #3
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Hi Few,

a copper ring surrounding the stator frame works well, but some people want to avoid the thickness of copper tapes. Some of them are up to 3 mil thick.

Another possibilty is the usage of silver-laquer as used for repairing car window heaters. put a short stripe of copper at one outer edge of the frame and brush a small stripe of silver around the entire stator frame and over the copper stripe.

Capaciti
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Old 6th July 2006, 11:52 AM   #4
SY is offline SY  United States
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Silver or graphite-loaded paint will do the trick, but... you'll find that it doesn't help the charging situation. Discharge/charge in a constant-charge ESL is limited by the series resistor. Moist weather sucks- it creates all kinds of unintended discharge paths, including across that resistor.

The auto heater paint is a good idea. The circuit repair pens will also work- a thin line is all that's needed.
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Old 6th July 2006, 12:24 PM   #5
Bazukaz is offline Bazukaz  Lithuania
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Hi,
If the resistor value is not extremely large , and the contact area is large enough , the charge time mainly depends on surface resistivity of coating.
I think that larger contact are should allow the diapraghm to charge faster.

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Lukas.
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Old 6th July 2006, 12:33 PM   #6
SY is offline SY  United States
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If that's the case, the speaker is badly designed and this mod will not help. Non-constant charge equals distortion.
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Old 6th July 2006, 05:07 PM   #7
Few is offline Few  United States
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Thanks very much for the helpful replies. They definitely give me a few things to think about. Here's one idea that came to mind: Has anyone tried to implement a sort of guard ring to reduce the charge leakage off the diaphragm?

Background: When building an op-amp-based integrator for a non-audio purpose I learned that the circuit was very sensitive to charge leakage along the surface of the circuit board and into the integrator's input. The solution was to surround the circuit board pad that leads to the input with a pcb trace that forms a conductive ring. By grounding that guard ring the charges migrating along the pcb surface are shunted off to ground before they can corrupt the integrator's input.

I wondered about trying the same thing on an ESL but applying high voltage instead of ground potential to the guard ring. Could you take the high voltage bias supply's output and send it two places: through the usual large resistor en route to the diaphragm (to ensure constant charge operation), and also through a smaller resistor and to a guard ring that surrounds the diaphragm and its electrical contact? The idea would be that the charge on the diaphragm wouldn't have a low potential region to leak to, even if humidity formed some unwanted partially conductive bridges at the diaphragm's perimeter. The reason for using the smaller resistor value between the high voltage supply and the guard ring is to provide some margin of safety but also ensure that the guard ring stays at the desired high potential. We don't need constant charge operation for the guard ring so a smaller resistor should be fine.

I haven't yet thought through how this would be implemented, but perhaps someone can point out why it's a stupid idea and save me the trouble.

Thanks again for the replies.
Few
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Old 6th July 2006, 07:12 PM   #8
Bazukaz is offline Bazukaz  Lithuania
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Hi,
As far as i know , the constant charge operation is caused by high resistance of the diapraghm coating itself , not the series resistor.Placing even a very large resistor in series with EHT supply does not prevent the charges from migrating through diapraghm's surface , if its conductivity is too low.
Placing a ruard ring over all the perimeter may help only in the case if leakage would happen to the edge of stator.So you could make spacers wider so that the electricity would need to travel through much larger distance.By the way , if the leakage increases in humid days , maybe your stators are not perfectly clean(like residuals of hand soap from hands , which would attract moisture , etc). ?
Leakage problems can be greatly reduced by building wire stators.

Regards ,
Lukas.
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Old 6th July 2006, 07:46 PM   #9
SY is offline SY  United States
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The resistor does insure constant total charge. The diaphragm coating resistivity is only concerned with the distribution of charge. If the diaphragm has reasonably high resistivity, the charge will be essentially evenly distributed even if there is some variation from spot to spot.

If a diaphragm is discharging from a spot it shouldn't, the speaker needs to be repaired, not modified.
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Old 6th July 2006, 08:00 PM   #10
Bazukaz is offline Bazukaz  Lithuania
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So , we need a very high value resistor ,probably in order of GOhms , for the best results ? I noticed that standard 20Meg resistor value is low , compared to diapraghm resistance , and would allow low-resistance diapraghm to be charged very fast.

Regards,
Lukas.
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