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Old 30th May 2010, 11:27 PM   #1
Few is offline Few  United States
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Default question about Sanders' PCB ESL stators

The patent application of Bloodworth and Sanders, found here, describes a method for fabricating electrostatic loudspeaker stators out of copper clad boards (printed circuit board material). One of the features they cite is the "complete encapsulation" of the conductors by the dielectric material. To do this, they use CNC machining to cut slots through the boards, and also cut back the copper trace so that there is a copper-free region between the edge of each trace and the slots that surround it (see figure, extracted from patent application). They then cover the copper traces with another slotted board to complete the encapsulation.

My question is: How important is the gap between the copper trace and the slots? I assume the gap is intended to provide insulation in the plane of the stator, but is that necessary? If not, it seems that a slotted printed circuit board stator could be made without resorting to CNC machining. I'd love to build a CNC router, but spending $2k to build the router is a bit hard to swallow. I can imagine much cheaper, albeit more labor intensive, methods for achieving the same ends as long as the gap between the copper and the slot can be eliminated without running into problems. For example, I have a cheap, plastic tile cutter with a diamond blade that does a great job of cutting PCB material. I can imagine using its blade to cut parallel slots through the boards without resorting to the complexity of a full CNC router.

Thoughts on the importance of the gap between the copper and the slots? The patent application states clearly that one of the main advances they lay claim to is the solution to the problem of insulating the edges of the copper traces, but is that really a big problem?
Few
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Last edited by Few; 30th May 2010 at 11:46 PM.
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Old 31st May 2010, 03:24 AM   #2
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Quote:
Originally Posted by Few View Post
The patent application of Bloodworth and Sanders, found here, describes a method for fabricating electrostatic loudspeaker stators out of copper clad boards (printed circuit board material). One of the

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Thoughts on the importance of the gap between the copper and the slots? The patent application states clearly that one of the main advances they lay claim to is the solution to the problem of insulating the edges of the copper traces, but is that really a big problem?
Few
The copper clad problem is a small thickness of the clad - high electrical field intensity at the edge - corona discharge.
That's why edges of the stator shall be round or rounded - it is noted in almost any esl design description.
Cutting the slots would not be the biggest problem. PCB itself with the buried copper layer will be the most expensive part
Besides that inexpensive pcb are usually limited in size - somewhat 18x18 in sq. And price does not come down unless you order hundreds of them..
Alex
P.S. It seems that much easier to make stencil and paint electrodes. Unfortunately that returns us to the root of the problem - insulation layer...

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Old 31st May 2010, 01:54 PM   #3
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I went to Pad2Pad and drew up a reasonably sized (like 30"x10") PCB that was just the outline of a rectangle (to be used as the spacer, not the stator itself). It was several hundred dollars for each one, and you would need 2 per speaker.

The uniformity would be nice, but this is more of a "professional" part and not something an individual would be able to rationalize.
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Old 31st May 2010, 03:30 PM   #4
Few is offline Few  United States
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I hadn't considered the thickness of the copper clad layer and the resulting electric field strength so thanks for pointing that out.

On the other hand, the cost estimates seem overly pessimistic to me. McMaster-Carr has 24" x 36" single sided copper clad boards for about $29 for 1/16" thickness and $38 for 1/32" thickness.

Boards without the copper cladding (to be used for the insulating layer) cost about $13 for 1/32" x 24" x 36", although that's for a paper and phenolic composite. Fiberglass and epoxy is about $23 for the same dimensions--not sure if you can get away with the cheaper material for the insulating layer.

Either way, four boards of each type would build two 1 foot by 6 foot stators. Considering the flatness of the resulting stators, combined with the ease with which segmentation could be introduced, I find the approach to be an attractive alternative. Of course I have no interest in convincing others that they ought to do it this way--to each his or her own. My original post was aimed at the question of the need for the gap between the copper and the slots. Thanks for helping me think through that aspect. If others have ideas, by all means chime in.

Few
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Old 31st May 2010, 04:51 PM   #5
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Hi,

i do not see any sense in designing and manufactureing something tricky.

Its Ok just to make a single layer pcb. The copper layer is on the outside, and the copper has a certain distance to the holes. To make it perfect, the copper layer is insulated with coating.

In the picture you can see such a pcb. If you look closer you will recognize, that the copper has some gaps. This enables segmentatioon of the stator.

Capaciti
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Old 31st May 2010, 06:25 PM   #6
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Quote:
Originally Posted by Capaciti View Post
i do not see any sense in designing and manufactureing something tricky.

Its Ok just to make a single layer pcb. The copper layer is on the outside, and the copper has a certain distance to the holes. To make it perfect, the copper layer is insulated with coating.
Hello,

Very nice looking stators. Thanks for sharing the pictures.
I did have two questions:

1) Can you share what material you are using for the coating over the copper? I understand you may not wish to share this information for company reasons.

2) Have you noticed any issue with the PCB stators tending to eat away the coating on the diaphragm due to increased corona from sharp edges of the copper? Beveridge describes it like this "...the edge of the thin copper sheet around each hole is like a knife blade. Even where the insulation did not break down, corona forms quickly at the edges around these holes...." He noticed that after years of use, the corona ate away at the coating in circles around the sharp edges of the holes in the copper layer. According to his patent, he used nylon as a coating over top of the copper trace.

Old Beveridge thread:
Harold Beveridge ESL's

Beveridge PCB stator patent:
Electrode for electrostatic transducer - Google Patent Search

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Old 31st May 2010, 06:43 PM   #7
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Bolsert,

1. The coating is from the german company Peters and is called DLZ 1600. Its a specific coating which builds up to 200 micron in one layer without droping into the holes (as you can see in the picture). Processing this coating is a nightmare. I did it on my own, but if i would have followed up the pcb-stator i certainly would have given this task to a coating specialist. It worked fine, but i cancelled further activities of pcb stator, because still a wire stator easily outperforms any other stator design for most aspects (costs, safety, reliability, easy, efficiency.......)

2. corona is an issue if the copperlayer has sharp edges and you are right, insulation wont help in this case. Therefore you need a certain edging of the copper. There are some pcb manufacturer which are able to edge in such a way , that the copper layer is softly rounded at the edges.

Capaciti
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Old 1st June 2010, 02:20 AM   #8
mavric is offline mavric  United States
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This is very interesting, quote from "rbrv" along time ago, a way of testing and a differnt idea for an esl, his quote."That's a good question about a reverse esl. I would propose that the first thing to do is to define what constitutes an esl.

The common use and widespread acceptance of the high resistance, constant charge membrane working between two stators is what most people think of as an esl. In these systems the membrane is passive and the stators do all of the work.

If we accept that definition, then a reverse esl would be to have a constant charge on each of the stators and the membrane carrying the audio. This would make the stators passive and the membrane would do all of the work. In that case, the amount of voltage necessary would not change.

In a Beveridge system, there is no passive component. Both the stators and the membrane are active. All three carry the audio. The benefit of this is that it reduces by half the amount of voltage necessary for the same amount of sound pressure. I think that this is still an esl, but certainly is not what most people think of as an esl.

As far as having the aluminum evaporate from the Mylar when used with conventional electrodes, I may be able to shed some light on that.

I have studied the loss of aluminum I found in the transducers my father made with the more conventional circuit board electrodes. I believe you are correct regarding the high charge density, and specifically that it is caused by corona.

Corona is formed at the surface of a conductor carrying high voltage. It still forms even if the conductor is well insulated. On my father's circuit board electrodes, the edge of the thin copper sheet around each hole is like a knife blade. Even where the insulation did not break down, corona forms quickly at the edges around these holes.

In studying the Mylar from at least 50 of these transducers, I have found that the aluminum coating started evaporating in small circles around each hole. In more severe cases, these circles had grown to the point of overlapping each other. In the most severe cases, virtually all of the aluminum coating was completely gone from the entire surface.

I have a highly polished copper pipe which is about one inch in diameter. It has a long wooden handle. I use it in conjunction with a Variac and a neon light transformer to check my electrodes for arcing. This set-up can produce up to 9,000 volts. I place plastic spacers along the sides of an electrode and then I slide this copper pipe back and forth along the spacers. When I do this in a darkened room, I can see the blue light of the corona forming between the electrode and the copper pipe. The more voltage I apply, the brighter the corona.

With the circuit board electrodes, the corona first forms as circles around the holes and other places where there is an edge of the copper sheet underneath the insulation. The corona forms at relatively low voltages at these edges. At higher voltages, this blue circle gets very bright and even develops into a white ring surrounded by blue corona.

When I do the same thing with the Epoxy composite electrodes which we made in the '70's, this corona forms a uniform blue light along the whole surface of the copper pipe. At higher voltages, the blue light is brighter and some white "sparkles" appear as well. With my new electrodes, these "sparkles" still appear, but at much high voltages. With them at extremely high voltages, the lovely blue corona just gets thicker and a brighter blue and is still perfectly uniform.

I have been told that corona is mostly empty space with a few highly charged particles moving very fast. I know that the aluminum coating on the Mylar is only a few atoms thick. It is my guess that every once in a while, one of these high energy particles hits the Mylar and knocks off one or two atoms of aluminum and that this can eventually remove the whole coating. That is what it looks like to me.

Rick Beveridge



i do not mean to intrude on the toppic, but that is very neat and very crrious. Mav
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Old 1st June 2010, 06:04 AM   #9
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As soon as AC voltge applied with or witout dielectric there will be discharge at some voltage level.
So ionised particles will move out or to the electrode, depending on the sign of the charge.
Electrons are small and does not damage electrode/dielectric.
The latter can't be said about ions. Older ones can recall ion traps in CRT.
Magnetron sputtering would be one of the technologies relevant to the phenomena observed - i.e. coating "disappearence".
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Old 3rd September 2010, 03:04 AM   #10
Few is offline Few  United States
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I'm either too stupid or too stubborn to let this idea go without convincing myself it doesn't have merit.

Latest thoughts:
  1. McMaster-Carr sells 1/32" thick (slightly less than 1 mm for those who prefer more practical units) fiberglass boards with copper cladding on both sides and sells them for a reasonable price. If the stators were made by slotting these boards, and then insulating the side that faces the diaphragm with another fiberglass board with similar thickness, might the double layer of copper reduce the "knife-edge = large electric field" issue? I'm assuming that both copper faces would have the same voltages applied. It seems to me that the sandwich of two copper layers would approximate a 1/32" thick solid copper layer and help avoid the electric field hot spots associated with a single thin layer. Is that wishful thinking?
  2. Can anyone explain to me why the insulation does not help reduce corona discharge problems at a given voltage? I would think that the dielectric would get polarized by the applied field and reduce the net electric field between the dielectric and the diaphragm (or the other stator). Of course if the intention was to say that you have to crank up the voltage to make up for the losses I just described, then that is another issue. But for a fixed applied voltage, would an insulating dielectric not reduce the corona discharge?

There seem to be several reports of Sanders' pcb-based ESLs producing very high SPLs and he "claims" extremely high reliability. I realize we shouldn't believe everything stated by the manufacturers when they're trying to sell their wares, but if the Sanders panels play loudly and reliably, that would seem to suggest this approach is viable.

Thanks in advance to anyone with the patience to help work through these ideas.
Few
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