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Building an Electrostatic Speaker
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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Thu Oct 06, 2011 1:39 am    Post subject: Reply with quote


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As promised some pictures of tightening the membrane, ignore any wrinkles you see, they will be stretched out.








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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Thu Oct 06, 2011 6:58 pm    Post subject: Reply with quote

Some days ago I spent some time putting the variable HV supply together and I noticed two annoying things:

1) The transformer doesn't seem to like to be driven with a square wave and it is squealing, a bit too annoyingly for my taste.
2) The H-bridge gets a bit too hot for my liking

After playing around a bit I found that the transformer really has to be driven by a sine wave, or at least something that almost looks like one so over the last few days I designed a new variable voltage supply. It uses the same square wave generator, a two stage lowpass to filter the square down to a sine and an audio driver chip to drive the transformer. The high voltage stack has been unchanged. I even made room for a heatsink on the board in case the large copper area isn't enough to keep the driver chip cool enough.

I have the new design and layout attached.



HV_VAR2_PCB.pdf
 Description:
PCB layout

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 Filename:  HV_VAR2_PCB.pdf
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HV_VAR2_sch.pdf
 Description:
Schematic

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 Filename:  HV_VAR2_sch.pdf
 Filesize:  43.44 KB
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mightym



Joined: 04 Jun 2011
Posts: 6
Location: Okrahoma


PostLink    Posted: Sat Oct 08, 2011 12:44 am    Post subject: Reply with quote

Thanks for the detail Kim, The fog is slowly starting to lift...

John

As I understand what you are doing, the film is more, or less tightened top and bottom ( in the vertical ), and then the outer edges are stretched to take out any remaining wrinkles, then start again until you are satisfied no wrinkles?

The Mylar you use is aluminum coated, yes?
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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 08, 2011 4:33 am    Post subject: Reply with quote

John,

I indeed start in the vertical but ultimately it is stretched as much in the vertical as it is stretched in the horizontal. I will have more pictures tomorrow as I will finish the first one.

The mylar is aluminized, that's why it looks like a mirror.

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 08, 2011 6:13 pm    Post subject: Reply with quote

Stretching almost done


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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sun Oct 09, 2011 12:14 am    Post subject: Reply with quote

More pictures:

Closeup of membrane connection



Overall shot



With charge ring



Closeup shots




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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sun Oct 09, 2011 7:06 pm    Post subject: Reply with quote

And this is how it is going to look with the front mounted



What I did learn was that I definitely need the front spacers in order to keep an equal distance. After the HV leaks off that will be next, they do play though

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sun Oct 09, 2011 10:00 pm    Post subject: Reply with quote

So I started on the segment spacers for the front stator and so far I have been using square 1/8 acrylic rods that I still had from the ESL-0.5 built. I only had 5 left so I opened up the new shipment and guess what, they are round not square as ordered.

Arggghh, I wonder how long that is going to take.

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 15, 2011 11:00 pm    Post subject: Reply with quote

Well ultimately I decided to cut my own spacers, the picture shows the front stator with the new spacers and 2 layers of a silicone conformal coating just hanging around and drying in the garage.


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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sun Oct 16, 2011 6:55 pm    Post subject: Reply with quote

I put the first complete speaker together, the picture shows it in relation to the ESL-0.5. To hide the stretching tape I used a silvery gray tape that nicely bridges the color between the shiny silver of the membrane and the black of the stators. Most of that tape will be covered by the powder coated strip used to hide the screws anyway.

They were actually playing while taking the pictures, it sounded very good. I will try to make some measurements tomorrow night when the house is quiet.





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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Mon Oct 17, 2011 12:26 am    Post subject: Reply with quote

After listening to both of them a bit I can make the following observations backed up by my wife.

1) The tonal quality between the ESL-0.88 (large one) and the ESL-0.5 ("small" one) is remarkably similar
2) The ESL-0.88 definitely goes down further than the ESL-0.5

The measurements tomorrow will provide a more objective answer.

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Tue Oct 18, 2011 2:58 am    Post subject: Reply with quote

So tonight I measured the frequency response for the first time, it looks like this



I have to say that it looks quite good. When comparing it to the response of the ESL-0.5 as shown on page 4 of this thread it can easily be seen that this one does not have the broad dip between 150Hz and 350hz. It can also be seen that the exponential spacing nicely avoids the resonance peak.

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 22, 2011 7:02 pm    Post subject: Reply with quote

This morning I went back to the original variable high-voltage supply and found a few fixable errors in it.

1) The square wave that the oscillator produced didn't have a 50% duty cycle meaning that there was DC going into the transformer. It becomes a whole lot better with a few component changes which reduces an annoying buzz in the transformer and it keeps the H-bridge a lot cooler.

2) The transformer doesn't quite like to be driven by a square wave, but I can resonate out the leakage inductance on the secondary side. I don't have the exact capacitor I need but it works good enough for a test hack.

Speaking about a test hack, I put together the variable HV-supply and the new 1:75 Plitron transformer and am playing music again. I have to do some measurements but the speaker efficiency is a lot better that with the old 1:50 transformer.



Be careful HIGH-VOLTAGES, don't touch the picture in the wrong place Laughing

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AnalogRocks
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Joined: 08 Mar 2006
Posts: 22556
Location: Toronto, Ontario, Canada

TV/Projector: Sony 1252Q, AMPRO 4000G


PostLink    Posted: Sat Oct 22, 2011 7:23 pm    Post subject: Reply with quote

beun wrote:

Be careful HIGH-VOLTAGES, don't touch the picture in the wrong place Laughing


Damn I touched it with wet hands...

Mr. Green

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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 22, 2011 7:28 pm    Post subject: Reply with quote

Didn't I say be careful? Kids ... they never learn
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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Thu Oct 27, 2011 3:01 am    Post subject: Reply with quote

With the help of the customer I have been making more frequency response measurements. In the plot below there are three overlayed plots measured with different resistors in series with the transformer primary, namely 1.1 Ohm, 0.7 Ohm and 0 Ohm.

I am getting the impression that the amplifier doesn't quite like it when there is no series impedance, at least not trying to produce high level high frequency signals. I didn't notice anything weird on music. The response without any series resistance is remarkable flat though.

I will do some experiments but my guess is that the series resistor will end up around 0.4-0.5 Ohm.


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garyfritz



Joined: 08 Apr 2006
Posts: 7949
Location: Fort Collins, CO


PostLink    Posted: Thu Oct 27, 2011 3:41 am    Post subject: Reply with quote

I wonder how many of those bumps are caused by your room? Too bad you don't have an anechoic chamber handy. Have you ever tried hauling a speaker to some quiet country location and measuring the speaker response *outside*?
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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Thu Oct 27, 2011 4:46 am    Post subject: Reply with quote

Many of the bumps and valleys below 300Hz are caused by the room especially the dip at 200Hz, I can easily change them by hanging some damping material behind the speaker, open a door or move them around. I am still planning to place them in the opening of the garage door and measure them there.
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garyfritz



Joined: 08 Apr 2006
Posts: 7949
Location: Fort Collins, CO


PostLink    Posted: Thu Oct 27, 2011 11:59 am    Post subject: Reply with quote

It would take a helluva lot of damping material to do anything to the response at 200Hz or below. Garage might help but it will also create a big echo chamber. Out away from everything, set on grass instead of concrete, would be a lot closer to an anechoic chamber.

But that's just to get the best-looking and most accurate measurement, so your chart is comparable to the big boys' charts. For an in-room measurement, that's a beautiful response curve.
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beun



Joined: 28 Jun 2006
Posts: 669



PostLink    Posted: Sat Oct 29, 2011 9:03 pm    Post subject: Reply with quote

I just finished polishing the second frame and since I haven't posted a picture of that here it is. As can be seen I use a standard rotary polisher and Meguiars swirl remover and machine finish.


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