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oublie 30th July 2009 02:55 PM

wire stator design
 
hi folks,

wire stators i've a couple of questions.

1. wire tension how much or little.
2. wire thickness is it important i.e. thick vs thin
3. where do you get wires that will deal with a couple of kv i've been using wire rated for 600v for headphone bias connection but what about the big stuff.

4. why has nobody built a panel with variable stator to diaphram thickness in other words make the s/d? change from 1mm to 2mm along the width of the panel. feed the relevant voltage to the relevant wires to provide the sound required this would in effect do the job of both a wide spaced bass panel and narrow spaced tweeter panel but without the extra construction effort?

bolserst 31st July 2009 03:33 PM

Re: wire stator design
 
Quote:

Originally posted by oublie
hi folks,

wire stators i've a couple of questions.

1. wire tension how much or little.
2. wire thickness is it important i.e. thick vs thin
3. where do you get wires that will deal with a couple of kv i've been using wire rated for 600v for headphone bias connection but what about the big stuff.

4. why has nobody built a panel with variable stator to diaphram thickness in other words make the s/d? change from 1mm to 2mm along the width of the panel. feed the relevant voltage to the relevant wires to provide the sound required this would in effect do the job of both a wide spaced bass panel and narrow spaced tweeter panel but without the extra construction effort?

Since nobody with more experience has chimed in yet, I'll take a stab at this.

1. wire tension: I hold as little tension as possible, just enough to pull the wires straight, while gluing the wires to the cross bars. Remember, whatever tension you are holding will be transferred to the cross bars once the glue has dried and you release the wires from your stretching jig. Too much tension and the crossbars will bend and sometimes come unglued from the spacers.

2. Wire thickness: For me the choice of wire diameter is more a construction/availability choice rather than a performance choice. Thinner wire is cheaper and easier to tension, but you need more wires and your crossbars need to be closer together to provide enough support. I have used between 18 -22 guage solid copper wire with 0.40mm PVC insulation.

As long as 1) the distance from the outside of the insulation to the diaphragm is the same, and 2) the % open area is the same then the SPL output will be quite similar for different wire sizes using the same HV bias and drive voltage. Most people use 40% - 50% open area.

3) Wire insulation: from what I have read, the most important thing is that the insulation be PVC. As insulations go, it has a rather low resistance, so most of the HV bias shows up in the air gap rather than wasted in the insulation; even in high humidity. PVC also has a rather large dielectric constant, so essentially all of the signal drive voltage shows up on the outside of the insulation rather than being wasted crossing it.

A 600V rating works fine since the majority of the voltage is dropping across the air gap to the diaphragm, not the thru the insulation. I prefer irradiated PVC as it is tougher, less prone to being nicked, and doesn't get soft and shrink back when soldering the way the standard soft PVC does.

4) Variable wire spacing can, and has been done, but usually it works out better if separate diaphragms are used for the two spacings. If one diaphragm is used, and you have a section of 1mm spacing that you drive with just high frequencies, and the rest of the panel has 3mm spacing and you drive it with low frequencies, all is well until low frequencies near the fundamental resonance of the diaphragm are reproduced. Then, the whole diaphragm moves with large excursion, even if the signal isn't being sent to the wires driving the 1mm spaced section, the diaphragm here will move with nearly as large excursion as the driven 3mm section; usually hitting the stator wires in the 1mm spaced section.

The other thing to consider is that gaining efficiency for high frequencies is not really the biggest problem for ESLs. Since most ESLs are dipoles, the low frequencies roll off at 6dB per octave. With typical panel widths of 12" - 18" this starts at 300-500Hz. So you need 6dB - 18dB more output at low frequencies than at high frequencies depending how low you plan to have your ESL response go.

Hope this helps...

Few 1st August 2009 03:23 AM

I second Bolserst's advice, but of course everyone puts their own twist on it. Here's where I might add a few variations to what you've already heard:

1 & 2) I'm leaning toward more wire tension in my current wire stator project, rather than as little as possible, but because I haven't completed my project I don't feel cocky enough to claim you ought to follow my lead. I'm using double build magnet wire with nylon insulation--about 24 AWG and 45% open area. It's taken awhile to devise a satisfactory way to stretch the wires, but I think I've now got it worked out. Anyway, if I were to do it again I'd go with finer wires rather than heavier ones. I think 24 AWG will be fine, but I don't think I'd want to go heavier.

I found that I had all sorts of "why doesn't anyone do it this way?" questions before I built my first or second ESL. Since that time I've found that it often turns out that the reality of trying to construct these things has a way of making some of the more creative solutions seem less than practical. I certainly encourage you to try new and creative solutions--I'm hoping to learn from you!--but I've also found there are also good reasons many people converge on similar solutions to the same problems. Bolserst's advice does a good job of explaining some of the issues, such as those surrounding the variable spacing design. I think you'll find the "without the extra construction effort" part quickly dissipates when you start to carefully consider the design and take all the details into account. On the other hand, if you come up with a creative solution that the rest of us haven't thought of before, by all means share it! We'd be very appreciative. There's no doubt that there's still room for some creative design solutions.

Best of luck,

Few

Calvin 1st August 2009 07:10 AM

Hi,

1) it depends ,-)
One way is to use stranded litz wire. Since this wire is soft it has to be kept under constant tension.When building the stator frame is sligthly bowed. You will need a stator frame with high mechanical strength and lots of closely distanced supporting points, like the often used louvre. Straigthening the frame tensions and straightens the wires. Acoustat used this method and it is described in Fikier´s little book too. The advantage is that winding the wire is easy because of its softness and it will become perfectly straight when tensioned.
Personally I don´t like the idea of considerably large forces ´stored´ within the wires and frame construction. Using soft wire which has the tendency to wobble around unless the free ´swinging´ distance is very small will lead to ugly optics, which only can be camouflaged with cloth.
The other way is to use single stranded wires. Here tension is applied only to straighten the wires after winding. The tension has to be so high that the copper just starts to ´flow´. The wires straighten perfectly flat. Then You can release tension completely and glue the wires to the supporting structure. Since the wire is hard the distance between supporting points can be larger and allows for optically more pleasing open designs. The possibility of breaking glue joints between wires and support is low because there is no stressing mechanical tension.

2) it depends ;-)
The thicker the wire the more distance is allowed between supporting points. H07VU for example spans up to 100mm of distance. This is often the same distance that supporting spacers on the diaphragm need. A fine example of this ´coincidence´ are the audiostatics. Thick wire comes with larger distances between the wires and as such larger variations of the electrical field in close proximity to the conductors. They work well with rather large d/s values. Thinner wire accordingly spans less distance, but it allows for smaller stator-diaphragm distances.
Since with most wires the insulation grows in thickness as the wire diameter grows You should choose the wire thickness after the voltage needs.

3) the voltage withstand capabilities of insulated wires is much higher than the rated voltage. From the thickness of the insulation and the used material You can roughly calculate the maximum withstand voltage.
A good insulator for ESL-usage has a high withstand voltage, a value of surface and volume resistivity in the slightly dissipative range, low creep an track and a high value of the dielectric constant epsilon. Of course mechanical strength and resistance against aging, breaks and last but not least the glueability have to be kept in mind too.
The most critical situation is when touching the stators, because the stators are grounded through the audio tranny and Your body closes the current loop. If You can´t touch the stators the insulation may be chosen thinner.
PVC is a often used material because its cheap and easily available in lots of colours and sizes.
Other materials like PU or Kynar (a Nylon-derivative) might be even superior but do cost more.

4) if the membrane is allowed to swing freely over its complete area the smallest d/s defines the maximum dynamics. In this case different d/s don´t make sense, because in the area of higher d/s You´d loose considerably on efficiency.
If the movement of the membrane is restricted by spacers, different d/s are possible, if the signal to the smaller d/s´ed panel segment is highpass-filtered. But that would mean the same as building two different panels.

jauu
Calvin

bolserst 8th August 2009 08:34 PM

magnet wire
 
Quote:

Originally posted by Few

1 & 2) I'm leaning toward more wire tension in my current wire stator project, rather than as little as possible, but because I haven't completed my project I don't feel cocky enough to claim you ought to follow my lead. I'm using double build magnet wire with nylon insulation--about 24 AWG and 45% open area. It's taken awhile to devise a satisfactory way to stretch the wires, but I think I've now got it worked out. Anyway, if I were to do it again I'd go with finer wires rather than heavier ones. I think 24 AWG will be fine, but I don't think I'd want to go heavier.


Few

A word of caution when working with magnet wire...
Unlike PVC insulation, the double build coating on magnet wire does not take kindly to being stretched. With PVC wire, you can stretch the wire 3% or so as Calvin mentioned to straighten and cold-work (harden) the copper wire without much concern for the integrity of the PVC insulation. Not so with magnet wire....at least in my experience using high quality Essex magnet wire. Even stretched just 0.5% I had problems with multiple fine radial cracks forming in the insulation. The completed panels would arc at high volume levels. I couldn't see the cracks with my eye, but they would draw an arc when probed with HV. Once located I fixed many of the cracks with corona dope, but over time more developed.

It was a pretty panel(red magnet wire), but I considered it a learn from failure experience.

I hope you have better luck, or have better magnet wire insulation.
Keep us advised on your progress.

bolserst 8th August 2009 10:47 PM

Re: magnet wire
 
Quote:

Originally posted by bolserst

I had problems with multiple fine radial cracks forming in the insulation.

this should say circumferential cracks...for some reason I can' t seem to edit the above post.

Few 8th August 2009 11:14 PM

Quote:

Even stretched just 0.5% I had problems with multiple fine radial cracks forming in the insulation.
Hmmm...that's pretty disconcerting. Well, I guess I'll find out. Thanks for pointing out the potential (unintended pun) problem. I'm too far into the construction to back out now, but I'll bear your warning in mind.

Few

oublie 20th August 2009 10:12 AM

Thanks for all the info guys.

Some of the info you have provided will be very useful particularly in regard to d/s spacing and maximum dynamic range.

Calvin 25th August 2009 08:45 AM

Hi,

I´m surprised to hear about the cracks-problem. Magnet wire is wound with constant mechanical pull and bending. So a little percentage of stretch should be fine---in theory. Did You contact Essex wire about this?

jauu
Calvin

bolserst 25th August 2009 07:30 PM

Quote:

Originally Posted by Calvin (Post 1906570)
Hi,

I´m surprised to hear about the cracks-problem. Magnet wire is wound with constant mechanical pull and bending. So a little percentage of stretch should be fine---in theory. Did You contact Essex wire about this?

jauu
Calvin

Actually, I did. They specified a recommended tension range for each gauge of wire when winding coils and motors. I don't have the exact numbers handy, sorry.

I used a small pull scale and applied the maximum recommended tension for the wire I was using to one wire to see if it was enough to permanently lengthen and straighten the wire. It was not. It did stretch the wire, but when tension was released, the wire returned to the original length. I had to increase the tension about 30% before permanent stretching occurred.

My general construction technique for wire stators is to wind all the wires as tightly as possible on a jig and then stretch all of them at once, enough to straighten and strengthen(cold work) the wires, then realease most of the tension before glueing wires to crossbars. For a 60" long panel, I needed to stretch the Essex wire about 1/4" to permanently straighten the small waves & kinks in the wire. For PVC wire 1/2" - 3/4" is typical for uniform straightening.

As a reference, Janszen stretched his wires ~6% according to USpatent 2896025.
http://www.google.com/patents/about?...BAJ&dq=2896025
(column 4, line 40)

If/when I use magnet wire again, I would plan to pull the wire tight, but not stretch it. This should avoid the possibility of cracks forming in the insulation.


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