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Old 25th October 2010, 07:22 PM   #11
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Quote:
Originally Posted by Calvin View Post
[...]The voltage driven sim always ending around ~120dB@20kHz. [...]
Try varying the D/S spacing...

Sure, as with any sim, one needs to realize that it is just a graphical representation of an equation which was derived as a simplified model of reality with a number of assumptions taken. At least the website explains in detail what the simplifications and assumptions are, so people can judge for themselves...
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Old 25th October 2010, 07:46 PM   #12
markusA is offline markusA  Sweden
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Like Calvin said, interesting sim but I've never seen such curves irl?
It's hard to get anything usefull from it when you don't know what to trust.
It would be very nice indeed to get a guesstimate for the sensitivity based on the most common esl parameters but that sim's very odd.

Since starting this thread I've dusted of my old physics books and actually found some interesting stuff. Equations I've seen in audio contexts suddenly make more sense.
There's still a lot of stuff that's unacounted for but I'm getting there.

To reconnect somewhat to my original question.
Current- vs voltage drive.

Is this statement true and a good summation?
ESL's are capacitors and as such they can never hold a greater voltage than the peak voltage across them.

And maybe...
The amount of current pumped into them is irrelevant as long as you fill the minimum requirements and you can't raise voltage with current capacity. (Except by using other means like transformers etc)
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Old 26th October 2010, 04:53 AM   #13
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Quote:
Originally Posted by markusG View Post
Is this statement true and a good summation?
ESL's are capacitors and as such they can never hold a greater voltage than the peak voltage across them.
As a very, very, very simplified model you could say that an ESL panel is a capacitor, yes. Of course such a model is even worse than that sim, because a capacitor will never produce any power, acoustic or other.

I think your statement is circular... Basically you're saying "a capacitor can never have more voltage across it than the peak voltage across it." That seems obvious to me.

Remark that you can say the same thing about an inductive load driven by a perfect voltage-drive amplifier. The amps job is to shunt any back-EMF produced by the inductor. Just as it is the amps job to source and sink any reactive current flowing through a capacitive load. In both cases, if the amp cannot meet that goal, the result is compromised.

So perhaps you mean to say "a capacitor will never generate a back-EMF voltage when driven by an imperfect voltage source"?

Quote:
The amount of current pumped into them is irrelevant as long as you fill the minimum requirements and you can't raise voltage with current capacity. (Except by using other means like transformers etc)
If voltage drive is your target, yes you could say that. Actually what the amp is doing is pumping a certain amount of charge Q (depends on the signal voltage) from one stator to the other and back every period. The peak value of current is Q times frequency, which explains why ESL impedance is so low at high frequencies.

Many ESL systems will fail to meet this reactive current requirement due to a combination of inadequate current sourcing capability in the amp and bandwidth limitations in the step-up transformer.

Kenneth
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Old 26th October 2010, 05:26 AM   #14
markusA is offline markusA  Sweden
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What I'm trying to wrap my head around is why a high voltage drive is necessary for a DD amp and why you are forced to use transformers with low voltage sources.
It might include some huge simplifications on my part but I'm looking for the easy explanation.
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Old 26th October 2010, 06:27 AM   #15
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What we ultimately want is high sound pressure, so we need a large electrostatic force acting on the charges of the charged diaphragm. The force acting on a unit area of diaphragm is the product of the charge density Q on the diaphragm and the (AC) electric field strength E between the stators. To maximize Q, we must maximize the polarization voltage. To maximize E, we can either increase the signal drive voltage V or decrease the D/S spacing d, since E=V/d. But decreasing d reduces the maximum mechanical excursion of the diaphragm, so it limits max LF sound pressure. Therefore, we still must maximize V.

So I guess, the ultimate reason why ESLs are inherently high voltage devices, is because of the high compressibility of air... If air were less compressible, it would take much less force, and hence less diaphragm excursion, to get the same pressure.

Concerning the current vs. voltage issue:
In theory a capacitor could be charged up to an arbitrary voltage by a current source. But one needs a current source that just "keeps on going", even in the face of the growing voltage on the capacitor. Unfortunately, this is exactly where electronic current sources fail. They cannot handle a load whose voltage exceeds their supply voltage.

A mechanical current source (charge conveyor belt) could do it (as in a Van de Graaff generator) but unfortunately, those work only for DC.

Kenneth
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Old 26th October 2010, 07:43 AM   #16
markusA is offline markusA  Sweden
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Thanks, that's pretty much what I had gathered but I was having difficulties putting it into words.

It would explain why a high voltage source is needed.

I suppose the next step involves estimating how high a voltage we need? Maybe the topic for a new thread? Involving all the scientific stuff of course.
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Old 26th October 2010, 08:38 AM   #17
405man is offline 405man  Scotland
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This will give you some idea of what is required to directly drive a full range electrostatic

Where are the 833 amps?

Stuart
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Old 26th October 2010, 09:26 AM   #18
Calvin is offline Calvin  Germany
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Hi,

ESLs are high voltage devices because they are high impedance devices ( several kOhms to several 100kOhms, at least with typical dimensions and in the audio frequency range, compared to the typical 2-20Ohms of dynamic speakers). Similarities occur to tubes and FETs vers. bipolar transistors.

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Yes thatīs a Manīs amp..... a powermanīs, like Tim Allen is.....isnīt that frightening?

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Old 26th October 2010, 09:29 AM   #19
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The regrettable part is that 90% of the time, only 10% of the voltage is needed. This is because of the large crest factor (ratio between peak voltage and rms voltage) of the typical music signal.

When I first tested my big full range ESLs (~3.5mm D/S spacing!) I used a solid state amp and some 50Hz toroidal transformers, which generated only about 500Vrms or so. I had to turn the amp all the way up, but it sounded really loud! Unexpectedly loud.

Of course the catch is that I didn't have any headroom left for peaks, so on everything except vocals or flute, the speakers sounded like ****.

Morale of the story: beware of this trap, the system might sound good at low levels, but be sure to test with extremely dynamic music too, and verify the peak reproduction.

For a hybrid ESL+dynamic woofer, a DD amplifier which can reproduce 100% of the peaks is certainly a possibility. For full-range ESL you'll find you always end up with a rail voltage requirement which is... unpractical. Unless you can live with a low acoustic output power, that is...

The choice of rail voltage for a full-range DD amplifier is thus really a compromise between feasibility on the one hand and how much of those 10% peaks you want to accommodate on the other hand.

ESL design is one long chain of trade-offs

Kenneth
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Old 26th October 2010, 01:09 PM   #20
Calvin is offline Calvin  Germany
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Dīacor ;-)
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