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Old 24th October 2010, 11:37 AM   #1
markusA is offline markusA  Sweden
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Default Current vs voltage drive ESL?

Inspired by another thread with a discussion concerning direct drive amps.

What's the difference in driving a ESL with a voltage amp compared to a current amp?
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Old 24th October 2010, 03:37 PM   #2
nac134 is offline nac134  Canada
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Unless Im way off - I believe that using voltage drive with an electrostat is to bypass the step up transformer in the signal path. Typically, an amplifier has way too much current and far too low voltage to drive the panel - so there is a step up transformer in the speaker. Tube amps suffer the opposite problem - they produce too much voltage and not enough current - so they use a step down transformer on the output. So with a tube amp running an electrostat, you have a step down transformer, followed by a step up transformer. So you can do away with both, running the bare signal from the amp right to the panels. Don't try it without consulting the proper authorities.
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Old 24th October 2010, 05:41 PM   #3
markusA is offline markusA  Sweden
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I'm sure you're right but I was wondering more along the line why a current amplifier can't be used to drive esl's?
U=RI and all...

Since I'm a no0b with the electronics bit there's a lot for me to learn.
However...
Since the stators aren't touching can there really be a current running through the circuit?

The definition for an electric field found in my physics book reads
E=x(V2-V1)/a
E being the electric field [V/m], V2 and V1 the electric potential [V] of the stators, a the distance [m] between the stators and x, well I'm not really sure what the x is doing there?
Please correct me if I make an error. It would appear the electricfield can exist without a current as long as there's a difference in electric potential.

If I remember things correctly it is the change in the electric field that's causing the charged diaphragm to move?

P=U^2/R
P being the power [W], U the Voltage [V] and R the resistance [omh]. In our case R isn't really R but Z. This would explain power needed by the esl.

Still I feel like I'm missing something really essential.
How could there not be a current? It really makes my head hurt.

Please, can someone explain this to me?
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Old 24th October 2010, 08:47 PM   #4
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An electrostatic speaker can be modelled in the first approximation as a capacitor.
Therefore there is no continuous current through it but there can be alternative current whenever the voltage on the capacitor changes.
You can review the chapters about RLC circuits in the physics textbooks to understand the power needed to charge and discharge a capacitor at a certain voltage and a certain frequency.

An ideal capacitor doesn't dissipate power. If you want to have a better model, you need to add a dissipative subcircuit. A very simple one would be a resistor in series with the capacitor.
Maybe someone with more knowledge about the ESLs can talk about better models for them.
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Old 25th October 2010, 04:22 AM   #5
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The place to start thinking is this: a monumental acoustic sound of 130dB is one electric watt (please correct me if I am wrong). ESLs make lots of sound without needing much power to speak. Cone speakers are very inefficient (that's a cue that cone speakers are not a smart way to make sound).

Great problems driving an ESL with a transformer and only because they are so wonderful even when poorly driven do they sound so much better than anything else. The transformer output looks screwy to the ESL panel and the transformer makes a screwy load to a conventional hifi amp.

So, you can make a high voltage direct drive amp with a resistor as the load (and by the way, an ESL strapped across that resistor) or you can hand-tinker with transformers.

Correction for earlier poster: ordinary tube amps make only a small fraction of the drive voltage needed by an ESL. You can ditch their regular 10k ohm to 8 ohm matching output transformer and substitute a mild step-up transformer. Not a bad way to power ESLs.
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Old 25th October 2010, 07:48 AM   #6
markusA is offline markusA  Sweden
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Yes, now we're getting somewhere.
I had missed the obvious, esl's are capacitors and should be treated that way.
A capacitor can never hold a higher charge than the voltage across it can supply. The current flowing only flows while charging the capacitor. This way it won't matter if the amp can put out 1 or 1000 amps as long as the voltage is to low.

Bypassing the panel with a resistor sounds like a interesting idea but I wonder if it'll really work? Current will flow continously and there will be a voltage across the resistor. Put enough current through it and the voltage will be high enough to charge the panel.
However the panel is frequency dependant and the reactance will vary. Current division between resistor and panel will cause voltage fluctuations, possibly not so great for driving the screens? I don't know? I'm just thinking out loud here?
Bringing up the voltage across a resistor would also waste a lot of energy in the form of heat, not a very efficient way of doing it?
Still I don't know the inner mechanics at work here and it might be a viable of doing it?
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Old 25th October 2010, 11:43 AM   #7
Calvin is offline Calvin  Germany
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Hi,

as with all audio gear, compromises dictate the end-result.
what current drive promises is:
- a linear frequency response compared to a 6dB-rise with voltage drive
What is usually and too easily forgotten and often not told are the premises and disadvantages of current drive which are:
- cd (current-drive) is not applicable to the entire audio frequency range
- it applies only to the far field range. Under near field conditions, Headphones e.g, linear response is achieved with voltage sources
- the membrane size must be small compared to the radiated wavelength, so it doesnīt apply for example to line-sources
- small membrane areas mean high impedance values, mean high voltages, mean high transformation factors of the audio tranny
- if implemented with a voltage source feeding the panel via resistors (as in the Quads) the voltage demands are seriously increased, hence transformation factors of ridicolously high values required
- in praxis cd doesnīt make equing obsolete as the schematics of all Quads proove.
- since the load defines the current and voltage demand from the amp, a current amp would require the same supply voltage and current range as a voltage amp

So current drive is not per se advantageous in driving speakers, neither dynamic nor electrostatic. One must closely look if and how a claimed Pro applies in praxis and if the unavoidably occurring Cons donīt eat up any possible improvement.

jauu
Calvin
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Old 25th October 2010, 11:45 AM   #8
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Default Current vs. voltage drive

Hi,

try to dig up the old articles from Peter Walken... He shows that the frequency response is flat when the supplied current is proportional to the audio signal. Sadly, the ESL is reactive, so when you drive it with a voltage source instead of a current source, you get a rising response for higher frequencies.

There is a simulator to play around with here: Electrostatic Loudspeaker (ESL) Simulator

It is possible to make a special amp with current-sensing feedback to accomodate the ESL, or you can use a line-level equalizer.

But when you use electrical segmentation (RC sectionizing), then if you do it right you can restore the flat frequency response for voltage drive. This is because the higher the frequency, the smaller the radiating area then becomes. Using this system, you wouldn't need the above tweaks.

HTH.
Kenneth
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Old 25th October 2010, 03:13 PM   #9
oshifis is offline oshifis  Hungary
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Quote:
Originally Posted by markusG View Post
I had missed the obvious, esl's are capacitors and should be treated that way.
ESLs are capacitors with rather high loss (tangent delta, dissipation factor - that dissipation is acoustic energy we are interested in).
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Old 25th October 2010, 03:38 PM   #10
Calvin is offline Calvin  Germany
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Hi,

well the simulator obviously sims something......but has anybody actually ever measured such curves in praxis? .... me definitely not! The voltage driven sim always ending around ~120dB@20kHz. I wouldnīt trust a sim thatīs results are clearly so far off of the praxis.

jauu
Calvin
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