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What is really needed to drive an ESL?
What is really needed to drive an ESL?
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Old 2nd January 2018, 11:46 AM   #1
gentlevoice is offline gentlevoice  Denmark
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What is really needed to drive an ESL?
Default What is really needed to drive an ESL?

Hi all,

I have for some time now been contemplating a planar loudspeaker and from a SQ point of view I consider the ESL to likely be capable of delivering the "best" SQ (whatever that may be ) due to its design features.

However, in most all cases a transformer is inserted between the amplifier and the ESL thus reducing bandwidth (and more?) both top and bottom of the ESL. I am only considering a constant charge ESL design.

I would be interested in playing quite loud - i.e. in a very large (up to 400 square meters) room - and thus I reckon that either a very powerful amplifier is needed, or an efficient ESL design.

Considering an efficient ESL design I am aware of two options being open (there may be more):

A.: The bias voltage is set to be high, and/or

B: The D/S spacing is kept low. The latter may reduce the available diaphragm excursion but I am considering a diaphragm altogether with a couple of m2 size (~ 2.2 m2 - big, I know!). For a low D/S spacing a reliable way of stabilizing the diaphragm (so that it doesn't become excentered towards either of the stators) is assumed to be in place.

So - just to get an idea about what might be needed I calculated what the "moved" air volume would be of a 14" membrane, corresponding to an appr. ~15" woofer, moving 3 cms peak-to-peak. This corresponds to appr. 1000 cm2 woofer surface area and appr. 3000 cm3, or 3 dm3, moved at this 3 cm p-p excursion. Quite loud I reckon if such movement is assumed available throughout the audible range ... ?

Similarly, for a 2.2 m2 ESL membrane (22000 cm2) this corresponds to a peak-to-peak excursion of appr. 1.36 mms. That is 0.68 mms on either side of the membrane centerline ...

Which leads me to a question for those of you familiar with ESLs:

How much voltage/current is actually needed to drive a static charge ESL in practice? Here I am thinking about the voltage & current fed directly to the stators and not the voltage/current input to a transformer.

I hope some of you may be able to give some idea about this ... please when/if replying also state stator size, D/S spacing, diaphragm impedance, diaphragm bias voltage, or .. ?? that you consider relevant in order to realistically assess the voltage/current needed to drive an ESL ...

Hope this is accessible to reply to ... - & thanks for any feedback ...

Cheers,

Jesper
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Old 2nd January 2018, 01:21 PM   #2
Chris Daly is offline Chris Daly  Australia
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Hi Jesper
I would learn as much as you can from this reprint document written by Peter Walker
originally in Wireless World :May 1955

Wide Range Electrostatic Loudspeakers P.J.Walker Wireless World May 1955

Secondly get to hear a good set of ESL57's, noting the design requirement
for a curved panel and how that is going to be achieved

Thirdly assess amplifiers for them
The Quad ESL - Christian Steingruber

Another notable manufacturer of ESL panels and speakers is Martin Logan
similarly get to hear what they can do.

Hope that helps.
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Old 2nd January 2018, 05:00 PM   #3
sumotan is offline sumotan  Indonesia
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Hi Gentlevoice,
How did you you come to the conclusion that the step up trafo is limiting bandwidth ?
Like tube amps if trafo is well design, bandwidth is not compromise at all. Amp wise, I believe it's more to do with the design then anything else. I'm using a 100 watt AB amp
with my ML Odyssey which plays pretty loud.

Cheers
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Old 3rd January 2018, 12:28 AM   #4
golfnut is offline golfnut  New Zealand
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Hi

You really need to download a copy of Baxandall's paper on Electrostatic loudspeakers - I'm sure, like me, you will find it a revelation. there are several copies around on the WWW - see The wire electrostatic Loudspeaker page - Introduction for example.

The maximum OP from the ESL is almost entirely determined by the area and the low cutoff frequency - stator-membrane spacing is almost irrelevant - see Baxandall for detail.

Also, ESLs are a pressure source, conventional speakers are a volume-velocity source - the displaced volume on an ESL membrane will be quite different from a conventional cone.

For all practical purposes, the ESL behaves electrically like a capacitor. A large full range ESL of 1 m^2 has a capacitance typically about 1.5 nF, and the capacitive load is a serious difficulty. Consider a 1:100 transformer with 1 ohm primary resistance, the source impedance seen by the ESL is 10 k (100^2 x 1 ohm). Hence the RC filter formed by the resistor and ESL has a cutoff frequency of 10.6 kHz - hence problem. The problem gets much worse with higher step-up ratios. So the primary winding resistance (including speaker leads) must be very low.

Also, consider 3 uH output inductance from amplifier and speaker leads (e.g., 1 uH from the amp and 3 m of speaker leads at 0.7 uH/m) then the cutoff frequency of the LC filter is 24 kHz. To build a transformer for such an ESL is very difficult because there is so little headroom without reducing bandwidth below 20 kHz. With higher step-up ratios the transformer design is impossible.

So the problems are primarily with the capacitive load of the ESL, not the transformer per se. This can be fixed by using passive equalisation of the ESL. For a point-source ESL, the LC transmission line segmentation used by the QUAD ESL-63, converts the ESL impedance to a pure resistor - much friendlier load (Technically a difficult solution too - see Baxandall for detail). For a line source, the RC transmission line equalisation described by White (also on the wire esl site or pm me for a copy) reduces the capacitive load by a factor of 20 X, again a much friendlier load. This solution is easy for DIY, only requires resistors.

The bad reputation of ESLs for killing amplifiers comes from people trying to drive large capacitive ESLs directly through the transformer e.g., using active equalisation. The impedances have to be very low to avoid the bandwidth limiting effects, and the slightest problem will result in massive currents and a suicidal race between the fuse and the OP transistors.

regards
R
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Old 3rd January 2018, 08:42 AM   #5
gentlevoice is offline gentlevoice  Denmark
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What is really needed to drive an ESL?
Hi all ... & many thanks for your feedbacks ;-)

... BTW before replying ... I notice that both Chris & golfnut are from New Zealand ... maybe there's some interest in electrostatics there ... ?

@Chris Daly: As it is I actually have a copy of the Peter Walker article and have read it a couple of times. It is a quite interesting paper, however, besides detailing some of the theory of electrostatics (like e.g. the bandwidth vs efficiency correlation) I didn't remember it being specific about ESL efficiency (could be, though, that my memory lacks here).

In any case I was hoping that someone had actually measured the voltages going to their panels (together with some information on the design) so that I could get an idea about the actual requirements for driving an ESL.

@golfnut: Thanks for outlining some practical technical aspects of ESL design - and some of the work-arounds for compensating for the capacitance of the ESL assembly ;-)

Regarding the Baxandall paper I will just consider ordering the book from the library - I think it is available here in Denmark.

And then I'd like to "re-phrase" from #1 that my main reason for asking this question - what is actually needed to drive an ESL - is that I am curious as to if it is at all feasible to direct drive an ESL - i.e. build a (superb!) dedicated amplifier to drive the ESL without the transformer in-between. Thus omitting the band limiting of the transformer etc. ...

I realize there may indeed be challenges in designing such an amplifier but knowing what the approximate required voltage/currents needed are may give an idea about the magnitude of such challenges. Again, I will need to play loud so SPLs around 115 dBs are likely ...

Anyone knowing about this?

@sumotan: Hi & thank you also for your input. But as I write above I would like to omit the transformer, if possible.

Cheers,

Jesper
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Old 3rd January 2018, 09:20 AM   #6
lcsaszar is offline lcsaszar  Hungary
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Hi gentlevoice, if your goal is a couple of m2s in a large room, you may go for many smaller sections, like 8 x 0.25 m2 each. A transformer will be needed for impedance matching, but not the transformer will limit upper and lower corner frequency. Lower is limited by the dipole principle (front-to-rear cancellation) and higher by the diphragm thickness. If you use e.g. 8 sections each 0.25 m2, it will be 2 m2. Then you can have 8 transformers, one for each section, and combine the primaries series/parallel for better matching. Study how Acoustat achieved large surface using several smaller panels (2+2).

As for omitting the transformer, there are serious arguments against it: you are dealing with lethal voltages. The driving audio voltage is several hundred volts (yes it could be easily delivered by a tube amplifier, but then you would need huge coupling capacitors in terms of capacity and voltage, at the end the size and cost won't be less than using transformers), and there is the bias voltage of several kilovolts. If you use 8 x 50W transformers, their size could be really small.

Last edited by lcsaszar; 3rd January 2018 at 09:28 AM.
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Old 3rd January 2018, 06:44 PM   #7
bentoronto is offline bentoronto  Canada
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What is really needed to drive an ESL?
I suppose there is no theoretical obstacle to any extension of ESL powers. But like with cone drivers, the trade-offs are challenging: size, movement, corona discharge, beaming.......... I'd guess dreams of low freq extension and loudness have to be eased.

Dayton-Wright ESLs were capable of great loudness, maybe KLH 9s some too. The Dayton-Wrights cells were enclosed in a heavy gas in an acoustically (almost) transparent box, that brought important benefits of various sorts.

I kind of don't like transformers with their idiosyncrasies. I built and used a direct-drive amp for a few decades. Certainly the best sound I've ever heard.

B.
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Old 3rd January 2018, 09:07 PM   #8
gentlevoice is offline gentlevoice  Denmark
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What is really needed to drive an ESL?
Hi again ... Icsaszar & bentoronto - thank you both for your replies.

@bentoronto:

Quote:
I kind of don't like transformers with their idiosyncrasies.
... This is along my line of thinking and the reason why I asked my question in #1.

However, I've searched some more in other diyaudio threads and noticed that Calvin in this thread & post mostly answered my question:

Any Direct Drive ESL Amp projects someone could share?

So it seems that the voltage & current demands are quite substantial. Things to ponder ...

Thanks again for replying ... I think I now know what I need to know.

Cheers,

Jesper
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Old 3rd January 2018, 10:41 PM   #9
MarcelvdG is offline MarcelvdG  Netherlands
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My colleague Frank Verwaal studied the theory of electrostatic loudspeakers before building his own and also made his own Matlab scripts for simulating the SPL, see Elektrostatic Loudspeakers

By the way, with his ESL design 2 kV peak was not enough for an ordinary living room.

Last edited by MarcelvdG; 3rd January 2018 at 10:47 PM.
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Old 4th January 2018, 08:30 AM   #10
gentlevoice is offline gentlevoice  Denmark
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What is really needed to drive an ESL?
Hi Marcel ... Thank you also for considering my question and replying.

Quote:
... made his own Matlab scripts for simulating the SPL, see Elektrostatic Loudspeakers
Incidentally I actually found this webpage of yours yesterday evening while searching for information about ESL power requirements. As it is, however, I am not familiar with Matlab so I would not be able to use his simulation ...

Quote:
By the way, with his ESL design 2 kV peak was not enough for an ordinary living room.
Ok, very useful information, thanks! ... that really is my worry ... that ESLs are so power hungry that driving them is quite a challenge. A DD amplifier is my best guess but can e.g. the PSU be made sufficiently stable at such high voltages - and will it be able to deliver sufficient current while keeping the voltage stable ...

Anyway, I might just ponder this ... and otherwise consider my question replied.

Cheers

Jesper
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