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Old 19th April 2015, 11:37 PM   #1
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Default MOSFETS and direct drive ESLs

Mentor Pass,

I stumbled on an old (July 2002) DIYaudio thread regarding use of MOSFETS to direct drive Electrostatic Loudspeaker panels:

ESL amp - switching amp?

In the thread, you noted:

"If you can get those MOSFETs to switch at the voltages
and speeds needed to direct drive ESLs, then you will
become an audio divinity, and temples will be erected in
your honor. And I will visit them."

That was 13 years ago.
Are we there yet? With the current generation of SIT and SIC MOSFETS (or even IGBTs) can a direct drive, tube-less, transformer-less quality amp be built with existing technology? I kinda envision a "stack" of high-voltage MOSFETS to achieve ESL voltages (5-6 kv or so).....

Is it time for the divinity to appear? (And do you have a schematic?)

Greedy boyz want to know..........

Last edited by CanAm Man; 20th April 2015 at 02:40 PM.
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Old 20th April 2015, 12:36 AM   #2
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We have Mosfets which will withstand 1700V (Silicon Carbide) which is a
definite improvement in our prospects.

So I can imagine balanced systems which would deliver DC coupled
+/- 1500 V or so peaks across the stators.

Do you think that would do it?

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Old 20th April 2015, 05:37 AM   #3
SyBorg is offline SyBorg  United States
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This could be close. A common step-up ratio for the input transformers in ESLs is 75:1 to 100:1. With the Vsupply of an amplifier at +\- 30 to 50V this means we are about half way there at full power, and all the way there at reasonable listening levels.
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Old 20th April 2015, 06:40 AM   #4
EUVL is offline EUVL  Europe
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This one can take single 3200V rail and output +/-3000V at its differential outputs :
Semisouth JFET's: What are they good for?

And here is a lower voltage version for ESL headphones using currently available devices :
Pass STAT Headphone Amplifier Concept

And if you are willing to use MOSFETs, there are many devices that can go to 2500V and even 4500V these days, i.e. 5000V or 9000V rail.
IXTH02N250 IXYS | IXTH02N250-ND | DigiKey
IXTF02N450 IXYS | IXTF02N450-ND | DigiKey

With TO247 packages, you can dissipate 30W per device.
That would be 12mA at 2500V Vds. You can alway parallel devices to get more bias.


Patrick
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Old 20th April 2015, 02:34 PM   #5
SyBorg is offline SyBorg  United States
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The load that an ESL panel would provide to this amplifier is right in line with these devices.

There are two main types of ESL panels and the loads differ. In an unsectioned panel, the front and back stators create a capacitor with air as a dilectric. A 12"x48" panel would have a capacitance of about 500pF. The intrinsic SPL response of this type of panel has a 10dB/decade upward slope.
To counteract this, a feed resistor is used to create a low-pass RC filter. Typically this resistor is placed on the primary of the transformer, but in this case there would be no transformer so the feed resistor would be in the range of a few hundred K to a couple of M Ohms.

For a segmented ESL, the panel is divided into many vertical sections, with these sections separated by resistors to form a large RC ladder (see diagram below, from user bolserst). By having the highest frequencies on just the thinnest center section, horizontal dispersion is much improved. Again, there is a feed resistor, but is is of a much lower value. In my case, the R is about 350K and the C is about 70pF, making the feed resistor about 150K.

With a +/- 3000V supply and 150K of feed resistance, we are looking at about 20mA drive current required. Based on EVULs post, this is in line with what these devices can handle if paralleled.
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Old 20th April 2015, 02:34 PM   #6
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Thanks, gents.

I agree with you. I've done some cursory research on semi advances, and I think we are at the "starting point" for a direct-drive semiconductor ESC amp. Pretty exciting stuff.

Time for me to look at some specific semi data sheets, and press on from there. If you'all have any suggestions for circuits, I'd be interested. I think I'll need to move further threads to the Solid State or Exotic Speaker forums, unless you can see a Class A ESL amp design that would be practical.

We live in exciting times. I have some "retired" Martin Logan Sequel panels, and would love to use them, along with a good tuned port woofer or sub design. I doubt if I have the "smarts" to develop a quality amp design, but I still have a dream to try.
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Old 20th April 2015, 03:19 PM   #7
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You might have to log in to see this... esldiy.com

IXYS depletion MOSFET differential amplifier
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Old 20th April 2015, 05:01 PM   #8
Calvin is offline Calvin  Germany
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Hi,

Quote:
Is it time for the divinity to appear?
Certainly not ... that amp will still be a devilish machine

Against an amplifier for dynamic speakers, where the loading speaker is rather irrelevant, the requirements for a DD-ESL amp vary widely.

Basically it depends on the ESL panels efficiency and type of construction.
Fullrangers are inherently low efficiency and as such can be ruled out, as voltage and power requirements are way too much.

Leaves us with hybrid panels, which could be divided into electrically segmented and non-segmented. Both of which may be driven directly to decent SPLs with amps of >2kV supply lines.
The non-segmented panel though confronts the amp with such high capacitances that the current and power demand towards the upper audio bandwidth become rather non-practical.
The electrically segmented hybrid panel seems the only one left where a direct drive amp could actually make some sense.
Depending on the segmentation it may require just 1/10 of the current and power to drive a same-sized non-segmented panel.
It remains questionable though if there is any benefit against a decent transformer driven panel.

jauu
Calvin
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Old 20th April 2015, 08:03 PM   #9
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Well, I have a mile roll of mylar and a gallon of coating and a 10KV supply for
bias. Maybe one of these days Jam will help me build some panels.

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Old 20th April 2015, 08:51 PM   #10
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Quote:
Originally Posted by Nelson Pass View Post
Well, I have a mile roll of mylar and a gallon of coating and a 10KV supply for
bias. Maybe one of these days Jam will help me build some panels.

Gee..... I can see a whole new Nelson Pass product line emerging..... Joining "First Watt" will be "First Kilovolt".......!

When can I DIY one???? Can I build a pair for less than a case of Petrus??

My mind is boggled.....
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