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Old 22nd October 2012, 03:54 AM   #41
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Quote:
Originally Posted by maudio View Post
Back on topic


Ciss almost 1000pf, that is a bit high. One of the most important parameters to look for is low gate capacitance (Ciss). You have to stack multiple mosfets to handle the voltage and to do so you need a resistor divider driving the gates. Those resistors together with Ciss limit your bandwidth. Lowering the value of those resistors makes your dissipation skyrocket.

The 1000V IR IRFBG20 is such a nice mosfet for this application because it has only 500 pf Ciss.
The BFC40 from Semelab has 550 pf and will even handle 1500V. Semelab also has the BFC60 with as little as 40 pf but that will only handle 100mA/20W and that is just not enough for a 4kV amp. Ideally would be a mosfet in between those 2 but I have never found one. Come on guys, start producing a bfc 50

The higher the current rating the higher Ciss will be. So you need to get the smallest available mosfet that will handle the power.
Here it comes SJDT170R1400 - SEMISOUTH - JFET, SIC, N-ON, 1700V, 3A, D2PAK | Farnell United Kingdom
Don't forget zeners, just in case.
The only decent thing to decouple such potential would be decent trasformer or sigma-delta ADC/DAC with opto in between. Tube based 8kV DD monster was posted quite a time ago.
BTW 5W power supply with few pF between in-out is much easier than most people think...
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Old 22nd October 2012, 07:16 AM   #42
maudio is offline maudio  Netherlands
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nice one!

now we only need to find a matching p-channel fet so we can start building that DD amp
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Old 13th November 2012, 03:14 PM   #43
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I was looking for FET's today and I found a few that look pretty good.
Some are more costly and some not so bad.

In various power ranges and all available from Digikey.
Some are even Depletion mode types.


http://ixapps.ixys.com/DataSheet/DS1...V02N250_S).pdf

http://ixapps.ixys.com/DataSheet/DS1...V03N400_S).pdf

http://www.st.com/internet/com/TECHN...CD00158273.pdf

http://www.st.com/internet/com/TECHN...CD00159991.pdf

http://ixdev.ixys.com/DataSheet/DS10...1R6N100D2).pdf

http://ixdev.ixys.com/DataSheet/98812.pdf

http://ixdev.ixys.com/DataSheet/DS98...A-P05N100).pdf

http://ixdev.ixys.com/DataSheet/DS10...P08N100D2).pdf

http://ixdev.ixys.com/DataSheet/98545.pdf

Cheers !

jer

Last edited by geraldfryjr; 13th November 2012 at 03:17 PM.
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Old 14th November 2012, 12:16 AM   #44
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Quote:
Originally Posted by geraldfryjr View Post
I was looking for FET's today and I found a few that look pretty good.
Some are more costly and some not so bad.

In various power ranges and all available from Digikey.
Some are even Depletion mode types.

SKIPPED

Cheers !

jer
You've missed the point - no silicon mosfet even lateral one can outperform carbide's capacitances per se, moreover jfets are believed to be better in terms of distortions introduced: Nelson amps.
As they try to penetrate a market - you can easily get some - just wait for maturity and all you'll be getting generic smps oriented parts or worse IGBTs...
Sadly IXYS staff is very often made out of unobtanium as it was mentioned before, good luck of getting some. Being small guys they do not like small guys like diyers
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Old 14th November 2012, 12:50 AM   #45
jcx is online now jcx  United States
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Default better figure of merit doesn't help if the only available device is 10x too large

but what current production alt semi devices are available with reasonalbe I rating for ESL - the few I see are for 10s kW power switching and the die are just too big for the job

Semisouth goes Dodo ; what now ?

Last edited by jcx; 14th November 2012 at 01:09 AM.
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Old 14th November 2012, 04:59 AM   #46
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All of the ones I listed are in stock at Digikey/
Most are under $3 except for the first two.
Sic Fet's would be nice but they can't be had from a normal vender and there are problems with Semisouth now.

The current demand for an ESL depends on how much capacitance it has.
My little panels of less than 50pf (35pf) would roughly require about 100watts at 5kv peak and up to 400watts peak to fully drive them up to +10Kv peak.
10kv peak is an awfully lot of drive voltage.
However, I was able to do this for a short while with my little panel.

Other than that I don't have any exact figures at the moment.

Even at 5Kv peak my panels gets in excess of 105db at 1 meter and a larger panel would do much better than that.
But the bigger the panel the more power is required of course.

I think I have found it to be around 1000watts to fully drive a 1' X 4' panel at 5Kv to 6kv rms I believe.
I forget what the D/S was that I used,I think it was .062" for that estimation.

Since ESL's have a rising response with increasing of frequency the largest voltage swing at the lowest frequency of operation is what is most important and is what well set your usable maximum SPL.

When eqaulized to be flat the drive voltage at 200hz will be -20db lower at 20Khz.

This pretty much keeps the power demand the same across the whole bandwidth for 200 to 20Khz as an example as the capacitive reactance falls at the same rate as the frequency rises.
As the voltage is halved as per octave due to the -6db per octave equalization, at the same rate, the power demand stays the same.

Even with a gate capacitance of say 650pf it would still only require less than a watt of drive power even with a 500khz sine wave considering a full 15v swing on the gate in which in most cases will not be required.

Yes, many of the FET's are optimised for switching but this doesn't mean that the can't be used for linear circuits.
I called IRF and talked to an engineer a few years ago on this and that is exactly what he had told me.
He said that all of the FET's can be used in linear circuits just some a more suited for switching circuits than others.

I tried every type of FET I had in my circuit from IRF510's to IRF840's and IRFZxxx switchers, They all worked just fine in the linear circuit.
I will be exploring this again more sometime very soon.
Right now, I almost have a new little panel finished.
The details of its construction can be found here,

A Segmented Stator Desktop ESL

It is of a segmented stator design and should have even less of capacitance and current demand on an amplifier than what a regular panel would be.

jer

Last edited by geraldfryjr; 14th November 2012 at 05:11 AM.
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Old 14th November 2012, 05:32 AM   #47
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Here is a recent thread of similar if not the same topology that I have been messing with,

8 Watt Hifi with few parts

And the THD figures have been shown to be very clean.

I didn't have any way to measure THD when I built my circuits but I was able to listen to them using a Piezo driver and as well as monitor it with my scope and it was very clean with music as well as with Sine and Square waves.
I have no doubt in mind that some thing like this would work nicely for ESL's especially a smaller one at least.
This topology has been done before and is also used for ESphones as well.
Here is a design by Neil S.Mckean that is the same as well,

Do It Yourself - Electrostatic Speakers - Project: ESL H.V. Amp by Neil S. Mckean

Cheers !!

jer
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Old 15th November 2012, 12:29 AM   #48
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Quote:
Originally Posted by jcx View Post
but what current production alt semi devices are available with reasonalbe I rating for ESL - the few I see are for 10s kW power switching and the die are just too big for the job

Semisouth goes Dodo ; what now ?
Too little too late... there are other coming in like Infineon...
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Old 30th May 2013, 07:23 PM   #49
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I just received a few HV FET's today from STMicroelectronics to start working on my direct drive amplifier design.

Here is what I have so far,

STW4N150 1500v 4A N-CH TO-247

STF6N95K5 950V 9A N-CH TO-220FP

STP21N90K5 900V 18.5A N-CH TO-220

STTH812DI ULT Fast 1.2Kv Diode

STTH512DI ULT Fast 1.2KV 5A diode

And I should have a few IRFBG30's on their way as well as various other samples.

ST also has some 1700V FET's but I didn't see them until after I put in my order so I will get some next time.

Plus I also got a few high current drive opamp's and assorted precision opamp's as well.

I already have an old Heathkit SB-101 power supply that provides a good source of filtered 900Vdc to start with and for use with my DIY ES headphones.

Then from there I will build a higher voltage switcher for bigger design's and FET stacking types of configurations.

My goal is to get between 6 to 7KV peak voltages although even 1500V will make my little desktop panel ESL comfortably loud as well.

My Desktop panel design is less than 50pf so little current will be needed, Although my end goal is to drive at least a 400pf panel to 6 to 7Kv peak voltages to start with and from there who knows.

jer
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Old 30th May 2013, 07:37 PM   #50
jcx is online now jcx  United States
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parasitic C of the multi-Amp power fets will be much larger than your desktop panel

you should probably look at IXYS HV Mosfets for a better match of die size to your application


for the 400pF panel 1 Apk should be fine for listening to music - even if thats not adequate for 20 KHz full amplitude sines



and before you start - could I interest you in some Life Insurance?
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