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Electrostatic Amp THD Specs/Measurments for the Full Audio Band ?
Electrostatic Amp THD Specs/Measurments for the Full Audio Band ?
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Old 13th November 2017, 12:08 PM   #1
MrMagic is offline MrMagic  Greece
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Default Electrostatic Amp THD Specs/Measurements for the Full Audio Band ?

I can't find a single electrostatic amp characterized with THD specs up to 20Khz or measured by a reviewer. All THD specs I've seen, are only for 1Khz -which was fine in the 50's, but not today, especially when we are talking about High End. Do you know of any such specs / measurements?

The reason is, I'm trying to develop an "impossible" electrostatic headphone amp. Initial goal: ultra low distortion <0.01% at the full audio spectrum, ultra low noise, high output voltages -about 1400Vpp (500Vrms) -solid state, DC, direct drive. Complementary output (which would be the easy solution) is out of the question, as there are no PNP bipolar or P-channel mosfets capable to withstand such voltages. Simply using two N-channel mosfets is an option but highly unlikely (or too difficult) to achieve such low levels of distortion at such a high voltage span, and at least up to 20khz. So I'm researching other topologies, even some completely exotic ones, that I'm currently trying.

Which is why I would like to know what has been achieved so far in terms of THD levels for the full spectrum -as a reference.

Last edited by MrMagic; 13th November 2017 at 01:02 PM.
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Old 14th November 2017, 06:57 AM   #2
EUVL is offline EUVL  Europe
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Why would you need 500Vrms for headphones?

And can't you use 900V PNP in cascode to get 1800V max ?


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Old 14th November 2017, 07:06 AM   #3
jcx is online now jcx  United States
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save some time by reading Gilmore's ES amp threads Headphone Amplification - www.Head-Case.org
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Old 14th November 2017, 08:11 AM   #4
EUVL is offline EUVL  Europe
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Fully complementary solid state is much more interesting.


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Old 14th November 2017, 09:00 AM   #5
MrMagic is offline MrMagic  Greece
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Quote:
Originally Posted by EUVL View Post
Why would you need 500Vrms for headphones?
Two reasons:
1) I want it to be able to drive anything on the market (there are a couple of amps with a similar output)
2) I want to drive my current DIY electrostatic headphones to the maximum and make another version with an increased space between the diaphragm and the stators in order to get a lower frequency response.

Quote:
Originally Posted by EUVL View Post
And can't you use 900V PNP in cascode to get 1800V max ?
Nope. You can cascade more than one transistors to make a high voltage one, but you can't have more output voltage sweep than the voltage rating of the transistors, because then each transistor will "see" the full supply voltage difference which will be much higher than their ratings and all of them will get fried.


Quote:
Originally Posted by jcx View Post
save some time by reading Gilmore's ES amp threads Headphone Amplification - www.Head-Case.org
Thanks. BTW, I just saw a post of his on head-fi.org about electrostatic amps' voltages and he mentions the ratings as "peak to peak stator to stator" by adding each output peak-to-peak and thus reaching double the actual voltages -which doesn't make sense. For example for +-750volt supplies he mentions 3000V "stator to stator" which is wrong. Even stator-to-stator the pp value is the same.
Electrostatic Amplifiers Voltage ratings | Head-Fi.org

Last edited by MrMagic; 14th November 2017 at 09:12 AM.
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Old 14th November 2017, 09:06 AM   #6
MrMagic is offline MrMagic  Greece
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Quote:
Originally Posted by EUVL View Post
Fully complementary solid state is much more interesting.Patrick
It is just easier but it can't be done at such voltages.
"Interesting" is to provide the "ultra" specs that are desirable, with a different topology, because it is a challenge.

Last edited by MrMagic; 14th November 2017 at 09:09 AM.
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Old 14th November 2017, 10:31 AM   #7
EUVL is offline EUVL  Europe
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http://www.electro-tech-online.com/a...amp-jpg.58872/


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Old 14th November 2017, 11:08 AM   #8
MrMagic is offline MrMagic  Greece
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Quote:
Originally Posted by EUVL View Post
You were right, I stand corrected, thanks for the link. I misunderstood the concept. If all transistors on one side are on, the voltage difference of 1600 volts will be divided to the transistors of the other side based on the resistor ladder.

Still, I wouldn't consider this solution as elegant -it's ugly
And it requires triple the active parts.
Not to mention they play a dangerous game. Humidity or other factors could shorten the life of the output stage and if one fails, then most likely half the output transistors will fail.
So I will leave it as a last resort in case a more elegant solution fails to provide the "ultra" specs.

P.S.
That topology would be nice to drive large ESL speakers of 10,000 volts or so -although I've thought of something completely different without the risks of a frying avalanche.

Last edited by MrMagic; 14th November 2017 at 11:12 AM.
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Old 14th November 2017, 11:33 AM   #9
EUVL is offline EUVL  Europe
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If you use 2x 2SA1968 etc., it is still very simple and elegant.
Say you use +/-800V rails, you can put a 900V ZD across the C-E of the cascode device as added protection.
Under normal operation, the BJTs will not see full output swing.
So the operating voltage is a fraction of its max. rating.


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Old 14th November 2017, 02:07 PM   #10
MrMagic is offline MrMagic  Greece
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Quote:
Originally Posted by EUVL View Post
If you use 2x 2SA1968 etc., it is still very simple and elegant.
I prefer not to use abandoned parts from an ex-company (Sanyo) that was bought by ONsemi but they are no longer produced -except from a ghost company that I still can't find after searching for half an hour. Unless I'm missing something.

Aside that, its PNP version -2SA1967 at 900V can only withstand about 5ma max continuous current which is too little for my purposes.

Powerful PNP/P channel transistors and mosfets simply do not exist in voltages higher than about 300 volts for bipolar and 500 for mosfets.

Last edited by MrMagic; 14th November 2017 at 02:11 PM.
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