[Muamp]

Hi Coinmaster,

Yes, you can increase the space between the stators to reduce the capacitance and then increase the p-p voltage.
The maximum p-p voltage at headphone stators is normally about 1300v p-p (460v RMS). In my very first panels I made, (from perforated steel) I originally used spacers of 1.5mm each side with an audio transformer energiser which went up to about 2200v p-p. And of course the large ESL speakers have bigger spacers and use much higher voltages.

David.

 

[coinmaster]

Instead of increasing ac voltage, how bout increasing stator voltage? It's easier to increase DC voltage than get higher AC voltages with low distortion.

 

[jgazal]

Instead of increasing ac voltage, how bout increasing stator voltage? It's easier to increase DC voltage than get higher AC voltages with low distortion.

Do you mean increasing bias voltage in the diaphragm/film or increasing dc voltage rails in the amplifier?

 

[coinmaster]

I'm not sure how you are biasing it but I was going to keep the diaphragm at ground level and put bipolar voltages on the stators. So increasing the spacing and voltage of the stators should allow the same result with less capacitance as long as the air dialectric isn't broken.

 

[jgazal]

I was going to keep the diaphragm at ground level

How do you charge the diaphragm resistive coating?

 

[coinmaster]

Ground is a charge potential just like any other voltage.

 

[jgazal]

Ground is a charge potential just like any other voltage.

I don't understand.
Perhaps I should let others chime in.
Nevertheless, just one more question.
In the circuit you mentioned, if diaphragm bias is at ground potential, does the charge in the coating vary as you apply your higher gain AC signal to the stators?

 

[100VoltTube]

I believe the general goal is at least 300v p-p, but it is nice to have headroom. Most gilmore designs use ±400v rails or more, and can swing almost to the rails.

Determining efficiency and capacitance and so forth is all just math. It's irritating math, but if you want cold, hard values, there's no substitute

 

[coinmaster]

If the diaphragm is attached to ground then no, it stays at ground potential unless I apply the signal to the diaphragm which would certainly make things much easier to design in so many ways. Unfortunately I need to design a pair of stats using ground potential on the diaphragm because I'm going to build an ES/ribbon hybrid and that is the only practical way to do it without major capacitance issues and construction difficulties. Then again maybe increasing stator distance isn't such a great idea after all since the electromagnetic field of a ribbon is supposedly equal to only a half turn of a coil and I'm not sure how much current I'm going to require for that as it is. I could stack the magnets in series but then that would be more mass obstructing air waves. Then again such a headphone design should have ludicrous sensitivity so maybe I don't need to go all out on both ends, or maybe I should. Bah, so many variables.

 

[100VoltTube]

In the design @coinmaster is talking about, as I understand it, the diaphragm is at 0v. The highly resistive coating keeps the entire diaphragm at that charge as it moves. The stators are biased, probably at about 600v. that bias works just like the bais in a normal electrostatic headphone. A good way of thinking about it is coulomb's law:

"where ke is Coulomb's constant (ke = 8.99×109 N m2 C−2), q1 and q2 are the signed magnitudes of the charges, and the scalar r is the distance between the charges. The force of interaction between the charges is attractive if the charges have opposite signs (i.e., F is negative) and repulsive if like-signed (i.e., F is positive)." (from wikipedia)

That is why diaphragms are biased, and why there are 2 stators.

 

[100VoltTube]

BTW, you can do single sided very easily with a tube like the QK329 right at the beginning of the signal path. Unfortunately, I'm not aware of anywhere that sells that tube. There may be none left.

Edit: It would seem that they come up on Ebay sometimes. One sold for $226.15
There goes that idea

 

[chinsettawong]

You can also make your amplifier more powerful in order to overcome the high capacitive load.

 

[100VoltTube]

Here's an odd idea: A headphone with 2 drivers per side, placed one behind the other, both sealed together. The close one would have very thin mylar for detail, and the outer one would have thick mylar for punch in the bass. They would be wired in series, so the capacitance would be huge, but a powerful amp. like a KGSSHV, should have no problems driving them.

 

[coinmaster]

Yeah I was thinking about this as well. The added bonus would be a push pull effect giving it more SPL.
The problem might be that the thicker mylar won't be able to keep up with the thinner mylar which may smear the sound. It's possible that the dual drivers themselves might be enough to achieve better bass but who knows.

 

[chinsettawong]

Hi 100VoltTube,

If you go back and read my posts, you would see that I actually did try the two stators two diaphragms design a few years ago. At that time, the result was quite disappointing. The sound was kind of echoing and didn't sound good. I didn't know why.