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Old 27th January 2010, 08:23 PM   #1
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Default Single stator electrostatic headphone.

Hello DIYs,

I'm considering building a single-stator electrostatic headphone. I have noticed, however, that some say that such a design is inherently unlinear because of the differences in the electrostatic field force as the membrane moves back and forth. Apparently this is because the force acting on the membrane varies by the distance to the stator.

This surprises me, though, as I would guess that the variation of the electric field will be quite small given the sub-millimeter movements of the membrane. My reason for saying this is that the electric field emanated by the membrane - or the stator - to me appears to be more comparable to infinite flat surfaces than to a point source, the latter having square variations in force on the membrane (i.e. my guess).

I would assume that a single stator (constant charge) would exhibit very low electric field variations since the field is more or less the same. Or have I missed something?

I have attached a drawing that hopefully illustrates my thoughts.

Any of you have ideas on this?

Greetings

Jesper
Attached Files
File Type: pdf SE electric field illustration.pdf (8.1 KB, 99 views)
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Old 29th January 2010, 08:35 PM   #2
godfrey is online now godfrey  South Africa
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Hi Jesper

For a single-stator ESL, you can't have constant charge on either the membrane or the stator.

You only get constant charge with constant voltage, but then there's no output.

There's a nice explanation of the fields and forces here: http://dept.physics.upenn.edu/~uglab...ric_forces.pdf

The important bit (at top of page 2) is that the force on the membrane is proportional to the square of the voltage between it and the stator, and inversely proportional to the distance between them.

So e.g. with a bias of 500V and a signal of +-50V, you get about 2.5% 2'nd harmonic distortion. Movement of the membrane will add a bit more distortion too.

It might sound good anyway (a bit of low-order distortion isn't a disaster).

There's another problem: if the membrane's stretched too tight you get no bass, but if it's too loose it gets sucked onto the stator unless the bias voltage is low, in which case sensitivity is also low.

Why not just build a push-pull version? It's no harder, maybe even easier than single-sided.

Perhaps you want to drive the headphone with a single-ended signal e.g. the output from a single triode?

This can still be done with a normal two-stator headphone by only applying the audio to one stator and earthing the other stator.

Regards - Godfrey
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Old 30th January 2010, 08:37 AM   #3
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Hi Godfrey,

- and thanks for your reply. Hmmm ... First it made my head spin ("why could it be that a single stator had no output with constant charge ...?") but I guess the reason is that the output power available to the electrostatic system is dependent upon the resistor in series with the constant charge membrane (right?) And if this is about 1 Gohm, the power is virtually zero, thus no output.

My reason for not wishing to use two stators is that I'd like to have as few "obstructions" between my ears and the membrane. But I'd like to ponder your idea about two stators, one of the grounded. And, yes, I would like to drive it from a single-ended source.

Thanks again for your reply - very clarifying (supposing my comments above is correct)

Jesper

Last edited by gentlevoice; 30th January 2010 at 08:41 AM.
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Old 30th January 2010, 10:15 AM   #4
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Default electrical low cut-off frequency?

Hmmm...

BTW - can I ask you, or maybe another DIY'er reads this, how the electrical low cut-off frequency in an electrostatic headphone with two stators, and constant charge is calculated?

Would it be reasonably correct to say that it's a highpass filter formed by the combination of the capacitance between either of the stators and the membrane (parallel capacitance), and then the resistance of the charge resistor?

That is, if the capacitance between either of the stators and the membrane is e.g. 180 pf and and the membrane's charge capacitance is 2 Gohms, then the low cut-off frequency is: 1/(2*pi*2 Gohms*180pf/2) = 0,88 Hz ?

Best regards,

Jesper
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Old 30th January 2010, 10:16 AM   #5
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by gentlevoice View Post
My reason for not wishing to use two stators is that I'd like to have as few "obstructions" between my ears and the membrane.
You'd have to be very careful not to let the membrane touch your ear, otherwise the membrane is obstructed and your ear gets zapped

I've been thinking of building two-stator headphones too. I'm starting to think the idea of single-ended drive with two stators might be a good one.

Partly for safety but also because with the earthed plate against your ear, the ear-to-plate capacitance won't affect the signal.

Cheers - Godfrey
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Old 30th January 2010, 10:24 AM   #6
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Hey Godfrey - appreciate that you consider my health . I was intending to put something in between the membrane and my ears, though. Having 500 volts just next to a very sensitive part of me might not be that conducive to a relaxed listening experience

Cheers

Jesper
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Old 30th January 2010, 10:48 AM   #7
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by gentlevoice View Post
That is, if the capacitance between either of the stators and the membrane is e.g. 180 pf and and the membrane's charge capacitance is 2 Gohms, then the low cut-off frequency is: 1/(2*pi*2 Gohms*180pf/2) = 0,88 Hz ?
Firstly that should be: 1/(2*pi*2 Gohms*180pf*2) = 0,22 Hz because the membrane sees both capacitances in parallel i.e. 180 + 180 = 360pF.

However that is not a low-frequency cut-off in terms of frequency response. It is the frequency below which the membrane no longer operates at constant charge. So above that frequency, distortion reduces with increasing frequency.

There is no low-frequency electrical cut-off in an electrostatic headphone, only a mechanical one.

There will most likely be a low-frequency electrical roll-off in the drive circuit, though. For example, the stator is normally connected to earth with a resistor and the signal is applied to it through a capacitor. So with a 100K resistor and 100nF capacitor, the roll-off will be about 16Hz.

Regards - Godfrey
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Old 30th January 2010, 10:52 AM   #8
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by gentlevoice View Post
I was intending to put something in between the membrane and my ears, though.
... and as long as you're putting something there, it may as well be another stator.
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Old 30th January 2010, 11:01 AM   #9
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oh - yes parallel capacitors ... C1 + C2 ... it slipped - thanks.

F
Quote:
or example, the stator is normally connected to earth with a resistor and the signal is applied to it through a capacitor. So with a 100K resistor and 100nF capacitor, the roll-off will be about 16Hz
Why is this - to protect from electrostatic discharge if the membrane touches the stator?
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Old 30th January 2010, 11:35 AM   #10
godfrey is online now godfrey  South Africa
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No, I was thinking of the DC decoupling at the output of the amplifier.

For example if you want to drive the headphone from the plate of a valve, the valve plate will have a few hundred volts DC on it, and you'd want to block that from the headphone.

With transformer coupling it would not be needed, but then the transformer itself would have a low-frequency roll-off.

In general, there is likely to be an electrical low-frequency roll-off somewhere - in the amplifier and/or the coupling between it and the headphone.

The electrical roll-off is easy to control though e.g. by choice of components.
The mechanical roll-off due to membrane tension is normally the one that determines the final result
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