Biasless ESL?

[bentoronto]

As far as I know, there are billions of electret microphones of every quality right up to the highest out there. I'm pretty sure some headphones were electret (Koss?) and have always wondered why they didn't become as common as in mics. Or why they weren't used for stationary systems. Anybody know?

Of course, when you read about them, it sounds like a cross between a permanent magnet and a perpetual motion machine.

[alexberg]

Unfortunately, most of the mics have built-in Jfet for front-end/decoupling of cable capacitance. Otherwise one may buy a zillion of surplus mics and make decent HF

[kavermei]

Originally Posted by bentoronto

[...] have always wondered why they didn't become as common as in mics. Or why they weren't used for stationary systems. Anybody know?

Not speaking with authority here, but my gut feeling would be cost -- I think electret films are very expensive per unit area, and considering that a speaker needs so much more area than a mic or a pvdf sensor...

Another guess (again, just guessing, would be nice if someone could shed more light on this): maybe the charge imbalance attainable with electret films is much less than what can be attained by coating & electrifying the film, leading to a much lower sensitivity for electret ESL speakers, all other factors being equal?

Kenneth

[Calvin]

Hi,

some reasons for electrets not beeing used extensively in stationary equipment are the amount of charge, the stabilty and longevity of the stored charge and membrane thickness, i.e weight.

jauu
Calvin

[alexberg]

Electret films on avail have specific charge around 10^-4 Q/sq.m. Usually teflon based. PVDF is piezo material as well and not that expensive.

[bentoronto]

Anybody like to try to integrate these electret posts? Sounds like a non-definitive "Yes, but...." situation.

Is the world due for another try at electret ESLs?

[Calvin]

Hi,

I know of manufacturers trying to improve elecrets with regard to longevity and amount of charge. My latest infos were that they are testing a film thats charge would be equivalent to 2kV bias.
If that film is low enough in weight and features sufficient mechanical strength and endurance a large electret apeaker panel would hold a few advantages over the classical way of biasing. There woud be no flashovers between stator and membrane and charge distribution over membrane would be very even. There would also be no need for a wall conncection to power the ESL.

jauu
Calvin

[bentoronto]

Thank you for summary. 2kv is a useful bias voltage esp. if you can have closer stator spacing (in the absence of HV bias).

No small benefits to get rid of HV bias, instant turn-on, perhaps pre-fabricated electret force could be modulated across membrane for various purposes, etc.

I suppose it moves the construction away from DIYers.

[Calvin]

Hi,

why should it dis-DIY? It´d be just a matter of sourcing the film. But that´s a problem with any other film too, nowadays that the production of certain films has ceased like some Hostaphan and Mylar types.
With regard to building things would become easier and safer, especially no handling with high voltage supplies -apart from the audio tranny..
But I´m afraid that weight-per-unit of the film would be too high. Also the mechanical strength probabely won´t be sufficient for hard mechanical tension, which would ask for different methods to achieve high values of stiffness with low tension.

jauu
Calvin

[markusA]

I have a question or two about this technique.
Let's assume we have a ESL with a 3kVpp signal at full swing.
When we attenuate the signal (we don't want the speaker att full full spl all the time) with a max attenuation of 60dB we get a 3V drive signal.
Now, sourcing the HV bias from 3kV is easy, what do you do when you only have 3V available?