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peranders 14th September 2002 10:21 PM

Electrostatic speakers, real ones and...Martin Logan
 
2 Attachment(s)
Hi!

Can anyone explain the difference between a electrostatic speaker with high resistance diaphragm and with low like Martin Logan? Is it an advantage with "sticky" electrons.

Is this an old Quad?

gilid 14th September 2002 11:13 PM

I thought M-L had a high-resistance diapragm - I used to own a CLS mark 1, where the diapragm was coated with carbon powder, resulting in a high resistance coating (approx 100 kohm, if my memory serves me well)

MRehorst 15th September 2002 02:30 AM

A high resistance diaphragm is used for ESLs to maintain "constant charge" operation. By having a very high resistance, the charge on the diaphragm is prevented from moving toward the center of the diaphragm when the diaphragm is deflected toward one stator or the other by the audio signal.

If the charge were allowed to move (low sheet resistance diaphragm), at extremes of excursion all the charge would move to the center of the diaphragm so only the central area of the diaphragm would be driven and the result would be distortion.

I believe Martin-Logan uses a vacuum deposition process to put an extremely thin metallic coating on the diaphragm. Whatever the method used, it is a very high resistance coating.

MR

MRehorst 15th September 2002 02:32 AM

No, it's not one of the Quad 57s. Not an ESL63 either. Could be one of the newer models (I haven't seen their guts), or someone else's speaker altogether.

planet10 15th September 2002 03:59 AM

Quote:

Originally posted by MRehorst
No, it's not one of the Quad 57s. Not an ESL63 either. Could be one of the newer models (I haven't seen their guts), or someone else's speaker altogether.
The newer models are close enuff to the 63 that you'd recognize a family resemblance, so these are another maker. I assume the damped part in the middle are tweeter panels?

dave

cyclotronguy 17th September 2002 06:01 PM

Graphite
 
Over the years graphite has been the conductive coating of choice for many ESL makers. Graphite it a poor choice for everyone but the makers, guaranteeing a 100% failure rate. High voltage on the system will eventually drive the carbon (graphite) off as carbon dioxide.

Cyclotronguy

MRehorst 17th September 2002 08:44 PM

Eventually like when? I am still waiting on drivers I built 12 years ago to stop working.

No I haven't had them biased continuously for 12 years, but the force produced by the electric fields in the speakers is very weak compared to the force used to rub the graphite into the film.

The graphite coating will outlast foam speaker surrounds, belts and lubricants used in tape machines, esoteric CD players, and turntables. It will outlast most pots, many switches, most electrolytic capacitors, vacuum tubes, and a lot of other materials found in audio systems.

Graphite works just fine and lasts a long, long time.

MR

MRehorst 17th September 2002 08:47 PM

Uh, as to carbon dioxide...

I've never heard of graphite being converted to CO2 in a high voltage field. Do you have any sort of technical reference for that?

MR

gilid 17th September 2002 09:08 PM

graphite coating
 
Yes,cyclotronguy is right - I had the stators on my Martin Logan CLS's replaced twice. Once was for delamination, and the other time was most likely due to the carbon issue. That is why ML went to the molecular deposition process (incidently, this is done for them by PPG corporation, using the same process used to deposit heating films onto aircraft winshields, for anti-icing protection).

It is also tricky to achieve a uniform resistance using carbon, since it is not a controllable process - it is easy to end up with more carbon on one portion of the diaphragm.

cyclotronguy 17th September 2002 09:32 PM

Sorry, I no longer have access to the documentation; it belongs to my previous employeer.

Part of their normal operation required frequent fabrication of high purity carbon into electrostatic deflectors. The carbon and documentation came from a small branch of Union Oil that supplied the carbon in various forms.

Rate of re-dox was a time, charge, pressure, mole relationship; proportional to time and charge, inverse for pressure.


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