Panel Coatings

This little debate gets a section of its own, since it is seemingly never ending. Rather than go straight on to look at specific coatings, let's take a look at the resistance requirements of a coating clearly spelled out in the original patent(s) by Williamson and Walker, based on Hunt's mathematical insights. Each sub-section of the debate has its own page, just click on the menu below to see as much detail as you can stand.

Patents & Panel Coatings

From U.S. Patent 3,008,013 we have:

"Provided the charge on the diaphragm is substantially incapable of changing during the period of one half cycle of the lowest frequency being handled, a simpler method of connection may be adopted, and the polarizing supply may be returned to either end of the transformer secondary instead of the centre tap."

The reason for rendering the charge on the diaphragm "incapable of changing" is taken from Hunt's paper and book in which he shows that constant charge operation reduces harmonic distortion (in a nut shell).

Now, also from U.S. Patent 3,008,013 :

"The electrical resistance of the diaphragm is determined by three considerations. In the first place the resistance must be high enough to maintain substantially constant any charge carried by any elemental area of either side of the diaphragm when the diaphragm is vibrating at an audio frequency."

So, here we have a hint that the diaphragm was coated on each side, and that treating the resistivity as a "lumped" quantity , and inserting series resistance in the EHT circuit is not on, as far as the designers were concerned. This attention to detail is probably one reason why modern ESLs that use low resistance diaphragms and high value series (charging) resistors never seem to have the mid-range purity of the Quad Electrostatic Loudspeaker.

Moving on in the next paragraph of the patent mentioned above:

"This reduces non-linearity due to the quadratic nature [F.V. Hunt] of the forces which are involved if the diaphragm potential is fixed. Due to the fact that in any practical loudspeaker the diaphragm does not operate as a rigid piston because of the necessity for supporting members and because acoustic impedances are not always uniformly distributed over its area, the same degree of improvement is not obtainable by the insertion of resistance external to a conducting diaphragm." So, here is a fairly damning indictment of the use of external (to the diaphragm) charging resistors to achieve a high resistance path and a low distortion sound. A high resistance membrane ensures good efficiency and a quiet background also.

The second reason for the resistive properties of the diaphragm to be as they are:

"The resistance must also be high enough to prevent injurious sparking in normal operation. So that if a spark should tend to occur between a small area of the and one of the plates, the discharge current flowing in the areas of the diaphragm adjacent to the discharging area causes the potential of the discharging area to be reduced below the value required to maintain the discharge. Only a very small charge is dissipated before the voltage becomes thus reduced and the spark is accordingly rendered harmless."

Note that the designers say in "normal operation". You can arc the thing easily if you put more than 35VRMS on the audio transformer terminals.

The designers third reason for a particular resistance of the diaphragm:

"Thirdly, the upper limit to the value of the resistance is determined by the necessity of allowing the polarizing charge to spread uniformly across the diaphragm and must be low enough to allow sufficient current to flow to maintain the electrostatic charge despite leakage from the central part of the diaphragm through the air by ionization. A resistance value of 1000 megohms per unit square has been found to satisfy all these requirements in loudspeakers of normal size; however, resistance values ranging from several hundred megohms to several thousand megohms per unit square may be used. "

Do these guys ever get specific? Yes, in G.B. Patent 815,978, lines 69 to 71, they clearly state:

"A range of surface resistivities of the order of 2 x 109 to 1 x 1012 ohms per unit square is found to satisfy the above requirements."

That is pretty clear.

So, now you know why we need a high resistivity membrane to make a very high quality electrostatic loudspeaker. Accept no substitutes.

How then, did they get the high resistance, and what was the coating? Well, (you guessed), here is another quote from G.B. Patent 815,978":

"A suitable material for the diaphragm is nylon, the surface of which, owing to its affinity for moisture, has the above stated resistance over the relative humidity range 95% to 38%."

Of course, we know that the diaphragm in a Quad is made substantially of polyester film. This is simply because you cannot get nylon to stretch and stay stretched to the required tension. So, what did Williamson and Walker do? They applied a nylon coating to the surface of the polyester to get the range of resistivity required. This, in terms of the engineering constraints in the 1950s was the only realistic way to go in a "producible" speaker. The nylon they chose had to be easily applied to a thin membrane, not be too thick and add much mass of its own. They chose CALATON® made by I.C.I. However, you can apply any soluble nylon and it will work just as well. Even my DIY nylon coating achieves this goal.

Apply it to both sides to ensure uniformity of charge on both sides, since it does not give a very even finish,as many refurbishers, myself included can attest.

Specific Coatings

Specific - actual materials that I can use for God's sake - why do you think I'm reading this at all, man!! OK, you can coat a polyester film with:

Probably a few weird and esoteric, and possibly erotic things that I've never heard of, and probably don't want to hear of.
To work, and go on working, you need a substance which can:

                        
      o Achieve the resistance range stated above for the Quad ESL
      o Retain its resistivity for many years.
      o Stick to the diaphragm for many years.

Graphite

Graphite, as just that - graphite - will not, no matter how hard you rub it in to achieve the resistivity range required, or anywhere near it. It must be applied with great care, and reasonable force(!), to grind it into the plastic. It gives uneven results, and you need a good surface resistivity meter to know when the coating is uniform in resistance. It contaminates the work area, and the stator, if you leave any whatsoever lying around the place. It does stay on the diaphragm well, because you grind it in. Electrically, its main fault in this application is that it is just far too conductive.

Hand Soap

Yes, just the stuff you buy in the supermarket. You may as well buy the cheapest you can get, as long as it has Sodium Lauryl Sulphate in it. This provides a conductive film from the residual anions and/or cations in the soap. The water, originally in the product will cause the conductivity to be higher, but this will evaporate. This is not a molecular film of water, but substantial water in which the components of the soap are dissolved. My experiments with soap film show that it does not achieve the optimum range of resistivity for Quads, and that it falls/rubs off the diaphragm very easily. The latter phenomenon is not so great when you want to vibrate the thing at audio frequencies.

Wall Paper Paste

(some carboxy methyl cellulose) Similar to soap in effect. A sticky mixture that you can dilute and achieve a uniform coating easily. This puts it ahead of graphite in the handling stakes. However, as we all know, it is designed to stick to porous surfaces and is in water solution. Hence, it has the same problems with falling off the membrane as Hand Soap.

Carboxy Methyl Cellulose (pure)

Information from Tony Fitchett, Wellington, New Zealand indicates that pure carboxy methyl cellulose, made up properly in a lab to 3% solution provides a long term resistivity of 1012W per square. Better adhesion of this material may be due to the lack of additives used in wall paper paste.

Indium Tin Oxide

A good choice if you can handle the chemicals involved. With practice I am told that you can manipulate the amount of oxide in the polyester resin to achieve just about any surface resistance that you like. Not really recommended for the casual refurbisher, but very good, nonetheless. Sheldon Stokes claims that he uses this mix. I suspect that it may be just Tin Oxide/Resin mix. There is a (now obsolete) water-soluble polester resin based glue that you can add Tin Oxide to. It was known as Zelec™ , but as far as I know it’s now unavailable.

Tin Oxide / Poly Resins

This sort of thing is commercially available, or used to be, in industrial cements/adhesives, like ZELEC. This is simply Tin Oxide mixed in a water-based polyester resin.

Soluble Nylon

What Quad originally used on the diaphragm of the Quad ESL. It has an affinity for atmospheric moisture and retains a molecular (not bulk) film of moisture in the vicinity of the diaphragm, and this is what carries the current, in the main. There are also large triboelectric differences between the materials used in the Quad ESL (Saran®, Mylar® and Nylon® ) such that charge tends to stay closer to the diaphragm when a combination of these substances is used. This prevents the electron "cloud", "field", from becoming too voluminous (not much really) and causing the panel to hum, as more than a usual amount of current flows from diaphragm to stator across the air gap and into the audio transformer and back to the EHT unit. It is possible to make your own soluble nylon or maybe buy some from DuPont in the form of ELVAMIDE® See - Nylon Coatings in Detail