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The Original Quad Diaphragm Coating
A Direct Replacement
This article describes the broad details of a method for restoring or replacing the original Quad ESL diaphragm coating to its original specification. The description is mostly non-technical. Issues of materials obsolescence are addressed, and a number of apparent peculiarities in the original design explained. The methods described can be implemented with minimal, readily available equipment and chemicals. All surface resisitivity measurements quoted in this document were taken with a fully calibrated SR meter to ASTM D-257 standards. The accuracy is +/- 10%, and the repeatability is +/-5%.
Although almost anything, within reason, that can be spread on plastic can be used as a coating on an electrostatic speaker diaphragm, and work after a fashion; the original Quad ESL is particularly optimized in this respect, both in the chemistry and the choice of substrate material. Any old coating will not do the job quite as well.
This is not to say that other coatings (e.g. graphite) cannot be used with success in original Quads. They can, and when properly implemented they produce excellent results also. However, these substitutes, by and large place extra drainage on the EHT power supply, which is already marginal in respect of current drainage.
The original Quad speaker is a true marvel of subtle optimization within the materials / cost parameters of its day. Indeed, no polymer commonly available in production today would provide a markedly better material in their construction. I have only just begun to realise how subtle some of these optimizations are. The actual choice of which coating to employ in the original Quad ESL was/is part of this optimization.
Alas, as all of us know, the original coating is no longer available to Quad, or anyone else. The original substance is not a secret. It was called CALATON and was made by I.C.I. (U.K.). This material was a soluble form of Nylon. It could be dissolved in water/alcohol solution. The resulting solution was painted (literally) on the original Quad Mylar or Saran diaphragms that made up the moving element in the Treble and Bass panels respectively. The surface resisitivity, initially, achieved is 10 10 to 1014 ohms per square. Surface resisitivity is dependent on coating thickness as follows: R(surface) = R(volume) / t where t is the thickness of the coating.
Did Quad just put soluble Nylon on the membrane or was there an additive of some kind? A key question, obviously. Can soluble Nylon still be obtained anywhere in the world? - another key question. Will it actually work? The best question of all. Let's see about the answers to these questions then.
Soluble Nylon is still made by TORAY Inc. of Japan under the trade name of AQ-NYLON. When queried by e-mail regarding the product they are silent as the grave. Soluble Nylon was used in artifact restoration until the 1970's. It is somewhat disastrous in that application as it turns out, and its use has been discontinued. This may be why most previous manufacturers don't produce it anymore. Do not despair, there is a way around this problem, as I will describe later in this article.
Did Quad add "doping" materials to the membrane material? This question can be answered by Quad, OR, by spectroscopic and gas chromatographic analysis. OK - out the door, down the road to the chemistry department. Ask the professional chemists to "pretty please analyse this old piece of Quad diaphragm plastic and what's on it". Offer free beer and other sundry bribes, wait for a week, and see. The answer? "Yes, it's Nylon and polyester, but we can't say if it has had a significant amount of additive. It might just be dirt." Great guys(!) What can you expect after 20 odd years though? So, maybe Quad did, and maybe Quad didn't "add" something. This issue was sidelined for later examination, as and if necessary.
Firstly, unlearn, as I had to, the idea that static electricity means "stationary electricity". Think in terms of "nett charge separation", since this is actually the phenomena and also what we require. It is also nice if the charge does not move about on the membrane creating spurious capacitance variation and other bad happenings. F.V. Hunt's original mathematics shows that if capacitance variation is kept to <8% then distortions of all types are >0.5% at audio frequencies. Impressive, in a speaker.
The next basic lesson to learn (or re-learn) is about the Triboelectric Series, (see below).
· Asbestos <---- Acquires More Positive Charge
· Rabbit Fur
· Human Hair
· Cotton <-- ZERO
· Sealing Wax
· Hard Rubber
· MYLAR (P.E.T. in treble panels)
· UV Resist
· Acetate Rayon
· SARAN (P.V.C. in bass panels)
· Silicone Rubber <-- Acquires More Negative Charge
We can see that Nylon is near the top of the series, and both Mylar and Saran are much further down. In effect, this means that electrons tend to leave the Nylon and the Mylar tends to accept them. This produces a definite negativity in the Nylon. A close contact between Nylon and Mylar, as in a coating, guarantees that the electrons will stay very, very close to the Mylar "side" of the pair. Quad coated the diaphragm on BOTH sides. Perhaps now, we can see why. The Nylon surface tends to be positive and the Mylar tends to be negative. There is NO bulk current flow to speak of because of the generally insulative nature of each substance. It is a "nett separation of charge ". The migration of charge through the bulk of the membrane is slow by any normal electrical standard. If a Quad membrane was coated with Nylon on one side only, then one side of the diaphragm would tend to be in slight negative charge deficit with respect to the other side. Quad then provided a small trickle charging source at 1500 and 6000 Volts for the respective diaphragms to provide the "make up" of charge carriers in the system. I am sure that this is more obvious to us now, in an age of semi-conductors. In 1954-55, it was very clever indeed.
So, it was not simply a matter of placing a high resistance coating on the membrane, but a PARTICULAR kind of high resistance coating that is triboelectically positive with respect to Mylar or Saran for the most desirable effect in this system. The EHT supply is designed to work "hand-in-glove" with this specific combination of materials.
A coating such as graphite will work, but excessive current drainage on the power supply will cause the membrane to oscillate (hum) at the line frequency. This is BAD. The hum can be overcome by putting large resistances in the EHT power supply line to a panel. This limits the current drain, and the charge, as a result is more closely confined to the surface of the membrane, eliminating the hum. Sound quality is still excellent, and this is a solution proposed by both Sheldon Stokes (U.S.) and Andrew King (U.K.). It is workable and expedient, but does not exactly restore the speaker to original condition. Other benefits accrue when these resistors are added and both Andy and Sheldon have these described in detail on their web pages. I highly recommend them as essential reading, and will not elaborate further here.
The use of a large value charging resistor does not guarantee an absolute minimum of distortion at all sound levels across the normal audio spectrum, however. This is a consequence of the curvature of the membrane under dynamic conditions. As a result the capacitance will vary across the membrane - closer to the plates at the edges than in the centre. This means that a series resistance, large or otherwise, will cause variable RC time constant across the membrane. For small membrane excursions, this is negligible, so the technique is an excellent, cost-effective one for treble panels which (of course) experience low physical excursions. The ONLY way to effectively solve the problem is to place the high resistance on the diaphragm, so that a very high resistance is "in series" with every individual capacitance, so to speak. Standard calculations indicate that a minimum of 109 ohms per square is sufficient. The Triboelectric necessities of life raise that value somewhat (Nylon is used) in the narrow confines of this particular speaker. Conveniently, this enables the designer to lower the diaphragm to stator spacing and increase the efficiency of the device overall. This is another good reason, in addition to low distortion, for a very high resistance coating.
Maybe TORAY Inc. will reply to my e-mail about the soluble Nylon, maybe not. Please feel free to contact them, they are on the web. If available at a reasonable price, it would be a valuable resource.
DIY Soluble Nylon?
Yes! Nylon is “soluble” in Phenol/Water, Phenol/Methanol and 90% Formic Acid to name a short list of chemicals. An organosol is formed. Strictly, this is a kind of suspension, but let's not quibble about that since it won't be a problem in practice.
Materials : 25 grams Phenol (White, Crystalline solid, AR grade preferred) 150 ml water (tap water will do, distilled or demineralised is better) Nylon Fishing Line (not gel spun polyester!)
N.B. Phenol should be handled with care. Wear gloves, and mix it in a well-ventilated work area. Phenol vapours are slightly flammable at elevated temperatures.
- Heat the 150 ml of water in a glass beaker to about 50 degrees Celsius. This temperature is not critical. We want pretty hot water but not boiling.
- Add the Phenol crystals to the water in 5 gram lots with constant stirring. Wait for each to dissolve.
- Chop up about 5 grams of NYLON fishing line (not critical again), into 1 cm pieces and drop into the solution about 0.5 grams at a time. Keep stirring with a glass rod over heat, but avoid boiling the solution.
- Stir for 15 to 20 minutes until all the Nylon "goes into solution".
- Put the solution aside to cool to room temperature. You will see it separate into a thin dense layer, and a thick less dense one.
- Store this 'organosol' in a glass bottle with a resistant plastic stopper or glass stopper.
The “solution” made above is in fact an ‘organosol’ which is a kind of dual phase suspension (I'm told). It will, given a short time at rest, begin to separate into two obvious layers. The amount of Nylon you have dissolved is indicated by the thickness of the lower layer. The thicker it is, the more Nylon you have in 'solution'. Hence you must SHAKE WELL before and during use. This shaking process creates a suspension of phenol/Nylon in phenol/water as small droplets.
N.B. Phenol is also known as 'Carbolic Acid', and was used as the first hospital antiseptic/disinfectant. It is mildly corrosive. You should keep it away from skin and eyes as much as possible at these concentrations. It has a pronounced and distinctive smell, and prolonged vapour inhalation (hours and hours) can be harmful. It is very harmful if you drink it!! Don't be put off, normal care will avoid accidents.
An alternative to aqueous phenol is Methyl Alcohol and Phenol. This vapourises from the work surface better and faster, but I prefer to avoid one more potentially flammable chemical and dry the coating with a hair dryer.
You can apply the material produced above in a fairly direct way to the Mylar membrane as follows:
- Set out the work area with the Mylar diaphragm to be coated, using your preferred method.
- Shake the bottle of organosol you have produced above until it appears as a uniform, white, milky coloured liquid.
- Put some gloves on!
- Open the garage window!!
- Wet part of a paper towel by closing the mouth of the bottle off with the towel, and shaking until the organosol wets the folded towel.
- Wipe the wetted towel in a side-to-side motion across the width of the diaphragm, and move down the diaphragm in an overlapping zigzag fashion until you cover it. I recommend that you recharge the towel at least twice when doing a Bass panel diaphragm and once when doing a Treble panel. Your mileage may vary - as they say. The material will sit on the polyester as a streaky set of lines of droplets. It is as if you had a very bad windscreen wiper on your car. This is OK!
- Dry the membrane as you go with an ordinary hair dryer set to LOW heat. We don't want ot heat shrink the Mylar by accident at this point.
- The Nylon coating will dry to a whitish, streaky, thin-in-parts finish.
- Re-coat if you like, but don't expect it to ever look "even" or "uniform" - It won't.
The Nylon will adhere to the Mylar like tar to the proverbial tomcat. Guess what? It looks exactly like the original "sloppily applied white-paint-like substance" that people talk about when re-building old Quad panels and membranes. The smell of Phenol will linger for days! Don't worry about this, since when the panel is re-sealed inside the dust covers with a good tape all around, you won't notice an odour at all. You could also leave it in a protected out-of-doors area for the smell to dissipate for a day or so too, and then seal it up - take your pick.
DO COAT BOTH SIDES OF THE DIAPHRAGM! How do I know this?
Does It Work?
Yes. However, you must re-install the EHT supply bolts exactly as they were. The bolts I refer to are the two 12mm x 2mm bolts that hold the EHT wire onto the panels (Treble or Bass). As it happens, they just punch straight through the diaphragm, and this is what Quad intended - plus the fact that it is a lot faster during manufacture!
The idea is not to achieve a good, solid contact with the membrane. Do not use little bits of foil, et cetera., to do this. With a different membrane treatment, this may be OK, but not in this case. These fine (2mm dia.) bolts sit in the centre (approx. and automatically) of a 3 to 3.5 mm diameter rivet and panel hole. I suspect these two rivets are a mechanical device to cause simple automatic centring of the bolts. The bolts touch the diaphragm physically, but we are not trying to create a classical electrical circuit here. This is a very, very high resistance contact, at 'best'. A cylindrical radiator (bolt) is placed so that it radiates electric field to the surroundings across a very tiny air gap (in the main), and this supplies the "make up" charge on the diaphragm. The triboelectrical differences between the Nylon and polyester take care of the rest. In other words, don't try to set it up in terms of a classical current flow. It doesn't work like that, and trying to make it work like that is extra effort and counter productive.
A diaphragm coated in ”pure” Nylon as done above will take a long, long time to charge fully - sounds just like a Quad ESL again, doesn't it? In fact, too long. We know that the original Quad membranes would charge to a useable level in one to five minutes. Although we also know that if Quads are left on, the sound continues to improve for days, especially the mids and highs. I strongly suspect that this is due, in the case of the original, factory diaphragms to partial coating loss with age, and the fact that the distribution of charge in the Nylon becomes more “even” with continuous charging on a “good” membrane. In any case, this led me down the path of additives that Quad might have used to speed up the initial charge rate. To cut a long story short - to get an acceptable charge rate - add some anions permanently to the Nylon coating. In practice, doping the membrane with anions is simple. Take about 10ml of the organosol you prepared previously. Add 4ml of hand soap containing Sodium Laurel Sulphate. I had this done in a lab, but I have done it with a household product with the same result.
Coat a diaphragm in the same way as before with this “doped” organosol. Dry well using the hair dryer as before. Measure the surface resistance! If it is anything, anywhere below 1010 ohms per square, polish the surface off with a dry paper towel, as hard as you like and measure again. If this fails to raise the resistance, then 'strip' the surface with the original 'undoped' (no soap in it) organosol to raise the resistance again. The surface will increase in resistance to 1010 to 1012 ohms per square. It will NOT be uniform across the entire membrane, and it doesn't need to be. It isn't in the factory membranes either, should you be wondering. In fact completely removing and drying the soapy Nylon material seems to be sufficient. This absolutely minimal contamination seems to be locked into the Nylon coating for the life of the Nylon - a long time.
What you now have is an essentially insulative, by modern ESD standards, layer contaminated with anions, sitting on another insulative layer.
The effect is PERMANENT. I have completely 'stripped' the soap-loaded membranes with organosol until the surface measures 1012 ohms per square. These membranes still charge rapidly and improve with time. They sound just like the originals.
Don't be tempted to brush large amounts of the Nylon organosol onto the Mylar. For one thing, if you don't know what fibres are in the brush, it may dissolve! Secondly, a very high phenol accumulation in a given spot for a few minutes can weaken the Mylar. I actually had to use a 95% solution to do this, and this recipe is a lot less than that. Wiping and drying immediately will fix the entire coating as permanently as the original - so expect only about 20 years service from it.
Don't clean the EHT bolts with penetrating oil or silicone lubricants if they are a little corroded looking. I made this mistake - duh! This dramatically reduces their ability to create the small, uniform electric field where they 'contact' the membrane. Brush them with a small wire brush, and 'degrease' with isopropanol or other alcohol. In a pinch - buy a few new THIN bolts.
Typical Diaphragm Data
(as measured during trials of various diaphragm materials)
I sincerely appreciate the work of Sheldon Stokes and Andrew King as presented on their web pages, which attracted my recent interest in this subject. Having owned some Quads for 20 years, I can only say that Sheldon and Andrew's info has been invaluable. I hope my small contribution will interest them in return.
I also acknowledge some very useful insights provided by H.J. Wintle concerning charge trapping insulators and some fundamental notions about 'the real nature of static electricity" provided by William Beaty.
To Andrew King - thanks for answering my e-mails.
To Rob Flain at Quad Service (UK) - thanks for answering my e-mails.
Copyright © May, 2000 Gary J. Jacobson
This work may be used for non-commercial purposes with acknowledgment of the author.
No commercial publishing allowed without express written permission of the author.
All rights reserved.
After writing this document, I came across some important facts about nylon. One of these in particular is the fact that Nylon is hydrophilic, and tends to attract water from the air, is rather significant. The upshot of this new information is that you can apply the above coating as nylon dissolved in phenol/water, and let it dry. There is no need to add the soap at all, and strip it out. I was drying the diaphragm coating with a hair dryer, but I now let it air dry, since that leaves residual water, and the panel works immediately, charging in about 45 seconds up to a minute or so - quite acceptable. The previous procedure - using soapy organosol, and then stripping it off was not significant in so far as the soap was concerned, but merely the water content!
A re-read of the original patent by Williamson and Walker confirms that they chose nylon partly for it's water absorbing tendencies, and the moisture film on the coating, however slight aids conduction at the level required. Furhtermore, this is self-adjusting relative to the air humidity as the air drys out or becomes more humid. The tribo-electric effects certainly exist, but there is no evidence in Williamson and Walker's papers that they considered this as a factor when choosing nylon.
So, the conclusion is, finally, that the original Quad Electrostatic coating was pure nylon, nothing more.