Frequently Asked Questions ... the man said ... and that's what they are. I have composed the collection here from various questions asked in e-mails and other communications over the years. If you have a question that is not "Frequently Asked" then you can get a lot of people to ask the same question and it will then appear here. This is not a static document. Alternatively, if it is a less frequently asked question you can look in Parts , Fixit, Hard Core, Album or History. If you just cannot find your answer(s) anywhere on the site, then please e-mail me. The question can be about the Quad ESL, or the Quad ESL '63.

The listing below is somewhat prioritised, in order of most frequently asked, to least. However, this is not strict!!


What coating did Quad use on the original Quad ESL diaphragm?
A substance called CALATON. CALATON was a form of Nylon, which had its surface modified such that it would go into alcohol/water solution at the reflux temperature of Methyl Alcohol. CALATON was made by I.C.I. (U.K)

Was the diaphragm coated on one side only or both sides?
The diaphragm was coated on both sides
How do I know?
It is written down in GB Pat. 815,978 (OCt. 19, 1955), lines 25 to 33, in which Walker and Williamson state:
"The electrical resistance of diaphragm 21 is determined by three considerations. In the first place the surface resistance must be high enough to maintain sufficiently constant charge on both sides of the diaphragm of an elemental area despite the variation of capacitance of this elemental area due to the audio frequency vibration of the diaphragm." ... and, I’ve actually looked at one (or two ;) )

To maintain this strictly on "both sides of the diaphragm" they coated both sides of the diaphragm! Dr Stokes and I have had a small disagreement about the matter of the capacitance. He believes that the secondary and tertiary effects as expounded by F.V. Hunt are negligible, and that the capacitance should be a lumped (integrated) quantity. The designers of the original Quad ESL apparently thought otherwise. For a detailed explanation of this argument see the Hard Core section of this site.

Can CALATON be obtained anywhere, anymore, these days?
The short answer is - NO - commercially - See Area 51

Can a commercial replacement be sourced?
Yes. Soluble Nylons are available from DuPont - ELVAMIDE, and from TORAY Ltd. (Japan) - AQ-Nylon. I have been able to source a small amount of ELVAMIDE. See Area 51

Did the designers intend to use just Nylon as a coating, or was there something special in CALATON?
The answer is YES - The designers intended to use pure Nylon as the coating. In fact, they intended to use a membrane of pure Nylon (only) at one point in the design process.

How do you know? (invariably follows the previous question).
Peter Walker told me....and you....and everybody else in the known world. It is referred to quite clearly in U.S. Patents 3,008,013 (July 15, 1955) and 3, 008,014 (Sept. 12, 1957). Also in Great Britain Patent GB 815, 978 (October 19, 1955). The designer's state therein (lines 73 through 77, page 3):

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

The original reference that David Theodore Nelson Williamson and Peter James Walker used was: "Fibres, Plastics & Rubber", by W.S. Roff, Butterworth's Scientific Publication, London, 1956, pages 22, 27, 77 and 312

What is the Treble Panel Diaphragm made of?
The Treble panel diaphragm is made of polyester film, or more technically, Polyethylene Terypthalate, or PET film. This comes in several commercial guises including Mylar (Dupont), Melinex (I.C.I.) and Hostaphan (SKC, et al)

.How thick is the membrane in the Treble Panel?
6 microns in the factory fresh product. This was tensilized Mylar, made by DuPont. Ordinary, non-tensilized film as made in the 1950s and 1960s was not of sufficiently high tensile strength to be useful in the speaker, and wouldn't retain its tension for very long. We have sourced tensilised 6-micron P.E.T. to the same specification (made to order) as the original used by Quad. This is the ideal diaphragm replacement material for a treble panel. See Area 51

How thick is the membrane in the Bass Panel?
The Bass Panel membrane is 12 microns thick in all factory fresh original Quads. The Bass panels were made of SARAN, which is a PVC film, not unlike sandwich wrap film. We have also been able to source some 12 micron PVC which is virtually identical to the original. See Area 51

What is the resistivity of the diaphragm coating?
The original diaphragm coating had a resistivity of 2 x 109 to 1 x 1012 ohms per unit square. This is coincident with the surface resistivity of Nylon - surprised?

How do we know?(as per previous question)
It is in the original patent issued in Great Britain (GB 815, 978) which states in lines 69 through 71: "A Range of resistivities of the order of 2 x 109 to 1 x 1012 ohms per unit square is found to satisfy the above requirements." {To find out what the “above requirements” are here, you will have to read the Hard Core section of this site}

Can't I just insert a large value charging resistance, and have a lower diaphragm resistance? Dr. Stokes, Roger Sanders and others say that the diaphragm coating resistance doesn't matter too much, and I should just insert two 22 M resistors in the treble panel power supply and a 22 M Ω resistor in the bass panel supply and all will be 'cool man'.

OK, if you like, and the panels will sound pretty good. However, here is a quote form the original Williamson and Walker Patent G.B. 815,978 (lines 40 to 48) which says otherwise:

"Owing 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."
Another extract from the Provisional Specification attached to Patent G.B. 815,978 (lines 60 to 74), says, "...the damaging effect of such a discharge may be reduced by inserting a resistance of high value in the connection between the polarizing source and the diaphragm; this resistance has the effect of preventing the full polarising voltage from being applied continuously once a spark current develops but does not prevent a spark from occurring. With loudspeakers of all but the smallest size, however, this arrangement is not satisfactory because the capacitive charge stored in the loudspeaker is considerable and sufficient in itself to cause damage should a spark occur."

Here is yet another subtlety that modern designers of electrostatics ignore, inserting a large value charging resistance in the belief that it "cures all" - nope.

Can I get a coating like this on the diaphragm myself? Yes. Various graphite and metal oxide-in-resin coatings have been used to do this. They are all a 'bit fiddly'. Graphite can be made to work satisfactorily, but it is the least simple of all to apply well, and a resisitivity meter is essential to the process to ensure a uniform and correct resistance. If you would like to try a DIY alternative to the original coating, then you can read - “The Original Quad Diaphragm Coating - A Direct Replacement” by Gary Jacobson. If you can't hack all that chemistry, see Area 51

What are the panel dust covers made of?
Apart from a little bit of a wooden frame, they are covered with 6 or 12 micron film of the type used in the original panel on the Treble and Bass Panels respectively. You can use 12 micron film on both types of panel with some small sonic degradation in the Treble Panel. Treble Panels do require 6 micron film in the diaphragm and dust covers to give their very best

Where can I get 6 micron tensilised Mylar?
You can't get it anymore, since Du Pont have stopped making that specific product. The machinery they used to make it is gone too, so don't beg on the door step. You can get equivalent tensilised P.E.T. on this site - Area 51 - Mylar® is a registered trademark, that’s all.

What can I replace 6 micron tensilised Mylar with?
The short answer - 12 micron Mylar. It is nearly as good, and produces a little detectable sonic degradation in the treble panels, but you must get it to a higher tension to make the diaphragm resonance supersonic. See Hard Core, for the formula that shows a relationship between diaphragm tension, diaphragm mass and resonant frequency. Alternatively, SKC in the U.S. make a 7.5 micron and even a 4.5 micron tensilised polyester film, if you would like to buy 8 ton (I asked) minimum of the stuff!. Also, you could try 6 micron Hostaphan (very promising), sold in the U.K. by GoodFellow, see the Parts section of this site for details. Also, I am offering tensilised 6 micron P.E.T. ® on this site too, for a lot less money than Goodfellow do! See Area 51

Where can I buy 12 micron Mylar? (less than a ship load)
You can get it from Barry Waldron at the ESL Info Exchange. Barry sells 10 x 4 feet sheets for about $USD10 last time I looked. I've used it, and it is good stuff.

What is the thickness of the Quad ESL '63 diaphragm?
The Quad ESL '63 diaphragm was originally mooted by Peter Walker in an AES lecture in June 1979 to be 3.5 micron thick melinex (polyester film by I.C.I.). [See AES Paper ]

Can I buy Stators for the panels somewhere?
Originals are only available second hand. This can be a very good deal, if you can re-build them, or know someone who can (no, I don't do this for anyone, except me!). You cannot explicitly buy stators at all, but you can buy the made up panels from Quad, Musicweidegabe, Germany, and (yes) they have no web site, so look at the Parts section of this site for the e-mail address, and other details.

What is that gray paint on the Stators?
Gray paint, solvent based. Same stuff they sprayed on the Bass Panel dust covers to stop them showing shiny plastic through the front grille, (Source : Rob Flain, Quad U.K.)

What is the correct Treble Panel diaphragm voltage?
1.5 kV +/- 7%(from the Quad Repair Manual)

What is the correct Bass Panel diaphragm voltage?
6 kV +/- 7% (from the Quad Repair Manual)

Why don't I get 6kV when I measure the EHT's Bass Panel pin?
Are you using an electrostatic voltmeter? (Probably not) If you are using a probe attached to an ordinary DVM then expect to read somewhere around 3.9 to 4.2 kV with no bass panels connected and a perfectly good EHT supply. Why? The probe does not have a high enough input resistance and draws too much current from the small Cockcroft-Walton ladder in the EHT. This draws the voltage down to the level stated above. So- DON'T PANIC!!

What is the correct voltage on the EHT with good bass panels connected?
IF you are using a probe for measuring the EHT then 4.30 kV is just about right. Why? Well, you're drawing down the voltage because you're using a probe, but the EHT now has a little bit of resistive loading - the panels.

What sort of voltages will I see if I measure a bad bass panel?
Anybody's guess really. However, if you have "leaky" bass panels, then anything under 3.6kV is suspect. If you get readings much higher than 4.30kV then there is likely to be an EHT fault, probably a blown capacitor - no, they are not "good for life". For a more reliable supply, replace them with 3kV rated ceramics.

What special tools are required to work on the speaker?
For a complete list see the Parts Section of this site.

What is the big brown / gray can on the rear-left of the speaker?
That is the audio step-up transformer and matching network. The most reliable, and hardest to replace part. It is very clever design indeed, with a balancing choke and other niceties such as are missing completely on modern transformers of the type. Treat it with respect and care, and it will last another 40 years.

What is the small box and transformer on the right hand rear of the speaker?
This is the mains transformer and the little box "on top" is the EHT (Extra High Tension) step up ladder sealed in bees wax. The output of the mains transformer is around 610 Volts AC with enough current to kill(!). The very high voltages are delivered at minuscule current levels, and are paradoxically quite safe. It's the amount of energy it delivers that can stop your heart, not just the voltage.

How do I know the EHT voltages are correct in my speakers?
See above for the values of the Treble and Bass Panels. You will need special measuring equipment to measure the high voltages involved, see the Parts Section of this site. Do not use ordinary digital voltmeters, they will not work, and they may damage the EHT supply by draining too much current.

Help! Why is the EHT voltage so low?
Possible causes include: Faulty EHT block, low mains supply voltages, faulty mains transformer, 'leaky' panel(s). Turn the speaker off, and disconnect (de-solder) both panel lines from the EHT supply. The thick and thin red wires together on the left hand side of the EHT as viewed from the rear. If the voltage now comes up to specifications, then you have a leaky panel, somewhere. If the voltages are still low, then you have a mains transformer fault, or a low mains supply condition. See the Fixit Section of this site, for information on repairing the panels, fixing 'leaks' and replacing the mains transformer.

What is a 'leaky' panel?
Loss of voltage on the panel (to under specification), caused by the ionization losses at rivets, joins and connections. It can also be caused by corroded peripheral tapes or insulation breakdown, see the Fixit Section. The "loss" is loss of electrons to the surroundings. You would be surprised what will conduct at 6000 Volts!!

Can I get High Voltage Wire somewhere?
Lots of places. For these sources, see the Parts Section.

What sort of resistors can I use?
In the audio transformer, only high voltage resistors are suitable. The kind used in T.V. and other CRT focus circuits, of appropriate value, will work nicely. These are about $1.50 each, rated to about 10 - 15 kVolts (plenty). However, the audio transformer is the last thing you should touch. Elsewhere, more ordinary high voltage types will work fine. In the EHT supply block a perfectly ordinary 1/2 W 22M and 330k ohm resistor will do. See the Parts Section.

What sort of capacitors can I use?
In the EHT supply, the appropriate value as per specification, rated to 2.5kV minimum operating voltage. In the audio transformer, use the value per specification, rated to 6kV (4kV minimum). Parts Section.

What sort of diodes can I use in the EHT?
Bog standard 1N4007 is satisfactory. The original factory fitted diodes will be very likely very leaky if the unit has not been in for service for 15 to 20 years. If your EHT is faulty, this is one (cheap) possibility.

What can I use to repair the conductive layers on the Stators?
This is not very often asked - maybe folks don't know that there is a conductive layer there? Treble panels are coated with a conductive layer on the inside of the stator and bass panels on the outside. In both cases, they are painted over with that gray paint. You can use silver loaded paint for small areas ($AUD18 / ml), or the paint that is used to repair windscreen heater tracks (various brands). Parts Section and the Fixit Section of this site.

What Amplifiers can I use with the Quad electrostatic loudspeaker?
Any amplifier you like - is the trite answer. How long it will last before the smoke escapes is anyone's gues. This is not a one paragraph answer, and so I have a whole page devoted to this very topic. Please see the Amplifiers page for details on this question.

Should I use a special solder?
If you like to spend a lot of money - go ahead. It won't make a blind bit of difference to the sonics of the speaker.

Can I change the grille colour?
Get thee behind me Satan. OK, so you don't like Gold or Black or White (Quad, Germany). Scrub it all over with a soft wire brush, lightly! Then wash all the smut off with a good car washing detergent. Let dry, very thoroughly. Prime the whole of each side of the grille metal with an etch primer that is safe to use on aluminium. Then spray finish in your favourite colour. Then, go to confession, or whatever your religion dictates, oh evil one.

What are the dimensions of the original Quad Electrostatic?
34.5 inches wide by 31 inches high (including the feet) by 10.5 inches deep, taking the curvature into account.

Can I buy a whole "new" old Quad Electrostatic anywhere?
Yes, second hand, rebuilt or from Quad, Germany (which is to say, second hand, and re-built). See the Parts Section.

Is the original Quad Electrostatic a 2-way, or 3-way speaker?
It has 3 divisions to the matching circuitry, so, strictly speaking, it is a "3-way" speaker.

Is the original Quad Electrostatic a "phase correct" speaker?
You bet. The first of the line, in fact! The directivity was slightly altered to suit stereo after the first 400, or so came off the production line. Peter Walker (1994) speaking to Ken Kessler :

"And when stereo came along, you had to have two of 'em. A bit big for that but it worked very well. In fact after we made 400 of them we modified the directivity because of stereo. The directivity pattern was made for mono - not quite the same as stereo. Serial number 409 I think it was." I'll take PJW's word on that, even if he styled himself a "...silly old man of 78..." in that same interview. I hope I will be half as "silly" at 78, mate

Do you re-furbish Quads?
I, Gary Jacobson, being of sound (sort of) mind, and body, do solemnly declare that I do not do this for anyone, except me. I would not deprive those of you who want this adventure. Well, very occasionally, after a lot of discussion, I do a few, now and then.

How long does a Quad play after the mains are disconnected?
The answer depends on the condition of the panels and the EHT block (mainly). If the panels do not leak and the EHT is up to specification, then about 1 to 2 hours, with the level decreasing accordingly. If you've got a low resistance coating (e.g. graphite) then the time to decay to zero level will be less.

Where can I get a power indicator LED?
Wake up and smell the flux. That's a neon glower, with built-in dropping resistance. Quad couldn't have used a LED in the earliest speakers - wasn't invented, was it? Later on, they chose not too - had a large supply out back, I guess. If you want a neon light(!), see the Parts Section.

Wasn't the original film in the Bass Panel dust cover self-tightening or something? So, how come you're saying heat shrink film is "original"?
OK, the answer is in two parts. First of all, after about 1980, or so, Quad used a heat-shrinkable dust cover film. The important thing, about any film, is the gauge - it cannot be too thick, and 12 micron is alright on a Bass Panel dust cover. Some folks say it's OK on Treble Panels, but you really should use 6 micron there. The "self-tightening" thing arises from the Quad factory practice of bending the front dust cover frame inwards (concave) and then mounting the film (reasonably taught) and then when the frame was bent to the convex shape of the front of the panel during mounting, the restoring forces in the wooden frame kept the dust cover stretched. Heat shrinking is faster and better in my opinion. Quad use it in all their speakers now too. Practical Tip: If you want to take advantage of the restoring forces in "oppositely bent" frames, just swap the back frame with the front frame when recovering! [ Thanks to Stewart Penketh in Canada, for this very useful, practical tip. ] Dust Cover Film - Area 51

Can I use double-sided tape to hold the diaphragm in place on the rear stator?
A conditional 'yes'. The condition is that you use only Scotch Brand 3M tapes which employ high performance adhesive types 467 or 468. Other tapes lose their grip over time. The tape must be ultra-thin of course. Several tapes made for the computer industry and aircraft manufacturing industry are of a suitable type. Actually finding a supplier - now that's tricky.

What are the New Panels from the Quad Factory like?
First of all, they're not made by Quad at the factory at all. They are made by One Thing Audio in Coventry, England, or by Quad, Germany in Koblenz and have been for several years. So, they are nothing new. The ‘One Thing’ coating is a compound (probably polyester glue) containing graphite. The Quad, Germany coating is soluble nylon. The membranes are either 12 micron or 6 micron depending on who you ask. Rob Flain at Quad says "6 micron", but I know that Ron Best, One Thing Audio's technical brain, uses 12 micron film in all bass panels and 6-micron in treble panels. They undoubtedly work, and work well. They are just not like the originals, that's all.

Who has the best panels in the world?
Wayne Picquet and Stewart Penketh have the best rebuilds in the world right now. See my Links page and contact them directly.

What is the resistance of the Quad '63 Diaphragm coating?
This is between 107 and 108 ohms per square. The coating is a kind of carbon lacquer.

How high above the floor should the speaker be mounted?
Originally designed to stand on three small wooden feet that held it about 4~5inches of the floor and tilted backwards, the original speaker can (of course) be mounted on stands that raise it higher. I have stacked Quads that have the lower of the pair in each stack at ~35.5cm (14 inches) above the floor. This can, in some rooms, improve the width of image and height of image at the expense of some depth. However, the depth can be regained by using adequate acoustic treatment within the room. Peter Walker had a definite opinion about the height of the ESL ‘63 above the floor also.