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Refurbishing Classic Quad Electrostatic Loudspeakers

Andrew Back
Open source (hardware and software!) advocate, Treasurer and Director of the Free and Open Source Silicon Foundation, organiser of Wuthering Bytes technology festival and founder of the Open Source Hardware User Group.


January 3, 2019 16:07

Did you know that Gradient produced a subwoofer "Gradient SW-63 for your Quad speakers. You can read about this here There is however no picture on the english page. But on the Finnish page is a picture
Anders Weckström the engineer (see picture) has his own webpage I knew Anders some 40 years ago when we all where very interested in designing speakers. There is probably some firsthand information if you try to contact.
Here is some more information:

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January 3, 2019 08:48

great article!

something to be checked out, for sure

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August 28, 2018 09:44

You can just replace the whole HV supply with a tiny DC-DC converter module available via ebay for about $5. ( It will take a DC input between 5 and 15V and deliver 2-6.5 kV out. Use an LM317 variable regulator to supply the variable DC input ( A safe wall wart will supply the input to the variable regulator. If you want to simplify further, a regulated 12-13V switching supply will drive the DC-DC converter to 5.5 kV. You must use a HV resistor at the output of the sup[ply to prevent low frequency distortion.

Those speakers usually have broken diaphragms. Those can be replaced by getting some 6 um polyester film, stretching it tight on a pneumatic stretcher, and then gluing with 3M Scotchgrip 4693H contact cement. Coat the diaphragm with Licron antistatic spray.

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August 28, 2018 09:45

Wireless World had a constructional article to build electrostatic headphones. I can not recall if the amplifier was also part of the project - probably not?

There are some design ideas you can use on the link below. It won't work at 6,000V but it can be adapted to 600V pk-pk. The design is not a high frequency or a high capacitance driver.

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August 28, 2018 10:36

Many thanks for this article Andrew. I've been meaning to refurbish the voltage multiplier boards in my own pair of ESL57s for some time. So having read your article and inspired to do it, I went to order the parts you have specified. The Vishay 10nF caps are on backorder for delivery on 19 September. I can live with that. But the rectifiers are also on backorder with a delivery date of 24 may 2019!

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August 28, 2018 09:46

An excellent article Andrew. I own two pairs of these speakers and have so far refurbished the bass panels and re skinned the treble panels but not yet attempted the ht supplies as mine all appear good. I found the Fluke 75 with it's Fluke 40kV probe put too much load onto the ht supplies to give accurate readings but did, as you point out, provide an indication od major weaknesses. I run mine throgh a pair of hand built all valve stereo power amps with about 15W per channel so overload is not a problem. Few people get the chance to hear a pair in good condition. They have to be heard from the sweet spot, equidistant from each speaker and directly on axis. If listened to from any other position they usually sound awful and I suspect this is the reason for their sometimes poor reputation. Stacking a pair increasss overall SPL by 3dBs and by 6dBs in the bass register according to Peter Walker in a 70's article. This is largely borne out in listening tests, though the bass will never be floor shaking. What bass there is is beautifully detailed and even and tight as a drum. The treble can be a bit reticent too. The utter clarity of the midrange is what cannot be forgotten or subsequently lived without. The combination of ultra light diaphragm and driving force applied directly over it's total surface gives a transient attack and freedom from resonance that becomes readily apparent when listening to all but the finest conventional speakers.
Thanks for sharing an article that shows that some things from the depths of time still offer state of the art performance today. Walker's Wonders!

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August 7, 2018 07:46

Interesting article. What about the ESL 63 refurbishing? Shall be a lott to speak about. I have just started a new project about mating the ESL 63 with OTL amplifier as well as refurbishing them. Ill be back with details.

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August 7, 2018 14:42

@VICTORDASCALESCU From what I read the ESL63 can be more prone to failure/degradation and is obviously much more complex. I hope one day to pick a pair up for comparison. Look forward to hearing more about your project!

August 3, 2018 07:40

That is a very interesting article Andrew, thank you. The history followed by your refurbish is a nice combination. I wonder if they had been invented and produced now you would stand any chance of repairing them ?

August 6, 2018 10:09

@Boss thanks! Well, Quad do still make ESLs, but with the ESL63 they moved to a more complex affair with panels that were segmented with concentric rings to which the audio signal was applied with increasing levels of delay. On one hand an improvement in various respects, but with the cost of much added complexity and many swear by its predecessor. I'm sure the current models sound fantastic and are even more complex in construction. So there is something to be said for the elegant (relative) simplicity of the ESL57 design and how it lends itself to more easily being repaired.

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