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stokessd 8th June 2019 07:59 PM

Quad 63 (and later) Delay Line Inductors
There was some interest in another thread about the modern Quad inductors that form a portion of the delay line in the 63's and newer Quad electrostatic loudspeakers.

Strangely, the British electronics in these speakers is quite robust. But over the 24+ years I've been fixing quads, i've seen just about every failure you can have. I've only seen a couple damaged inductors and they were the results of massive overdriving of the speaker. Below is my knowledge of these inductors, although I haven't really dug into them very deeply (both metaphorically and physically) which you'd want to do if you were going to reproduce them.

The inductors are air core units with a second shorting winding that increases the inductance dramatically beyond what a single coil of this size and wire gauge is capable of. The inductance is about 2.7H each if my measurements are correct. The attachment below shows my method and results. (NOTE: The DCR is in Kohms not ohms as noted on the sheet. And I went back and re-measured the inductor resistance and got the same numbers as the first two inductors; I beleive all inductors are identical). I used a function generator, a resistor and a voltmeter to measure the inductor by forming an AC voltage divider. Using the fact that and inductor has an impedance of: Z = 2 * PI * f * L, I was able to calculate L because I know, Z, and f.

As seen in the pictures, the copper coil is sitting in a plastic "bobbin" with three leads which are soldered to the delay line circuit board. The entire bobbin, and actually the entire delay line circuit board is encapsulated in bee's wax which makes these things easy to work on.

Two of the leads are connected via a buss bar inside the bobbin, and the third (middle) lead runs from the center of the coil. The shorting coil is attached to both the left and right attachment points on the buss bar (forming a short through that bar. And the main inductor coil is attached to one side of the buss bar, at the outside of the coil and the other end is attached to the center lead coming from the middle of the coil.

I don't have a feel for how the shorting coil is arranged relative to the main coil. I'd think it would be bifilar wound, but I have never completely unpotted one of these to check it out.

The way I've seen these fail is that the very fine wire breaks near the solder connection at the buss bar. It appears that the wire overheads and melts, but on come specimens the wire looks thinned which doesn't make sense with a pure melting failure mechanism.

To fix them, I melt the wax around the buss bar and in the center of the coil and using a short piece of jumper wire, I re-solder the fine magnet wire to the jumper and then the jumper to the buss bar. I use the jumper wire because without un-potting the entire coil, the wire where it breaks is too short to reach the buss bar.

Here's a couple pictures of the inductor including one through my stereo-microscope eyepiece. Taking a picture with my cell phone through the eyepiece is a real kludge but works reasonably well to document things.


MarcelvdG 8th June 2019 09:54 PM

2.7 H air cored? Wow!

You have probably known this for decades, but according to Peter Baxandall (in chapter 3 of Loudspeaker and Headphone Handbook, J. Borwick, ed., Butterworths, London, 1988), the shorted windings are meant to damp the transmission line for high audio frequencies, so treble is only produced by the middle of the loudspeaker. That has something to do with preventing edge effects due to the finite size of the ESL-63.

esl 63 10th June 2019 11:36 PM

Reading about the induktor, it is basically 18H, but to work ideally it was calculated that they needed a "lossy" induktor so a shorted induktor was added. And the net result became 18H*(22/132)=3H

esl 63 11th June 2019 08:52 AM

The placement of the delay induktors is not a coincidence either... well nothing that Peter Walker did was. Each Induktor is affecting the nearby ones, all is taking care off in the design, measured and calculated and solved! Peter Walker, soo ahead of it´s time.

bolserst 14th June 2019 08:23 PM


Originally Posted by stokessd (
…The way I've seen these fail is that the very fine wire breaks near the solder connection at the buss bar. It appears that the wire overheads and melts, but on come specimens the wire looks thinned which doesn't make sense with a pure melting failure mechanism.

Thanks muchly for posting these pics!
Your mentioning the wire looking thinned, makes me wonder if the wire was unintentionally stretched beyond yield tension before or after wires were soldered terminated, and “necked” down. The increased resistance at that point would certainly lead to local heating. Alternatively, it is not uncommon for thin wires to fail near terminals when the solder joint is poor(ie of high resistance).

Hans Polak 2nd March 2020 07:40 PM


Thanks for the information.
May be I can integrate this in my replacement circuit diagram.


legopart 2nd March 2020 11:41 PM

why quad using wax?

stokessd 3rd March 2020 02:43 PM


Originally Posted by legopart (
why quad using wax?

Why not?

Easy to apply, safe, effective, cheap. The only downside is that the melting point is low.


Hans Polak 9th March 2020 02:14 PM

2 Attachment(s)
Hi Sheldon,

I've tried to capture the ESL63 in a LTSpice simulation.
Maybe you're interested in seeing the results, resp. freq. range and step response for the 8 sections and the impedance as seen from the Power Amp.


MarcelvdG 9th March 2020 10:57 PM

How do you model the inductor losses and parasitic capacitances?

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