Symmetrically segmented wire-stator ESL's

Everything else is two dixie cups and a string 😎



Above:  
My latest ESL design; the Jazzman Mk III, 
and the Jazzman RiPol Sub


Introduction

Greetings all from the Jazzman,

It still amazes me that an average Joe with practically no electronics experience can build a speaker at home that rivals the high-end commercial offerings, and bests most of them.  And building the actual driver from scratch takes DIY to a whole new level.  

Roger Sanders' Cookbook enabled my first ESL build in 2008, and my latest speakers would not have been possible without ESL gurus Calvin, Rod White, and Steve Bolser (a.k.a. CalvinGolfnut and Bolserst at diyAudio) sharing their knowledge on the diyAudio Forum.   Calvin's advice with the transformers, Rod's white paper [1] on electrical segmentation, and Steve's Segmented ESL Calculator and gift for explanation made it easy to derive the segmentation scheme and resistor values.  
Roger, Calvin, Rod, and Steve-- you guys are the best!  

I'm not a business and I don't build speakers to sell (don't ask) 
but I will share my drawings and all that I've learned with anyone who's interested. 
I'm pleased to share with you my DIY electrostatic loudspeaker projects.
Charlie Mimbs
Savannah, GA
jazzman1953@gmail.com

[1] Wide-Range Electrostatic Loudspeaker with a Zero-Free Polar Response, D. R. White, JAES Volume 57 Issue 10 pp. 822-831, Oct. 2009

                  
The Jazzman MkIII 
 

















This speaker is the crown jewel of my 14 years designing and building ESLs. Its 69 hardwood details are solid rift-sawn red oak with interlocking joinery, and nothing is hidden but the grill magnets-- even the electronics are visible thru the Plexiglas safety covers.
Building these speakers is a ridiculous amount of labor but the results are magnificent-- beautiful and reliable, with downright spooky imaging and clarity. 
The only difficult material to source is the UL1061 wire; which is typically a 10,000 ft. min-buy.  That much wire will build six pairs of speakers, so I arranged a group buy with several other builders.  
Materials costs, especially the oak, are up significantly in 2022. Amortizing the wire cost (only counting the 1,500 ft needed) the cost per pair is about $1,300 and 125 hours labor, excluding jigs.  Required tools are a table saw, a router, circle jig, shop made finger-joint jig, wire stretching jig, diaphragm tensioning jig, and a flat work table.  


RiPol Sub 
Bass that rises from nowhere and recedes back to nowhere: 










MkIII speakers with magnetic grills:


Jazzman MkIII Playing: 



Mervyn's Eros Clone speaker build - June 2019:
The exquisitely crafted speakers shown below were built by my online collaborator Mervyn Tims.  Segmented wire panels mated to a 10" Aurum Cantus woofer in a compact transmission line built with amazing skill-- has to be a killer combo.

I'm dying to hear them!  


  












































My friend Martin and I built this speaker together, for Martin.  We call it the 'Audi' because the speaker pair needed to fit into the back/trunk of Martin's 2008 Audi car for transporting.  This speaker became the prototype for the Jazzman Mk III. 

The 'Audi' H-baffle hybrid ESL



































Video of the Audi speaker playing at Carverfest 

Video of wire stator build







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The Basics
How do they work?


















If your hair has ever stood on end while unloading a clothes dryer, then you've felt the same force that drives an electrostatic loudspeaker (ESL).

An ESL is a push/pull sonic motor; consisting of a thin plastic diaphragm suspended in an electric field between two conductive screens called stators.  


A high-voltage DC power supply puts a biasing charge on the diaphragm, and the output from an audio amplifier, routed thru a voltage step-up transformer, puts the driving AC voltages on the stators.  And the ultra-light diaphragm responds with exquisite fidelity.

Stators can be made from just about any conductive material, including perforated sheet metal, conductive-coated plastic, wires, metal rods or even bug screen.  


My ESL Journey:
My first ESLs used flat, perforated metal stators which gave great slam and imaging but beamed like crazy.  

The next generation stators used segmented welding rod conductors and had switch-selectable wide and narrow dispersion modes. These had a nice, balanced sound but were just butt-ugly.  

Finally; my newest panels are more finely segmented for optimal dispersion and they have beautiful oak lattice supported wire stators that both look and sound like fine musical instruments. 

Flat perf-metal panels are by far the easiest to build and they sound fantastic in the sweet spot.  They are also directional to the extreme, prone to arcing if the stators are not perfectly prepared and coated, and they require a very stable amplifier to drive their all-capacitive load.  

Welding rod panels can be electrically segmented for wider dispersion and balanced response, and when segmented with a resistor network, their part resistive/part capacitive load is easier on amps.  The downsides are time-consuming to build and should be coated for arc resistance.  
    
Insulated wire panels like the ones shown on this page are now my preferred choice.  They are very reliable and easily segmented for tailored dispersion, balanced response and easy load.  They are also time-consuming to build; requiring a support lattice and a stout jig to stretch the wires.  

I opted for single strand wire which, once stretched, holds itself straight without having to be in tension.  Some builders prefer to use stranded wire, which does not have to be stretched, but it needs a bit of tension to keep it straight. 


More to come on electrical segmentation, but first; my personal 
home stereo speaker:

Jazzman Mk III

























































Dimensions:     67”H x 14.5” W x 15"D, weight 55 lbs  
Woofer:            Peerless SLS 12 in H-baffle OB
Stators:             Segmented wire type, 10.5” x  46.5” active area 
Wires:               UL-1061 / 20AWG / SRPVC / solid copper
Wire spacing:   11 wires/inch, 42% open area 
Diaphragm:       6-micron Mylar C / Licron Crystal coating
Bias supply:      3.2kVDC Cockroft-Walton cascade  
Transformers:   (2) 50VA 230V/2x6V toroidal, 76:1 ratio 
Crossover:        DBX Driverack V360, 265Hz @ 24db/Oct


Dispersion
Wavelengths shorter than a speaker’s radiating width tend to beam rather than spreading out (this is why tweeters are small; because treble wavelengths are short).   Flat-panel ESL’s beam like crazy because they have to be large to offset their limited excursion and dipole roll-off.  The resulting 'head-in-a-vise' sweet spot is great for solo listening at the focus but not so good for entertaining guests, where wider dispersion is needed.  

The typical way to widen an ESL’s dispersion is by curving its panel, and thereby it's radiated wavefront.  A segmented ESL uses a flat panel and curves its wavefront electrically, using discrete stator conductors driving discrete zones on the diaphragm.

Below is a post by Steve Bolser on the DIY Audio Forum which compares the dispersion patterns of unsegmented flat panels, curved panels and segmented flat panels: 


More About Electrical Segmentation
The segmented ESL panel uses symmetrically arrayed wire groups powered by frequency/phase attenuated drive signals to function as a line source projecting a cylindrical wavefront.                  

Each stator has 90 insulated copper wires arrayed in 15 six-wire groups which are apportioned into eight discretely powered electrical sections; consisting of a single center wire group comprising section one, and seven left/right paired wire groups arrayed symmetrically on either side, comprising sections 2-8 (see Schematic).  

The center wire group connects directly to the amplifier interface and receives the full audio bandwidth above the crossover frequency.  The left/right paired wire groups are powered in series from the center wire group, and thru resistors inserted between the wire groups.  The resistors couple with the wires' capacitances to form a series of low pass filters which progressively chop off the upper frequencies toward the panel edges.  The resistor values were derived using this Excel spreadsheet: Segmented ESL Calculator    

As driven by the segmented stators; the diaphragm radiates the top treble band from only a narrow vertical zone at its center, and the left/right paired zones on either side radiate progressively lower frequency bands toward the panel edges.  In this way, the widths of the radiating zones do not exceed the radiated wavelengths, so all frequencies spread out uniformly, rather than beaming.  

Below:  Mk III Wire Lacing & Segmentation Scheme  
  


Guidelines for Symmetrically Segmented Wire Panels (hybrid ESL):   

·                Diaphragm-to-stator gap (d/s):  0.063” (+.015/-.000)  
·                Span between diaphragm supports: 70-100 x d/s
·                Wires:  18-22 AWG single-strand copper, PVC or XLPVC insulation only
·                Max span between wire supports (wire gauge/inches): 22/2, 20/3, 18/4 
·                Wire O.D. (insulation included) should not exceed d/s
·                Gap between wires (insulation-to-insulation) should not exceed d/s
·                Ideal (max output) open area:  42%  (gap/OD)
·                [Down to 12mm] more/narrower wire groups = wider/smoother dispersion
·                Transformer(s) power/ratio (each speaker): 100-160VA / 50-100:1
·                Bias voltage: 1.8kV – 4kV
·                X- over frequency/filter slope:  >200Hz / 24db


Stretching the Stator Wires
Once the panel design is set, the next step is building a jig to stretch the stator wires.  

Stretching the single-strand copper wires to plastic deformation  renders them perfectly straight and ensures they remain straight when subsequently relaxed.  1% elongation is sufficient, and so the wire loops were stretched from 48” to 48.5”.   

An important feature of the jig is its reference platform which sets the stator wires, wire glue lines, and frame rails perfectly in-plane flat, to ensure that the critical diaphragm-to-stator gap will be perfect.     

Stretching ninety 20-gauge wires at once requires a strong jig and about 4,000 pounds of force.  The jig has a ¾ MDF platform mounted in a stout frame cut from yellow pine 4x4’s.  The wire loops wrap over .063 diameter x .250 length steel pins inserted half depth into drilled 3/16” aluminum plates.  The pins are angled 3 degrees from vertical to hook the wires.  One pin plate is stationary and the opposite moveable pin plate bolts to a 3/16 steel subplate that’s welded to two 3/4 x 12 all-thread jack rods.  Turning the coupling nuts on the jack rods pulls the movable pin plate to stretch the wires. 

Before stringing the wires, wax paper coated with liquid car wax is laid down over platform to prevent the wires from bonding to the jig when gluing the support lattice onto the wires.  The car wax helps the wax paper peel away cleanly from the completed stator

Note:  For .052 diameter wires spaced 11 wires/inch; the gap between wires would be about 1mm (.042).  However; 1mm diameter pins proved to be too weak to resist bending over, so had to step up the pin size to 1/16 (.063).  To set the correct wire spacing during glue-up, I used 3D printed comb guides.  On prior speaker builds, I used 5/8-11 TPI all-thread rods to set the wire spacing.  
 


Below: Stator wires on the stretching jig

































Stator Support Lattice
The wires are supported by an interlocking oak lattice which is assembled and glued down over the wires, in the stretching jig.  

Before gluing the slats over the wires, most of the wire tension was relaxed to prevent preloading and warping the stator.  E6000 glue was used for both the wood-to-wood and wood-to-wire bonds.

First; the wax paper and vertical lattice rails were laid down in the jig, then the 3D- printed wire guides were installed, and finally the interlocking horizontal slats were glued down one-at-a-time, over the wires.

An earlier method to hold wire spacing used lengths of 5/8-11 TPI all-thread rods as comb-guides (see photo below).   


Below:  Stator jig with 3D-printed wire guides



Below:  Alternate method-  all-thread rod wire guides


Below:  Oak support lattice assembled over wires







Below:  Completed stator 







Diaphragm Spacers
Double-sided tape spacers bond the diaphragm to the stator and set the diaphragm-to-stator gap (d/s) at .063”.  

The vertical lattice rails are flush with the wires and their spacers are .063 3M double-sided urethane foam tape; 3/4" wide on the side rails and 3/8" wide on the center rails.  

The horizontal end rails anchor the wire end-loops with glue bonds.  The end spacers consist of (1) .047" thick x 1" wide x 10.5" length polycarbonate shim bonded onto the wires with E6000 glue, plus (1) layer of .015" x 1" wide 3M UHB double-sided foam tape over the shim (0.062 total).  

The double-sided tape spacers bond the diaphragm to the stator instantly with minimal fuss. 


Diaphragm 
The 6-micron Mylar C diaphragm is vertically sectioned into equal thirds for stability and tensioned to 1.0% elongation using a pneumatic bike-tube jig.  The jig is an MDF platform sized two inches longer and wider than the stator and 2 inches high with all edges rounded over to .50” radius, sanded smooth and dusted with baby powder to prevent snagging the delicate diaphragm film.  A 700mm x 35mm Schrader valve type bike tube is stretched around its perimeter.  

The Mylar film is wrapped over the jig and secured on the backside with double-sided tape.  Inflating the bike tube with a hand pump tensions the diaphragm.  Tension is gaged by first marking reference points on the diaphragm exactly 12 inches apart using a fine tip felt pen.  As the tube is inflated, the target elongation is reached when the distance between the reference marks reaches 12.125 inches.  

For my panels' span between supports, 1% elongation provided enough tension to prevent driving the diaphragm into the stators at high volume, yet kept the drum head resonance low enough to set the crossover frequency below the ear-sensitive midrange region.  

Since the amount of elongation/tension required to stabilize the diaphragm is dependent on the span between supports, the elongation would vary for different spans-- 1% would not work for all panels.

The stator is then pressed into place over the diaphragm to affect the bond.   


Diaphragm Coating
The diaphragm must be made conductive enough to hold a biasing voltage yet resistive enough to slow the charge migration across its surface.  

Next; the periphery edges of the diaphragm were masked off with painters tape and the Licron Crystal ESD conductive coating was spray applied in one “just wet” coat and allowed to dry for eight hours before assembling the panels.  The coating dries to a pale blue-gray, almost clear coating about 2-microns thick with E7-E9 resistance.   

Video:  Diaphragm tensioning and panel assembly

Below: Bonding stator to diaphragm on bike tube jig



Below:  Bonded diaphragm ready for conductive coating



Charge Ring  
The charge ring is ¼ inch wide copper foil tape applied to the periphery of the rear stator, centered on the foam tape spacers.  The wire lead from the DC biasing power supply is soldered to it and when the front and rear stators are mated together, the charge ring contacts and conducts the biasing voltage onto the diaphragm. 

Below:  Rear stator with spacers & charge ring
 
























Below:  Completed front & rear stators ready for assembly





















RC Segmentation Network (Jazzman MkIII):
The RC (resistor/capacitor) filter network tailors the panel's dispersion pattern electrically, by attenuating the frequencies and phasing of the music signals driving the separate wire groups.  
  
The wire segmentation scheme and network resistor values were derived using Steve Bolser's Segmented ESLCalculator spreadsheet.  From the spreadsheet options, I chose Symmetric Config 2, both stators segmented, in eight electrical sections, and a low-cutoff of 260Hz.  For my panel the spreadsheet calculated 107kΩ for R, with R/9 resistance on the section one wire groups, 0.75R on section two wire groups, and R on sections 3-8 wire groups. 

It’s common practice to move/reflect the section one resistances to the primary side of the transformer to protect against core saturation (see Schematic, damping resistor R1).  Reflecting section one's R/9 resistance across an ideal transformer would divide the sum by the turns ratio squared (4.6Ω in this case).  However, placing this much resistance on the primary side of a real transformer, interacting with its winding resistance, leakage inductance, and winding capacitance, would result in a significant roll off of high frequencies down into the audio band.

The transformer's winding capacitance adds to the load capacitance and its leakage inductance combines with the load capacitance to generate an ultrasonic resonance peak in the frequency response and rapid roll off above it.  Coincident with this response peak is an impedance minimum which can be a difficult load for the amplifier.  When series resistance is added on the primary side it dampens this resonance peak.  However, as previously noted, too much resistance over-damps the resonance, rolling off the audible highs. 

The spreadsheet assumes an ideal transformer is used, so it doesn’t calculate the effect of resistance on the primary side of a real transformer.  
The general guidance is to omit the section one resistors on the secondary side, add a 1Ω series resistor on the primary side and give it a whirl.  My panels sounded really good with this initial setup. 

From there the only tuning, if any, is adjusting the series resistance on the primary and/or the first two stator sections to dial in the treble response.  Less resistance increases treble and visa versa.  


The schematic and parts list show the spreadsheet values except with section one resistors omitted and reflected as 1Ω on the primary.  I think this would be optimal for most listeners. 

All remaining resistors on the secondary side are 2W, 500V in series.  Wattage/voltage are highest across the first resistors and decrease down the line.  Based on R=107kΩ; Multiple resistors are ganged to spread the load, as follows: 

Section 1:                      none  (reflected as 1Ω on TFMR primary)
Section 2 (.75R):          (4)  20kΩ  
Sections 3, 4:                (2)  30kΩ + (1) 47kΩ
Sections 5-8:                 (1)  56kΩ + (1) 51kΩ


Below:  RC network resistors (Jazzman MkIII speaker)





Below: Segmentation Resistor Connections









Schematic (Jazzman Mk III Speaker): 








Amp/ESL Interface (same on all my designs)
Each stat panel has an interface to its amplifier; consisting of a high voltage DC power supply to bias the diaphragm, and a tandem pair of step up transformers to convert the amplifier’s output to the higher voltage AC required to charge the stators.  Unlike most commercial hybrid ESLs which include a passive crossover network in the interface, my speakers use an active digital crossover upstream of the amps, and the interface can be much simpler.  

Each interface uses (2) 50VA 230V/2x6V toroidal transformers wired in tandem, with the 6V windings in parallel as the primary and 230V windings in series as the secondary; giving a 76:1 winding ratio. The DC biasing supply uses a floating ground that’s center tapped between the transformers 230V windings.

The DC biasing supply is a simple, reliable half-wave rectifier and voltage multiplier outputting 3.25kV.  It’s powered by 115VAC mains current into a 115V/230V transformer and diode/capacitor ladder with a 20MΩ charging resistor at the output.  The charging resistor helps stabilize the charge on the diaphragm and limits the potential current that might otherwise sustain any arcing to the stators.     


Below: Amp/panel interface (Jazzman Mk III)






Bass Section - Jazzman Mk III 
The Peerless SLS 12" woofer sits on an open baffle with triangular sides, which form a tapered H-baffle.  The system is a 3-way using a digital crossover feeding (3) stereo power amps.   A pair of Ripole subs cross over to the Peerless mid-bass woofers at 60Hz using a 24db/oct LR filter, and the mid-bass woofers crossover to the ESL panels at 250Hz using a 24db filter.   I haven't tried running the Peerless woofers without the subs but I believe they would be adequate to normal listening volume.   With the Ripole subs unloading the Peerless woofers on the bottom end, these speakers can really ROCK.  



Power & Control -  Jazzman MkIII Speaker
The speaker pair is vertically bi-amplified using a DBX Driverack Venu 360 digital crossover feeding a pair of vintage Carver TFM-25, 225 watts/channel stereo amplifiers.  The PA2 Driverack is also a good option (less $$ than the Venu 360).  


The general guidance is to set the crossover frequency is least two octaves above the diaphragm’s drum head resonance with a 24 db/octave filter slope or at least one octave above resonance with a 48db/octave filter slope.  My diaphragms resonate at about 90 Hz and the crossover is set at 250 Hz using the Driverack's 24db/octave Linkwitz-Riley filter.  

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Below: My First ESL - The Beam Splitter

The transmission line woofer box/frame was built in 2008.  
The original perf metal stators were updated to segmented wire type in 2015.

























Bass Section - Beam Splitter Speaker 
The woofer box is a single-fold, tapered transmission line stuffed with 0.5lbs/Ft3 of polyfil.  The line’s sectional area is 125% of the woofer’s cone area at the front, tapering to 100% of the same at the terminus.  The cabinet is ¾” MDF sheathed in 5mm red oak plywood and the panel frame is solid red oak.  

To minimize the woofer box’s profile and footprint, its volume extends upward, behind the stat panel, and its frontal surfaces are angled to form a V-shaped “beam splitter” which deflects the panel’s rearward sound out the open sides of the speaker rather than back to the diaphragm.   

When designing the bass section I followed Roger Sanders’ lead and opted for a transmission line enclosure and a woofer with low moving mass, low QTS, and low inductance coupled with a very strong motor magnet.  The ideal matching woofer doesn’t exist of course but low inductance takes priority and the Aurum Cantus AC250 MKII I chose works pretty well.  




Below:  Bob Carver loves my speakers!
               Cellphone Video from Carverfest 2016





















Videos:

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