Acorn Mk 2 Overview

Acorn ESL Description


The Acorn MkII Electrostatic Loudspeaker kit has been developed with the knowledge and experience gained over many years with our other successful electrostatic speakers, the Acorn MkI, ESL III, ESL IIIb, ESL IV and 505 bookshelf panels.

While the Acorn MkII ESL has been designed primarily for the home constructor, there have been no compromises made to performance or sound quality to achieve this aim. Indeed, the performance of the Acorn is up amongst the very best of the commercially available electrostatic loudspeakers, regardless of cost.

As well as being a top shelf audiophile loudspeaker, part of the design requirement was for the Acorn to become part of a high-end home theatre system.

It is extremely important that effects speakers sound “the same” as the front speakers to avoid Arnie turning into Donald Duck when he exits stage left!! To this end it is considered essential that all speakers used in a high-end theatre system, possibly with the exception of a sub-woofer, use the same electrostatic principle for operation.

As the Acorn is a modular design, it is a simple matter to use 2 of the panels that make up half of one Acorn speaker for the rear effects. This means that exactly the same tonality, transient response and low-level detail retrieval are retained by the effects speakers.

If deemed necessary, our 505 ESL panels can be used for the centre front position, which again has the same tonal signature as the front and rear effects speakers. This serves to pinpoint central images for those sitting off axis to the front main speakers.

While the Acorns do have good bass response in their own right, a sub-woofer is still necessary for the very low frequencies engineered into most movies to enhance the special effects.

It now becomes clear why these speakers are called the Acorn; “from small Acorns large Oak trees grow”!!

The final requirement was for the design to be easily constructed at home with minimal equipment and no special jigs. This design goal has been met.


Transducer Layout

There are four electrically separate sections laid out in an asymmetric, mirror-imaged array on a single flat plane. The bass panel is separate to the mid / treble panel and is joined to them using a plastic H section extrusion, effectively forming a single panel. The narrow treble section is normally offset to the inside edge of the speakers forming a distinct “Left” and “Right” handed loudspeaker, however, experimentation is encouraged by turning the mid / treble panel upside down and placing the treble section next to the bass. In some rooms this may improve off axis response.

This still results in a left and right handed speaker as the bass panels should be located on the outside (closest to the side wall) of each speaker.

The bass panels have an array of node points scaled to distribute panel resonances which eliminates the “one note bass” often encountered.

The 4 sections of the combined loudspeaker panel are fed from a specially designed 3 way crossover with each section having its own transformer dedicated to that section only. This helps to reduce intermodulation distortion and gives excellent control over each section.

In addition to this, the treble section has a very narrow (around 8 mm wide) super tweeter strip running up the centre of the treble stator. This super tweeter strip is fed high frequency signal which then feeds the side strips via 2x surface mount resistors (SMD’s) on each stator. In normal operation the super tweeter strip is capable of delivering up to 35 kHz with good dispersion, the sections either side of the super tweeter strip start to deliver sound at around 18kHz and go down to around 6kHz before the mid-range panel takes over.

As a narrow strip is emitting the high frequencies, the sweet spot is wide with excellent off axis performance. When correctly positioned in the room, the stereo image never collapses into one speaker, even when listening as far off axis as a side wall.

Given a good recording and source equipment, the soundstage presented is three-dimensional with distinct separation between instruments, tangible depth and height. Width extending past the outer boundaries of the speakers is apparent on those recordings in which this effect has been engineered. Frequency response is extended and flat.

Load Presented

As with our other designs, it was considered essential to make this speaker a zero-shunt capacitance design.

As an example, ESL panels, which have air gap spacers that are simply glued onto a conductive stator, may be simple to build but there is a serious trade-off. The area of stator that has the spacer covering it is non productive - it cannot drive the diaphragm, BUT the amplifier is still driving the capacitance it presents.

This means that on one speaker the size of the Acorn, there would be over 2750 square centimetres of stator doing no work at all, but still presenting a load to the amplifier. To get this into context, this represents a “dead” panel 1 metre long by 27.5 cm wide. This means that the speaker must be significantly larger to achieve the same output. This increases the capacitance, which in turn requires a more powerful amplifier to drive it.

The method adopted in the Acorn is for the printed circuit board (PCB) stator to have the copper surface etched away from the air gap spacers and every other section that is not directly required to produce sound. This means that all the conductive copper on the stator is contributing to the output with the exception of a few small tracks used to connect sections together.

Because in the MkII Acorn the bass section is separate to the mid treble sections, diaphragm tension can now be optimised for the individual panel requirement instead of being a compromise of low enough resonant frequency for bass but high enough to maintain stability in the closer gapped mid and treble panels. If desired, heavier gauge film can be used on the bass section to allow deeper bass extension.

Stator Design

The gauge of PCB material that is used in all of the Acorn Mk II stators is 1.6mm thick FR4 material. This gives sufficient mass and stiffness to allow operation at all frequencies of interest.

Whilst using stators of thicker material does not cause high frequency roll-off to any serious degree (that is until they get to around 2mm thick), they are unable to retrieve low-level detail completely. This is due to the air mass that is contained in the individual holes. Effectively the holes form a small resonator, which destroys low-level information. The frequencies that this occurs are linked to the ratio of the thickness of the material versus the diameter of the hole and are too complex to thoroughly explore here (even if I understood it fully). The slots in the Acorn stators have been optimised to allow excellent detail retrieval without requiring super thin stators to achieve it combined with optimum open area for excellent sensitivity. The thicker stator material also provides sufficient stiffness to avoid the need for additional stiffening members glued to the outer surfaces.

The bass and mid-range section stators have 5 asymmetrically spaced posts in the central vertical axis to clamp the panel together down the central axis of the bass and mid-range sections. The panel is also clamped with plastic through bolts down the dividing sections between the treble and mid-range sections. This, combined with the channel section that clamps the outer edge of the panel, makes for a very stiff structure with accurate stator to diaphragm spacing over its entire area.

The stators are coated with 2 layers of solder-mask during their manufacturing process, this effectively insulates them.

Air Gap Spacers

Current versions of the Acorn MkII use a fibreglass pultrusion for our air gap spacers. Pultruded products are very stiff and dimensionally accurate. To maintain low levels of distortion it is important that the gap between the diaphragm and the stators is kept the same on each side, using a pultrusion ensures this. An additional benefit is the pultrusions we use have a radiused edge, this avoids the diaphragm flexing over a sharp edge which can cause fatigue and failure.

Stator Damping

The PCB material used for the stators is acoustically inert so does not “ring” when producing sound. It is not necessary to coat them with additional damping material.

Diaphragm Material

We have used a very light but strong 3.8 micron polyester film for many years on our other projects with excellent sonic response and durability. However, the design of the Acorn MkII has allowed us to advance this process further. As the Acorn MkII has two separate panels for bass and mid / treble, we can use 2 types of film to advantage. The bass section uses 4.2um Mylar C and the mid / treble section uses 3.2um Toray film. The thicker film used on the bass panel allows the same resonance for higher tension without sacrifice of transient response while the thinner film on the mid / treble panel delivers extremely accurate mid and high frequencies combined with very fast transient response. The use of this film combination results in amazingly fast transient response and low-level detail retrieval. When auditioning this material in the Acorn MkII most listeners will comment on the extra detail that they are hearing from familiar recordings, which they have not heard before from their own systems.

Acoustic Damping

The Acorn MkII is designed to operate with a grille cloth in place. The cloth serves two functions, it prevents dust and insects getting into the works and it provides diaphragm damping at resonance. Operation of the speaker without cloth is not recommended but some clients prefer to have nothing in front of the soundwave for critical listening. It is permissible to have the front grille cloth easily removed for serious listening but the rear cloth should be left in place to provide damping. Operation with the cloth removed from both sides will likely result in uneven bass response.


Tensioning the Diaphragm

This is the scary bit that causes most home constructors some concern.

Over the past 20 years we have developed a technique to tension the film that does not rely on time wasting tensioning frames etc. Our method is simple and quick and gives reliable, repeatable results. This is not a heat shrink method, which can have some unpredictable results.

All the builder needs to do is provide a smooth, flat surface large enough to place two 930 x 250 mm Acorn bass panels side by side with about 100 mm clearance all around and a narrow gap (3mm) between them. A sheet of glass such as an old shower screen or window is good for this. ESL’s have a poor WAF (wife acceptance factor) as it is so think twice before using the favourite glass coffee table or dining table.

The tensioning gauge and all materials required to successfully and easily apply the diaphragms are provided in the Acorn kit. Tensioning and applying the diaphragm to two bass panels (or two mid / treble panels) takes about 30 minutes. The work-surface area needs to be large enough to get 2 bass panels or 2 mid / treble panels side by side so they can be covered in the same operation, this saves time and materials.

Conductive Coating


Due to demand from our customers, we have developed a low mass, clear conductive coating that replaces our Opaque material. This coating is not an ionic solution such as used in static dissipants and does not rely upon ambient moisture to make it conductive. It is humidity independent meaning that the weather will not affect its performance.

Our latest coating also has good UV resistance and does not deteriorate with elevated temperatures.This coating has been extensively tested in both high humidity areas and the very dry areas of the Australian desert regions with excellent results. The surface resistivity is typically around 500 to 1000 megohms per square when fully cured. This means that charge migration will not occur, even at the lowest frequencies of interest.

This gives good bass performance and reliability without sacrificing sensitivity.

Charging the Diaphragm

As the Acorn ESL has panels with different air gaps between the stators and diaphragm, two polarising voltages are used to maintain the correct sensitivity for each section. The bass section has the larger gap therefore requires a higher polarising voltage.

The Acorn Mk II has separate sheets of film covering bass and mid-range / treble sections, all that is necessary is for the coating to be applied to the diaphragm using the applicator provided in the kit.

Each panel has two separate copper high voltage “charge rails” that run up each side of the panels to provide charge to the conductive coating.

These copper charge rails are fixed to the panel half that does not carry the diaphragm (the rear half panel). When the two panel halves are brought together, the copper foil contacts the conductive coating and transfers charge onto it.


EHT Supplies


New EHT supplies have been developed for the Acorn. This design uses a small switch-mode inverter, which drives a voltage multiplier stage.

The supply has a tapped output, which provides the two voltages required for the Acorn plus an additional output for sub bass panels. More on this later.

The EHT supply is driven by an adjustable regulated dc supply, which is powered by a standard 12 volts dc Plug Pak. The EHT supply is part of the power supply circuit board and is normally housed in the base of each loudspeaker. This means that there are no mains potential or high voltage wires leading to the loudspeakers, just 12 volts dc and the normal speaker cables from your amplifier. It is a simple matter to optimise operating voltages by the turn of a single trim pot on each power supply board. The regulator eliminates any variations in polarising voltage due to mains voltage fluctuation.

The latest version of the EHT supply has a 25 turn trim-pot and two test points to make adjustment of the high voltage very easy to do. This makes assembly easier and, due to the use of the 25 turn trim pot to adjust the output, setting the voltage applied to the panels is a breeze. Two 12 vdc mains adaptors are supplied in the kit, one for each speaker. The mains adaptors are CE approved and come with multi region adaptors for the USA, Japan, EU etc.


Audio Transformer

The audio transformers we use are another proprietary design of 1:200 turns ratio for the bass section and 1:100 each for the mid-range and treble sections. The bass and mid-range transformers are too large to mount on the power supply board so are mounted on a separate board with the power supply / crossover placed between them.

The treble transformer is a PCB mount design and is mounted on the crossover board.

These are a multi section, interleaved design with a power handling capability of well over 100 watts.

Low distortion and faithful reproduction of signal sources such as square waves were a priority.

Using a dedicated transformer for each section of the panels reduces intermodulation distortion levels, which are already very low. It also allows a single amplifier, bi-amplification or tri-amplification to be used. The new crossover board is fitted with individual inputs to each discrete section that allows for this arrangement. Single, bi or tri wiring is also possible using this system.

The transformers are specifically designed for electrostatic loudspeakers with very high voltages envisaged; a second priority was placed on internal insulation and high voltage enamel for the winding wire. The high turns ratio gives sufficient drive voltage, even from lower output amplifiers, without putting too high a demand on current.

However, amplifiers with good current capabilities are necessary for high sound pressure level listening.


Panel protection

The new crossover board is also provided with a series of VDR’s connected across the panel outputs. These VDR’s are effectively out of circuit until their threshold voltage is reached, at that time they start to conduct, effectively shunting the excessive voltage and protecting the panel from over-voltage. This system is designed to prevent occasional high energy peaks that may damage the panels, it is not designed to act as a maximum output limiter that is working continually.


N.B. The latest crossover board has tracks to by-pass the crossover components and easily enable an active crossover to be used. We strongly recommend that the speaker be operated with the passive crossover in place first to establish a benchmark before using an active crossover.

In many cases the passive crossover was considered to sound “better” than the active approach.