Acorn Electrostatic Loudspeaker Kit

The following Electrostatic Loudspeaker Kits are supplied for home use only and may not be used for commercial gain unless authorised by ER Audio in writing. All photographs, drawings and text contained in the kits are subject to copyright laws and may not be reproduced or copied in part or in full without written permission from E R Audio.
Acceptance of any kit implies acceptance of these terms.

Design Philosophy Acorn Electrostatic Loudspeaker

Some design aspects of the ESL loudspeaker described below are subject to patents pending for Immersion Technology International PLC.

The Acorn has been developed with the knowledge and experience gained over many years with our other successful electrostatic speakers, the ESL III, ESL IIIb, ESL 4 and ELS 5.

While the Acorn 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 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.

A curved ESL speaker has been designed 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 three electrically separate stators laid out in an asymmetric, mirror-imaged array on a single flat panel. The narrow treble section is offset to the inside edge of the speakers forming a distinct “Left” and “Right” handed loudspeaker.

The mid-range section is placed more or less centrally followed by the bass section on the outside.

The bass panels have an array of node points scaled to distribute panel resonances which eliminates the “one note bass” often encountered. These node points also reduce anti-node formation at certain frequencies, which result in “dead” or redundant sections of diaphragm.

High and some mid-range frequencies are filtered from the bass panels by feeding them via a high value resistor, which, in conjunction with the capacitance of the panel, forms a low pass filter. The same technique is used with the mid-range section.

As a narrow strip is emitting the high frequencies, the sweet spot is reasonably wide with very good off axis performance. When correctly positioned in the room, the stereo image never collapses into one speaker, even when listening greatly off axis.

Given a good recording and equipment, the soundstage presented is three-dimensional with distinct separation between instruments. 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 and, as the design is essentially crossover-less, there are no signal modifying (destroying) capacitors or inductors to shape it to be acceptable.

Load Presented

As with the ESL III, 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 grid 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 grid 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 has to 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 stator to be supported away from the air gap spacers on a CNC machined non-conductive plastic panel. This means that all of the stator is contributing to the output.

The plastic support structure has been CNC machined from a light but stiff plastic and allows for the bass, mid-range and treble sections to be set at the optimum depth very accurately. Close tolerances are essential in any ESL design as variation in depth can lead to distortion components and diaphragm instability.

As the stators are recessed into the support structure, a single diaphragm can be used across the surface of the panel. The spacers that remain between the stators create the three discrete sections.

Stator Design

As a support structure was to be used to reduce shunt capacitance, this gave the opportunity to use the thinnest but stiff material available for the stators.

The gauge of steel that is used in the treble and mid-range sections is only 0.6mm thick. The bass section has 1mm thick material to improve stiffness.

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). However, it is our experience that thin grids simply sound better so we adopted this approach.

The bass section stators have 5 clearance holes punched down the central vertical axis to allow small plastic posts, which are machined into the plastic panel, to pass through. These posts are used to firmly clamp the panel together down the central axis of the bass section. 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 whole area.

The stators are powdercoated with a high dielectric strength coating that effectively insulates them.

Stator Damping

Thin metal stators tend to “ring” when producing sound so it was important to provide some damping to the grid to deaden the colouration that would occur. This is especially important where large areas of unsupported stator are exposed.

Fortunately, our experience with the ESL III and the use of a polyurethane material, which gave superb damping in this speaker, was very successful in stopping ringing. We have therefore adopted the same material and method in this design.

Diaphragm Material

We are now using a thinner “faster” material than the previous film. This material is made by Dupont and has been proven to give exceptional sonic performance as well as being tough and durable. This film is 3.5 microns thick.

The use of this film results in amazingly fast transient response and low level detail retrieval. When auditioning this material in the Acorns (and ESL III’s for that matter), 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.

Tensioning the Diaphragm

This is the area that causes most home constructors some concern.

Over the past 15 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 the 920 x 440 mm Acorn panel with about 100 mm clearance all around. 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 one panel takes about 30 minutes.

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.

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 in excess of 1000 ohms 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.

Getting the charge onto 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 has a single sheet of film covering bass, mid-range and treble sections, fortunately, the coating we use can be easily applied where needed so it is a simple matter to leave a gap in the coating to allow the coating over the bass section to be electrically isolated from the coating over the treble / mid-range section.

Two electrically separate copper high voltage “charge rings” that run around the perimeter of the separate stators provide charge to the two separate areas of conductive coating.

These copper charge rings are applied to the panel half that does not carry the diaphragm. When the two 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.

The EHT supply is driven by an adjustable regulated dc supply, which is powered by a standard 15 volts dc Plug Pak. The EHT supply 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 15 volts dc and the normal speaker cables from your amplifier. As the regulator drives both speakers it is a simple matter to optimise operating voltages by the turn of a single trim pot. The regulator eliminates any variations in polarising voltage due to mains voltage fluctuation.

Audio Transformer

The audio transformers we use are another proprietary design of 1:90 turns ratio.

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

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

As these were 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 turns ratio of 1:90 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.

Acorn Electrostatic Loudspeakers Finished in Australian Marri Timber Enclosures

Panel Dimensions: 1860 x 445 x 25 (WxHxD) Enclosure Dimensions 2100 x 480 x 90 (at narrowest point)

Acorn Electrostatic Loudspeaker kit

The Acorn ESL kit is available in two options.

Option 1 has the stators already bonded into the machined support structure and all electronics are pre built, tested and matched. This eliminates the most time consuming, messy and tedious part of the construction. All that is required to complete the panels is for the diaphragms to be fitted and made conductive (not a difficult job with the procedure we have developed) and for the diaphragm charge rail to be fitted. The panels can then be assembled, fitted into their enclosure and wired up.

Option 2 is a full DIY kit requiring the stators to be bonded into the support structure and all electronics assembled onto their circuit boards.

Please note enclosures are not supplied in this kit, however, plans are provided in the construction manual.

Option 1 AUD $3625.00 (US $2975.00)
Note: Domestic sales add 10%GST

Option 2 AUD $3200.00 (US $2625.00)

What do you get in the box???

The picture above shows the Acorn ESL kit in its packing crate.
The photo’s below show the contents in finer detail.

The photo above shows the audio transformers, speaker cable connection plates, EHT supplies, voltage regulator, plug pak,connection terminals etc cable and balance resistors.

Above - Panel components, showing stators pre-assembled in support panel,
edge channel and a roll of diaphragm film.

Above - Construction materials consisting of adhesives, rollers, applicator and brushes, tension gauge, rubber gloves etc, plus a roll of copper foil tape for the EHT charge ring and a construction manual on CD Rom.

Right - a completed Acorn panel, there are 4 of these in an pair of Acorn speakers

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Review 1 Review 2 Review 3