Electrostatic Loudspeakers: High End HiFi You Can Build Yourself - ZVEIL

Electrostatic Loudspeakers: High End HiFi You Can Build Yourself

by Jenny List August 03, 2016

If you have an interest in audio there are plenty of opportunities for home construction of hi-fi equipment. You can make yourself an amplifier which will be as good as any available commercially, and plenty of the sources you might plug into it can also come into being on your bench.

There will always be some pieces of hi-fi equipment which while not impossible to make will be very difficult for you to replicate yourself. Either their complexity will render construction too difficult as might be the case with for example a CD player, or as with a moving-coil loudspeaker the quality you could reasonably achieve would struggle match that of the commercial equivalent. It never ceases to astound us what our community of hackers and makers can achieve, but the resources, economies of scale, and engineering expertise available to a large hi-fi manufacturer load the dice in their favour in those cases.

The subject of this article is a piece of extreme high-end esoteric hi-fi that you can replicate yourself, indeed you start on a level playing field with the manufacturers because the engineering challenges involved are the same for them as they are for you. Electrostatic loudspeakers work by the attraction and repulsion of a thin conductive film in an electric field rather than the magnetic attraction and repulsion you’ll find in a moving-coil loudspeaker, and the resulting very low mass driver should be free of undesirable resonances and capable of a significantly lower distortion and flatter frequency response than its magnetic sibling.

Speakers Driven By Static Electricity

If you’ve ever felt your hair lift as you take off a synthetic-fibre jumper, you will be familiar with the phenomenon of electrostatic force. Coulomb’s Law in action, the force resulting from a significant difference in the accumulated electrostatic charge between garment and wearer. Harnessing this force in a speaker would require a charge that varies at audio frequency creating the force on a sufficient surface area to create sound waves, by proximity with a conductive electrode maintained at a constant opposite charge.

A practical electrostatic loudspeaker. By Akilaa (Own work) [Public domain], via Wikimedia Commons.
A practical electrostatic loudspeaker. By Akilaa (Own work) [Public domain], via Wikimedia Commons.
A practical electrostatic speaker uses a flexible film that is as thin and light as possible as its moving component. A partially conductive coating is applied to it — just conductive enough to allow it to accumulate charge, but not enough to allow that charge to flow away freely. Two acoustically transparent conductive electrodes are suspended next to the film, one on either side. These electrodes can be wire mesh, an array of parallel wires, or a perforated metal sheet.

The film is held at a very high constant static charge by the application of a multi-kilovolt DC supply, while the electrodes on either side of it are supplied with very high voltage audio frequency in an antiphase push-pull configuration. This push-pull feed on either side of the film ensures that the forces on it are the same when it moves to either side of its center position. If there was a single electrode on one side of the film it would introduce distortion because the half of the cycle when the film was furthest from the electrode would receive less force than the closest.

In the majority of electrostatic speaker designs, the high voltage audio is created using a step-up transformer from a conventional audio amplifier designed for moving-coil speakers. It is possible to create an audio amplifier with an output in the multi-kilovolts, but the designers of such amplifiers face challenges from the high voltage itself and from the availability of suitable devices capable of delivering low-distortion audio at those voltages.

The Drawbacks

A Martin Logan electrostatic speaker on the left next to a moving-coil speaker. By Bownose on Flickr [CC BY 2.0 ], via Wikimedia Commons.
A Martin Logan electrostatic speaker on the left next to a moving-coil speaker. By Bownose on Flickr [CC BY 2.0 ], via Wikimedia Commons.
You might think that with low distortion and amazingly flat frequency response there would be no stopping the electrostatic speaker. But as with so many things there are weaknesses to these devices. They work in both directions, for a start. Half the sound the produce comes from the front, while the other half goes off somewhere behind the speaker where you can’t benefit from it. You can’t box them in with an infinite baffle or a ported enclosure as you can with a moving-coil speaker, and if you try to reflect too much of the backward-facing sound in your direction you’ll soon be face to face with its being out of phase with the sound coming from the front.

Once you’ve reconciled yourself with your speaker’s propensity for sending sound backwards, you then have another problem to contend with. Electrostatic speakers are very directional, so while you can set them up for a perfect stereo image in one spot, a relatively small movement can take you out of it. Various attempts have been made to broaden their spread with differing degrees of success, some manufacturers produce curved panels while others attempt to supply the grid of electrodes from a series of delay lines to create the effect.

Finally in the litany of electrostatic speaker woes, these speakers are not so good when it comes to bass reproduction. Electrostatic speakers will therefore often come with a moving-coil bass unit as a companion to the electrostatic panels, and will usually incorporate a separate bass amplifier and active crossover. Because the radiative patterns of cone speakers and the electrostatics are different, the bass-to-treble balance will vary as you move around the room, further accentuating the sweet-spot issues.

Build Them

Of course, general information about electrostatic speakers is all very well, but this is a site for hardware hackers. You want to see real examples, and maybe have a go yourself. In which case we won’t disappoint you, with electrostatic speaker builds from [Mark Rehorst], [Jazzman], and [Ken Siebert].

The materials required for home electrostatic speaker construction are fairly straightforward, with maybe one exception: the plastic film that forms the moving part. [Mark Rehorst] has an exhaustive list, and makes this important point: “Buying stuff specifically for ESLs is like buying parts for Ferraris. The seller knows you expect to be robbed so they try not to disappoint you“. The world of high-end hi-fi works to different economic rules it seems, so sourcing the same components from suppliers in other industries is the way to go.

Ken Siebert's high voltage bias supply.
Ken Siebert’s high voltage bias supply.

The transformer is an example with plenty of scope for stretching your budget. Audio transformers are expensive at the best of times. Happily there is a cheap alternative; most home builders use standard mains transformers connected in reverse. It has to be said though, as soon as you put a transformer in an audio circuit its performance is only as good as that of the transformer, so there may be some compromises in this component.

Any thin flexible plastic film can make a noise in an electrostatic speaker, but for best performance the thinner your film, the better. 5 micron thick Mylar seems to be the preferred choice.

Once you have your film, it needs a slightly conductive coating. It mustn’t be too conductive: charge must accumulate but not flow away too quickly. Some electrostatic speakers will take a few minutes to reach their maximum volume for this reason, as the charge moves very slowly to fill the panel. Older designs used graphite powder rubbed onto the surface of the film, while more recently they use spray-on coatings intended for static protection in the electronics business.

The fixed electrodes can be made from a variety of materials. [Mark] uses perforated aluminium, while [Jazzman] and [Ken] use a grid of wires, and in one case welding rods. A well known commercial design uses a very large perforated printed circuit board with the electrodes etched in a carefully designed pattern to try to broaden the angle of the finished speaker.

The high voltage bias supply is usually generated with a voltage multiplier chain from an AC transformer, though more recent designs may use solid state inverters. Normally about 4 to 6 KV DC is required for this task.

[Jazzman]'s film stretching table.
[Jazzman]’s film stretching table.
Once these components have been sourced, there only remains any woodwork necessary to make the frames that hold it together, and copper tape and wiring to ensure all contacts are made. Building the speaker is then a fairly straightforward workshop task, with one tricky moment: stretching the film over its wooden frame. It needs to be under slight tension, as any slackness or wrinkles will cause distortion. The technique used by most home builders is to stretch it over a frame with a bicycle inner tube around its edge, and then to inflate the tube which draws the film taut.

If you have got this far in an electrostatic speaker build, you should have something fairly special: there is very little engineering that the commercial manufacturers can do to make it much better. Listening to an electrostatic speaker can be something of a startling experience, but beware of superlatives. The hi-fi industry has a special kind of mumbo-jumbo with a whole vocabulary of pseudoscience to make its adherents feel good about eye-watering price tags, and electrostatic speakers are something they have placed on a special pedestal all of their own. You probably won’t be disappointed with your speaker build, but you’ll know it has the flaws detailed above and you’ll put up with them for the bragging rights.


Filed under: Curated, Featured, musical hacks, Original Art, Skills, slider

Jenny List
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