Mechanical Design of ESL 57's

In this section I will cover the mechanical design of the speaker and how to dismantle the speaker should they need attention.

The ESL 57 is an old loudspeaker. This may sound like an obvious statement but it must be stated now so that the problems that are associated with the speaker are more fully understood. I cannot praise the design enough. In its day it was a world first and as such deserves commendation. However, what I have always found strange is that over the whole period of production very few design changes were made to improve the performance of the speaker or to make it more compatible with higher powered amplifiers. The design itself leaves a lot to be desired. At times it seems that Quad designed the prototype which did the job but the prototype was never engineered to a point where it could be considered a mature design which was fit for higher volumes of production. Enough of my criticism, on with the speakers construction.

The speaker itself is built on a wooden frame which is braced in both bottom corners to prevent flexing. The front of the speaker has an expanded aluminium grill attached to prevent electric shock. This front grill is attached using screws along the bottom edge of the grill. Along the bottom should also be an earth point. The grill is also fixed down the sides with staples. These are found by first removing the wooden side rails. The top of the grill is held in a slot which runs the width of the speaker. The rear grill is held in place with screws around its edge and an earth tag on the bottom. Sometimes the rear grill can become clamped under two of the screws which hold the audio transformer in place. The rear grill has damping material stuck to it. This material is integral to the speakers design and should not be removed. The front grills were supplied in one of two colours - black or gold.

The whole speaker sits on three wooden feet. These are marked on the bottom of the feet with either an L an R or with nothing for the rear foot. Each foot is held in place with four screws.

Once the front and back grills have been removed the inside of the speaker will be visible. Depending on the age of the speakers they will either be dusty or very dusty inside. High voltages act as dust magnets.

The view from the front of the speaker will show the treble panel in the center with the two bass panels either side of it. The panels themselves are held in place by two aluminium brackets at the top and bottom of the treble panel. Care should be exercised when removing the screws which hold these brackets in place as the dust cover on the treble panel can be punctured. When the speakers left the factory they had a dark grey paint sprayed onto the bass panel dust covers so that they couldn't be seen through the grill. Strangely Quad masked off the treble panel so these never had grey paint applied to them. The panels were sprayed in situ so it is not uncommon to see some overspray.

The view from the rear of the speaker will show the panels as per the front view. Additionally, you will also see the audio transformer on the left of the speaker and the power supply on the right of the speaker. The central treble panel should have felt pads to prevent rear sound radiation. This is also integral to the design and should not be removed. Many people have reported improved performance by removing the felt pads. I believe this is solely due to the performance of the whole speaker deteriorating over a long period of time. By removing the felt pads more treble is radiating out of the speaker which gives the impression that the speaker is improved.

The Power Supply


The power supply module is actually two separate parts namely the mains transformer and the voltage multiplier. The job of the mains transformer is to step-up the supply voltage to 610V for the voltage multiplier. The voltage multiplier takes the supply voltage and multiplies it up to the required voltage for the treble and bass panel diaphragms. These are 1500 and 6000 Volts respectively.

There are two changes which Quad made to the power supply as a whole over the speakers life. Firstly, to overcome new safety legislation the mains input plug was changed from a round 3 pin Bulgin plug/socket to a 3 pin IEC connector. Secondly (and more important) are the changes that were made to the voltage multiplier. I have seen 4 different designs of voltage multiplier. The earliest type is encased in a solid epoxy block which renders the unit unserviceable and if found to be low on output is a scrap item (inside the multiplier is a series of diodes which fail over time). Later Quad fitted the multiplier inside a small plastic box. This was then filled with wax to prevent corona discharge. This makes this unit serviceable. Of this wax type, I have seen two varieties. Both are serviceable. The latest design is a single PCB with all the components mounted on the board. The whole assembly is dipped into wax and left to dry. These are also serviceable. They are currently available new from Quad.

To perform any service work on the power supply it is probably best to remove the whole unit from the speaker. The unit is held in with four nuts and bolts. However, Quad attached the front and rear grill earthing wire to part of the power supply unit and this will have to be unscrewed from the assembly. There are three connections from the voltage multiplier will have to be un-soldered. The bass panel supply will be a fat red wire, the treble panel supply will be a thin red wire and the third wire is the ground for the audio transformer. This is a thin black wire. All three will need to be un-soldred before the unit can be removed. Take care when using a soldering iron inside the back of the speaker - there is nothing worse than seeing a blob of solder drop straight through the bass panel dust cover on its way through to the diaphragm and beyond. Result - knackered diaphragm. Use a piece of paper or card to prevent this happening. Once the unit has been removed it can be tested to see what output voltage it is giving. If it is found to be low further investigation will be necessary. This is explained in the refurbishing section.

The Treble Panel

More often than not, the reason I see a pair of speakers is because the treble panel has been arced. This happens when the voltage difference between the stators exceeds 2200 volts. An arc jumps the gap and renders the panel useless. Occasionally a small arc will only produce a small hole in the diaphragm and this may not cause too many problems. Generally though an arced panel renders it unusable and should be replaced. An arced panel can be identified most easily by playing music through the speaker. Any crackling or distorted sound can most likely be blamed on a faulty treble panel. In a small number of cases distorted sound can be caused by bass panel problems. The panels all get the audio signal from a single point i.e. the audio transformer. In bass panels which have insulation problems arcing can occur between the high voltage diaphragm supply and the audio signal on the stators. This can be mistaken for an arced treble panel. Only experience can tell the difference. The other way of telling if you have an arced treble panel is to look for arcing and sparks from the panel when music is played through the speaker. In a darkened room this is quite easy although you may have to get on the floor and look up through the louvers in the grill in order to see it. With the front cover removed, arcing is usually very obvious as an area of black soot.

Removal of the panel is relatively easy. The first task is to disconnect all the wires to the panel. The high voltage is removed from the power supply as detailed above. Whilst in the back of the speaker, look at the felt pads that should be on the back of the treble panel and make sure there are no staples going through the felt and into the black wooden uprights which support the panels. If there are any remove them. The audio signal wires will need to be disconnected from the audio transformer. Remove the four screws holding the audio transformer and un-solder the wires. Make sure you know exactly where they came from as Quad didn't use different coloured wires to distinguish the front stator from the rear. Also make a note of how the wires are fed through the rubber grommet in the side of the transformer housing and then through a short piece of insulation before being connected to their solder posts. The panel is actually removed from the front of the speaker. Both aluminium brackets will need to be removed and retained. The panel is removed by first sliding the bass panels in toward the center of the speaker. The bass panels do not need to be disconnected to remove the treble panel but it would be useful to have some ones assistance while you are removing the treble panel. Also, what will have probably happened is that the sticky tape which holds the dust covers on the bass and treble panels will have probably deteriorated and the back edges of the bass panels will be stuck to the front edges of the treble panel. Carefully prise them apart. Once the bass panels have been slid in a little they should be free to come out and once they are the treble panel can be removed. Be careful when removing the treble panel as loose wires flying around can puncture a dust cover. Also in each of the two black upright supports there should be two small spikes on which the treble panel sits. They are there to stop any sideways movement. The spikes penetrate the wooden frame of the dust cover. Be careful you don't puncture the dust cover on the spikes. The treble panel should now be out. I would advise that the bass panels are re-positioned back where they came from to prevent them from relaxing. They have probably been sitting in the same place for a number of years and you don't want the rivets which hold the bass panels together to become loose.

The construction of the treble panel is deceptively simple. In essence you have a 5 layer sandwich. The outer layers are the dust covers. These are simply a wooden frame on which a piece of film is glued. The next layers are the stators themselves. These are made from plastic. All of the stators I have seen are made from a red coloured plastic. Over the years the design of the stator changed very little. The only change I know of is to the way the cavities are produced in the stator. Early panels were made by a vacuum forming process where a flat sheet of plastic is heated until it is soft and then sucked down onto a former. This created the cavities in which the diaphragm moves. Later treble panels were made of thicker plastic and the cavities are machined out probably either by a milling machine or an engraving machine. All the panels I have seen are painted grey. This paint was applied very poorly and some always comes off when the sticky tape is removed. The paint on the outside faces of the panels is simply for cosmetic reasons (so you cant see them when the front grills are on the speaker). The grey paint on the inside surfaces of the stators acts as an insulator to reduce the likelihood of arcing. If any has come off the inside surfaces I would suggest that corona spray is used to insulate the exposed area. Right at the center of the sandwich is the diaphragm itself. The treble panel diaphragm is a polyester film and is 0.00025 inch (a quarter of 1 thousandth of an inch) or 0.006 mm (6 microns) thick. Once the dust covers have been removed you will see the treble panel naked. You will see all the rivets which hold the stators together, the audio signal wires, the high voltage wire for the diaphragm and insulating sticky tape around the edges of the panel. To get access to the diaphragm all the rivets, the high voltage fixing bolts and the insulating tape will have to be removed. This is the job I hate the most for two reasons. Firstly, if the panel is a few years old it is very likely that the insulating tape around the edge of the panel has deteriorated. This will mean that the adhesive on the tape has either turned into a dusty powder or it will come away from the tape and be left on the stator. This means you will either be left with sticky fingers which is very annoying or with dust all over the place which is equally annoying. Secondly, all the rivets need to be removed. I use a small pair of pliers but you can devise whatever method suits you the most. I have tried drilling them out but this doesn't work as the rivets are usually loose and so they just spin. I have also heard of people using a small grinder but this generates a lot of heat on the rivet and could melt the stator. Once all the rivets are out you can split the two halves of the panel. Sometimes they may appear to be stuck together but they aren't. Inside you will see several things. The diaphragm itself which is covered on both sides with the white conductive compound, four white strips of masking tape and probably a hole through the diaphragm and a black area where the arc happened. You will probably wonder how they ever worked. I have never seen a consistent coating on a diaphragm. They always look like someone used an old rag to apply the coating. You may also wonder how the diaphragm is charged up as there is no direct connection between the high voltage supply and the diaphragm. Also you might notice that the holes where the rivets went through the diaphragm seem badly cut. The way the panels were made was that the stators were prepared ready for the diaphragm. Glue was applied to the edges of one stator and the coated diaphragm was lowered onto the stator, clamped and the glue was allowed to set. When set, the other stator was laid on top and the whole thing was riveted together. Quad didn't think that just banging a rivet through was a problem but this caused the holes in the diaphragm to have rough edges and was a potential point for a split to occur. The diaphragm voltage at no point has a direct connection to the diaphragm. Instead, there is conductive paint applied to the inside surface of the stators around the periphery of each cavity which carries the voltage. This is then painted over so I can only assume that the diaphragm gets its voltage by some field effect (anyone with a more scientific explanation of this, please drop me a line). The audio signal comes in on the wires from the transformer, goes through the solder tag and onto the conductive paint on the area where the holes are which conducts the audio signal.

The Bass Panels

I mention the treble panels first because these are usually the first to fail. Everything which has been stated about treble panels is true for bass panels with a few exceptions.

The panels are constructed almost identically. They are a five layer sand which with the same layers as the treble panel. The main differences are that the conductive paint for the audio signal is on the outside surface of the stator, the diaphragm voltage is carried inside by thin strips of aluminium foil, the connections to the panels are made via solder posts and the bass panel diaphragm is made from something called saran wrap which is basically PVC cling film. This is double the thickness of the treble panel diaphragm at 12.5 microns or half a thousandth of an inch.

Usually failures in treble panels are easy to spot. This is not the case with bass panels. Bass panels don't fail in the same way as treble panels. They don't arc. The stators are at least 1/4 inch apart and it would take about 18,000 volts to jump that gap. Bass panels usually suffer from insulation problems. This can be identified by measuring the on and off load voltage of the power supply. This would identify any leakage while no music signal were present. As soon as an audio signal is applied you can suddenly have voltage swings of 1000 volts quite easily. With the diaphragm at 6000 volts already thats a potential difference of 7000 volts so its not surprising that the insulation can fail. Usually this is either some rivet heads arcing to the dust cover or arcing to the conductive paint for the audio signal. The only way to cure them is to strip the bass panel down and have it on test. This involves un-soldering the three solder posts, cutting through the tape holding the dust covers on and removing the covers from the panel. This is a fiddly job at the best of times. Its not helped when the three wires from the solder posts get stuck and won't go through the holes in the board which supports the solder posts. The panel will need to be tested in its naked state to see where the insulation has failed. See the refurbishing section.

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