White Paper On Cabling For ESL Loudspeakers

Article By Roger Sanders
InnerSound, Inc.

 

  Electrostatic loudspeakers (ESLs) are different. The load they present to an amplifier and speaker cables is quite unlike that of conventional magnetic loudspeakers. To a loudspeaker cable, they appear as a capacitor, while magnetic loudspeakers appear as a combination of a resistor and inductor. It therefore is not surprising that cables for ESLs have different requirements than those for magnetic loudspeakers. Cables have inductance, capacitance, and impedance. Cable manufacturers juggle these parameters to get the cables to sound the way they want. Let's look at these elements more closely and see how they should be optimized for ESLs.

An ESL is driven by a high-voltage, step-up transformer. This transformer is inside the speaker and converts the relatively low voltage of an amplifier to the several thousand volts needed to drive an ESL. Unfortunately, all transformers have leakage inductance. This inductance interacts with the capacitance of an ESL to form an L/C (inductance/capacitance) resonant circuit. This produces an undesirable, high-frequency peak in the frequency response of the ESL.

It is essential that this resonance be kept well above the audio spectrum to prevent the sound from being excessively "bright." Since the capacitance of the ESL is fixed, the only way to get the resonance high is to build a transformer with very low leakage inductance. Designing and building very low leakage inductance transformers that will operate over a wide frequency range and at high voltages is extremely difficult. One of the reasons that some ESLs sound better than others is the design and quality of their transformers.

Inductance is a big problem with ESLs due to the L/C resonance described above. ESL manufacturers expend great effort to obtain transformers with low inductance. So it is vitally important that the cables have low inductance too. If the cables add a lot of inductance to the circuit, they can undo the transformer designer's best efforts.

Inductance

In a speaker cable it is largely determined by the area between the conductors. Most speaker cables have conductors that run side by side ("twin-lead") and that are separated by a small distance, so have moderate inductance. They do not have the low inductance desired for the best performance when driving ESLs. Some cables use many small wires that are woven together. This reduces inductance greatly, but at the cost of increased capacitance.

Capacitance

SHould be very low. This is not as critical as inductance, but it is important. Remember that an ESL is a capacitor, and amplifiers find capacitors very hard to drive. If the cable adds more capacitance, it only makes things that much worse for the amplifier. Capacitance is highly affected by how close the conductors are to each other. So to keep the capacitance low, the conductors must be widely spaced. Note that this is just the opposite of what we need for low inductance.

Many cable manufacturers deliberately add a lot of capacitance to their cables. For example, you will find a box at the end of MIT cables, which contains capacitors. Alpha Core (Goertz) cables are made as a sandwich with two ribbon conductors very close together, and this type of construction produces high capacitance and often, amplifier instability. Woven wires are close together so have high capacitance. These types of high-capacitance cables are best avoided when operating ESLs.

Impedance

This is the resistance to the flow of current in a cable. Most cables are designed to have low impedance so that they don't significantly reduce the damping factor of the amplifier. But some manufacturers deliberately use high impedance cables to alter the sound of the speaker by both interacting with the speaker's crossover and reducing the damping factor. When the damping factor is reduced, the amplifier cannot keep the woofer under good, tight control. The result is that the bass becomes "loose." In the case of an ESL, it is best to use a medium impedance cable as this will "damp" the L/C resonance and reduce its magnitude. Since the L/C resonance should be supersonic, this damping effect may not be audible. But reducing the resonance will make life much easier for the amplifier. Of course, if the ESL's transformer is poor, the L/C resonance will be in the audio range and damping it with a medium impedance cable will help smooth out the high frequencies.

InnerSound ESL Loudspeaker Cable Design

InnerSound's cables are uniquely designed to meet the needs of ESLs in three ways. They have low inductance, low capacitance, and moderate impedance. How is this done? Because the conductors need to be close together for low inductance, but wide apart for low capacitance, simultaneously obtaining low inductance and low capacitance seems impossible. But surprisingly, there is a solution to this problem. Coaxial cable construction runs one conductor inside the other. So electricity "sees" the conductors in the same place. This results in very low inductance.

InnerSound's coaxial, low-inductance design is enhanced by spiral-winding the conductors in opposite directions. This further cancels inductance. But what about capacitance? Doesn't a coaxial design place the conductors close together forming a high-capacitance cable? Not necessarily. The conductors can be physically separated by a significant distance using a thick, high-value dielectric to produce very low capacitance while maintaining ultra-low inductance. The impedance is determined by the size and length of the conductor. InnerSound sizes the conductors to obtain medium impedance in the typical range of cable lengths used by most audiophiles.

InnerSound Woofer Loudspeaker Cable

For driving the conventional, magnetic woofers used in hybrid ESL/woofer systems, the demands for low capacitance and low inductance are relaxed, although maintaining these parameters at low levels is still desirable. At the same time, the impedance needs to be low to maintain a high amplifier damping factor to achieve tight control of the woofer. InnerSound's bass cables meet these criteria by using dual pairs of coaxial cables. This technique drops the impedance to very low levels while maintaining low inductance and capacitance.

Interconnects

All interconnects are not equal. There are some very specific features that interconnects should have. InnerSound interconnects have superb performance and the finest features. But all the hype surrounding interconnects makes it very confusing to know what is important. The purpose of this paper is to explain the facts so you can make intelligent decisions. The facts can be quite surprising as you will soon see. There is no doubt that speaker cables can exert a small influence on the sound of your audio system. But interestingly, all well-designed interconnects sound identical.

The above statement sounds absurd, since interconnect manufacturers all claim that their products will make your system sound better. They also claim that different types of wire (copper, silver, oxygen-free copper, etc.) sound different, that skin-effect causes transient-smearing, and that dielectrics change the sound. So the idea that all interconnects sound identical is outrageous. Or is it?

Have you actually done a well-controlled test to verify their claims? I strongly urge you to do your own testing. Don't take my word for it. Do your own test and prove it to yourself. Fortunately, it is very simple and easy to test interconnects. Let me explain how. The idea behind the test is to make it possible for you to switch back and forth between interconnects instantly and repeatedly while all other components in your stereo system remain the same. You can then listen very critically for any difference in sound between the interconnects you are testing.

You cannot accurately test interconnects by listening to one set for awhile, then unplugging them, connecting another set, and listening again. Our "audio memory" for subtle details is too short to accurately remember subtle differences in sound in such a test, and we cannot check repeatedly to be sure of what we hear. So we are easily deceived. You must be able to switch instantly and repeatedly to hear any real differences between interconnects (or other components). You do not need any test equipment. You can use your preamplifier to do the switching. You will need a "Y" connector so you can connect the two interconnects under test (let's call them "A" and "B") to the same component -- probably your CD player.

Note that the "Y" connector is the same for both interconnects, so even if you believe that the "Y" connector somehow corrupts the sound, the same corrupted signal will pass through both interconnects so the test will still be valid. Remember that we are only listening for any difference between the interconnects, and you can hear that difference (if present) on any signal, even a corrupted and distorted one. Inexpensive "Y" connectors can be obtained from Radio Shack. If you want audiophile grade "Y" connectors, Sound Connections International (voice 813 948-2907) sells beautifully-built, gold-plated units at reasonable prices.

Connect one end of interconnects "A" and "B" to the "Y" connector. Do so for both channels. Connect the other end of interconnect "A" to one of your preamp line-level inputs (such as "CD"). Connect the other end of interconnect "B" to your tape monitor input. Do so for both channels. Be sure you don't reverse the channels. All line-level inputs on a pre-amplifier are identical, so it doesn't matter which ones you use. But by using the tape monitor, you can do the test by just pressing one button. The tape monitor inputs allow to switch back and forth between interconnects by toggling the tape monitor switch instead of having to press different input switches, or rotating a knob. Toggling a single switch is more convenient and makes it easy to do the test "blind" so you don't know which interconnect you are listening to. Doing the test blind is desirable so your personal prejudices don't influence the test results.

If your pre-amplifier does not have a tape monitor function, then use any two line-level inputs. If you have to use a rotary selector switch, use two inputs that are next to each other on the rotary switch so you can easily move back and forth between them. The test is done by simply listening to music while switching back and forth between the two sets of interconnects as much as you wish. The idea is to try to hear any difference between the interconnects. There is no time limit, you may switch whenever you wish and take as long as you want. The test is easiest to do if you have a remote-control preamp so you can sit in your listening chair and simply push the Tape Monitor button on the remote whenever you want to switch to the other interconnect. If you don't have a remote control preamp, then you may need an assistant to switch for you whenever you signal them to do so.

To do the test blind, press the tape button several times quickly so you get confused and don't know which interconnect you are listening to. If your preamp has an indicator light showing what you are listening to, then either put a piece of black electrical tape over the light or close your eyes while you do the test so you can't see the light. After doing this test, you will discover that all the hype surrounding interconnects is just that. The fact is that all well-designed interconnects sound identical. But please carefully note that I said all well designed interconnects sound identical! Some interconnects are badly designed and do indeed sound different. So just what is a "well-designed" interconnect?

First, the interconnect must be shielded. Shielding prevents RFI (Radio Frequency Interference) and EMI (ElectroMagnetic Interference) from corrupting the sound. RFI can take several forms with the simplest being a buzzing sound (usually caused from light dimmers), to actually hearing radio or TV program transmissions faintly in the background of your music. EMI is caused by magnetic flux lines cutting across the interconnect and inducing currents in it. This can take the form of hum if the interconnect is near an electrical transformer or motor, or will be crosstalk if the interconnect is near another interconnect that is active with a different signal.

Shielding is usually done by braiding a fine wire mesh around an internal conductor(s), making the interconnect coaxial in design. Although this mesh is usually adequate, there are small spaces between the wires in the mesh so that there is not 100% coverage. To obtain the greatest shielding, some interconnects are designed with a solid foil shield. This foil is prone to cracking and breaking if it is flexed, so the foil (usually aluminum) is often deposited on Mylar film that is wrapped around the wire to improve flexibility. But still, foil-shielded cables should only be used in stationary applications since frequent flexing will eventually crack the shield. Braided-mesh shielding should be used for interconnects in home audio systems.

The second requirement is that the interconnect have low impedance. High impedance can cause loss of output at both high and low frequencies depending on the loads presented by the components connected to the interconnect. And when the frequency response is restricted in this way, the effects are indeed audible. Buy why would you want to limit your system's frequency response? The third requirement is that the connectors at the ends of the wire be practical and trouble-free. This encompasses several factors:

1) They must not oxidize or corrode as this eventually will cause a high impedance contact and restrict the frequency response. Most connectors are gold-plated to meet this criterion, although solid nickel connectors also work well. If they are gold plated, it is important that the plating be of high quality so it doesn't easily chip or flake off.

2) The outside contacts of an RCA connector should NOT be tapered. If they grip only on the their tips, they can put great pressure on your components' jacks and can gouge or scratch their gold plating. Along these lines, it is best to avoid connectors that have clamping mechanisms that you tighten after insertion. These can put enormous pressure on your components' jacks and then the slightest motion can tear off the gold plating. And it is virtually impossible to tighten them without moving them. The best contacts are those that have precision-machined, parallel walls in the shape of a perfect cylinder. These produce smooth, even, firm pressure on the jack without damaging the gold plating.

3) The connector should have a strain relief. The purpose of the strain relief is to prevent tension on the interconnect cable from being transferred to the delicate connections inside the connector. This requires some kind of clamping mechanism so that the connector is solidly anchored to the outer covering of the interconnect cable, while the wires inside the connector are slack. Most RCA connectors don't have any strain relief. Some have springs around the cable near the connector to prevent excessive cable bending, but it doesn't prevent tension from damaging the internal connections. Some RCA connectors have a small metal strap inside the connector that is pinched around the cable, but this is weak and grips very poorly. The best connectors will have a clamp that can be screwed down and that gets a really solid grip on the wire's outer cover, but these are very rare.

4) The connector should have a tough, scratch-resistant, attractive exterior surface. Most are painted, and paint is easily damaged. Some are gold-plated. But gold is soft and easily scratched. The best have industrial hard-chrome surfaces. This type of coating has an attractive, matte silver color and is very durable.

5) The connector, particularly small RCA connectors, should have a "grippy" surface so that you can grasp it firmly to remove it so you don't have to pull on the cable. A thick ring is nice, but often interferes with other connectors that are close by. So the best option is deep knurling on the surface that produces a rough, easily gripped surface, without increasing the size of the connector.

6) The connector needs to be highly conductive to keep the impedance low. Steel is not a suitable material. Brass or copper should be used.

Amazingly, many very expensive interconnects fail to meet these basic criteria. In particular, many have no shielding at all! This is inexcusable in an expensive interconnect. The manufacturers of such poor interconnects only get away with this because most home environments have little RFI and EMI. But this is not always the case and there are many systems that are plagued with buzzing and other noises due to the lack of shielding. The owner is very frustrated that he can't get the noise out and never suspects that his exotic interconnects are the cause.

Some interconnects have very high impedance. This is because the interconnect uses extremely tiny wire. The manufacturers of such interconnects claim that very small wire prevents "transient smearing" due to "skin effect" or some other arcane reason. But the fact is, wire size and type does not affect the sound (unless the impedance is too high). There is no such thing as "transient smearing" in interconnects and "skin effect" does not alter the sound at audio frequencies. You discovered this in your listening tests. But some of these interconnects have several thousand ohms of impedance and can adversely effect the frequency response of your system.

Very few interconnects have connectors that meet the "practical and trouble-free" criteria outlined above. There are too many connector types to discuss here, but if you will examine them, you will see that few meet the criteria outlined above.

Manufacturer

InnerSound, Inc.
1700 Hwy 16
Whitesburg GA 30185
770 838 1400

Website: www.innersound.net