Are you confident in your choice of cable? Or do you become a quivering mass of jelly at the very thought of deciding between braided coax, foil screening, or conductive plastic? In an overview of what the market has to offer in terms of cable, Paul White stresses the importance of having the right connections.

One of the less glamorous aspects of live sound is the amount of cabling required to get a system up and running. While there is little that can be done to reduce the amount of cabling you need, we can offer some advice on the type of cable that should be used.

In a typical audio system, all the signal cables, other than those linking the power amplifiers to the speakers, should be screened. Screened cable can be recognised by its coaxial construction where one or more inner conductors are surrounded by a tube-shaped screen so as to minimise the effects of external interference. The purpose of the screen is to intercept electrostatic interference and drain it away to earth before it can affect the signals passing along the inner wires. In the case of an unbalanced cable, the screen also forms the signal return conductor; in a balanced cable, the signal is carried by the two cores and the screen does not constitute a part of the audio signal path.

The screen may comprise woven copper braiding, layers of multi-strand wire wrapped around the inner cores in a spiral-like fashion, a thin layer of metal foil, or conductive plastic. Each type of cable has its own strengths and weaknesses, which has implications for screening efficiency, flexibility, cable capacitance, ease of termination, and so on.

BRAIDED SCREEN COAX

Probably the most common type of screened cable is braided screen coax. Combining good screening efficiency with durability and a reasonable amount of flexibility, braided screen cables are available in a variety of thicknesses. However, they do tend to be time consuming to terminate. To make a connection, you have to strip the outer plastic sleeve, then either unpick the last couple of centimetres of the screen, or part the strands sufficiently in order to pull the inner conductors through the side of the screen about 1.5cm from the end. Once this is done, you have to twist together the strands of screen so that you can solder them. If the cable is heavy duty, you can end up with a fairly thick bunch of screen that has to be soldered to a relatively small connector.

The screening efficiency of a cable is partially dictated by the dimensions of any gaps in the screenings. The shorter the wavelength of the potential interference, the smaller the gap it can leak in through. Loosely woven screened cable may seem to be more flexible and easier to terminate, but it can be more susceptible to high frequency interference, such as is generated by the sparking electrical contacts inside motors and so on. It's also generally true that the longer the cable, the better the screening needs to be.

Because of their good screening properties, braided screen cables are often used as instrument or mic cables. Figure 1 shows the construction of a typical braided cable.

WRAPPED SCREEN COAX

Wrapped screen cable is similar in construction to woven screen types, except the wire screening is simply wrapped around the inner core rather than woven. The result is a flexible, easy-to-terminate cable. If the cable is bent, however, there is a possibility that gaps may open up in the screening, compromising its effectiveness. What's more, any movement of the screen relative to the core, caused by bending, tends to change the cable capacitance slightly, and in low signal applications (such as mic leads), this can generate noise — we've all had experience of mic or guitar leads that crack when they're moved or bent. Larger diameter types sometimes include a cotton filler to separate the screen from the core. In my experience, this seems to make the cable even more prone to kinking.

Because of these limitations, wrapped screen cable is best avoided. If you must use it, short runs only please, and don't use it for mic or instrument leads. Figure 2 shows how wrapped screen cable is put together.

FOIL SCREENING

Foil screening provides the best immunity to interference of all the screened cable types. Inside the wound foil screen (and in electrical contact with it) is an uninsulated length of wire used to make the screen connection. The foil screen is usually aluminium and quite fragile, so you can't solder directly to it.

Foil screened cable is also very easy to terminate. To make a connection, the outer insulation is stripped as normal. The foil screen is also peeled back and cut off to the same length as the outer insulation. The inner cables are then stripped and soldered as required; the exposed uninsulated screen connection wire is soldered to the screen terminal.

While this type of cable exhibits excellent screening properties, it is relatively stiff and it is possible to break the foil screen if very tight bends are made in the cable. Foil screened cable is therefore best suited to wiring the insides of equipment racks or other applications where little flexibility is required. Figure 3 shows the construction of a foil screened cable.

MULTICORES

A multicore usually comprises a number of small diameter, individually screened, twin core cables constrained within a common outer sleeve. Multicores are used in fixed installations and for connecting the mixing console to the stage box in conventional PA applications. Multicore that employs foil screening may be used for this latter purpose so long as the cable isn't bent further than the manufacturer's specifications permit.

There are two types of foil screen construction; one where the foil is wrapped around the inner core as a continuous tube, and the other where it forms an overlapping spiral. Spiral-wrapping is less likely to suffer damage through bending. It is possible to wind the cable onto a large diameter drum, but most users tend to coil it 'freehand' in a large packing case so as to avoid kinks or tight bends.

In mobile multicore applications, it is especially important to choose connectors with properly designed strain relief systems, and to fit these in accordance with the manufacturer's instructions.

Note that some users utilise multicores to feed signals in both directions — mic feeds from the stage and power amp feeds back to the stage. Although this is possible, it isn't a good idea as capacitive coupling between the conductors can result in sufficient crosstalk to make high frequency feedback a possibility. You may end up with an oscillation above the range of human hearing, in which case the first you'll know about it is when your tweeters no longer work — HF oscillations can burn out a tweeter coil in a second. Other than plugging the output of the bass bin amp into the HF cab by mistake, this is probably the most common cause of HF driver damage.

Conductive plastic screening is a relatively new innovation. The traditional metal screen is replaced by a sheath of carbon loaded plastic. Constructionally, the conductive plastic cable is much like the foil screened cable discussed earlier, and because you can't solder to plastic, the same uninsulated drain wire runs inside the screen. To make a termination, you strip back both the outer jacket and the plastic screen, then make the connections to the inner cores and the drain wire.

Most conductive plastic cables also use very soft, rubbery outer jackets available in a range of colours. Screening efficiency isn't generally as good as for the other types of cable because of the higher resistance of the plastic screen. It is therefore not advisable to use this type of cable for very long runs or in unduly hostile interference environments. It is, however, ideal for instrument leads and is also successfully used in the manufacture of some microphone cables. Figure 4 shows a cross section of a conductive plastic cable.

While hi-fi fanatics seem happy to pay fortunes for esoteric speaker cables, what really matters in live sound is that the cable is durable and has as low an electrical resistance as possible. The function of a speaker cable is to provide a low resistance path between the amplifier and the loudspeaker. If the cable resistance is significant compared with the impedance of the speakers to which it is connected, the power will be shared between the speaker and cable.

Take an extreme case: an amplifier capable of delivering 100 Watts into 4 Ohms or 50 Watts into 8 Ohms. If the cable has a resistance of 4 Ohms, the total of the speaker and the cable is 8 Ohms, so already the amplifier is delivering only 50 Watts instead of 100 Watts. Furthermore, the 50 Watts is shared equally between the speaker and warming up the cable, so only 25 Watts actually reaches the speaker. In reality, cable resistances are unlikely to be anywhere near this high, but you can see from this example why the cable resistance must be kept as low as possible — and low resistance invariably equates to thick cable.

Using inadequate cable will also reduce the damping factor of the amplifier — the damping factor is defined as the output impedance of the amplifier divided into the impedance of the speaker connected to it. This is important, because the higher the damping factor, the greater the amplifier's ability to sink any current produced by a loudspeaker voice coil when it overshoots its position and starts to work as a generator rather than a motor. By sinking the current produced by the driver in this way, the cone movement is effectively damped, producing a tighter, more accurate bass end. When calculating damping factor, the cable resistance must be added to the output impedance of the amplifier, so the higher the cable impedance, the lower the effective damping factor becomes.

Each type of cable has its own strengths and weaknesses, which have implications for screening efficiency, flexibility, cable capacitance, ease of termination, and so on. As a general rule, use the shortest, thickest speaker leads you can, make sure speaker cables are roughly the same length, and use low resistance connectors such as Speakons or XLRs. Jack connectors are a compromise, even on low powered systems.