Choosing Studio Cable

Tips & Techniques

Published in SOS February 1996
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Technique : DIY

When it comes to cabling, it pays to have the right connections. PAUL WHITE makes the introductions.

 

Buying the right pieces of equipment to put together a home studio is hard enough, but without proper guidance, wiring it all up can turn into a nightmare of such proportions that if Freddy Kruger were to turn up in the middle of it, you'd be glad to have someone to hold your soldering iron! In the past, Sound On Sound has looked at how different pieces of studio equipment are linked to form a recording system, and has also covered the potentially confusing area of patchbay wiring, but how do you choose the best type of cable for each specific task, and what do you do when balanced equipment has to be connected to unbalanced equipment? While I don't profess to have all the answers to the hundreds of possible questions on the subject, the following article should at least clear up some of the most common queries.

All cable used for carrying signals (other than that used for connecting loudspeakers to power amplifiers), should be screened, and screened cable can always be recognised by its coaxial construction. Simplistically, a coaxial cable comprises one or more inner conductors surrounded by a tube-shaped screen, the idea being that the screen intercepts any interference and drains it away to earth before it can affect the signals passing along the inner wires. The screen itself may be formed from woven copper braiding, it may simply comprise a layer of multi-strand wire wrapped around the inner cores in a spiral-like fashion, it may comprise a thin layer of metal foil, or it may even be made from conductive plastic. Each type of cable has its own strengths and weaknesses, dictating which applications it is best suited for, so I shall start by describing the most popular types used in the home studio.

BRAIDED SCREEN COAXIAL

The first type of coaxial cable I ever encountered was the braided-screen type, and this is probably still the most common type available today. Braided-screen cables are available in a variety of thicknesses and offer excellent screening efficiency, combined with reasonable flexibility -- however, they are time-consuming and difficult to terminate. To make a connection, you either have to unpick the last half inch or so of the screen, or part the strands enough so that you can pull the inner conductors through the side of the screen, about half an inch from the end. You then have to twist together the strands of screen so that they can be soldered to the appropriate terminal in the plug. It is due to their excellent screening properties that braided-screen cables are useful in situations where long cable lengths are needed, but where flexibility is essential -- for example, mic cabling in large studios, PA systems, or live recording rigs. Braided-screen cable is also used extensively in the manufacture of professional patch leads and instrument cables.

LAPPED SCREEN COAXIAL

Cable with a simple lapped wire screen is easier to terminate, simply because the screen wires aren't braided. One disadvantage in using this type of wire is that the screening properties vary depending upon how the screen is wound. Continued flexing can also compromise the effectiveness of the screen, as the cable is prone to damage through kinking. Large-diameter types sometimes include a cotton filler, but this seems to make the cable even more vulnerable (the same is true of some braided cables too). There are varying opinions as to the usefulness of this type of cable, but I tend to implement it on short- to medium-length cable runs, where some degree of flexibility is desired. For example, I might use it to wire up a tape machine, but I wouldn't use it as mic cable through choice. The thinner lapped-screen types are also useful in patchbay wiring, although I prefer foil-screened cable for this particular application.

FOIL SCREENING

For permanent wiring, where flexibility isn't a main requirement, foil-screened cable has a number of advantages. Firstly, it is very easy to terminate, as just inside, and lying in contact with, the wound foil screen is an uninsulated length of wire (sometimes called the drain wire), which is used to provide the screen connection. The screen itself is usually aluminium which can't be soldered to directly. To make a connection, the outer insulation has to be stripped as normal, then the foil screen peeled back and cut off to the same length as the outer insulation, leaving the screen wire to be soldered. The inner part of the cable has its own insulation as usual and is stripped in the normal way.

The other advantages of foil-screened cable are that it has excellent screening properties, it is relatively thin, and it holds its shape when tied into cable looms, making it ideal for wiring the insides of patchbays or equipment racks. It is, however, not suitable for applications where a lot of flexibility is required.

Foil screening is extensively used in the manufacture of multicore cables, where a bunch of individually screened cables is constrained inside a single outer sleeve. Multicores are used in studio installations, but may also be used in live applications as long as the cable isn't bent further than the manufacturer's specifications permit. There is usually a recommendation as to the smallest diameter drum onto which the cable may be wound. Note that there are two types of foil screen construction. In the simplest type, the foil is wrapped around the centre core rather like a continuous tube, but other types have the foil wound in an overlapping spiral. The spiral-wrapped type of cable is less likely to suffer damage when bent around tight corners.

CONDUCTIVE PLASTIC

Conductive plastic screening is a relatively new innovation where the screen comprises a single tube of carbon-loaded plastic. Like the foil-screened cable, an uninsulated drain wire runs inside the screen, so in order to make a connection, it is only necessary to trim back the conductive plastic screen to the same length as the outer insulation, then solder the drain wire to the appropriate connector. The inner cables are quite conventional.

The benefits of conductive plastic screening are high flexibility, excellent resistance to kinking and easy termination. This type of cable is also available in a wide choice of colours. Unfortunately, the screening efficiency of this cable isn't as good as for the other types, which limits its use to short or medium runs. It may also be unwise to use this type of cable in situations where the level of RF (Radio Frequency) interference is known to be excessive. Conductive plastic-screened cable is ideal for making up short, colour-coded patch leads, short- to medium-length instrument leads, and short to medium mic leads.

AND FINALLY

No matter how good your cable, an audio link is only as good as its connectors, so don't skimp on quality, and, equally importantly, ensure the connectors are soldered on properly. It is also important to make sure that any cable clamping system that may be fitted is used properly, especially if the cables are going to be constantly plugged and unplugged. Most connector failures occur as a result of stress on the cable due to inadequate or non-existent clamping, and this is particularly true of mains plugs, where a failure could be dangerous as well as inconvenient. The clamp should always grip the outer insulation of the cable.

Keeping cables tidy is another problem altogether, and there are several commercial solutions, such as plastic spiral wrapping or split hose-type cable tidies. Using multicores wherever possible, rather than separate bundles of cables helps keep things under control, and for large MIDI systems where most of the instruments and modules will have unbalanced outputs, ready-made jack multicores, such as the Hosa 8-way, are both effective and affordable.

Finally, whatever wiring scheme you implement, keep it flexible -- the chances are that you'll want to change it eventually.

 

SPEAKER CABLES

Hi fi purists make a lot of fuss about speaker cables, sometimes spending several hundred pounds a metre on specialist cables. The main function of a speaker cable is to provide a low-resistance path between the amplifier and the loudspeaker, so thin bell wire is obviously a bad idea -- not only will thin wire take some of your amplifier power and turn it into heat, it will also reduce the damping factor of the amplifier. Without getting too technical, the damping factor of an amplifier is its ability to sink the current produced when a loudspeaker overshoots its position and starts to function as a generator rather than a motor. This mechanism effectively damps the speaker movement, keeping it under control, thus producing a tighter, more accurate bass end.

The most pragmatic approach is to use the shortest speaker leads you can, make sure they are both the same length, and choose heavy cable. I'm unconvinced that there's a difference between multi-strand cable and solid-core cable, and I've yet to hear the difference between the oxygen-free copper and 'virtually oxygen-free copper' that most stock cable is made from, but if you think it's worth the difference in cost, don't let me put you off. If you're on a tight budget, 30A cooker cable works perfectly well, albeit a trifle ugly.

 

DIGITAL CABLES

Digital cables for use with S/PDIF signals need to be chosen with more care than audio cables -- if their impedance is wrong, a proportion of the signal will be reflected back into the cable from the destination termination, and these reflections may corrupt the signal to the point that it produces errors. (Digital cable is usually around 75(omega) impedance.) If you're making up your own digital leads, it's important to fit the phono connectors correctly, as these can also cause an impedance mismatch -- I would recommend that it is generally safer to buy the cables ready made.

AES/EBU digital signals are generally robust enough to travel short distances over conventional XLR mic leads, though for longer distances, good-quality cable is advisable.

 

CABLE CAPACITANCE

The electrical resistance of signal cables is rarely a concern, except over distances of several hundred metres, but the capacitance of cable can cause problems, especially in high-impedance circuits, such as those that exist between guitars and amplifiers, or between high impedance microphones and amplifiers.

Due to the close proximity between the inner cores and the outer screen of a coaxial cable, the cable acts as an electrical capacitor -- the longer the cable, the higher the value of capacitance. This capacitor effectively forms a high-cut, 6dB/octave filter when combined with the impedance of the circuitry attached to the cable, and the higher the impedance, the lower the cutoff frequency of the resulting filter. In practical terms, a guitar lead 20 or 30 feet long may introduce audible tonal changes (a guitar amp has a very high input impedance), whereas a low-impedance mic cable would need to be extremely long before any adverse effects became evident.

Although short cable runs are unlikely to produce audible capacitance-related effects, it's generally best to buy the lowest capacitance cable of the appropriate type that you can get. It also makes sense to ensure that cable runs are no longer than necessary.

 

BALANCED AND UNBALANCED

Most coaxial audio cables come with two central cores enabling them to be used to carry balanced signals, so if an unbalanced lead is being made up, one of the cores becomes redundant. My preference is to connect the spare core to the screen at both ends of the cable, but you could just as easily trim it off.

When connecting balanced equipment to unbalanced, or vice versa, there are certain wiring systems that can be used to reduce the risk of ground loops. In a fully balanced system, the screen is often left unconnected at the destination end of the cable, and if twin-core cable is being used, the same technique can be applied when connecting balanced equipment to unbalanced, or vice versa. This is how it works, assuming the co-ax inner wires are coloured red and black:

BALANCED TO BALANCED:
Connect the red and black inner cables to the Hot and Cold terminals as normal, but only connect the screen at the source end.

BALANCED TO UNBALANCED:
Connect the red inner cable to the Hot terminals as normal, but at the balanced (source) end, link the black inner cable to the screen, and connect to both the Cold and the Ground terminals. At the destination end, connect the red wire to the Hot terminal, the black wire to the Ground terminal, and leave the screen disconnected. (Note: if the balanced equipment does not have a fully floating output, the signal level will be reduced by 6dB when fed into an unbalanced load).

UNBALANCED TO BALANCED:
Connect the red wire to Hot as usual, but join the black wire and screen at the source (unbalanced) end and solder to the Ground terminal. At the balanced end, connect the red and black wires to Hot and Cold respectively, but leave the screen disconnected.

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