This started as a response to a question on the email forum “The Bottom Line.” When it looked too long I decided to post it as a web page instead. Still, please be aware that there’s a lot that can be said about cables, and this is just an overgrown email message.
What makes cables different?
There are two kinds of cable differences that can be discussed: the kind that can be measured and is known to make a difference in sound according to commonly accepted electrical theory, and the kind that cannot or isn’t. Whether the second kind (oxygen-free copper, cryogenics, blah blah) makes a difference, I won’t discuss here. Let’s stick with the objectively measurable differences for this article.
Wires differ in:
• the size of the conductors;
• the number of conductors;
• the spacing between the conductors;
• the “dielectric” material that insulates between the conductors;
• whether there is a shield and how it is constructed;
• the outer insulating jacket.
These differences in turn cause differences in their electrical characteristics:
• how much power do they waste through heat;
• how much capacitance is there between the conductors;
• how much noise is generated by movement of static charges in the insulator when you wiggle the wire (this is called “triboelectricity”);
• how much external interference can be rejected by the shield;
• whether the wire can carry a balanced signal (needs two identical conductors plus, typically, a shield).
In addition, instrument cables have connectors on either end, which may differ in various things like mechanical strength, precise size, contact material, means whereby the wire is connected to the connectors, and means whereby the strain between the connector and wire is relieved. These don’t affect the sound in principle, but they affect how soon the cable fails, which affects the sound in the real world.
The differences in wire are very important, because mics, instruments, and amps all produce different sorts of signal. The relevant aspects of the signal are:
• how much power is being conveyed
• the impedance of the devices at either end
• whether the signal is balanced or unbalanced with respect to ground
I’m not going to explain what those mean, this is already getting long enough!
The job of a cable is to carry a signal between two points, without letting in any interference along the way and without modifying the signal at all. No cable does this perfectly, so there are tradeoffs to be made.
Let’s talk about some specific kinds of cable:
1. Speaker cable.
Speaker signals are high power, low impedance, unbalanced. Because they’re high power, interference and hum are very small by comparison, so the wires can be unshielded, and triboelectricity is not a problem. However, speaker wires are often run for long distances, so capacitance can be a problem, because it can cause the amplifier to oscillate at supersonic frequencies and destroy either itself or the speaker.
That, by the way, is why you should never use instrument cable for your speakers: its typically higher capacitance means you are likely to cause problems. Even if you don’t actually damage something, instability will change your sound and can decrease the amount of usable power. Some amps are less prone to this than others.
So, the ideal speaker cable has big, low-resistance conductors, with low capacitance between them. Shields are unnecessary.
2. Mic cable.
A mic signal is very low power, low impedance, balanced. It is quite susceptible to external interference, which is why it’s balanced: the idea is that the same interference will affect both signal conductors identically, and the mic preamp then subtracts one signal from the other, hopefully eliminating the interference.
Because the source and load are low impedance (and mics aren’t as susceptible to instability as amps are), capacitance is not a big problem; but anything you can do to help balance the signal will reduce noise. So some mic cables use “Star Quad” wiring where there are actually four, rather than two, signal conductors; they are intricately braided together and then paired up at the ends so that they behave like two conductors that are very close together physically. Star Quad increases the capacitance, but it reduces noise.
So, the ideal mic cable has at least two and maybe four signal conductors, plus a shield. Capacitance and triboelectricity are not problems.
3. Instrument cable.
An instrument signal is low power (but more than a mic), high-impedance, unbalanced. Like a mic signal it is susceptible to external interference, but different sorts: it is more sensitive to things like fluorescent lights and neon signs, less sensitive to motors and power lines.
Because the source and load are high impedance, any capacitance in the cable creates a low-pass filter. That is, it reduces the high frequencies in the signal. For that reason, you want low capacitance. But also, the high impedance means that triboelectric signals, which would get drained away by low impedance, can be a problem: when you wiggle the instrument cable you can hear noise from your amp. To deal with this, manufacturers add layers of intermediate insulators that are actually somewhat conductive.
Instrument cables are typically connected to musicians, who move around on stage. So they want to be fairly lightweight, which tends to mean physically smaller wires, which increases the capacitance. In principle you could make an instrument cable as beefy and as low-capacitance as a speaker cable, in which case they would be interchangeable (if it was shielded), but who would want that?
There is one problem with using the shield as a signal conductor rather than just as a shield, which is that shields sometimes get frayed or broken over time. To get around this, some instrument cables use two signal conductors, surrounded by a shield (just like a mic cable), with one of the signal conductors connected to the shield. (This is also claimed to produce some benefit with regard to interference, but that is only true in the presence of somewhat unusual kinds of interference, I believe. I have not seen a benefit.) The problem with doing this is that it increases the capacitance, as well as making the cable stiffer physically.
So, the ideal instrument cable has one signal conductor, surrounded by a shield, and is fairly low capacitance. Triboelectric shielding is useful.