How Far Can You Run Analog Audio Cable?

The distance you can go on an analog audio cable is based on two factors: the capacitance of the cable and the source impedance of the device.

Capacitance

All cables have capacitance. Capacitance stores an electrical charge between the two conductors in a cable.

In a twisted pair, that's between the two wires in that pair. In a coax cable, it's between the center conductor and the shield. The choice of plastic in-between will affect the capacitance (better plastic, lower capacitance).

Capacitance has more of an effect on high frequencies. This is one of those areas where the use of video cable pays off.

Find out the capacitance of a potential cable. It's a good indication of cable quality.

Impedance

Every cable has an impedance, but it doesn't really matter what the impedance is until you are running very long cables or very high frequencies -- neither of which will be done with analog cable.

The box (CD player, tuner, preamp etc.) that feeds the line also has impedance. Since it feeds the line, it is the source of the signal and the impedance is, therefore, called the source impedance.

You can turn to the owner's manual (or the manufacturer) to determine just what the source impedance is.

The table below will then tell you how far you can go on with given cable until the audio signal begins to be affected. In fact, the distance shown is that whereupon an audio signal of 20 kHz (generally, the highest frequency you can hear) will drop 1 decibel (-1 dB), which is just barely perceptible.

Going the Distance

So the key question is: What is the source impedance?

You will find that most consumer devices have a source impedance of 10 kilo-ohms (10 kΩ).

With a freebie cable (that might not even be copper), the capacitance can easily be as bad as 50 pF/ft. or even worse. At 50 pF/ft., with a source impedance of 10 kΩ, you can only go a whopping 8 feet until you begin to affect the signal (-1 dB at 20 kHz).

If you ever tried to put in a 50-foot or 100-foot RCA cable between devices (feeding a signal from one end of a house to the other, for example) and it sounded muffled and unusable, this was the reason why.

It wasn't bad cable, necessarily; it was probably high source impedance. Changing to a better cable will increase that distance to almost 27 ft. -- not a whole lot better.

The solution is to find equipment with low source impedance. The other possibility is to find a balun manufacturer.

These baluns can change the source impedance from high to low and increase the distance you can go.

Steve Lampen is a multimedia technology manager for Belden Cable. He can be reached at .(JavaScript must be enabled to view this email address).