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Audio cable run length

How far can I run an audio cable?

This is a difficult question to answer, because it does not have any single one right answer.

There are a number of variables involved on detemining maximum cable length:

• What is the capacitance (pF/Ft) and conductor DC resistance (Ohm/1000 Ft) of the cable?
• What is the highest audio frequency you intend to pass?
• What is the maximum loss budget required at that frequency?
• What is the source impedance of the equipment which send the signal to cable?
• What is the load impedance of the system?

If you can answer these questions, you can mathematically determine how far your cable can go. The length of the cable can be modeled by thinking the cable to be a low-pass-filter formed by the cable resistance and cable capacitance.

               ____
         +----|____|----+-------+
                 R      |
 Source               -----       Destination
                   C  -----
                        |
         +--------------+-------+

This is a simplified model of cable, but gives you good enough approximation what is happening. If we add the source and destination to the model we get the full model you can analyze:

        ____     ____
  +----|____|---|____|----+-------+-----------+
  |    Rsource  Rcable    |       |
ideal                   -----    | |  
voltage          Ccable -----    | | Rload   voltage
source                    |      |_|           out
  |                       |       |
  +-----------------------+-------+-----------+

What are the practical lengths

For parcical home hif applications you can run a normal audio cable carrying a line level signal around 2-10 meters without any noticable signal loss. With longer cables you might notice some attenuation art highest frequencies of the sound.

In professional audio world you can run balanced audio signal up to 200 meters without much worrying. This length applies to both balanced line level audio interconnections and signals from dynamic microphones with balanced connectors.

In very long runs sometimes the signal attenuation is not the major problem, but the noise picked by the cable. This noise can be avoided by using well shielded cables, route then away from noisy cable (mains wires usually) and thinking careful the system grounding issues (avoiding groundloops).

Can analog audio cables used with digital audio signals?

Analog audio cables may be used for digital AES/EBU audio, but only for distances (less than 10 meters). The impedance of most typical analog cables ranges from 40 to 70 Ohms. This large mismatch from the nominal 110 Ohms (digital impedance) results in signal reflection and jitter causing bit errors at the receiver.