Microphone or Line level Cables

How to analyze what happens when using longer microphone or line levels cables. The following calculation are based on maximum or worst case senario. The numbers have been rounded off to for simplicity. In this example the lengths are in feet. To get the most out of audio system requires some mathematical calculation and there is no easy way to do it. Notice that two of the most common electrical formulas show up : Ohms Law & Reactance Formula.

Note: dBu figures are referenced to 0 dBu = 0.775V

LOW LEVEL SIGNALS:

There are three things that need to be considered when driving long cables:

a) The length of the cable.

b) The cable capacitance between the conductors.

c) The output impedance of the device driving the cable

You can calculate the high frequency cutoff (-3 dB ) by:

- 3dB Frequency = 1 / ( capacitance x Output Impedance x 2 x Pi)
or
- 3 dB Frequency = 1 / ( Cable length x Capacitance perunit length x output Z x 2 x Pi)

Example :
Let say you have 100 Ohm microphone on a 500 foot cable. Capacitance per unit length is 32 pf. therefore,
- 3 dB Frequency = 1 / ( 500 x 0.000000000032 x 100 x 2 x 3.14 ) = 100 kHz.

Reference :
Chuck Mc Gregor, Community Professional Loudspeakers.