Line Impedance

Author: N.H. Crowhurst

Use of line impedance for long lines

There is clearly a disadvantage to both high and low impedance for running long lines. For this reason, an intermediate impedance, in the region of 500 or 600 ohms, is usually chosen for making long-distance connections. It minimizes the possible effect of magnetic and electric induction, and avoids high-frequency losses that occur at high impedance and the attenuation due to line resistance that occurs in using low impedance. In input circuits, for example, a transformer is used so that the impedance of the microphone or pickup looks like 500 or 600 ohms at the transformer secondary. The amplifier has an input transformer that works correctly with a 500- or 600-ohm source. The impedance measured across the line between the two transformers is 500 or 600 ohms, and the line is said to work at an impedance of 500 or 600 ohms. Similar techniques are used with high-level output circuits as well. The impedance at which a line is being used is a characteristic of its termination, not of the line itself.

Left: this combination does not make best use of all available audio - loses 6 dB of useful gain. Right: this combination gives 6 dB more gain but restricts frequency response.

There is nothing particularly magical about one particular line impedance. The use of any middle-value impedance (150, 250, 500, or 600 ohms) merely minimizes the defects of either high or low impedances. It is, of course, good to use a consistent impedance in any particular system. Using a 150-ohm impedance and connecting it to a transformer designed for a 600-ohm impedance at the amplifier (or vice versa) will not make the best use of the available audio.