Part 12: Cables in the Power Amplifier/Loudspeaker Interface

The interface between a power amplifier and a loudspeaker through a cable is a critical point in a playback system. Unlike interconnects, which carry low-level signals, loudspeaker cables carry much higher voltages and currents. Loudspeaker cables thus react more with the components they are connected to.

Damping factor is an amplifier's ability to control the woofer's motion after the drive signal has ceased. For example, if you drive a loudspeaker with a bass-drum whack, the woofer's inertia and resonance in the enclosure will cause it to continue moving after the signal has died away. This is a form of distortion that alters the music signal's dynamic envelope. Fortunately, the power amplifier can control the motion; the degree of this control, or damping factor, is expressed as a simple number.

Damping factor is related to the amplifier's output impedance. The lower the output impedance, the higher the damping factor. When you connect a power amplifier and loudspeaker with cable, the cable's resistance decreases the amplifier's effective damping factor. For example, an amplifier's damping factor of 100 may be reduced to 40 by 20' of moderately resistive loudspeaker cable. The result is reduced tightness and control in the bass. Loudspeaker cables should therefore have low resistance and be as short as possible.

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