L pad

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An L pad is a special configuration of rheostats used to control the volume of a loudspeaker while maintaining a constant load impedance on the output of the audio amplifier.

This constant-impedance load was important in the days of vacuum tube power amplifiers because such amplifiers often did not work efficiently when terminated into an impedance greatly different than their specified output impedance. Maintaining constant impedance is less important to modern amplifiers using solid state electronics, so L pads are rarely seen today.

[edit] Operation

The L pad does this by having two separate rheostats connected in an "L' configuration (hence the name). One rheostat is connected in series with the loudspeaker and, as the resistance of this rheostat increases, less power is coupled into the loudspeaker and the loudness of sound produced by the loudspeaker decreases. The second rheostat is connected between the input and ground (earth). As the first rheostat increases in resistance, the second rheostat decreases in resistance, keeping the load impedance (presented at the input of the L pad) constant. The second rheostat usually has a special taper (function of resistance versus rotation) to accommodate the need for constant input impedance.