Speaker L Pad
A speaker L pad is a special configuration of rheostats used to control volume while maintaining a constant load impedance on the output of the audio amplifier. It consists of a parallel and series resistor in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction. To maintain constant impedance in both directions, a "T" pad must be used. In loudspeakers it is only necessary to maintain impedance to the crossover; this avoids shifting the crossover point.
A 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. This was only true of full range speakers. Most modern applications for full range speakers use tapped transformers. Maintaining constant impedance is less important to modern amplifiers using solid state electronics.
In high frequency horns, the L Pad is seen by the crossover, not the amp. L pads may not necessarily use infinitely variable rheostats, but instead a multi-position rotating selector switch wired to resistors on the back. Tapped transformers are not L pads—low-end manufacturers to the contrary; they are autoformers. L pads can also be used at line level, mostly in pro applications.
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