Resistors
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Driver sensitivities rarely match: a tweeter is often 3–6 dB louder than the mid-woofer it is paired with. An L-pad built from resistors attenuates that difference, while a Zobel network uses them to compensate for the rising impedance of a voice coil.
Wirewound resistors are affordable and robust but carry a little inductance; metal film and graphite types are non-inductive and therefore preferred in the treble path. Size the power rating generously: resistors below 10 W rarely belong in a passive crossover.