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A passive device is anything that doesn't contain any powered electronic circuitry. Examples of passive devices include isolation transformers, surge protectors and resistor-inductor-capacitor (RLC) type conditioners. Power re-generators and UPS systems are active
Resistor-inductor-capacitor (RLC) devices represent the vast majority of 'audiophile' power conditioners on the market. The simplest devices forgo any resistors or inductors and simply place capacitors across live and neutral. More complex devices are comprised of resistors, inductors and capacitors. All of these products are essentially passive filters similar to those found in the majority of loudspeakers.
Many of these products increase mains impedance due to the components placed in series with the electrical supply. This can have the audible effect of reducing dynamics as impedance acts as a brake to the delivery of current. The best passive conditioners can be quite good but cannot match a good isolation transformer in terms of noise reduction, low supply impedance and ability to delivery higher dynamic current than the wall.
Nyal Mellor, Founder, Acoustic Frontiers