High/Low converters are used to convert high-level signals to low-level signals, in other words, to connect an external amplifier to a head unit that does not have RCA outputs. Most modern amplifiers allow connection to the factory head unit via a high-level input, but higher-quality hi-low converters (especially active hi/low converters) are a significantly better solution. This is especially true when connecting a subwoofer, as passive high/low converters have higher distortion and cannot handle the lowest frequencies.