A passive DI unit typically consists of an audio transformer used as a balun. The term "passive" indicates that the unit does not require a battery or other power source to operate. This makes passive DIs less expensive, but it also means that they cannot include preamplifier features which enable the user to boost the gain. The turns ratio on a passive DI is typically chosen to convert a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar or electric bass) to the 100–200 Ω expected by the mic input of an audio mixer. Typical turns ratios are in the range of 10:1 to 20:1. Less commonly, a passive DI unit may consist of a resistive load, with or without capacitor coupling. Such units are best suited to outputs designed for headphones or loudspeakers.