Passive DI Units
A passive DI unit typically consists of an audio transformer used as a balun. The turns ratio is typically chosen to convert a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar) to the 100–200 Ω expected by the 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.
The cheaper passive DI units are more susceptible to hum, and passive units tend to be less versatile than active. However, they require no batteries, are simpler to use, and the better units are extremely reliable when used as designed.
Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems), a pad switch (to accommodate different source levels) and a filter switch for coloring the sound.
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