DI Box (Direct Injection)
Definition
A DI box (direct injection box) converts an unbalanced, high-impedance instrument or line signal into a balanced, low-impedance signal at mic level, which a console input can receive over a long stage cable. A passive DI uses a transformer and needs no power. Radial recommends it for sources that are already active, such as keyboards or instruments with active pickups. An active DI uses a powered circuit, fed by phantom power or a battery, and offers a much higher input impedance, which suits passive instruments such as a passive bass or a piezo pickup. A ground-lift switch disconnects the ground between input and output to break the ground loop that causes hum.
In the field
DI boxes are used to send keyboards, basses, acoustic guitars and playback computers down a long multicore without hum or loss of high frequencies. When hum appears because the instrument and the console are connected to ground through two paths, the ground-lift switch usually removes it.
Related lessons
Sources
- Radial Engineering, Comparing DIs : DI passive à transformateur sans alimentation, DI active alimentée · radialeng.com
- Radial Engineering, DI Questions : passive pour les sources actives, active pour les instruments passifs ; ground-lift · radialeng.com
- DI unit (Wikipedia) : conversion haute impédance asymétrique vers basse impédance symétrique au niveau micro · en.wikipedia.org
- Jensen Transformers (B. Whitlock), « Understanding, Finding & Eliminating Ground Loops » : boucles de masse et isolation par transformateur · jensen-transformers.com