2 Signal processing
2.3 DI - Direct Inject
In order to connect your amplifier to a PA or a studio mixer, a DI box is used. A DI device (which is often built into the amplifier) provides a level adjusted, electrically isolated, balanced connection. The electrical isolation is a way to prevent ground loops and related dangers. With many different devices, some with very high power consumption, connected to the mains system in a building, voltages appear across the mains wiring. These voltages contaminate the ground (zero volts) level, for instance when a signal cable (very low power) goes from the main mixer to the bass amplifier on stage, along with the ground conductor from the mains, where the power amplifiers get their juice from (very high power). This leads to a difference in ground level, and this is called a "ground loop". A very loud, system-wide hum is the result. The electrical isolation has the additional advantage of safety. When something blows, chances of you getting mains voltage across your body (and your equipment) are reduced.
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