Some devices, especially microphones, require an external power supply: the so-called phantom power. Therefore, mixing consoles and microphone preamplifiers will typically have a switch to activate this, usually labeled "+48V".
Phantom power usually has a voltage of 48 volts and a current of a few mA. This is completely safe for the BluBOX, since it is short-circuited by the transformer - just like a dynamic microphone, by the way.
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Conversely, this is the reason why the BluBOX needs its own power supply and cannot be operated with phantom power. Because the processor of the BluBOX needs a much larger current requirement for the convolution process, namely approx. 300mA.