A DI is a device designed to interface a bass (we’ll stick to bass applications here, although DIs are used all over the band) with a mixer or mic preamp. It converts a bass’s high-impedance, instrument-level signal to a balanced (for noise rejection), mic-level, low-impedance signal appropriate for a mixer. A passive DI uses a transformer to achieve this conversion, but the transformer type results in the DI having a low input impedance—making passive DIs less than ideal for passive basses, which like to “see” a high input impedance for maximum fidelity. A powered or “active” component (either a tube or transistor) buffers the DI’s input, raising its impedance and consequentially improving a passive bass’s tone. The active component can also color the sound considerably, particularly when it adds gain. A tube DI is simply a DI circuit that uses a tube to buffer the input (and in some cases, add gain) while also incorporating its pleasing audio characteristics.

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