Passive Direct Inject Box

This article describes a Passive Direct Inject Box. Such devices are commonly used for coupling speaker output into an amplifier, mixing desk, PC sound card etc. 

This DI box has the following features:

• optionally balanced output (useful for reduced hum and RFI on  long cable runs);
• switched 20dB attenuator; and
• an Earth Lift capability for reducing hum loops.

Whilst high quality microphone transformers costing hundreds of dollars can be used, an inexpensive (~$2 Jaycar MM2534) transformer performs fine in most applications and will deliver bandwidth flat from 10Hz to over 30KHz.

Figure 1: Passive Direct Inject Box circuit schematic.

The DI box is best assembled in a metal box, such as a small die cast aluminium box. Note that the output jack sleeve MUST be isolated from the metal box.

The output of this DI box can be balanced or unbalanced, depending on whether a TRS or TS plug is inserted in the output jack.

Figure 2: the prototype passive direct inject box.

Note the insulated output jack on the left hand side of Figure 2.

If you are looking for something a little better, before you go buying $200 microphone transformers, have a look at the Behringer DI-100 which sells here in Australia for around A$72 at the time of writing.