A DI box or Direct Injection box is a very useful piece of equipment used to connect keyboard, guitars and other audio signal source directly to the mixer. DI box function is to convert the unbalanced output of the guitar, keyboards and other musical instrument to balanced output. Connecting an unbalanced output to balanced input of the mixer often led to humming noise which is really unpleasant for ears especially when no one is playing the instrument.
The circuit here is based from website sound.westhost.com with some minor modifications. The circuit is pretty simple, input is connected to a non inverting buffer and then fed to non-inverting buffer. Both opamps have a voltage gain of 1 and both of them are connected to the balanced output.
Power is derived from the 48V phantom power from the mixer but in case your mixer doesn’t have phantom power feature, you can use a 9V battery to power this unit. But using battery is not a good idea not unless you are regularly changing the battery with new one every time. You can also use an external AC/DC power supply to power this DI box. Voltage of around 9-24V will work but I recommend 12V. Take note also to remove the zener diode if you decide to use external power supply. You can opt not to remove it but just be sure that your input voltage is less than the operating voltage of the zener diode.
Powering with 9V battery
To power it with a 9V battery, just remove R7, R8,and D1. Connect the positive of the battery to the positive terminal of C2 and the negative of battery to the negative terminal of C3.
I did the PCB design using eagle PCB. It is free to use with the limitation of 100mm x 80mm max board size and 2 layers copper, good enough for any small project. The board size is 5cm x 5cm single layer board. I decided to use single layer to make it easy for those who want to build this project on their own.
Read also: SIMPLE DI BOX CIRCUIT