The ‘DI’ in the title stands for Direct Injection, a term used in audio engineering. Many DI boxes have appeared in Elektor over the years and their function is basically to take a single-ended signal (i.e the signal and earth output from, say, an amplifier output) and convert it into a symmetrical (earth-free) signal suitable for the microphone input of a mixer. A readymade IC like the SSM2142 or DRV135 from Analog Devices and Burr Brown/TI is another alternative but the basic DI box function can also be built with discrete components using just two transistors as described here.