Why should musicians use a DI box rather than just plugging straight into the sound desk?
This article was originally written back in 2010 and we’ve substantially updated it since then with some help from sound tech professionals Simon Lewis and Tim Horton who you can also see in the video clip below (thanks to them both).
What do DI boxes do? (The simple explanation by Tim Horton)
We use DI (Direct Injection) boxes to enable long-distance connection of unbalanced signals (e.g. acoustic guitar, bass guitar, keyboards, iPods, etc) to the mixing desk.
These sources typically connect using a jack lead (1/4″ – big, or 3.5mm – small).
If you run these signals beyond 5m / 25-ft (approx!) without a DI there is a high risk of noise / interference.
When we connect via a DI box we can run the cables significantly further.
If your sound desk is close enough to the source, there is no need for a DI, although they can still be useful (e.g. earth-lift to prevent buzz).
We would, however, always use a DI box on the aforementioned instruments by default. Arguably, it doesn’t matter whether the DI is passive (no power required) or active (battery or +48v from desk). Grab whatever is to hand at the time!
What do DI boxes do? (The technical explanation by Simon Lewis)
Most of what is written about DI boxes presupposes that there is a mixer some distance from the instruments, and that there’s a multicore (most likely analogue) and that the multicore is connected to the mixer’s balanced mic input (because once upon a time they were the only balanced inputs).
We could then say that DI boxes:
Present a high input impedance to the instrument
Have a low output impedance to the mixer’s mic input
Change ‘instrument’ or ‘line level’ (or in some cases, ‘speaker level’) to mic level
Accept unbalanced signals and converts to balanced
Can ‘lift’ (break) the signal earth connection
In this scenario, a length of unbalanced cable from a medium or high impedance source would be very susceptible to interference and would most likely attenuate high frequencies as well. A DI close to the instrument would help solve both problems.
There are numerous caveats to this scenario, and plenty of cases where you can ‘get away’ with not using a DI box. However, most production people would take the line that using a DI is the better approach, so include one, even if an alternative method might work.
How do DI boxes work? (the guitarist’s view by Andy Chamberlain)
The most common DI box is the Passive Direct Box. This unit is commonly used to connect guitars, keyboards and other electronics that output at line level. Generally, by going through a DI, you loose a little bit of signal strength (from 3 to 6dB) which may affect your tone or volume. An important feature of many good quality DI boxes is a switch to lift the ground. The ground lift switch eliminates unwanted hums and buzzes by interrupting the ground loops by preventing the ground currents from flowing between the two devices. It disconnects pin 1 on the XLR 3 pin jack, which is connected to the braid or foil shield in the cable and acts as the ground point of the circuit. If that read like Chinese to you then it’s basically a switch that may get rid of mystery noise that can come into your sound from interference with other bits of equipment on stage.
These units either work from a battery or phantom power from a mixer. Active DI’s can handle and put out higher signal levels and generally have no signal loss. This is important for low level signals (guitar pickups, piezos), so the sound stays unaffected. Active DI’s are more easy to overload than passive DI’s. Be careful with high output line signals from keyboards
So, acoustic guitars, keyboards and even bass are ok to DI straight into the system but electric guitars need a slightly different approach. I was a guest in a church once that had bought a brand new Orange tube guitar amp and they wanted to hook it up to the system. So they unplugged the speaker from the amp and connected the amp to a DI. NEVER EVER do this. Tube amps need to ‘see a load’ which is provided by the speaker and if you run the amp without the speaker then you will fry all sorts of things. Needless to say the Orange was dead. Many guitar amps have a dedicated DI output now but the choice of speaker is an integral part of a guitar amp’s sound so its much better to mic it with an SM57 or similar. If you must DI you can buy a Speaker Direct Box. This is like a DI and goes between amp and speaker to keep that load in place but with some additional inbuilt EQ’ing to replicate the sound of different types of guitar speaker cabinets in the chain. Some sound techs like to place a standard DI after a guitarist’s pedals and before the amp. Whilst this will technically work I wouldn’t recommend it as it will sound pretty thin and nasty unless the guitarist is running some kind of amp simulator on his board.