If you have used a PA as a tech or stood on a stage as a muso, you probably know something about a ‘DI’. A Direct Input box is used to convert different input signals into a balanced, low impedance signal which goes to the mixing desk. In a later post I will better explain balanced/unbalanced and low/high impedances. Most desks and multicores use XLR connections for their balanced channel inputs.
A DI can be active or passive though most are active. Passive DI’s don’t require power and are generally lower quality than a good active DI. Active DI’s need power. Depending on the particular device, they can be battery powered or phantom powered (power provided from the mixing desk through the XLR audio cable), or some by external power supply plugged into mains power.
Basically an instrument or device using a 1/4″ jack (like on a guitar lead) needs a DI. Examples of this are line out from a bass amp, channels left and right out from a keyboard, and channels out from electronic drums.
Other uses can be for ‘line level’ gear such as cd players, computers and mobile devices such as iPods.
There are times when you definitely don’t use a DI. Too many times I have set up my guitar amp on stage with the friendly sound guy asking me to plug a DI box into my amp. Sometimes I even get a free explanation of how using a DI out of the amp head instead of a speaker cabinet will cut down stage volume and be better for everyone. I’m usually playing through a Laney VH100R all-valve, no-digital bells and whistles kinda amp which sounds horrid through a DI. Any guitar amp without digital cabinet modelling of some kind needs a speaker, which must be mic’ed up. This isn’t your guitarist being difficult, without a speaker a guitar tone doesn’t sound right.
Which brings me to the point that some guitar rigs can in fact go DI straight into the PA if they are designd specifically for it. Many amps will have a line out but only certain amps and effects will offer the proper sound without a speaker cabinet and are gaining popularity as the technology improves such as Line 6 POD units & Spyder amps, Fender Cyber amps and many others.
It’s common for bass guitar amps have a balanced XLR output built in, so a DI box isn’t necessary. Some bass players prefer to mic their cabinets rather than plug into the PA, this is another instance where a DI isn’t required (but you could use both methods and blend them using 2 channels, a cool way to get the most out of your bass tone!). In this case you would probably need a Shure 52 type of microphone which are generally used on kick drums and are better able to capture the low bass frequencies than vocal and instrument mics like the Shure 58 & 57’s.
If you are in doubt about how to get an instrument through your PA, it’s always worth your while to find out properly. Is there internet nearby? Can you ‘phone a friend’? Or does the musician know thier gear and have a certain preference? It’s always good to know the conventional way first for common instrument types. Then if you are in doubt about hooking in a specific instrument you can ‘work with’ the muso by asking intelligent questions about their preferences instead of asking point blank “how do ya plug in a keyboard mate?” and destroying any credibility and confidence you had gained with the band.
The bottom line is, hook in an instrument incorrectly and your tones and mix will be rubbish. Know how to do it right and be a PRO!