Buenos dias, amigos!
I recently provided the PA and did sound for a show, and the bass player showed up with no amp (or cable or tuner for that matter), and wanted to run through the mixing board. Of course he did not have a DI box either, and he had never even heard of one. He said that he always runs his bass straight into the mixer and never has any problems. Well, why would he need a DI box?
First off, in the good old days things were more simple. Guitar players had their own amps, bass player had their own amps and there was a PA for the singer. But as time went on things kept getting louder and louder on stage and in the audience (think Led Zeppelin or the Beatles) and there was no way an SVT was going to fill Yankee Stadium with meaty bass tone. So eventually more and more stuff, including the bass signal and keyboards, started going into the mixing board and the house sound system.
You will not notice much of a change in your sound if you are running a passive bass and a short cable to the mixer (ala Mr. Tejano Grunge that I referenced earlier). But if you run a really long cable (50 to 300 foot snake) there will be signal loss (particularly at higher frequencies) and the opportunity for noise goes up, especially around power transformers, stage lights and dimmers. If you are using an instrument with active pickups, it will distort and sound terrible.
If you think about this, this is not too surprising. If you take your little magnetic pickup and try to force it output voltage through a couple hundred feet of wire, you are going to lose a lot of your output. This problem is compounded when you split the signal and send it both the PA and your amplifier.
So some smart people figured out that if the signal was converted from high impedance to low impedance (a microphone is low impedance), it would travel over long lengths of cable with less signal loss. Unfortunately, low impedance signals are more susceptible to noise created by magnetic fields – and just think of all of the magnetic field created on stage by the amplifiers and lights. Old-school landline telephones also send low impedance signals over ridiculous lengths of wire with virtually no added noise. How do they do it?
They use balanced lines, which I am sure you have heard of if you have ever messed around with sound equipment. Balanced line split the signal into two equal parts, with one part in phase and the other part purposely inverted (out-of-phase). There is also a magnetic shield, which makes up the third pin of your standard XLR cable. Outside noise that passes through the shield is picked up equally by both wires. When the two signals come back together, the out of phase signal is brought back into phase along with the noise it has collected. The original signal is now in phase and the noise goes out of phase with its counterpart and cancels itself out. Neat!
So, a direct box (or DI box) takes care of all of this in one little component: It converts the high output impedance of the pickup to a low impedance signal and convert the unbalanced connection to a balance the line. Then you can plug it straight into a microphone line input and put your sound into the hands of the by running the board.
There are untold variations in direct boxes – active, passive, ones with preamplifiers, and some effect and tuner pedals even function as DI boxes and have XLR jacks built into them. This blog post will get too huge if I try to explain all of the different types that are out there.
By the way, a lot of you already have DI boxes, and maybe do not even realize that is what they are. You know the XLR direct out on the back of your amplifier? That is connected to a direct box inside your amp, which will convert your pickup output to a low impedance signal and to convert your unbalanced connection. Go figure…