For a long time I could not understand why so much noise was included in the signal; hiss, and hum, when recording acoustic guitar through the built in pick-up, or why the acoustic sound in my live performances varied from lacking punch to just being too soft, downright unclear/muddy and un-mixable. I own a good quality guitar, with a pickup system that gets rave reviews, so in theory, all one should do is plug and play, and the sound will always be of a professional standard, right? Various hypothesis stacked up in my mind:

• Perhaps my pickup was faulty. I worked around the problem by placing a mic near the sound hole. This is good practice for recording acoustic anyway. However, in live settings, especially the ones I typically play in, you have a small set, jostled in between other musicians, so you don’t get much set-up and tear-down time. The PA is probably sketchy with few inputs, and the venue might not have many options for mic’ing up a guitar. In any event, like all workarounds, this does not address the underlying cause, and I was not buying that there was a fault with the pickup.
• I thought that perhaps because I don’t have a particularly aggressive playing style, I was not providing enough natural amplification. That was not it, because even with more volume in the play, the poor signal quality was still there. Also, louder, harder playing all the time is silly because you can not serve the dynamics of the song.
• I tried a noise gate, a noise gate simply cuts off the signal at the noise volume threshold, so you still have the noise when the signal is there, and after the cut off you have silence, but the cut off means you lose all of the sustain.
• I tried experimenting with gain levels and equalizing the hell out of the sound, but if the signal is bad to begin with, any amount of voodoo at the desk will only achieve nominal improvements.

What was I missing?

I began a web-quest and trawled through sound engineer forums, reading one web article after another. I spoke to fellow musicians and the guys at my music shop. After sifting the wheat from the chaff, a single common denominator emerged. It seems that if you line an acoustic-electric guitar (or bass guitar) into a mixer, you need a DI Box, alternatively known as Direct Injector, Direct Input, or Direct Box.

In essence, a DI Box is a transformer. Without getting into too many details about how transformers work, quite simply, a transformer is used to step up (or step down) voltage through a process called induction. In our sound application, we isolate the signal from the guitar in order to increase it’s voltage, before sending it to the mixing desk. We take the high-impedance signal from the guitar, and transform it to a more powerful low-impedance signal. The balanced, low impedance signal is then lined into the mixing desk. As shown in the illustration, you run your high-impedance instrument cable with ¼ inch plug from your guitar to the input side of the DI box. You run a low-impedance 3 pronged XLR cable from the output side of the DI box to the desk.

Some DI boxes are “Active” which means that in addition to the transformer, they also incorporate a pre-amplifier, that boosts the gain after the signal is transformed. This pre-amplification allows you to keep the volume control on your guitar pickup at a lower level, because overloading your pickup will also introduce noise.

I eventually chose this active DI box because it seemed a good value for money. When I tried it out, the difference was incredible. A good, clear, loud, noise free acoustic guitar signal.

Reference:

http://www.musiciansfriend.com/resources/article/Direct-Boxes-DI-Buying-Guide/m710120