Frequently Asked Questions
About Acoustic Guitar

 

What is a DI box and when would I need one?

Derived from a excellent series of answers provided by Tom Loredo on RMMGA Aug. '99

A Direct Input box's main purpose is to:

  1. Change the topology of a signal from unbalanced (2 wires) to balanced (3 wires); which improves noise rejection.
  2. To buffer.  That is, to provide a high impedance ("high Z") load to the source, and a low-Z output to the mixer. Impedance has nothing directly to do with "level," but since the balanced inputs on mixers are typically mic inputs, most DIs reduce the level of the signal by 10 - 20dB to prevent distortion of the mic preamp stage.

There are two basic types of DIs: Active and Passive. 

Passive DIs (also called Converters) have input impedances of perhaps 10-50K ohm.   This is "high" compared to the impedance into the mixer's balanced inputs (typically 1K ohm or less), but rather low if you are loading a bare piezo pickup (which needs a few Meg ohm).  There are "good" and "not so good" Passive DIs.  Passive DIs use  transformers to do the impedance conversion and to create a balanced signal.  Transformers are notorious for affecting the tone of signals.  They can alter frequency response, and they also inevitably distort as signals get louder and the ferro-magnetic core saturates (cannot get more magnetic in response to increasing input signal).  Cheap transformers, such as those used in converters, may not have good frequency response, and will distort at lower signal levels.  Good transformers (the benchmark being those made by Jensen Transfomers) can easily cost $40 or $50 just for the bare transformer, hence the higher cost of a "good" passive DI.    Passive DIs are only a help if your piezo pickup already has a preamp (i.e. you have to put a battery in the guitar).  If not, you need an "active" DI.  

Active DIs typically have input impedances of a few Meg ohm, and thus are the only type suitable for use with a bare piezo pickup (no preamp in the guitar). 

Many guitar pickup systems are not a bare piezo; they have an onboard preamp that does the buffering already.  Its output is perfectly suitable for the "line" input of any good mixer (the 1/4" input, not the 3-pin XLR mic input).  It provides plenty enough output to drive the mixer fine even with a longish cable run.   However, in many live situations, its not the strength of the source that limits you, but the noise your cable picks up along the way (from fluorescent lights, dimmers, etc.).  If this will be a problem for you, then it is best to use a DI (a passive one is fine in this case) as close to the guitar as possible, and run a balanced XLR cable to the mixer's mic inputs. The cable will actually be carrying a weaker signal (ironically), but because it is a balanced signal, there is noise canceling in the mixer that can often get rid of the noise picked up on the cable run.  

If you are playing out a lot, in a variety of venues, you probably should seriously look into getting a DI --- you'll probably run into a noise problem at some point!  If you always play in the same few places, just give it a try with a straight cable; if you don't have problems with hum or other hash in your signal, don't bother with the expense of the DI. 

In addition to the DI functions already mentioned, there are some acoustic guitar "DI" boxes now available like the LR Baggs Para Acoustic DI and BBE Acoustimax that offer additional functions beyond buffering and topology conversion, such as EQ (tone controls), effects loops, and additional gain.  It has EQ specifically tailored to the problem areas of piezo pickups; a nice box for about $150.  It's really up to your own tastes whether these things are needed.  If you are working with your own mixer, many of these functions may be duplicated in the channel strip already via the mixer's EQ and insert jacks.  

Standard DIs and the Baggs DI just mentioned (as well as the SansAmp Para Driver DI and LR Baggs Gigpro) are designed for a "single" signal.   But the Highlander IP2 for example puts out a buffered (low impedance) pickup and mic signal on a stereo jack.  Other pickups like the LR Baggs Dual Source are selectable as either Mono (blended single signal) or Stereo (buffered pickup and mic signal on a stereo jack). Other popular choices are the Fishman Aura, Fishman Pro EQ, and Fishman Pro EQ 2.

You can use a "Y" cord and just send these to two channels (both channels are low impedance outputs from the on-board preamp) to your mixer.  But the mic signal is likely to be pretty weak, and thus could present noise problems.  To deal with this, you need some gain near the guitar; not a DI.   This is why there are special acoustic guitar preamps like the Presonus AcoustiQ for dual-source setups like this.  They provide a combination of functions of buffering, gain, EQ, mixing, and topology conversion.  Another popular one is LR Baggs Mixpro which can be clipped to your belt but still has a lot of flexibility.  Higher in price, quality, and functionality are the Pendulum stereo preamp, DTAR Mama Bear, DTAR Solstice, DTAR Equinox, Again, if you are gigging in a variety of places and relying on someone else's gear, you really should be carrying around at least a Mixpro with you.   If you are always using your own mixer, you can try the Y cord route and see if it works to your satisfaction; you don't "need" a DI or dual-source preamp in this case.