Derived from a excellent
series of answers provided by Tom Loredo on RMMGA
Input box's main purpose
- Change the topology of a signal from
unbalanced (2 wires) to balanced (3 wires); which improves noise
- 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
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 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
LR Baggs Para Acoustic DI
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
Standard DIs and the Baggs DI just
mentioned (as well as the SansAmp Para Driver DI
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 Pro EQ,
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
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,
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
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.