Original Article Copyright (c) 1988 JdB Sound, Acoustics
From JdB Church Sound & AcousticsThis information has be written for the layman and should not be used as technical information. Many terms and descriptions are simplified for educational reading and for informing churches of details they can better understand and pass on.Thank you.
What are they and why do we need them?
It's 10:00am Sunday morning. The invited Gospel group just showed up an hour late. You have 45 minutes to set up, do a sound check and rehearse the group long enough to know what kind of sound they are best known for.
One by one the performers enter with their instruments. This group is planning to use the church sound system. Rumor told them that this church had a very good system. You see one electronic organ, two electronic keyboards, one string bass with a pickup and one electric guitar with an amplifier head. Finally, you see an electronic drum kit.
At the front of the church you have 16 mic inputs. You need 5 vocal mics and 9 inputs for the instruments. That leaves you with a pulpit mic and a tape player input.
Fortunately, you were prepared. Earlier in the week you rented 1 speaker director, 5 passive direct boxes and 2 active direct boxes. The church already owned 2 passive direct boxes.
By 10:30, the sound check was finished with the soundman sitting at his
mixer in the pew and 10 minutes later the group finished their rehearsal
and floor monitor check. At 11:00am, service started and the group performed
very well. Most people were not aware that the group set up in only 45
minutes. Is this really possible? Ever since the 16mm film projector was
used in the church and connected to the sound system, churches have needed
a direct box (or DI box). DI boxes are used to change the output signal
from one source and change the level and impedance to match a microphone
level signal input into a mixer. The most common application of a direct
box is when connecting an electronic keyboard or similar electronics to
a sound system. The DI box allows you to connect into a snake or existing
mic lines and send the signal up to 700 or 800 feet away. By converting
line level signal to a balanced signal mic level, you also avoid RF problems
and crosstalk in the mic cables back to the mixer.
There are 5 quick and convenient type of boxes
Recently on the news groups, there was some mis-information being shared which I feel should be corrected. A person posted the question, "Can an active DI boost the signal from -20dB to +0dB. I want to boost the signal of my acoustic guitar pickup."
The response to this question was remarkable. What was very surprising was when I saw who was answering the questions.
First of all, lets look at the question. - The request was to know if an active DI can boost an audio signal from -20dB to +0dB. First of all, we should know what -20dB means.
In HI-FI, -10 and -20dB is the standard used to connect from stereo equipment to equipment. Part of the reason for this standard is that a lower signal has few problems with noise from RF and HUMs and at that level, it is cheaper to provide RF protections at -10dB. However, this signal is too low to manipulate for editing without adding noise that is inherent of all audio equipment and signals - even digital signals.
In Pro Audio, we use +4 as a standard. Part of the reason (among other things) for this higher output is to boost the signal high enough for a greater signal to noise ratio. Let me explain. A line level to line level signal often already has a signal to noise ratio of 60dB or more. A microphone can have a signal that is from - 80dB to +10dB. That is 90dB of dynamic range. If the low level -80dB signal is clean - that is little or no noise, by boosting it to +4dB means that when you split the signal for monitors, effects and recording outputs, when you change the signal with the channel EQ and then send the signal out of the mixer, your original signal should be (almost) noise free. (If you have a cheep mic, that can often be a problem when micing a person's voice at a distance.) In otherworldly, the hotter the signal, the better for Pro Audio- and all church sound system come under this label.
As a side bar. - Have you even connected a CD player directly to a Pro Amplifier and found that you could turn the CD up all the way without clipping the amplifier? It's because the maximum output of most consumer CD player are .75 Volts - which is the maximum output of a HIFI product. For Pro Audio - many pro amplifier are 1.75 volts (+4dB) or 2.83 volts (+8dB). This voltage difference and signal difference is the main reason why you can not mix HIFI and Pro Audio equipment. (Note, some lower prices Pro audio amps have switches for .75 volts for consumer use.)
Back to the Question - Many active DI boxes have switches that can cut a signal down. Most Active DI boxes have 0dB, -10dB and -20dB. Some DI boxes also have -45dB. These are pads. Pads are loads created with resistors and other components to cut the signal down when the input voltage is too high. A passive DI box will loose between 3 to 6dB of signal. For many sound sources, this is OK. Also, passive DI use transformers. All transformers have a unique sound. If your church has an NC above 42dB, the sound of the DI will not be noticeable. If the church has an NC below 42dB, then everything counts.
Active DI boxes offer 0dB signal loss and since they don't use transformers,
they add far less coloration to the original sound. In order for a Direct
Box to Boost a -20dB signal to +0dB, you need internal amplifiers like
a mixer has. To the best of my knowledge, there are only a few Active DI
boxes that have such abilities. Generally, they should be called Active
DI with Preamps. Most of the common Active DI boxes do not have this ability.
The solution for the guitar player is to use a guitar foot pedal pre amp that can also add Bass, mid and treble tone controls - then go into a DI box. The signal will be boosted before going into the DI box - as it should.
For other low level signal, you can use a Guitar pre amp or- you can use a unit like the Symetrix 202. It is a two channel pre amp with phantom power. It can be used as a mixer by itself or to boost a low level signal. It can handle almost any kind of input and convert it to 600 ohm balanced load. You don't need a DI box with this unit. it has a ground switch, a pad, a gain control and phase switch. Because it has two channels, you can mix two sounds, like a mic and guitar before sending it to the main mixer, or mix 2 acoustic guitars. The option is yours.
What prompted this writing is the fact that well known audio experts are either telling people that any active DI box can boost a signal, or they are accepting this info as fact. Yes, there are some active DI boxes that have a built in line amplifier to increase the signal which makes these unit more than a pure active DI and if you read the product label, it will tell you if there is a line amp included.In summary, a DI box should always be used as your first choice when connecting an electronic instrument to a sound system where you are using mic cables over 50 feet to the mixer. Use the in line transformers sparingly, especially if your don't know the output voltage. And yes, with practice, in 30 minutes you can connect up to 24 mic inputs with two people and finish a sound check. Check DI 1, DI 2, DI 3......
DI Box FAQQuestion from those who search the web.
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Copyright (c) 1996-2012 JdB Sound, Acoustics
Created Feb 9, 1996 - Updated April 1999 - Updated 2012