DI box?

[freebird1127]

Hey all,

Here's a question I think I should know the answer to but don't.  I hear people talking about / using DI boxes all the time.  Direct Input boxes. right?  But what do they do exactly?  What are they for, why would I need one?  I see them selling for like $200... nothing in that small of a box can be worth that much.

[timrand]

A DI box is typically used for stage , studio work.

A D.I. box should provide a high input impedance for connection to a signal source, and a low output impedance for connection to the microphone input of a mixing desk.  

Alot of people use it for Bass Guitar where they plug the bass guitar into the DI out of the DI into their amp and the XLR on the box feeds to the desk

[freebird1127]

hmm, I see... so what are the input/output jacks a DI box usually has?

[brett]

Direct Injection boxes are used to put either an instrument level signal or sometimes amp output into a mixer.  The reasons that you'd do this are to avoid the complications of miking an amp, and avoiding noise (both electrical and ambient).  Usually, a DI box provides a balanced output signal, and the option of connecting or disconnecting ("lifting") the earth link.

As you can imagine, they're not the most complex beasts.  Here in Oz, there's a kit available for one that includes phantom power, a tone section, and more, for about US$25 (and it includes a neato painted metal box too).

I built one of these kits a while back for an accordian player, who had a pickup in his squeezebox, but didn't want to mess with amps etc coz he's blind.  He just wanted to plug straight into the mixer and let someone else mess with the sound while he concentrated on playing.  So it was kinda useful for him.  (it took about 4 hours to build, and I charged him US$50, so I'm definately doin it for the love, not the money).  He was very happy with it.

cheers

[brett]

The input is usually from the instrument and the in-line output usually goes to an amp.  The XLR output goes to the mixer.

[brett]

If you're interested, Silicon Chip magazine had a DI article and project in 2002 or 2003.  Any big library should have a copy.

cheers

[niftydog]

normally has a 6.5mm jack input and an XLR output.  May also have a "loop thru" output of another 6.5mm jack so you can plug "thru" it and into your amp for on-stage sound.

Unbalanced, high impedance input.  Balanced, low impedance output.

Usually, not always!

Try running a 50 foot guitar lead to a mixing console and you'll soon discover why DI boxes are good!

Hummmmmmmmmmmmmmmmmmm....

[freebird1127]

Hahahaha okay okay I get it!!  So basically it's just a balun, yeah?
Thanks tons guys, really appreciate it.  Hail over to OZ.  Cheers.

[gtrmac]

There are also 2 kinds. Active, which usually uses opamps to balance/boost the signal, or passive, which uses a transformer. Good audio transformers are relatively expensive by the way.

[Ge_Whiz]

I must say I've never really understood why DI boxes for guitar or bass continue to use transformers or even balanced line cabling. A simple common emitter, common source or op-amp stage will give much better impedance matching and transformation, and the output impedance is so low (a fraction of an ohm) that hum and interference pickup is negligible anyway, even with ordinary single-core screened cable. After all, you don't need balanced - or even screened - cable for speakers, do you? (Okay, the signal level's higher. But the principle is the same.)

[freebird1127]

Good point Ge_Whiz...

I saw a few Balun projects in a book I have, several of you may have it as well.  The Audiophile's Project Sourcebook by G. Randy Slone (utter props, Randy).  I'm more than certain these would do the trick, they all employ a simple Op-Amp stage and some clever wiring.  If interest is expressed I'll scan the schematics and send them to Aron.

Let me know.

[airhole]

Hi, a bit of a newbie to this forum.

I think they use transformers because u dun need a battery for transformers.  

I think the point of using balanced cables is in the case of LONG wiring to the mixing console, one does not want flourescent or other em waves interfering with the signal, causing unnecessary noise.  :lol:

I may be wrong, but this is what I know. If I am wrong, would appreciate if someone pointed it outo me.

Cheers,
george

I must say I've never really understood why DI boxes for guitar or bass continue to use transformers or even balanced line cabling. A simple common emitter, common source or op-amp stage will give much better impedance matching and transformation, and the output impedance is so low (a fraction of an ohm) that hum and interference pickup is negligible anyway, even with ordinary single-core screened cable. After all, you don't need balanced - or even screened - cable for speakers, do you? (Okay, the signal level's higher. But the principle is the same.)

[downweverything]

yes a good passive transformer is worth its weight in gold in isolation alone.   i personally dont want somebodys piece of shit bassamp connected to the 2 $20k consoles im using. a passive di is the next thing short of an isolated splittler