DI box... what's involved, how to build?

[otokomae]

Hi guys, the other guitarist in my band has been looking to buy a DI box and suggested that shows would go more smoothly if I had one as well, which got me wondering, just what is a DI box, what's it really do, and could I make one myself?  Up to this point I've mostly concerned my self with making and modding distortion pedals for myself and friends (as well as a compressor here and there, and a reverb pedal I made, I think off a GGG layout.)  I've never even wondered about DI boxes before, so now I'm looking into it.  I might know more soon, but I usually start these type inquiries here, so if anyone can tell me a little about these things I would really appreciate it!!!  Also, is there a quality layout somewhere that I can use to make one myself?  Thanks guys!!!

[Noplasticrobots]

A DI box is a Direct Injection box. It's used to convert an unbalanced signal into a balanced signal so you can plug into a console, mixer, digital multitrack recorder, etc. I've never played a show in my life so I have no idea how that can help you unless you're plugging straight into the club's mixer, which seems a little odd.

[Mike Burgundy]

A DI does a couple of things, but why this should ease your shows I'm slightly confused about. Guitar sounds crap straight into the board, so usually people mike the amplifier which sounds a lot better due to the very different frequency response. For bass, it is very useful, same with most other electronics devices and electro-acoustics.
That said, there's tow versions: one with a transformer, one without but with electronics. I'll go into it some more tomorrow.

[Transmogrifox]

I'll also admit up front that I don't know exactly what's in them since I have never seen a DI box schematic, but I've got a good guess based on the theory of what they're for:

In a house sound system you can have some cable runs that are quite long and are susceptible to noise, not only thermal noise, noise from lights and electromagnetic (60Hz hum), but also RF interference as well.  The idea with a balanced line is to send a differential signal so you have pin 1 as reference signal, pin2 is signal at 180 degress phase, and pin3 is shield ground to hopefully "catch" most electrostatic noise.

When you take the output, you subtract the two signal line signals.  Any noise on the line is likely in phase on both lines, and is the same noise (except for the noise that gets into the signal path before entering the balanced line).  Consequently the noise is cancelled while the signal is added when you take the difference on the output.

The use of a DI box, then, would be to take a line out from a bass amp or out of a guitar amp modelling pedal, or whatever output you have that is not balanced and convert it into a balanced set of signals 180 degrees out of phase with the same output impedance from each side.

I know this could be done easily with a single transformer (just get a good quality audio transformer).  You would take the guitar or bass or whatever input single ended on the primary winding, ground the low leg.  Then you would take the secondary winding and connect one signal pin to one leg, and the other signal pin to the other leg.  It would be the most effective if you could match the output impedance of the transformer to the input impedance of the mixer.  You would then need a buffer for the input signal, then put in a series resistance to match the impedances.

Then to get your -20dB selector switch, you would likely do fine to put a resistor divider at the output of the buffer.

Hopefully somebody will chime in with more knowlege about DI's than my guess about them.

[swal]

I don't know about electronic di. But a transformer di looks like this inside http://photobucket.com/albums/c87/swal/di.jpg
There are no markings on the transformer, but looks like a cheap piece of junk. But the di workers perfect in my studio for recording bass out of a digitech bp50 of acoustic out of a boss ad3.

[otokomae]

Well, that does explain why I would use one.  At a lot of shows I've used a line out from my amp because if I mic it I have to turn the volume down to a ridiculously low level.  I've noticed that when I use it at a club with a DI, the line out always sounds better, and I thought it was just because it was changing my unbalanced line to balanced, which is what mixers usually like to see, and what carries 100 feet from stage to mixer better.  But I'd never really wondered what actually goes on inside those boxes before.  Anyway, thanks for all the help!  If anybody knows a lot about the best type of transformers to use, or a really good design, etc, please let me know!

Oh, and in a related coincidence, this was the "Tech Tip" that I received in my email today from Mucisian's Friend: 

http://www.musiciansfriend.com/srs7/g=home/content/doc_id=99511/src=3NL5LO?c_lid=b1

It's an article about what DI's do and how that work!  Kind of funny to get it today...

[humboy]

hi,
I'm not an expert at that either but I know that you should mind two things; in the first place to use a shielding enclosure, ideally steel, nickel
etc. And then you ought to use a transformer that has ideally a high impedance at the input and low impedance at the output ( as usual ).
IIRC a 10:1 transformer with something like 10k input should work for what you're trying to do.

hope that helped.

[nightingale]

Hello,
I use an echoplex on my vocals live [soundguys nightmare].
I loolked into diy DI box, but the isolating everything to me is a pain.
You can get your hands on a used whirlwind, or Pro Co, or Groove tubes passive Di box for pretty cheap, and spend your time on building something more interesting like a distortion box like you mentioned.

To me a DI box is more of a utility that an effect?

I have been using the new Groove tubes one with no problems at all, propably 60 shows or so.

hope this helps,
ry

[MartyMart]

I have a couple of "peavey" DI's that are just a transformer.... and quite a "meaty"
one at that  !!
They are "passive" but sound great FWIW

Marty.

[mathflan]

I built a Active DI box without transformer, just OPAMP with Sound westhost schematics.http://sound.westhost.com/

Got to my website http://sounddiy.free.fr, section " DI-Box".



just for informations:

PASSIVE DI-BOX: Just with a transformer,no need power supply( passive),just  convert  unbalanced inputs (such as from a guitar or synth) to balanced.
like http://photobucket.com/albums/c87/swal/di.jpg

ACTIVE DI-box: have a circuit with opamp and sometimes a Transformer at the ouput, need a power supply, convert  unbalanced inputs (such as from a guitar or synth) to balanced,give a more powerful sound,with good high frequencies, more attack...

[Satch12879]

A Direct Injection unit does two things for you:

1.  High to low impedance conversion
2.  Balanced output

Impedance matching is useful, as was mentioned, if you want to interface a guitar, bass, or keyboard directly to a mixing console in the hopes of getting a more robust signal out of your instrument.  The use of a balanced output concerns live sound situations where, combined with the low Z output, you have the advantage of running long lines say from on stage to the front of house with almost no noise or signal loss.  You have to be careful which one you use though.  Passive DIs using transformers should generally be used for isolation purposes or on signals that are already somewhat low Z, like from active guitar or bass pickups.  They also can be expensive because good quality audio transformers are expensive. Active DIs generally have a higher input Z so you should use them with passive pickups.  They require a bit more work overall.

Check out the Jensen Transformers website as they have quite a bit of schematics detailing the construction of both passive and active DIs.

[brett]

Hi.
The op-amp versions work very well.  The ESP site mentioned above (westhost) has a workable design.

I built one for a blind accordian player (seriously!).  He didn't want to cart an amp, etc around, but wanted to gig with his mates.  Worked well for him.  I used a kit based on an article in Silicon Chip magazine (used to be Electronics Australia).  I think you can buy Silicon Chip articles or issues online.  The article was "A DI box for musicians" or something like that.

cheers

[Mike Burgundy]

As said, a DI provides:
impedance matching
balanced output
isolation
and sometimes an added line-driver, to give the balanced out even more oomph.
Impedance matching I think is clear (http://www.geofex.com/Article_Folders/impednc.htm), balanced lines means you have no problems with all kinds of interference that the line picks up. It's THERE, but the nice thing is it gets cancelled out. If you know this, skip to the next paragraph. The trick is that "balanced" lines carry two signals, one the exact opposite (so 180 degree shift) of the other: the "+" and "-" signal. Both of these pick up hum, and all kinds of nasties, just like an unbalanced line would. BUT: the interference is IN-phase (exactly the same for both lines), while the signals you DO want are OUT of phase (opposite, right? Doesn't get more out of phase than that). At the recieving end, you flip the - signal. This means that both lines are suddenly IN-phase for the important signals, and OUT of phase for the interference. Add the two together, and you get a nice and strong signal, while the interference cancels itself out.

The mentioned ESP site has some good stuff. also look at the Jensen corp. website - they produce some of the finest (and expensive) transformers available.
If you ant to go budget, I'd stick with an active, no-transformer DI.
This can be as simple as a 1:1 buffer and a 1:1 inverting opamp (see http://sound.westhost.com/project51.htm)
Most studio and pro-audio equipment use balanced lines externally, so for ideas you can scrounge around in those schems too (for example the Rane audio database)
Nice to be added features are: 20dB pad, ground lift, phantom power.
The ground lift and phantom power are a pain to combine, though.

If you do want to go transformer, Per Lundahl reputedly has very, very good stuff and is a lot more affordable than Jensen.
Also see http://sound.westhost.com/project35.htm Thats a dedicated DI, with phantom, etc.

hih

[moosapotamus]

I gutted a pair of cheapo whirlwind boxes and replaced the innards with the active DI circuit (used the Lundahl traffo) at the top of this page...
http://web.telia.com/~u31617586/
I rarely touch either of my sansamp boxes any more. Sounds beautiful. Requires (48V) phantom power. Highly recommended!

Here's a link to the jensen site, too.
http://www.jensentransformers.com
Click on the 'schematics' link, scroll down to 'musical instrument application schematics', and check out the direct box schemes that use the JT-DB-E traffo. Good stuff!

~ Charlie