DI Boxes For Live Use: Questions...

[Robertt8]

ok, i'm not exactly sure where to put this, but I'm looking for a little info about DI Boxes when used live. we have two acoustic/electrics that plug straight into PAs when we play live, and every now and then it's not a pro set up (no sound guys), but they insist that we use DI boxes (which is what they always use with sound guys when we play. i think we just need to pick up a couple of them so we can quit sweating it.

what's the difference between Active and Passive DIs? and why would i need one over the other?

the other question would be... how needed are they? will your PA blow up if you don't have it? what the story with these?

Thanks

[Minion]

A DI will take an Instrument level signal and convert it to a Line level signal which is what most Mixer expect .... Without the DI you will get a Lower level signal with a Lot more noise when plugged into a Mixer or any Line input....

Some DI"s can also convert the unballanced High impedance Instrument signal to a Low impedance Ballanced Mic level signal so you can Plug it into the XLR Inputs on a Mixer or preamp.....

A Passive DI is made from Passive components and do not require any Power, these ones usually just contain a Audio transformer to change Impedances and create a Ballanced output....

An active DI needs a Power supply and it has active circuitry and usually has more features than a Passive DI.....

If you do a search on google from DI boxes you can see the differance in features......


Cheers

[orksnork]

a lot just use a 9v...like my sansamp bass DI

[boingoman]

DI boxes by themselves either cause no level change or a level drop, in the case of a passive DI. Some boost the signal, but they are few and far between.

All DIs change impedance. Actives provide a low impedance output for interfacing with a mic pre. So do passives. A passive DI's output impedance varies depending on what is plugged into them, but is always lower than what is plugged into them.

The lower-impedance circuit made by the DI and mic pre is less susceptible to noise. It is also balanced, which can take advantage of the noise rejection in a balanced circuit.

To the original poster- with instruments that have active electronics, using a DI is largely a matter of convenience. An active instrument likely could drive 100 feet of cable to a mixer's line inputs no problem, but most places use long XLR cables or snakes. Rather than using adapters at both ends and repatching the XLR into a line input, or running 100 feet of 1/4" cable, it's easier to use a DI. In a coffehouse with a powered mixer right next to you, a DI is mostly unecessary. But like you have noticed, lots of people just automatically have that as part of the chain, and may feel it's a must-do kind of thing. There can also be noise issues, which for various reasons DIs are usually pretty good at solving. So yeah, I'd get a couple.

As far as active or passive, well, that depends. With active instruments, you can go either way, and get a good sound, so luckily mostly your ears can be the judge. Passives don't need phantom power or batteries, which means they will work anywhere. Actives can be battery, phantom powered, or both. I'd try a few if you can, as there will be ones that sound better with your gear than others.

[Robertt8]

great info guys. that explains a lot.

thanks!

[boingoman]

Originally Posted by Robertt8

great info guys. that explains a lot.

Just skip Minion's first sentence, and have fun.

[RAK]

Originally Posted by boingoman

Just skip Minion's first sentence, and have fun.

Oh this again. His first sentance is just fine.
Level/Impedance, for the purposes of this discussion (and other discussions) they can be interchangable. I still maintain they go hand-in-hand.

I think what he said was perfectly acceptable, but then again, we've already gotten into this in another thread.

[boingoman]

Originally Posted by RAK

Oh this again. His first sentance is just fine.
Level/Impedance, for the purposes of this discussion (and other discussions) they can be interchangable. I still maintain they go hand-in-hand.

I think what he said was perfectly acceptable, but then again, we've already gotten into this in another thread.

No it's not. His first sentence says a DI converts from instrument to line level.

[Dr_simon]

Aren't DIs more commonly for converting form HiZ to LoZ in order to allow a HiZ instrument signal to go into the mic input of a desk without overloading it ?

I have several DI boxes which also have line level outs as well. Maybe this is where the confusion comes form !

[boingoman]

Originally Posted by Dr_simon

Aren't DIs more commonly for converting form HiZ to LoZ in order to allow a HiZ instrument signal to go into the mic input of a desk without overloading it ?

The basic idea was about impedance, yes, to change the impedance of the circuit. But not so much so the mic pre won't overload, rather that the impedances in the circuit are closer to where they want to be for good signal transfer, noise rejection, and keeping the tone of passive instruments. Balancing the signal doesn't hurt, either, for noise rejection.

Originally Posted by Dr_simon

I have several DI boxes which also have line level outs as well. Maybe this is where the confusion comes form !

Could be, lots of units like the Brick do double duty.

[RAK]

There are the "A/V" type DIs that have all sorts of I/O and convert levels.
Something like this, as an example:
http://www.radialeng.com/di-jpc.htm

Also, I've copied this from Whirlwind's site. Again, my point in this thread (and the other) when I was talking about "level" was that in trying to keep things simple, it's easier to talk about level/impedance in the same breath. DIs do both, that's all I ever said.

"A Direct Box can be in-DI-spensible
by Al Keltz

Direct boxes are often referred to as "DI" boxes. This stands for "Direct Injection" as their main purpose is to convert unbalanced and/or high impedance instrument signals into a format suitable for direct connection to a mixing console's mic input - without the use of a microphone. In 1981, Whirlwind developed and produced the first commercially available Direct Box in the audio industry. (See www.whirlwindusa.com/bbstor.html .)

A DI serves several basic functions:
It converts a high impedance signal to a low impedance signal (although it will also accept a low impedance signal from a preamp, keyboard, active pickup or other electronic device).


It converts an unbalanced signal to balanced.


It reduces a strong instrument or line level signal (and sometimes even speaker level signal) to a mic level signal suitable for connection to the mic input of a mixing console.


It isolates electronic equipment on stage from the mixing console, which can help eliminate interference and noise caused by electrical interaction or ground loops. It also blocks phantom power sent from the mixer so it cannot interact with the the device connected to the DI's input."

Here's the link, as there is more information including block diagrams of a passive DI, and a picture where they label "Instrument or Line Level In" and "Mic level out to mixer"
http://www.whirlwindusa.com/tech02.html

[RAK]

Originally Posted by boingoman

No it's not. His first sentence says a DI converts from instrument to line level.

That's true, now that I re-read it. I originally read it as a "DI converts from instrument level to MIC level" I guess I inserted the correct wording in my mind when I read it.
Thanks for pointing that out.

[riffy]

For live use scenario (which is the only way I use them)

You plug your 1/4" instrument output (keyboard, guitar preamp, etc) to the DI box from there you plug the DI's xlr low impedance output to your stage snake's xlr input and away it goes back 100' feet away to the house mic input board. Most DI's have a 1/4" pass through so you can send that to your local monitoring amp or whatever. For live use, get one with a ground lift.

[littledog]

I wasn't in on the earlier argument, but does anyone disagree with the following:

Most DI boxes, especially the inexpensive ones, DO convert from Instrument level to Mic level. They also convert from unbalanced to balanced at the same time. This means if you plug an instrument into a DI box at one end, at the other end you will be plugging into a mic preamp. As someone else pointed out, the noise-cancelling advantage of the balanced signal is most pronounced on longer cable runs.

Most DI boxes also have a through jack that lets you take the original instrument level signal and split it off to an instrument amplifier.

You can find DI boxes that output line level signal, but to my knowledge, most of them are more expensive than the ones that do just mic level.

So, which part of all this is controversial?

[RAK]

Originally Posted by littledog

I wasn't in on the earlier argument, but does anyone disagree with the following:

Most DI boxes, especially the inexpensive ones, DO convert from Instrument level to Mic level. They also convert from unbalanced to balanced at the same time. This means if you plug an instrument into a DI box at one end, at the other end you will be plugging into a mic preamp. As someone else pointed out, the noise-cancelling advantage of the balanced signal is most pronounced on longer cable runs.

Most DI boxes also have a through jack that lets you take the original instrument level signal and split it off to an instrument amplifier.

You can find DI boxes that output line level signal, but to my knowledge, most of them are more expensive than the ones that do just mic level.

So, which part of all this is controversial?

That all sounds good to me. In this thread (and an earlier one) I said pretty much the same thing, but didn't say anything about impedance. Boingoman said I was completely wrong, and that DIs change impedance. My point was they do both, and it's okay to talk about them in the same breath. In the older thread, I was just trying to keep things simple. Here's a line from the Whirlwind site about the IMP2, very simple passive DI box:

"The IMP 2 converts a line or instrument level unbalanced signal to a low impedance mic level balanced signal."

I also thought that the transformer in a passive DI made them better for noise cancellation properties than an active DI.

With some Active DIs you can also actually add gain (I'm pretty sure)

There are all sorts of A/V DIs that have 1/4", XLR, 1/8", and RCA connectors and do various level/impedance "conversions" Something like the Whirlwind pcDI, or the ProCo AV-1A

[mshilarious]

Originally Posted by RAK

I also thought that the transformer in a passive DI made them better for noise cancellation properties than an active DI.

Transformers generally have better CMRR, and lack the self-noise of an active unit. But a good transformer is expensive. A passive DI will normally also be a symmetrical device.

With some Active DIs you can also actually add gain (I'm pretty sure)

Indeed, but a transformer will do that as well.

[boingoman]

Originally Posted by littledog

I wasn't in on the earlier argument, but does anyone disagree with the following:

Most DI boxes, especially the inexpensive ones, DO convert from Instrument level to Mic level.

So, which part of all this is controversial?

Not just inexpensive ones, level drop is inherent in any passive DI.

And it's not a conversion to mic level, per se. It's a drop of about 20db in a passive DI. If the signal going into the DI is 20 above mic level, then the signal that comes out will be at mic level. A +4dbu line signal that goes into a passive DI will come out of the XLR at around -16dbu, which is way above mic level.

[littledog]

Good point Boingoman. Your descriotion is more accurate than mine. For instance, if you inputted a signal that was already at mic level into a DI box, it would come out the other side REALLY low.

And your point about +4 line level coming too hot is a good reason why many DI boxes have a pad option. The cheap Rolls ones, for instance, give you a (-20db) and (-40dB) switchable pad.

[RAK]

That's why I put "Conversion" in quotes. It's really not a good word to describe what's happening. I mean, all a mic/line switch really is (say like on a console) is a Pad.

[littledog]

Originally Posted by RAK

That's why I put "Conversion" in quotes. It's really not a good word to describe what's happening. I mean, all a mic/line switch really is (say like on a console) is a Pad.

Is that universally true? I was under the impression that on higher end consoles they were actually two seperate signal paths. I could well be wrong though...

[RAK]

Originally Posted by littledog

Is that universally true? I was under the impression that on higher end consoles they were actually two seperate signal paths. I could well be wrong though...

It depends on the channel strip. Not that I have block diagrams in front of me, but I'm trying to think back to some of the console's I've used. Something like on the SSL 4056 I believe does have two seperate signal paths, so the mic/line switch actually "enables" the input. Meaning if you're running something into the mic in, but you have it in "line" mode, you don't get anything. It's been a while since I've used an SSL though, so I could be confused.

On the other hand, something like an A&H GL3800, the mic and line inputs are on the same signal path, so in that case, it is just a pad switch.

[xstatic]

On many budget consoles the mic/line button doubles as a pad, but only for signal present at the XLR connection. It also is used as a switch to switch the signal source from the xlr connector to the 1/4" jack. On most desks you can have signal running to both the 1/4" line input and the xlr input. By defualt the xlr typically takes the signal selection. However, depressing the mic/line button will not pad the xlr signal in this case, but will actually switch which connector that channel derives it's signal from.

Any way I look at it, a direct box is always preferable to just running 1/4" in to a console with an instrument.

[RAK]

So here's the block diagram for the A&H GL3800 series.
As you can see for the mic/mono inputs the mic and line inputs meet at the "Line/Pad (-20dB)" switch

This is actually the analog desk I have the most time on, and while it's been a while since I've used it regularly (that's what day jobs will do) I seem to remember the button is just labeled as -20 dB Pad, and it Pads both the XLR or 1/4" input.

http://www.glseries.com/Downloads/gl...ockdiagram.pdf