Newbie question, Do I need i Direct Box?

Dear "DIY" community friends, tell if I'm wrong.

Playing live with my band we playing back track from computer using RME audio interface. Usually technician plugs signal from audio interface to the Direct Box and then it goes to the FH mixer.  I read that output impedance from audio interface is 150 ohm so my questions is do we really need a Direct Box? I mean the only reason I see for that is that it's isolating signal. So My Question is what is better to put there:
isolation transformer
150 : 150?
(If I'm correct i can plug 150 ohm in to the Mic input to the mixer, it can be less impedance but not more)
150 : 600 ohm
150: 1.5K


Which one ratio should I use??? I want to build king of box with those 2 transformers plug it to the output of audio interface and give to the guys already isolated signal so there would be less mess on the stage.

Thank you for the help?

Olegarich said:

Dear "DIY" community friends, tell if I'm wrong.

Playing live with my band we playing back track from computer using RME audio interface. Usually technician plugs signal from audio interface to the Direct Box and then it goes to the FH mixer.  I read that output impedance from audio interface is 150 ohm so my questions is do we really need a Direct Box? I mean the only reason I see for that is that it's isolating signal.

Click to expand...

A direct box also attenuates. The assumption is that your interface is connected to a stage box and then to a mixer's mic inputs, which would likely be overdriven by the interface's line-level outputs.

-a

Olegarich said:

Dear "DIY" community friends, tell if I'm wrong.

Playing live with my band we playing back track from computer using RME audio interface. Usually technician plugs signal from audio interface to the Direct Box and then it goes to the FH mixer.  I read that output impedance from audio interface is 150 ohm so my questions is do we really need a Direct Box? I mean the only reason I see for that is that it's isolating signal. So My Question is what is better to put there:
isolation transformer
150 : 150?
(If I'm correct i can plug 150 ohm in to the Mic input to the mixer, it can be less impedance but not more)
150 : 600 ohm
150: 1.5K


Which one ratio should I use??? I want to build king of box with those 2 transformers plug it to the output of audio interface and give to the guys already isolated signal so there would be less mess on the stage.

Thank you for the help?

Click to expand...

An electric guitar wants to see an impedance of at least 50kohms (and it won't sound very good, since it really expects 10 times that). A passive electric bass works rather well with a 20k:200r xfmr. Active instrument would also accomodate that.
With a 150:150, your electric guitar will sound like a mandolin.
I hate passive DI boxes for guitars, nothing good comes out of them...

abbey road d enfer said:

I hate passive DI boxes for guitars, nothing good comes out of them...

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You are my master and I wouldn't argue with that. I suppose this is generally because of the quality of the transformer to meet a target retail price. But some years back I bought a bunch of NOS (custom) Triad transformers with 700K: 7K+7K.  I am a clean player and this thing is just as transparent as it gets.

Hey guys, about what electric guitars you're talking? There is no electric guitar gonna be plugged in to that box. There gonna  be only audio interface outputs plugged in to that box and that's it.  It will stay in the rack always plugged in.  If Direct Box attenuates the with ratio transformer is better?  should i put some resistors?

output from audio interface is only 150 ohm.

Sorry Oleg, we got carried away. That's what happens when guitar players get involved.

You do not particularly report any problems and I would assume RME has balanced outputs. So why you think you need transformer (passive DI) isolation?

Olegarich said:

Hey guys, about what electric guitars you're talking?

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That's what happens when you write "Direct Box" in the title of a thread...

Yeah Rme has balanced outputs, just thought that isolating signal would be good idea or  I'm  wrong?

Olegarich said:

Yeah Rme has balanced outputs, just thought that isolating signal would be good idea or  I'm  wrong?

Click to expand...

What do you mean by "isolating"? If you believe a transformer has a magic that allows to make things that are connected to it completely independant, you are wrong. Indeed, transformer provide "galvanic isolation", which means that DC cannot pass through it (if wired correctly). Is it what you need? I don't think there's any DC at either side of your connection...

Abbry Road d enfer, so you think I can  plug straight from audio interface to FH console? any attenuation with resistors?

I don't think that transformers will give me some kind of magic,  just thought that it will isolate first of all from DC. For example if by some accedent on FH console  they will turn +48 phantom power, secondly I've read about ground isolation transformers so thought it also will work that way.

Olegarich said:

Abbry Road d enfer, so you think I can  plug straight from audio interface to FH console? any attenuation with resistors?

I don't think that transformers will give me some kind of magic,  just thought that it will isolate first of all from DC. For example if by some accedent on FH console  they will turn +48 phantom power, secondly I've read about ground isolation transformers so thought it also will work that way.

Click to expand...

Your OP was somewhat confusing, so yes, you must be careful no to turn phantom ON on the channels that recieve your interface; but, don't you have line inputs? They are DC protected and perfectly suitable for the kind of level that goes out an interface.

Olegarich said:

........Which one ratio should I use??? I want to build king of box with those 2 transformers plug it to the output of audio interface and give to the guys already isolated signal so there would be less mess on the stage.

Click to expand...

The real problem with this setup is too high output impedance of your audio interface so you can't use IMO  a real output transformer (150-150 or less) directly connected to the interface's output. You can try 600:150 or 600:600 transformer with "ground lift" function and also you should ask sound engineer to re-patch this channel (or two) to the line input on the FOH console.  This will work, I used it several times for audio cards and electric piano outputs. This setup sounds better to me, but how much it depends of FOH console, than using standard  chain (di-box - attenuation - FOH mic input).  Btw, this setup can't be used if there are mic splitters and separate mixer for monitoring on the stage.

Probably  I'm bad in describing,  what I wanted to say is:

AUDIO INTERFACE OUTPUT - GOES STRAIGHT TO THE MAIN CONSOLE MIC INPUT
(In this situation, if technician  will press phantom power on his console, the DC will go from  his console to my audio interface output and can cause some damage.

AUDIO INTERFACE OUTPUT - to  AUDIO 1:1 TRANSFORMER and then to the MAIN CONSOLE
(In this situation if engineer will press phantom power, 48 VDC will go to my Audio TRANSFORMER and will not reach my audio interface outputs, because it will isolate any 48 DC.

What i wanted to ask, is it OK to plug 150 ohm impedance in to the Main console MIC INPUT and if the signal gonna be to hot?

I use only outputs from my audio interface.

It is very likely that you will come to the situation that for some FOH guys your output will be too hot.
So I would simply stick to the DI situation.

Also stage techs don't trust musicans "DI"-outputs for several good reason.
So it is quite possible that some will refuse your, however good builed, output.

Simply build a cool DI. It will do the job. Work in 100% of the situations and you even can use it for other things when your interface is obsolete some day in the future.

Of cause it could be as simple as a passive step down transformer.

Do you know how a transformer ist reflecting impedances from prim to sek. ?
There is plenty of good reading here in the forum.
You can do it!
;D

Go for a step-down transformer. A 4:1 is likely to give the best performance/cost ratio. Since it will operate at line level, you probably don't need them to be mu-metal shielded; the shielding given by a good steel enclosure is generally enough - unless you put it on top of a power amp! That makes a huge cost difference.
You will need a ground-lift switch that connects or disconnect the ground from your interface to the ground of the FOH mixer. Just one, you don't need individual ground-lifts. Are you sure you'll never need a separate feed for monitors? You may want to consider a splitter xfmr. The Edcor XS1100 can be wired for 4:1:1.

Olegarich said:

Probably  I'm bad in describing,  what I wanted to say is:

AUDIO INTERFACE OUTPUT - GOES STRAIGHT TO THE MAIN CONSOLE MIC INPUT
(In this situation, if technician  will press phantom power on his console, the DC will go from  his console to my audio interface output and can cause some damage.

AUDIO INTERFACE OUTPUT - to  AUDIO 1:1 TRANSFORMER and then to the MAIN CONSOLE
(In this situation if engineer will press phantom power, 48 VDC will go to my Audio TRANSFORMER and will not reach my audio interface outputs, because it will isolate any 48 DC.

What i wanted to ask, is it OK to plug 150 ohm impedance in to the Main console MIC INPUT and if the signal gonna be to hot?

I use only outputs from my audio interface.

Click to expand...

You should fire that engineer because he should have never plug line outputs from an audio interface to microphone inputs. That's plain stupid and asking for a disaster to happen like someone accidentally turned on the phantom power and damage your interface. If the mixer doesn't have dedicated line inputs, then change it! (although I find it hard to believe it doesn't have any)

And to make my point even clearer, even this cheap compact portable mixer has dedicated line inputs. It's almost mandatory for a mixer to have a dedicated 2 track line input or stereo tape return or whatever the manufacturer wants to call it.

http://www.music-group.com/Categories/Behringer/Mixers/Analog-Mixers/502/p/P0576

metalb00b00 said:

You should fire that engineer because he should have never plug line outputs from an audio interface to microphone inputs. 

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Maybe it's a tad drastic, since many mixers are designed to accept line levels on their mic inputs.

That's plain stupid and asking for a disaster like someone accidentally turned on the phantom power.

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Then again, many equipment tolerate the application of phantom power on their outputs without damage. Some are really foolproof and can operate continuously in that condition, others would manifest their disapproval by emitting incongruous noises or releasing the magic smoke of life.

  If the mixer doesn't have dedicated line inputs, then change it! (although I find it hard to believe it doesn't have any)

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As I said, several mixers make a saving on having wide range mic inputs that tolerate levels up to +20dBu.

metalb00b00 it's not about that. Imagine you playin venue sharing stage with 6 bands having short time for line check.  So you gonna come and say here is my audio interface that need to be pluged in to your line input? when guys wired all snakes and stuff, well even if they will it gonna take you a bloody time that you could spend for checking sound. Thats why i want to make this bloody box to give them signal with right level and impedance so they could plug in to the same mic in and no messing around with repluging and all the stuff. 

metalb00b00 said:

You should fire that engineer because he should have never plug line outputs from an audio interface to microphone inputs. That's plain stupid and asking for a disaster to happen like someone accidentally turned on the phantom power and damage your interface. If the mixer doesn't have dedicated line inputs, then change it! (although I find it hard to believe it doesn't have any)

Click to expand...

Yes, most pro live-sound mixers have line inputs, which attenuate and block the phantom power. And most of these line inputs are on 1/4" TRS jacks.

But in the real world, if our friend shows up with an 8-channel line-out interface, and you want to patch that into the console line ins, you need 1/4" TRS male to XLR male jumpers on stage to connect the interface to the snake, and you need XLR female to 1/4" TRS male jumpers at the console end to connect the snake lines to the 1/4" line-in jacks.

I'm sure that most pro sound companies carry these jumpers (I have an eight-way mini-snake of each type). The local club won't have it, and if you're the middle band of a four-band bill, forget even thinking about this, because the patch and re-patch time will be too long.

The correct solution for our friend is to buy an eight-channel (or whatever) passive DI box thing from Radial or Whirlwind or whomever, put it in the rack with the interface, wire the interface to the DI box 1/4" ins, and tell the venue sound tech "just plug your eight XLRs into this" and everything will be fine. Do not make it more complicated than it needs to be.

Actually, the real correct solution for our friend would be for him to have a small submixer on stage. The little Mackie things even have a switch for the main XLR outs which sets them to mic level, for level compatibility with the FOH and monitor consoles. His interface outputs can go to that submixer, and he can hand off a stereo mix to the house. Done.

Olegarich said:

Thats why i want to make this bloody box to give them signal with right level and impedance so they could plug in to the same mic in and no messing around with repluging and all the stuff.

Click to expand...

While I'm a fan of DIY, in some cases it makes more sense to buy and be done with it: http://www.radialeng.com/r2011/prod8.php