Line level to mic live box with isolator, which better way?

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Olegarich

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Hello My Diy family, again I need your advice on box that  I need to build.

This is going to be used for live shows.
We use a lot backtracks on our gigs.  I have 8 channels coming out from audio interface (RME) to FOH.  Sometimes when we play in smaller venues ,they are out of direct boxes and besides that there is always mess with cables. So I decided to build a box that will match Line level to Mic level and there will be no need in DirectBoxes.

My Audio interface output is 150 ohm while Mic input on console is 5K

So I want to build attenuator with two 4.7K and 150 resistor.  So my questions is:
1. Do you think that it would be good idea to put transformer there to ISOLATE audio interface from FOH? If yes, can I use 1:1 transformer after resistors?

Is there any problems without using transformer?

2. If I'll use 150:5K audio transformer as understand it will attenuate line level to mic level automatically?

Again, Thanks for your advice guys, appreciate that a lot!



 
Olegarich said:
1. Do you think that it would be good idea to put transformer there to ISOLATE audio interface from FOH?
It depends on the quality of your mic inputs and how far the box will be from the mixer. In a reasonably clean environment, that should not be an issue.However, on a busy stage, there could be ground loops that would make the use of an isolating transformer necessary. I would try first to put the box very close to the mixer, so the low level signal has to run a very short distance.

  If yes, can I use 1:1 transformer after resistors?
Yes you can; however, the transformer may be sensitive to magnetic interference. You need a well-shielded type.

2. If I'll use 150:5K audio transformer as understand it will attenuate line level to mic level automatically? 
  The resulting attenuation is about 20dB, so a nominal line level (+4dBu) will generate a level of about -16dBu at the mic input. You need to make sure the mixer can accept such a high level.
 
Since it's eight channels...

Have you considered using ADAT? The optical connection is about the best isolation you can get.
 
Olegarich said:
So I decided to build a box that will match Line level to Mic level and there will be no need in DirectBoxes.

My Audio interface output is 150 ohm while Mic input on console is 5K

Since you have a low impedance line level signal from the interface wouldn't it be preferable to simply go into line level inputs on the desk ?
 
Since you have control over the level going to your interface why not just lower the level 30-40dB and go right into the stage box?
 
cyrano said:
Have you considered using ADAT? The optical connection is about the best isolation you can get.
Agreed. If you're already digital at the RME, why add the delay of another conversion? Just send digital over optical. The cable will be expensive but not as expensive as 8 transformers and 24+ conductor cable. And it will be completely transparent whereas attenuating (not sure why you would want to attenuate anyway) will just require amplification later which means more noise.
 
Martin Griffith said:
What's the encode/decode delay these days?

I don't have numbers, but it's negligible, as there is no decode/encode, it's all primitive digital. Less than 1 sample?

And it easily covers longer distances. Only, rugged, long optical cables get expensive. Standard cable  is around 25 € for 20 m. 50 m is around 100€. Rugged is about twice as expensive. Then there's the added cost of an ADDA. Cheap, if you consider a Behringer AD8200, less than 200 €.

Sill, a good 8 pair analog multi isn't exactly cheap either. And you have to add the price of connectors.

It's one of the things I like about ADAT connections.

And if you need more channels, there's MADI. Covers distances up to 2 km without "amplification". Same cable, but needs a pair of them. I wonder what 2 km of optical would cost...
 
Now that I think about it...

If you want to run ADAT over long distances, you should look at the converters from appsys.ch.

Small, inexpensive boxes that convert optical to twisted pair, so you can use UTP cables (up to 300 m). See:

https://appsys.ch/en/products/extenders/adx-16

I use a 30 m optical and I'm considering those when I need longer runs. Not as much because UTP is cheaper, but it's available in very rugged form too. And that's way less expensive than rugged optical. A 100 m rugged optical drum is around 1200 €. Rugged UTP is about 300 €.
 
I don't think the sound guy in your average club would be too happy about being handed an optical cable and a box and told to string it across the club to the console.
 
Hey guys thanx for advises. Probably it’s still will be better just to stay analog and just build that box. Some wenues already has loom cables in walls and they go directly to the console mic inputs. Sometimes you play outside where again it’s just a mic inputs box on the stage. I wonder even where would you put that cable when you play outside, on the ground? Probably now it’s better to make a box that will work in any situations, Which would be analog to my understanding. So probably the best would be to put 1:1 transformer as isolator and then put resistor as attenuator of line level to mic level.
 
Don’t know about your experience playing live, but quite often we have problem of connecting 8 channel playbacks, i wonder what would it be if you’ll ask to put adat cable to foh. The best would be to give them 8 cables which are labeled as tracks and no messing around
 
Olegarich said:
Hey guys thanx for advises. Probably it’s still will be better just to stay analog and just build that box. Some wenues already has loom cables in walls and they go directly to the console mic inputs. Sometimes you play outside where again it’s just a mic inputs box on the stage. I wonder even where would you put that cable when you play outside, on the ground? Probably now it’s better to make a box that will work in any situations, Which would be analog to my understanding. So probably the best would be to put 1:1 transformer as isolator and then put resistor as attenuator of line level to mic level.

Two boxes of Whirlwind Multi Director will cost you less than USD500.
http://www.1staudiousa.com/dirbox.html
 
Olegarich said:
Don’t know about your experience playing live, but quite often we have problem of connecting 8 channel playbacks, i wonder what would it be if you’ll ask to put adat cable to foh. The best would be to give them 8 cables which are labeled as tracks and no messing around
I suppose that is sensible. You could do what you say using 1:1 with a resistor network but you want balanced like 15k:300:15k or whatever. If the two resistors are not matched well your CMRR will suffer. And the DPDT should use the output legs as common so that all resistors are switched out when line level.
 
Don't forget DC blocking. The FOH console mike amps may well have P48 enabled, and it's not clear what that will do to your interface. A transformer will block P48, as will some electrolytic capacitors placed in series with pin 2 and pin 3 of the pad outputs. A value of 22-47µF will probably be fine. Yes, that sounds too large for a 5K load, but it will also reduce the distortion of those capacitors, and prevent the mike amp from becoming noisier through its input current noise into the source impedance defined by those coupling caps. They're also not so large as to suffer from leakage problems, so a quality 22µF or 47µF polar cap, rated for 63V or 100V, will work well.

As for an ADAT digital link, that will almost certainly fail, unless you're in a room that already has 10-20 DIs hanging around just for the fun of it. Where you gonna get the gargantuan light pipe cable? Where are you going to run it? Who's the clock master? Can you slave to the FOH desk? Wait - that's another sync cable from FOH so you can slave to FOH clock. Just use analog. Use reliable connectors and construction and it'll be fine.
 
I think most people didn't understand the OP question or necessities.

He needs to give the 8 outputs of the RME to the local sound crew on the stage.

The local crew will connect those 8 outputs to their stage box, or multicore cable that will bring the 8 signals to the FOH (front of house) console that will be distant from the stage.

Problems:
- Live consoles have mainly Balanced Mic inputs, RME has Balanced Line outpus (Line level to mic level)
- RME and soundcards outputs normally have TRS jacks, local crew will want XLR's (TRS to XLR)
- You will want phantom power protection for your RME line outputs  (Phantom protection)
- Isolation might not be needed in this setup, but better to have it (Isolation)

Solutions:

- Without isolation: you can build a passive box with TRS inputs, H pad attenuator (30 or 40 dbs), caps to block 40V DC, and XLR outputs

- With Isolation: DI box (active or passive), you can build 8 DI circuits inside one box that take care of all the issues. the Bo Hansen DI is a good circuit.

 
Whoops said:
I think most people didn't understand the OP question or necessities.
I think most people did understand the OP's question and constraints; a few erred.


Solutions:

- Without isolation: you can build a passive box with TRS inputs, H pad attenuator (30 or 40 dbs), caps to block 40V DC, and XLR outputs
Which has already been proposed.

- With Isolation: DI box (active or passive), you can build 8 DI circuits inside one box that take care of all the issues. the Bo Hansen DI is a good circuit.
That is a quite heavy answer; what's the need for active electronics to go from line level to mic level? The Bo Hansen DI includes a transformer for isolation. A simple stepsown transformer would do the job more simply and efficiently (no power needed).

And please note the OP asked specifically two questions. I believe they have been answered pretty well.
 
abbey road d enfer said:
I think most people did understand the OP's question and constraints; a few erred.

Not really, at least you didn't understand the situation when you said "I would try first to put the box very close to the mixer"

Also when  ADAT was suggested

Neither is practical in a Live gig, what is pratical is giving an XLR Out connector to the local crew

abbey road d enfer said:
That is a quite heavy answer; what's the need for active electronics to go from line level to mic level? The Bo Hansen DI includes a transformer for isolation. A simple stepsown transformer would do the job more simply and efficiently (no power needed).

I didn't suggest Active, I suggested Active or Passive.
A stepdown transformer is basically a passive DI box, without the need for Hi-Z input.

I suggested the Bo Hansen Di also, in case the OP wants to go active and because the Bo hansen DI is a DIY friendly woth al the info available, pcb's and an help thread

Abbey helping people is not a pissing contest, I just gave my opinions to help Olegarich, because working on Live gigs a lot I understand perfectly his situation, and I'm just adding to the thread.

Hope you respect others people opinions and help, as I respect yours
 
Whoops said:
Not really, at least you didn't understand the situation when you said "I would try first to put the box very close to the mixer"
What's wrong with laying out 8 mic cables from the converter to the interface box, this one being placed close to the mixer?

  Also when  ADAT was suggested
I didn't suggest that, it came later, and yes, I agree, it was not a very adequate suggestion. Now you never know, thinking a little outside sometimes gives inspiration for people that have a slightly different application.

  Abbey helping people is not a pissing contest, I just gave my opinions
  Hey, you're the one saying people are giving wrong answers!
 
abbey road d enfer said:
What's wrong with laying out 8 mic cables from the converter to the interface box, this one being placed close to the mixer?

Because is not really convenient in a Live gig situation, it's more practical for the local crew to receive the Mic level and XLR's already on the stage. You will have a faster setup and less hassle dealing with the local crews that sometimes are really good, other times aren't.
 

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