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wiring xlr's
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cilla.scope View Drop Down
Old Croc
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Points: 2954 		Post Options Post Options   Thanks (0) Thanks(0)   Quote cilla.scope Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 2:18am
Originally posted by kevinmcdonough kevinmcdonough wrote:

hey

BUT.........

what your looking for is to go the other way, from the unbalanced RCA to the balanced XLR. In this case, your going from two pins to three pins.
Just like before the main signal is always the main signal, so the centre pin of the RCA gets connected to the "hot" pin 2 of the XLR. No big deal.

You now have the barrel of the RCA which is connected to the shield wire. Now the point of the shield wire is to pick up any interference, radio waves or random noise and keep it away from the main signal, bus it off to earth.  But at the XLR end, pin 3 of a balanced XLR eventually gets blended into the main signal (look up what balanced is to understand why if your not sure) and so if you attach the shield wire here you have the possibility of introducing that noise that your trying to keep away back into the main signal, not good. So the barrel of the RCA should be connected through the shield wire to Pin 1, where it can get sent away to ground, and pin 3 should be left unconnected so as to not introduce anything unwanted into the signal.


Nice try, but, sadly, incorrect.

Hint: with a proper electronically balanced output or a traditional transformer balanced output you will get naff all out of the phono if pin 3 is left unconnected ... remember in a properly transformer balanced output, if you only connect the "hot" end of the transformer coil, and leave the "cold" end floating  you won't get anything at all out of the unbalnced end. "proper" electronically balanced outputs, confgiured to drives into 600R will correctly simulate this. You might well get away with it on poorrly designed electronically balanced outputs though.

Regardless of which way around it is going, screen should go to the sleeve of the phono and pin 1 and 3 of the XLR,   live/hot should go from pin 2 .. should work in all circumstances.
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cilla.scope View Drop Down
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Points: 2954 		Post Options Post Options   Thanks (0) Thanks(0)   Quote cilla.scope Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 2:23am
Originally posted by audiomik audiomik wrote:

might be worth considering the actual circuit operation of Balanced inputs.

Firstly a 'Balanced Transformer coupled input':
for an XLR input 'hot' (XLR pin 2) is connected to one end of the center tapped primary winding of the input Transformer and 'cold' (XLR pin 3) to the other end of this same winding.
Ground (XLR pin 1) is connected to a center tap so for an unbalanced source signal in order not to short half of the input transformer winding then use pin 2 of the XLR as 'hot' and pin 3 as 'cold'. In this case there should be no connection to pin 1 of the XLR input connector. Alternatively 6dB of gain can be achieved if pin 3 of the XLR is left open circuit and pin 1 only is used for the ground/screen of the unbalanced signal source.
 


Errm  .. sorry, have to disagree there.

The screen is NOT connected to the centretap, in fact, usually, there is no centre-tap unless the device is phantom powered. The normal configuration for a transformer balanced input is simple the two ends of the primary, one to pin 2 , the other to pin 3. If you leave pin 2 or 3 diconnected, you will get naff all. You shold be able to measure several megaohms between pin 1 and pins 2/3 in a normal, non-phantom, balanced input.
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audiomik View Drop Down
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Points: 2962 		Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 2:31am
just to pick up on this point on getting unbalanced signals from electronically balanced outputs, some circuits are specifically designed to provide a 6dB increase in output signal when used in unbalanced mode by shorting one of the outputs to ground locally at the output (usually the '-' or cold)..... this then retains the full signal level weather balanced or not.

Probably not used though on low cost kit
Mik
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audiomik View Drop Down
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Points: 2962 		Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 2:39am
Re: The screen is NOT connected to the centretap, in fact, usually, there is no centre-tap unless the device is phantom powered. The normal configuration for a transformer balanced input is simple the two ends of the primary, one to pin 2 , the other to pin 3. If you leave pin 2 or 3 diconnected, you will get naff all. You shold be able to measure several megaohms between pin 1 and pins 2/3 in a normal, non-phantom, balanced input.

Normal configuration for 'floating' balanced line isolated inputs using transformers is what I refer to.
Do agree that in some instances the CT is left open circuit, however with long cable runs this can cause problems with CMRR..... - 'voice of much experience'!
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kevinmcdonough View Drop Down
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Points: 3751 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 9:44am
Originally posted by cilla.scope cilla.scope wrote:

Originally posted by kevinmcdonough kevinmcdonough wrote:

hey

BUT.........

what your looking for is to go the other way, from the unbalanced RCA to the balanced XLR. In this case, your going from two pins to three pins.
Just like before the main signal is always the main signal, so the centre pin of the RCA gets connected to the "hot" pin 2 of the XLR. No big deal.

You now have the barrel of the RCA which is connected to the shield wire. Now the point of the shield wire is to pick up any interference, radio waves or random noise and keep it away from the main signal, bus it off to earth.  But at the XLR end, pin 3 of a balanced XLR eventually gets blended into the main signal (look up what balanced is to understand why if your not sure) and so if you attach the shield wire here you have the possibility of introducing that noise that your trying to keep away back into the main signal, not good. So the barrel of the RCA should be connected through the shield wire to Pin 1, where it can get sent away to ground, and pin 3 should be left unconnected so as to not introduce anything unwanted into the signal.


Nice try, but, sadly, incorrect.

Hint: with a proper electronically balanced output or a traditional transformer balanced output you will get naff all out of the phono if pin 3 is left unconnected ... remember in a properly transformer balanced output, if you only connect the "hot" end of the transformer coil, and leave the "cold" end floating  you won't get anything at all out of the unbalnced end. "proper" electronically balanced outputs, configured to drives into 600R will correctly simulate this. You might well get away with it on poorrly designed electronically balanced outputs though.

Regardless of which way around it is going, screen should go to the sleeve of the phono and pin 1 and 3 of the XLR,   live/hot should go from pin 2 .. should work in all circumstances.



interesting.  I'm not an electrical engineer by any means and have never read up on the actual transformer side of things so bow to your knowledge, but this is the way I have always done it (only when going from an unbalanced output such as a DJ mixer or CD player etc into a balanced input) into everything from berry desks all the way up to big boy Midas desks and XTA crossover etc, (so i assume not cheap inputs) and never had a problem.

k
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cilla.scope View Drop Down
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A decent electronic balanced to unbalanced input, should not produce any output with only one leg connected.  The cheap units simply feed each pin into an op-amp (one inverting, one non-inverting input) and take the difference of the two signals. This is OK to a point, but doesn't correctly mimic the current flow in a true balanced circuit (ie, a bit of unblanced current injected into the "hot" pin should flow through the input circuit and back out the "cold" pin.  Not a problem on short runs, but can seriously mess up the CMRR on longer feeds and produce noise in some situations.

The top class inputs (have a look at  the input circuit on Sony broadcast kit) will not produce any output, as the op amps are cross connected on the input pad to mimic this current flow,   if you leave pin 2 or 3 floating, nothing comes out.  They also take the trouble to do this on their output circuits,  if you ground pin 2, pin 3 goes up 6db.  I have seen similar circuitry on other top-end gear, and as I said, on traditional transformer balanced gear it works this way.

On kit with cheap input circuits, you will get away with it, personally I would ground the pin anyway, for the day it gets plugged into somethig with a transformer input and no one can figure out why it works in this bit of kit and not in this bit of kit ...
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Louder than loud View Drop Down
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just wire them all together 
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If you leave pin 3 floating on the input of a soundweb green they can blow the input op-am. Not seen this on any other piece of kit but I always ground pin 3.
"These amps go up to 11"
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audiomik View Drop Down
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Points: 2962 		Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 3:07pm
I remember that Switchcraft used to do XLR line connectors with small slide switches built in - used to find them useful for a switched attenuator when needing to better match signal levels....
Think if these are still available then the switch could be wired to provide an on/off link between pins 1 and 3, thus solving the potential compatibility problem between the different inputs being described in this thread.

These were the sort of useful bits, together with the really useful Switchcraft kits to interconnect between many different things that I used to buy at Future Film Developments in Wardour Street. I understand they are no longer there but might still be trading somewhere else.

By the way, does anyone know if those Switchcraft 'adapter kits' are still available anywhere?
Mik
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Originally posted by Louder than loud Louder than loud wrote:

just wire them all together 

problem solved.  
REFLEX ALL THE WAY.... (however, im playing with horns again...) That ok Mister Valiant? :)
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cilla.scope View Drop Down
Old Croc
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Points: 2954 		Post Options Post Options   Thanks (0) Thanks(0)   Quote cilla.scope Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2010 at 8:07pm
I thought FFD moved to Uxbridge some time ago ...  still, they were always a purveyor of decent bits,  used to use them for jackfields and musa patch panels and the like.
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