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Test XLR audio source for true differential signal?



 
 
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  #1  
Old June 23rd 19, 11:52 PM posted to rec.audio.pro
Bob
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Default Test XLR audio source for true differential signal?

Given a XLR audio source, what is the easiest way to test if it's a
balanced and differential signal vs a non differential signal?

Is there a tester for this? A special setting on a mixer to listen to
each of the two differential signals individually? Or do I have to make
my own special XLR adapter to pass through only one of the two
differential signals at a time?
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  #2  
Old June 24th 19, 01:48 AM posted to rec.audio.pro
Mike Rivers[_2_]
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Default Test XLR audio source for true differential signal?

On 6/23/2019 6:52 PM, Bob wrote:
> Given a XLR audio source, what is the easiest way to test if it's a
> balanced and differential signal vs a non differential signal?


It's differential if you get signal between pins 2 and 3. Balanced is
more complicated. The requirement for a connection to be balanced isn't
about voltage, it's about impedance relative to a common point,
typically pin 1. There are several configurations that are balanced. You
can test it using a multimeter.

If it's a transformerless output and the connector is wired in the
conventional manner, there will always be a signal between pins 1 and 2.
There may or may not be a signal between pins 1 and 3. Once you've
determined that pin 2 is "hot" you can measure the resistance between
pins 1 and 3. If it's zero, then the connection is not balanced. If it's
more than just a couple of ohms, then it's probably balanced.

If it's a transformer output, it's fair to assume that it's balanced. If
the transformer doesn't have a grounded center tap, you can get
confusing readings between pins 2 and 3 to pin 1.

Here's an article I wrote many years ago about balanced and unbalanced
connections:

https://mikeriversaudio.files.wordpr...ed_revised.pdf



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For a good time, call http://mikeriversaudio.wordpress.com
  #3  
Old June 24th 19, 03:15 AM posted to rec.audio.pro
geoff
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Posts: 1,501
Default Test XLR audio source for true differential signal?

On 24/06/2019 12:48 PM, Mike Rivers wrote:
> On 6/23/2019 6:52 PM, Bob wrote:
>> Given a XLR audio source, what is the easiest way to test if it's a
>> balanced and differential signal vs a non differential signal?

>
> It's differential if you get signal between pins 2 and 3. Balanced is
> more complicated. The requirement for a connection to be balanced isn't
> about voltage, it's about impedance relative to a common point,
> typically pin 1. There are several configurations that are balanced. You
> can test it using a multimeter.
>
> If it's a transformerless output and the connector is wired in the
> conventional manner, there will always be a signal between pins 1 and 2.
> There may or may not be a signal between pins 1 and 3. Once you've
> determined that pin 2 is "hot" you can measure the resistance between
> pins 1 and 3. If it's zero, then the connection is not balanced. If it's
> more than just a couple of ohms, then it's probably balanced.
>
> If it's a transformer output, it's fair to assume that it's balanced. If
> the transformer doesn't have a grounded center tap, you can get
> confusing readings between pins 2 and 3 to pin 1.
>
> Here's an article I wrote many years ago about balanced and unbalanced
> connections:
>
> https://mikeriversaudio.files.wordpr...ed_revised.pdf



Or is he assuming it is balanced and differential, but anxious about the
'accuracy' of the differentiality ?

geoff
  #4  
Old June 24th 19, 05:04 AM posted to rec.audio.pro
Phil Allison[_4_]
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Posts: 381
Default Test XLR audio source for true differential signal?

Bob wrote:
>
> Given a XLR audio source, what is the easiest way to test if it's a
> balanced and differential signal vs a non differential signal?
>
> Is there a tester for this? A special setting on a mixer to listen to
> each of the two differential signals individually? Or do I have to make
> my own special XLR adapter to pass through only one of the two
> differential signals at a time?
>


** You could do that, but still not know the answer.

There are several common forms of "balanced" output.

Variously known as "floating", "impedance balanced", "earth cancelling" and full differential - ie with two identical, reverse phase signals.

The "floating transformer" one is easy, there being is no resistive path from pins 2 or 3 to ground and a low resistance ( ie 50 ohms ) from 2 to 3.

The last one will be revealed by using adaptors like you ask, if both signals are there it will also be 6dB stronger than otherwise when in use with a "balanced " input.

The other two are not so easy and will need reference to a schematic or some test equipment.

FYI Hum rejection comes from the *input* being differential and the use of twisted pair cables.


..... Phil


  #5  
Old June 24th 19, 06:12 PM posted to rec.audio.pro
[email protected]
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Posts: 604
Default Test XLR audio source for true differential signal?


>
> FYI Hum rejection comes from the *input* being differential and the use of twisted pair cables.
>
>
> .... Phil


Yes to the OP,
if your question is actually am i getting the correct common mode rejection using this source feeding a particular input, we can devise a test for that.

mark

  #6  
Old June 24th 19, 08:23 PM posted to rec.audio.pro
Scott Dorsey
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Posts: 16,590
Default Test XLR audio source for true differential signal?

In article >, Bob > wrote:
>Given a XLR audio source, what is the easiest way to test if it's a
>balanced and differential signal vs a non differential signal?


Measure signal with your dmm. Are both legs driven? If both legs are
driven, assume it's really balanced.

If only one leg is driven, it could be impedance balanced, or maybe not.
Measure resistance from the un-driven leg to the ground pin. If it is zero
or infinite, you have an unbalanced source. If it is a couple hundred
ohms, then someone has attempted to make it impedance-balanced. The value
you measure is the effective output impedance of the undriven leg.

Now..
Put a test signal into the input, measure the output with a dmm. Then see
how much shunt resistance to ground you need to add for the signal drop to
half the open-circuit value. This is the effective output impedance of the
driven leg.

If the two impedances match within a percent or two, you have an impedance
balanced output.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 




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