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[Peter Wu]

Given an XLR balanced audio output, how can you test whether it is truely balanced output or not?

If it's an output from a mixer it most likely is truely balanced. But if it's output from a mic or some adapters (e.g. 1/8" to XLR adapter), I'm not so sure and would like to be.

[Scott Helmke]

Use an XLR to 1/4" TRS adapter and plug in a pair of headphones.  If you hear the signal in both ears, then the output is balanced.  If you just hear it in your left ear then it is only "impedance balanced". 

[Kevin Graf]

And just why would an "impedance balanced" output stage not be a balanced output stage?

Bill Whitlock (retired Jensen Transformers) wrote the book chapter on balanced interconnect systems and "passive/impedance balanced" is one of the systems.

[John Roberts {JR}]

Given an XLR balanced audio output, how can you test whether it is truely balanced output or not?

If it's an output from a mixer it most likely is truely balanced. But if it's output from a mic or some adapters (e.g. 1/8" to XLR adapter), I'm not so sure and would like to be.

First we need to agree about a definition of what "truely balanced" means, it can mean different things to different people.

Passive impedance balance is a completely competent modern solution.

Do you have a problem you are trying to resolve, or avoid?

JR

[Ivan Beaver]

Given an XLR balanced audio output, how can you test whether it is truely balanced output or not?

If it's an output from a mixer it most likely is truely balanced. But if it's output from a mic or some adapters (e.g. 1/8" to XLR adapter), I'm not so sure and would like to be.

As JR says, it depends on what you call "balanced".

It could have differential drivers, or impedance balanced or transformer balanced.  They would all be balanced, but vary in output level, isolation etc.

One test is to put in a sine wave, turn it up a bit.  Measure from pin3 to pin 1 and from pin 2 to pin 1.  If you get a voltage reading each way, then it is truly balanced (not just impedance balanced).  If you only get a voltage reading in one position, it could be balanced, or unbalanced.  Now measure the resistance from that pin (that did not read a voltage) and see if it is around 0 ohm or around 100 ohms.  If it is 100 ohms, then it is impedance balanced.
If it is 0 ohms, then it is unbalanced

[John Roberts {JR}]

As JR says, it depends on what you call "balanced".

It could have differential drivers, or impedance balanced or transformer balanced.  They would all be balanced, but vary in output level, isolation etc.

One test is to put in a sine wave, turn it up a bit.  Measure from pin3 to pin 1 and from pin 2 to pin 1.  If you get a voltage reading each way, then it is truly balanced (not just impedance balanced).

I am repeating myself, there is nothing wrong with impedance balanced... and "truly balanced" is not a technical term...

JR

If you only get a voltage reading in one position, it could be balanced, or unbalanced.  Now measure the resistance from that pin (that did not read a voltage) and see if it is around 0 ohm or around 100 ohms.  If it is 100 ohms, then it is impedance balanced.
If it is 0 ohms, then it is unbalanced

[Peter Wu]

As JR says, it depends on what you call "balanced".

Sorry for using a non technical term.

For me, truely balanced means if I attach a very long cable between this output and a mixer, it would not pick up noise. Or should I say the noise it pick up would cancel out.

So impedance balance works for me. If I bought (or am given) a new piece of equipment, I want to make sure it won't create noise problem with long cable run before I put it in use.

[John Roberts {JR}]

Sorry for using a non technical term.

OPs get a pass...

For me, truely balanced means if I attach a very long cable between this output and a mixer, it would not pick up noise. Or should I say the noise it pick up would cancel out.

Impedance balance with a proper balanced differential receiver should cancel out any common mode noise.

So impedance balance works for me. If I bought (or am given) a new piece of equipment, I want to make sure it won't create noise problem with long cable run before I put it in use.

In theory there is a slight benefit to having both legs hot due to 6dB more signal but in practice this is not a real concern for anything other than extreme environments.

I can not offer a simple measurement to confirm the balance of a given output, but it is safe to ASSume that most modern XLR outputs are at least impedance balanced since that only cost the manufacturer pennies. 

JR

PS: for TMI back last century I tested the CMRR of some star-quad mic cable by looping it over a fluorescent light fixture...  and comparing it to standard mic cable... It passed with flying colors.

[Mike Caldwell]

Sorry for using a non technical term.

For me, truely balanced means if I attach a very long cable between this output and a mixer, it would not pick up noise. Or should I say the noise it pick up would cancel out.

So impedance balance works for me. If I bought (or am given) a new piece of equipment, I want to make sure it won't create noise problem with long cable run before I put it in use.

How long is the "very long" cable?

[Steve M Smith]

For me, truely balanced means if I attach a very long cable between this output and a mixer, it would not pick up noise. Or should I say the noise it pick up would cancel out.

That requires a differential input stage to benefit from the noise cancelling common mode rejection properties of balanced line. The signal does not have to be equal and opposite on both lines, They just have to present the same impedance. The input stage will do what it always does and amplify the difference.

I have some ART DI boxes which work like that. The circuits on pins 2 and 3 are identical, but the input to one of the circuits is grounded whilst the other goes to the input socket. They work fine as the impedance is the same on both output pins.

Now measure the resistance from that pin (that did not read a voltage) and see if it is around 0 ohm or around 100 ohms.  If it is 100 ohms, then it is impedance balanced. If it is 0 ohms, then it is unbalanced

Or it could be actively impedance balanced - see above.


Steve.

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