Balanced line

[Steve Eddy]

Originally posted by Nelson Pass
Actually it's a fine circuit. The - line may not be driven,
but it does experience the same noise pickup as the
+ line, and will give good rejection going into a balanced
input.

Well, the - line isn't SEPARATELY driven as it would be with an active solution. But the - line has the same signal on it as it would if it were, just that the signal level would be half that of the active solution (providing the active solution is separately driving both lines at the same level).

So for example if you had an unbalanced output driving the line with say, 1 volt, and you converted to balanced by driving the second line with the same signal only inverted, you'd measure +1 volt on the + line and -1 volt on the - line.

If you converted to a balanced output using the passive solution, you'd measure +0.5 volts on the + line and -0.5 volts on the - line.

And not to be too much of a stickler here, it's not the line that rejects the noise. That's done by the differential input. What balancing does is help keep common-mode noise common.

If an interference field impinges upon each line identically (and techniques such as twisted pairs helps assure that they do impinge identically), if the impedance of each line with respect to ground is the same, then the voltage induced across each line will be the same in terms of magnitude and polarity and subtracted out at the differential input.

If the impedances are not the same, then the voltage induced across each line will be different, and the differential input will amplify the difference between the two.

I think you don't see it often because using it requires
understanding of the nature of balanced lines, and also
because the public has been educated to expect both lines
to be actively driven.

Yup. Though I wouldn't call that "educated." Mislead would be more like it. And unfortunately in this industry of ours, it happens all too often and is looked upon (sadly quite rightly in many respects) as a technological backwater.

But hey, rightly or wrongly, it's still a lot of fun.

se

[Steve Eddy]

Originally posted by Peter Daniel
Another audio mystery revealed. I will try this circuit in my next project. Thanks Steve for sharing it with us.

My pleasure. Hope it works out for you (and any others)!

se

[dice45]

Steve,
thanks for sharing!

[Steve Eddy]

Originally posted by dice45
Steve,
thanks for sharing!

Oh, I didn't mean to share it with you. MEANIE!

Awwwww, I didn't really mean it. You can use it too. I love ya, ya big lug.

se

[Bob2]

Steve

Being of limited knowledge when it comes to this stuff, what would the circuit look like to convert an amp to a balanced input using your balanced output.

Bob2

[Nelson Pass]

Originally posted by Steve Eddy


If you converted to a balanced output using the passive solution, you'd measure +0.5 volts on the + line and -0.5 volts on the - line.

Sorry, I don't see any mechanism on your passive schematic
that would create this effect. The - line does not carry signal,
only noise.

[Peter Daniel]

Originally posted by Nelson Pass


The - line does not carry signal,
only noise.

So I new there was a price to pay after all. Of course it can't carry a signal since the signal is going through the ground.

So how does it really compare in reality, passive versus active retriving of a balance line?

[eLarson]

Why add more circuitry to convert unbalanced to balanced when the preamp in question already does this? If you feed the balanced output of the preamp to the balanced input of a power amp, you are good to go.

You could go to http://www.jensen-transformers.com/ln_in.html and get one of those if you absolutely must have the unbalanced to balanced conversion prior to your preamp.

But for the record, I wouldn't bother.

Erik

[Steve Eddy]

Originally posted by Bob2
Being of limited knowledge when it comes to this stuff, what would the circuit look like to convert an amp to a balanced input using your balanced output.

Unfortunately, at an input, it's not such a simple matter as it is with an output.

At the input, it's not just a matter of balancing the impedances of the two lines, but also of providing a differential input so that any common-mode noise on the lines can be rejected (which is the raison d'etre of balanced interfaces in the first place) and depending on the existing input circuit could involve much more significant changes in the circuit itself rather than just tacking on a few parts.

So without a specific circuit to work from, there really is no generic answer as with outputs.

se

[jleaman]

im going to try the esp projects. and see how they work.. is there a sound difrence between

balanced line in xlr compaired to rca in .. will i hear the difrence ..?