0
\$\begingroup\$

I have an audio source input, that I am putting through the circuit below to create a balanced audio signal.

enter image description here

There will be one of these for each channel, left and right.

I have been asked to provide a "balanced or unbalanced switch", so that a user can flip the switch and have either balanced or unbalanced audio out.

But, as far as I can tell, if you want unbalanced audio from this set up, you would just take the audio from pins 1 and 2, and dont connect pin 3. This gives you an unbalanced audio output.

If I added an extra stage to combine the balanced audio to unbalanced, then I would just be converting back to the original input signal.

Or am I missing something here?

Is there some way to add an "balanced or unbalanced switch"?

\$\endgroup\$
1
  • \$\begingroup\$ "If I added an extra stage to combine the balanced audio to unbalanced, then I would just be converting back to the original input signal." As the answers below point out, you don't need an additional amplifier stage to go back to unbalanced and the signal has the potential benefit of lower impedance leaving your circuit than leaving the original source. \$\endgroup\$ – Transistor May 23 '20 at 19:04
1
\$\begingroup\$

Firstly, that's a differential output, not balanced.

A true balanced output would couple interference on one leg to the other, improving common mode rejection at the receiver.

To test : short either leg to ground. If the amplitude on the other leg doubles, it's balanced. This has two consequences :

  1. the differential amplitude doesn't change due to a short on one leg
  2. You could unbalance the output simply by shorting one leg to ground.

However, differential may be good enough for your purpose, and it's not unusual outside pro audio.

As you say, hot + ground comprises an unbalanced signal.

But if you want to make an unbalanced output on pins 2 and 3, you could simply disconnect teh cross-coupling resistor from the -ve leg, leaving that amplifier as a voltage follower with 0V input.

The trivial way to generate a balanced output is to use a transformer; the secondary provides a fully floating balanced output.

For an example of a transformerless balanced output driver, see the SSM2142 which can be replicated with opamps and matched resistors if you like.

Some further discussion in this Q&A: attracted quite a few answers, of which mine was only one.

And also this one

\$\endgroup\$
6
  • \$\begingroup\$ I see a lot of similar circuits online for creating "balanced audio". If this is not "true balance", can you give an example of a circuit that is true balanced? \$\endgroup\$ – R2D2 May 23 '20 at 19:00
  • \$\begingroup\$ I disagree with this Brian. Wiki definition of a balanced line. \$\endgroup\$ – Andy aka May 25 '20 at 10:24
  • \$\begingroup\$ @Andyaka fair enough, I didn't feel like repeating the whole story. But I'm still unclear which part you disagree with (or all?) \$\endgroup\$ – Brian Drummond May 25 '20 at 10:32
  • \$\begingroup\$ I disagree with what constitutes a balanced output. I would say that the op's circuit is a balanced output but you say otherwise. \$\endgroup\$ – Andy aka May 25 '20 at 12:34
  • 1
    \$\begingroup\$ I do take your point about a transformer having perfect impedance balance and potentially very low driving impedance (hence it can do the extra things). At the end of the day it's just a definition seen in one persons eyes and not another's and I'm not getting bent about it. \$\endgroup\$ – Andy aka May 25 '20 at 13:17
1
\$\begingroup\$

Balanced uses XLR connectors and unbalanced uses RCA connectors, they also use different cables, so I wonder what the switch's purpose would be. It makes no sense to switch a balanced signal to a RCA, or an unbalanced signal to a XLR.

if you want unbalanced audio from this set up, you would just take the audio from pins 1 and 2, and dont connect pin 3. This gives you an unbalanced audio output.

Correct.

Note "balanced" does not mean two signals of opposite polarities. It means that both signals in the differential pair have the same source impedance, which gives best common mode rejection at the receiver. If both signals (hot/cold) have opposite polarity, headroom and SNR are maximum. But if you just use one hot signal and set the other (cold) to 0V, you still get to keep all the advantages of balanced connection with regards to EMI/noise rejection, only sacrificing a bit of headroom which never was required in the first place since the unbalanced signal source levels are at line level, thus lower that what is specified for XLR balanced...

So, to convert a signal from unbalanced to balanced, connect:

  • Source signal to Balanced Hot

  • Source ground to Balanced Cold at the source

You don't actually need opamps.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.