Electrical – balanced to unbalanced audio switch

audiobalancedunbalanced

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

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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"?

Best Answer

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