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How to implement a symmetric audio output on a small audio device using a single supply battery voltage only.

I like to drive symmetric XLR inputs, and use a symmetric connection for noise suppression on long cables. I have a DAC sporting two channels, thus I can create a positive and negative audio signal directly.

But how to connect the output?

  • Is it ok to always have the symmetric lines with some DC offset in respect to GND? I guess not, because there may be adapters to asymmetric inputs that will just short the negative line to GND if I'm understanding that right.
  • So is it safe to assume that a symmetric input will place my circuit in a safe DC range by it's own, so I can just connect positive and negative, and leave GND open?
  • Or is it still needed to place capacitors, and if, on which lines? Positive and negative only? Or just on GND?
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    \$\begingroup\$ Do you mean "differential" when you say "symmetric?" \$\endgroup\$
    – JYelton
    May 3 at 20:30
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    \$\begingroup\$ @JYelton Perhaps the OP wants to produce an output that is "compatible" in some way with whatever qualifies as standard XLR inputs (such as found in the Zoom H4n) from two DAC outputs using some unspecified "magic" in between? \$\endgroup\$
    – jonk
    May 3 at 20:36
  • \$\begingroup\$ As the answer below says, XLR is not about being symmetric, it's about being balanced impedance. And since it is audio the inputs are likely AC coupled so your output can be too, so even if DC bias is removed your DAC output can have DC bias. Just make sure you latch both DACs at the same time, or the signal will have intra-pair skew. \$\endgroup\$
    – Justme
    May 3 at 21:29
  • \$\begingroup\$ I just gave it a try, just connecting hot and cold to the DAC output and GND, and no GND / shield connection on the XLR connector at all. Seems to work quite well on a balanced mains-powered audio mixer input. In comparison to an unbalanced input with GND - GND connection which had a of noises if the audio device was touched or unpowered this is actually quiet. So for this setup, cold - hot - only connection works fine. I will try adding equal DC decoupling to the cold line too, as suggested. \$\endgroup\$
    – dronus
    May 27 at 19:29

1 Answer 1

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There's a difference between:

  • Symmetric, where both positive and negative halves of the differential signal have opposite voltage

  • Balanced, where both positive and negative halves of the differential signal have the same impedance

The desirable noise rejection properties of the XLR balanced connection come from the latter, ie having the same impedance on both wires. The symmetric voltage only gives you +6dB headroom, and a tiny bit of distortion cancellation in some circumstances, that's all. So you don't need to do it.

The simplest way to add a balanced output to your device is:

  • Connect the "hot" wire to the single-ended audio output, usually the output of an opamp.

  • Connect the "cold" wire to ground on your PCB at the point that is the reference ground for the audio signal.

Basically, the balanced receiver substracts voltages on both wires, which cancels noise picked up on the way. This requires noise to appear as common mode, which means noise should be the same voltage on both wires to be canceled. And this requires equal impedances on both sides, so if you include a DC blocking cap and a series resistor to protect your opamp, you should add the same components on the other wire in the pair.

The XLR shield should not be connected to a point that will cause current flowing in the shield to flow into sensitive ground nodes on your pcb. Ideally, it should be connected to the metal enclosure, so current flowing in the shield will be shorted to Earth via the chassis instead of creating noise voltage in the ground of your PCB.

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