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I've just learned about balanced and unbalanced audio connections and so I started trying to figure out which pieces of my equipment have what.

My Roland U-220 sound module appears to have unbalanced outputs (based on reading the schematic). I have a Behringer UMC404HD (sort of a mixer with a computer interface) with balanced inputs and outputs. Finally, I have two TOA TD-1 amp/speakers with unbalanced inputs (except a single balanced MIC input).

I can connect the U-220 to the UMC404HD and then connect the UMC404HD to the speakers using TS cables and the sound comes out fine (OK, I'm not a nitpicky sound engineer and I'll admit my standards might not be high).

Can someone work me through the signal path to explain why I actually get sound out of the speakers?

For example, the U-220 outputs are T and S. On the UMC404HD, T -> T, S -> R & S. I'm guessing that since S is the ground voltage (is that right?), the UMC interprets R as a 0v signal, inverts it and sums it with T. The net result is that T is unchanged, although with less volume that if the setup were balanced at both ends.

From the UMC4040HD to the speakers, it's less clear. I assume the UMC outputs the audio to T and inverted audio to S. But because I'm using a TS cable, the inverted signal shorts to ground. So T -> T, R & S -> S. I'm totally guessing that shorting R & S delivers a signal that is equivalent to just S. The speaker receives the audio signal and ground, which is just what it wants.

Various articles on the web which discuss building adapter cables and adding resistors suggest that the situation is a bit more complex.

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For example, the U-220 outputs are T and S. On the UMC404HD, T -> T, S -> R & S. I'm guessing that since S is the ground voltage (is that right?), the UMC interprets R as a 0v signal, inverts it and sums it with T. The net result is that T is unchanged, although with less volume that if the setup were balanced at both ends.

Not quite. With balanced connections there are three conductors: Ground, Signal +ve, Signal -ve. With fully balanced connections you actually only require signal+ve and signal-ve - Ground is used for both shielding and for a ground base for phantom power. (A correct understanding of this can help with reducing ground loops, but I digress).

When mapping a TRS or TS connector to balanced conductors, you are mapping the following:

  • Tip -> Signal +ve
  • Ring -> Signal -ve
  • Sleeve -> Ground.

When there is no ring, the Sleeve is increased in size and thus the mapping is as follows:

  • Sleeve -> Signal -ve, Ground

Signal-ve and Ground are bridged, which is the correct way of unbalancing a balanced signal.

From the UMC4040HD to the speakers, it's less clear. I assume the UMC outputs the audio to T and inverted audio to S. But because I'm using a TS cable, the inverted signal shorts to ground. So T -> T, R & S -> S. I'm totally guessing that shorting R & S delivers a signal that is equivalent to just S. The speaker receives the audio signal and ground, which is just what it wants.

Same applies here. You need an unbalanced output for the speakers (unless you are using active speakers with balanced inputs).

  • Tip of Jack is the signal
  • Sleeve of jack is the ground.
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  • For part 1, you say my description is not quite right. The sleeve (from the U-220) sends ground to the where the UMC might expect -ve. I said that the UMC will simply invert this signal and sum it with +ve, yielding a signal with half the volume of a balanced connection. You say that -ve and ground are bridged. What does this mean to the UMC in terms of how it affects the final signal?
    – freixas
    Jan 3, 2020 at 22:58
  • For part 2, you say I need unbalanced outputs, but I don't have those—I have balanced outputs from the UMC. The speakers are unbalanced and the cable is TS. It works, but I am no closer to understanding why. One possibility is that the UMC can detect that the cable is TS and send unbalanced output, but I have no evidence that it does this (unless this is how all balanced outputs are designed).
    – freixas
    Jan 3, 2020 at 23:03
  • -part 1- it's more about ensuring that your understanding of the signal flow is correct. When you bridge the -ve and ground any current that is on the -ve signal is routed straight to ground, so the -ve signal line effectively becomes a ground line. There is still signal on the +ve conductor which is basically how UMC gets it's signal.
    – Mark
    Jan 4, 2020 at 0:08
  • -part 2- ok so the output jacks are TRS? This is exactly the same situation. You use a TS jack so the jack sleeve bridges RS and thus you are left with +ve signal on Tip, -ve signal grounded via the RS connection to ground. There's no 'detection' in play here - simply jamming the TS jack into the TRS socket will be enough to bridge the Ring and the Sleeve mechanically. This is how these connectors are designed.
    – Mark
    Jan 4, 2020 at 0:10
  • Let's revisit part 1. The UMC has a balanced input, which means that it will sample the plug at 3 points. It expects +ve, -ve and ground. It actually gets +ve, ground and ground. My thought is that circuit designer doesn't have to design for this. If ground is equivalent to a -ve signal of 0v, then inverting 0v and summing it with +ve results in a signal 1/2 the strength of what it would be if there were a proper -ve. Yes?
    – freixas
    Jan 5, 2020 at 6:08

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