Noiseconformist wrote:I can't see, why a few milliohms would cause significant crosstalk, given the relatively low currents flowing.
There was a couple of situations that came up in that thread on Facebook. Now that particular quote was to do with the original poster's desire to distribute 0V from a single PSU to multiple locations and cases. The current travelling along 0V connections can be quite large in this instance. There are several reasons for this but where you have one PSU and multiple cases the main reason for this is usually the current imbalance between the negative and positive rails going through that one connection. If the +15V is taking say 1A and the -15V taking 750mA, then 250mA will have to flow through the 0V back to the power supply.
250mA travelling through, say, 10 milliohm would give a voltage drop of 2.5mV. That might not sound a lot but that's a noticeable 3 cents of pitch deviation on a VCO. And 10 milliohm is probably considerably less resistance than the poster's DIN plugs and the lengths of inevitably small diameter wire he'll be using to fit into the tiny DIN plug's housing. The bigger the resistance the greater the voltage drop across that connection.
The other situation mentioned in my posts on that Facebook thread referred to multiple modular cases each with their own PSU. This is less susceptible to 0V errors but they can still occur. The power supply transformers often have a little leakage by which the mains voltage is capacitively coupled into the secondary winding. This can create a small 50Hz/60Hz (and more often its harmonics) current on the 0V connections if the 0V isn't earthed at the PSU which it wouldn't be with a double insulated line lump like the Yamaha PA-30. Thankfully, in a well designed line lump these leakages are really small and can be safely taken away with the audio connections. But they are still there and will still create a small amount of noise on the audio output as soon as you connect your modular up to an earthed item like an amp or mixer.
Return currents from audio signals will also disturb the 0V. These are quite small, but each 10V output signal into 50Kohm input will create a current of 200uA that needs to be returned along the 0V connections. These can add up quickly in a large patch.
The upshot to all of this is to reduce the differences in voltage across the whole 0V connection system. And the best way to do this in an unbalanced audio system is to reduce the resistance within the 0V network. That is, big wires, chunky connections and small distances.