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Hi
[Assuming the inputs are perfectly balanced, then the output of each opamp will be 180 degrees opposed. If you took those two outputs and summed them together, there would be no signal, because they cancel each other, yes?]
No signal relative to 'ground' BUT a there is still signal current flowing through the summing resistors.
[Now, if the transformer was replaced with 2 5k resistors in series, and you fed the output of the opamps (180 degress opposed) into each end of this network, what would you see at the centre tap of the two resistors? It's a summing point, yes?}
It is a 'summing point' only to the extent that the potential difference relative to 'ground' would be zero. Mr Kirchoff's law is upheld.
[Transformers are impedance convertors, so in reality, with a 10k load and 1:1 ratio, the primary looks like 2 5k resistors in series with a grounded "no signal" (voltage wise) centre tap with respect to the 180 degrees opposed signals. If the signals were in phase, it's a different story.]
Assuming the outputs of the amplifiers are 180 degrees in opposite phase then a 'centre tap' would 'appear' to be grounded. If however the signal is NOT balanced going into the transformer then the centre tap will still have half the overall voltage which is presented to the windings on it but this would NOT be zero volts relative to 'ground'. Take for example the case if the inputs (and by implication the output of one of the amplifiers) is UNbalanced then the voltage you would see on the centre tap is half the 'active' voltage, relative to ground.
In this situation, joining the centre tap to ground would be a bad idea as by transformer action the active half would try to force current back into the inactive amplifier.
There is nothing mystical as you say but you have to consider where 'real' electrons are flowing and they certainly can't from a mystical node part way down a transformer winding, it HAS to come out of the 'ends'. A transformer is a current operated device and only cares about the current flowing through the wire, not where it is referenced to in the wider world.
Matt S
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