How can a signal backfeed and create a crosstalk in a passive summation circuit?

Question

...a passive summation circuit can be a simple way to combine signals but one disadvantage is that the summed signal will backfeed into each of the sources. This could be an issue if you wanted to use the source unmixed as well as combined - you would get crosstalk.

The amount of crosstalk will depend upon the impedance of the source and a low enough output impedance this wouldn't be noticed. (from Designing voltage summer without op-amp)

Can anyone explain this further? How can a signal backfeed?

Answer 1

In an active summer the voltage at the summing node is extremely close to zero but in the passive summer the voltage at the summation point is your output voltage and is non-zero.

This voltage will cause a current to flow though the summation resistors and into the source. If the source has a non-zero output resistance there will be a voltage generated at the output of the source that is caused by the other inputs.