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I don't think you understand the concept as much as you think you do

They do see the same signal? Are you sure about that?? :)
I think you'll find that only at one single frequency do the woofer and tweeter receive exactly the same signal.

Yet another person who doesn't understand the concept. My goodness.

They never receive the same signal at any frequency. At the frequency where the amplitudes are equal, the phase is different.

And I don't know why you insist on pushing the crossover analogy in this thread. It really is irrelevant. Your point seems to be that two interconnects hanging off a switch box makes two parallel LCR networks, and hey, a crossover has two parallel LCR networks too! Therefore they must be comparable. Well, no they are not. The crossover is designed to be a frequency dividing network with cutoffs in the middle of the audio band. The interconnects are not. Neither is the switch box. And the source impedances are vastly different, and the loads are vastly different too. There is literally nothing comparable in your analogy.

I suggest to read up on voltage sources.
Also read up on the specific requirements for this test called out by the engineers who have advocated it. You do need to satisfy some requirements for it to be valid.

The primary requirement is: for this "separation" of networks to occur and a valid A/B comparison to result, you do need to have a real-world source that is pretty close to a theoretical voltage source. For interconnect testing something like a zero ohm headphone amplifier would work well. For power amplifier testing, a solid-state amp with very low output impedance. If you satisfy that requirement in the conventional two-way speaker system example I mentioned, you'll find that disconnecting either the woofer/network or the tweeter/network individually will not alter the operation of the other in a significant way. When both are connected, the two legs will have minimal/no interaction with each other.

In either case, the characteristics will lump.....but only as seen by the source....which should be a voltage source. In that case, regards the loads, the networks will operate as if the other were not connected.

This is basic stuff fellas.

That's totally irrelevant because the output stage of a source component or pre-amp is far, far from an ideal voltage source. Typical output impedance values for components with solid state output stages vary from a few 10s of ohms to a few hundred ohms. For tube gear it's typically a few hundred up to a few thousand. And these impedances are reactive too, with output transformers and/or caps around the output for isolation, DC blocking, and filtering.

Let's go back to basics. An ABX test is a statistical hypothesis test. The hypothesis is that two different line level interconnects produce audibly different results during music playback from an audio system. A valid ABX test of the hypothesis can only be performed if the design of the experiment preserves or duplicates the conditions under which the hypothesis could be true. If your experiment depends on using a source that behaves as an ideal voltage source and isn't representative of a typical audio source component or pre-amp, then you aren't testing the hypothesis.

Using a typical audio pre-amp driving a typical audio power amp, you can't hand wave away the fact that this particular test setup makes the parallel capacitance of A the same as the parallel capacitance of B which equals the sum of the two interconnects. The open-circuited interconnect hanging off the switchbox may also be conducting RFI into the circuit under test. Even the switchbox itself is a problem. The mere presence of the switchbox in the circuit could invalidate the test unless its impedance is a couple orders of magnitude less than the cables under test. And you would have to be sure that the switchbox doesn't introduce a path for RFI or EMI to enter the circuit, and the switchbox has to preserve the relationship between the grounds of the source and sink components.

The QSC and AVA ABX comparators, for example, are essentially passive pre-amps. When you put one of those things in the circuit, you are not testing the original hypothesis.

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