Two different master clock - clock cable - streamer systems. (Think SOtM master clock and SOtM sms200ultra Neo). All equipment is the same. All cables are from the same manufacturer, the same termination and the same length. All impendances from the in and out connections and cables match.
The *only* difference is in the first system, all the impendances are 50 ohms. In the second system all the impendances are 75 ohms.
Question: which system sounds better? The first system (all 50 ohms)? The second system (all 75 ohms)? Is there no difference between the two? Why?
I can’t think of any reason why 50 ohms would be preferable to 75 ohms at the lengths that are involved in a home audio system, given that everything else is equal, as you indicated. So my instinct would be to go with 75 ohms, since as you indicated most audiophile-oriented unbalanced digital cables are 75 ohms.
Regarding Teo’s comments just above, his cables are of course very unique and I have no knowledge of how or if conventional transmission line theory may apply to them. But in the case of conventional (non-liquid) digital cables, regarding ...
50 ohm vs 75 ohm is a transmission line technology concept... improperly applied to these very, VERY short lengths of cable.
... a generally accepted rule of thumb is that a cable should be treated as a "transmission line" if the length is greater than 1/10 of the wavelength of the signal, and beyond this length reflections on the line resulting from impedance mismatches can become important enough to affect performance. And I should emphasize that this rule of thumb is not specifically related to audio, where extremely tiny effects such as timing jitter in the sub-nanosecond area may be audibly significant.
The OP’s master clock generator is unusual in that it generates the clock in the form of a 10 MHz sine wave, rather than as some approximation of a square wave. As a rough ballpark approximation let’s assume that a conventionally designed digital cable he might choose has a propagation velocity of 75% of the speed of light in a vacuum. That would be about 225 million meters per second. The wavelength of a 10 MHz sine wave in that cable would be about 225 million meters per second divided by 10 million cycles per second (i.e., 10 MHz), which is 22.5 meters. One tenth of that is 2.25 meters, or about 7 feet.
So in general, i.e., for most non-audio applications, maintaining a good impedance match at that signal frequency would be considered essential for cable lengths exceeding about 7 feet. In high quality digital audio applications I would expect that threshold to be considerably tighter, which is to say that I would expect impedance matching to be critical for considerably shorter lengths than that. And in the much more usual case of clock signals that are approximations of square waves, rather than the sine wave that is generated by the OP’s particular equipment, the signals involved would have significant frequency components associated with their risetimes and falltimes that are **much** higher than 10 MHz, and so in those more usual cases I would expect that threshold to be considerably tighter still, likely in the area of just 1 or 2 feet. And very possibly even less, depending on the risetimes and falltimes of the signal provided by the specific source. (For those who may not be familiar with this terminology "risetime" and "falltime" refer respectively to the amount of time a digital signal requires to change from its lower voltage state to its higher voltage state and vice versa. Or more specifically, those terms are generally defined as the amount of time required for the signal to change from the 10% point between its two voltage states to the 90% point, and vice versa, since what happens when the signal is close to those voltages is usually not important).
The bottom line: Keep impedances matched, if you are not using one of Teo’s cables!