Phono Box DS3 B output impedance

The jury is still out on whether the "impedance" of the cable, which when we say it is 50 or 75 ohms for a single-ended cable (with RCA terminations) or 110 ohms for a balanced cable terminated in XLRs, should be added in, in calculating the ratio between the input and output impedance of two devices connected to each other by cable. Lanx is absolutely right to say that cables have "impedance", but when we use that term for cables, we are really referring to a quantity more properly called "characteristic impedance". Characteristic impedance is the square root of the cable inductance divided by its capacitance (L/C). We express this fraction in "ohms". By this definition, cables with high capacitance have low characteristic impedance. Obviously, if you just take an ohm meter and measure the resistance across any IC, it will be only a few ohms, which can be ignored in our calculation of the ratio of the input to output Z. It is or was my belief that characteristic impedance only becomes significant at frequencies way higher than the audio spectrum, e.g. at MegaHz or GigaHz frequencies OR if the cable is very long, much longer than what you are likely to be using. But maybe I am wrong. Maybe we will find out.

Pro-ject answered me that the impedance of the XLR output was of 100 Ohms. So assuming that the impedance of the cable itself is between 75 and 110 Ohms, we should be fine with a 1,66 kOmhs input impedance.

Regarding the way we should count the impedance of the cable in the system, I can't answer at all but I would be curious to learn if someone can help.

They are talking about "characteristic impedance". Whether that matters at audio frequencies and with typical 3 to 6 foot long ICs is the question. The more I read about it, the more I am convinced it does not matter. Here is a practical rationale: If the characteristic impedance of the cable matters in calculating the source impedance at audio frequencies (less than 20KHz), then a typical LOMC with an internal resistance/impedance of 10 ohms would be seen by the phono stage as a 110 ohm source impedance (assuming a "100 ohm" cable). Many load such cartridges at 100 ohms. If the cable characteristic impedance is important in that calculation, you have a 110 ohm source driving a 100 ohm input. This would result in about 50% of the signal voltage going to ground. Maybe that’s OK, but when you then increase the input Z of the phono stage to, say, 1000 ohms or even 47K ohms, which many do use even with such LOMC cartridges, there should be very obvious increase in gain that goes with that change from 100 ohms to 1000 ohms or 47K ohms. In my own experience that does not happen, and I have never read that anyone else has had that result. This is just speculation on my part.

Found this quote from the late Al Margolis, who was either an EE or a physicist and who was in any case brilliant, on the "cables" forum in 2009:

"Characteristic impedance, being part of what are called "transmission line effects," is (at least for typical interconnect lengths) generally considered to be utterly inapplicable to audio frequencies. Note that I limited the statements in my first post above to cables carrying analog audio, not digital signals, video, or rf."

And found this quote from Atmasphere (Ralph), posted in 2008 on the Cables forum. Here he is speaking of speaker cables, but it's the same parameter, characteristic impedance shortened to "impedance" when speaking of ICs that are 50, 75, or 110 ohms in their characteristic impedance. I put his significant words in bold italics:

"The second thing to understand is that all speaker cables have a property known as ’Characteristic Impedance’. This is not the resistance or impedance of the cable, rather it is the property of the cable such that it will have its best performance when terminated by a specific impedance. For example, a cable with a characteristic impedance of 8 ohms will perform best when terminated by an 8 ohm load. Its actual DC resistance will likely be quite low as will its AC impedance, however.

Case closed, so far as I am concerned. You need not be concerned about cable (characteristic) impedance. I did not steer you wrong.