At what distance are balanced XLR cables needed?

"The 99.99999% figure quoted for purity is outright preposterous"

Nordost uses 99.999999% pure copper.

"BTW this is why you don't see copper wire that is Teflon insulated."

I just look at my Satori speaker cables that are 6N copper in Teflon. You can get many Nordost cables that are 8N copper (silver plated) in Teflon insulation as well as Neotech hookup wire that are 7N continuous cast copper in teflon (no plating). Many Audioquest cables (like Colorado or Gibraltar) are 6N copper in Teflon.

I know about microphone standard but am unfamiliar with preamps designed to have 600ohm output impedance to drive 600ohm load.

"Actually if you work the math the opposite is true- with no current at all then the minor resistances, odd diode effects and the like will become more prominent."

I don't see why - where there is no current resistance is unimportant. 1ohm total (of all impurities) resistance with 40k load impedance divides voltage by 1/40001 but with 600ohm load it is 1/601 divider. If there is any diode effect then junction voltage increases with the current.

Kijanki... I have a nice bridge in Brooklyn. Would you be interested?

Atmasphere... Question... 600 ohms is a practical low output impedance for tube circuits. (Some are 1000 ohms or more). But solid state circuits can be much lower eg: 50 ohms is common. Is the 600 ohm value a maximum?

Eldartford, FWIW our preamp has an output impedance much lower than 600 ohms. A lot of tube preamps do as well. The trick is: what is their output impedance at 20 Hz or 5Hz. With most tube preamps the output impedance at these frequencies will be much higher than stated. Its another way of saying that if you have a 600 ohm load, these preamps will no longer play bass right.

The 600 ohm value comes from the fact that spaced conductors reach a maximum characteristic impedance of 600 ohms in free air. The reason this was adopted is due to the phone companies: before balanced line existed, there was no such thing as true long distance calling! Transcontinental and intercontinental calls became possible due to the increased resolution of balanced operation.

Even if the cable is only a meter long this increase in resolution is audible.

So the 600 ohm value is a practical maximum. Microphone often run much lower- my Neumann U67s run at 150 ohms.

These days any professional or semi-pro audio gear for music or recording has to support this standard. It is only in high end audio where there is still mystery surrounding this subject. And, not surprisingly, it is something that cable manufacturers, for the most part, would rather you not know. I see their business as safe, however, since there are still single-ended cables to be made, as well as speaker and power cables. And as long as the high end industry remains ignorant regarding the standard, there will be a market for 'exotic' balanced cables too.

Atmasphere... So the 100 ohm output, balanced, from my Behringer DEQ2496 is OK. Whew!!

Your preamps are no doubt better than the average tube gear. 600 ohms is what I always saw with the mid fi stuff I used.