At what distance are balanced XLR cables needed?

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.

Output impedance is not that important within certain limits. My Benchmark DAC1 has output impedance values from 60 to 1600ohm dependent on output divider jumper position. Benchmark recommends for 1600ohm output maximum cable length of 26ft - assuming typical 32pF/ft and 0.1dB drop at 20kHz. With my cable (6pF/ft) I could go 139ft.

Kijanki, with the 1600 ohm parameter selected, it assumes a high impedance at the other end as there would be distortion if it were actually driving a load of a lower impedance.

You will hear big differences in cables with it set up this way.

As a general rule of thumb, the output impedance should be about 1/10th of whatever it is driving. So the 60 ohm position *should* be able to drive a 600 ohm load just fine. And FWIW, many solid state preamps can do that. Its the tube units where this is a challenge.

So if you set the unit to 60 ohms and then place a 600 ohm resistor between pins 2 and 3 at the input of the amplifier, the result is that you will not be able to hear differences between interconnects, regardless of cost.

I have a McIntosh C2300 preamp feeding a McIntosh MC402. They are 16 feet apart. I have used both Balanced XLR connections and RCA single ended connections. The balanced has sounded better every time. By the way the RCA Single ended cables were supposed to be more highly regarded and definitely way more expensive.

So, to answer your question, I found 16 feet the distance in my system.

I would suggest using the very best cable and connectors available, and if that means using XLR's, by all means use them.