Balanced cables and transmission loss

[jaguarsg]

i like to save money by making my own mogami cables. how long can i make them without losing an audio quality?

[Phiz]

I've seen people on here argue that you should shoot for 20 feet for a microphone cable. However lots of people, especially in live sound work, often route mic level signals for hundreds of feet. Give the impedances of modern equipment, the worst you will experience by going to long is a very slight roll off of the highs.

Here are some rough worst case numbers. Assume we can treat this as a simple RC series low pass filter. -3dB point is f=1/(2*pi*R*C). Assume a 600ohm microphone will contribute the majority of the resistance, thus we don't need to worry about the resistance of the cable. C for Canare quad cable is 220pF/m (this is a high capacitance cable, and thus a good worst-case). Solve for f=20kHz

length= 1/(2*pi*R*c*f), where c is cap/m, so C=c*length
length=60.3m, or 197 feet.

So at 197 feet, with a quad cable, you would have a slight but probably noticeable attenuation of your highs.

[nclayton]

Not to mention most mics' actual source impedances are pretty significantly lower than 600 ohms, particularly condensers that might have source impedances in the single or low double digits. i don't know who's recomending 20' mic cables, but that seems a little crazy. Maybe 20 feet is a good upper limit for guitar cables, but for low impedance mics the rule is pretty much just to make the cable as long as you need to in order to get from the mic to the mixer. If you can hear the difference between a 20 and 50 foot mic cable it's probably due to N rays.

Ned

[dayvel]

Judging by what I read on audio boards, we're experiencing a serious spike in N ray activity.