Signal Loss Occurs in different ways. Signals cables loss before amplification can occur faster than loss on speaker cables after amplification. However there are factors that can change the equation one way or another, frequencies, type of conductors, level of processed signal (Pre amps), amplification of signal, gauge of wire etc. For regular DJ type systems Signal cables (Before amplification) of up to 30 feet is negligible. For Speaker cables, up to 100' is hardly noticeable.
I hate going back this far...but this thread popped to the top of the new posts list.
The above information is mostly wrong.
On the pre-amp side of the signal chain, there are low voltage signals. The cables can be "balanced (three conductor, or two conductor plus shield)" or "unbalanced (two conductor)". In general terms, xlr connectors (e.g. mic cables) are balanced. In general, 1/4" TS connectors, 1/8" stereo connectors, and RCA connectors are unbalanced. 1/4" TRS could be two unbalanced signals (e.g. stereo L & R) or one balanced signal...there is no good way to tell in the general case. See the Rane tech note for more.
http://www.rane.com/note110.html
Unbalanced signals are prone to picking up outside noise and outside electrical interference. The longer the cable, the more likely noise it to be introduced. In general terms, noise from the cable can be held to reasonable amounts in 10ft or shorter cables. I use unbalanced patch cables to connect music sources (e.g CD players, computer sound cards) to the mixer...and all these cables are as short as practical. Signal loss is not really a concern, when compared to the noise potential.
Balanced signals are much more resistant to noise and outside electrical interference. With balanced signal cables, it is possible to send a signal through hundreds of feet of cable with no significant signal loss or noise introduction. My personal best is 800ft. I was able to measure a 0.01v (the limit of my volt meter) loss across that distance. From the mixer to the amps, all my signal cables are balanced xlr.
After the amp, the signal is a high(er) voltage. This connection is two "unbalanced" two conductor. Here, resistance losses in the cable length become a concern, along with inductance in particularly long cables. In general terms, to keep the transmission losses to a reasonable level, the cable resistance should be <5% of the load. For an 8ohm speaker, that means the wire should have less than 0.4ohms of resistance. For a 2 ohm load (e.g. four 8 ohm speakers in parallel), the wire should have less than 0.1ohm of resistance. Using that rule of thumb, you can calculate the maximum length of a speaker cable based on the load and the wire gauge. There are many helpful websites with pre-done calculators.
In general terms, a 12 ga cable, at 50ft length (100ft of conductor, round trip) is the longest speaker cable that can be safely used with a 2 ohm speaker load. If you keep the load higher, or the cable shorter, so much the better. To be conservative, all my speaker cables are 12ga, and less than 25ft long. I place the amps near the speakers and run balanced signal cables from the mixer to the amps.