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Microphone Month

More on balanced cables and noise

InSync reader David P brings up more issues pertaining to our recent thread on balanced/unbalanced cables and noise (see inSync issues 3/4/99, 3/12/99, 3/23/99, 3/24/99, and 3/25/99).

“It’s my understanding that unbalanced lines such as instrument cables lose about 1 dB of signal for every 18.5 feet of cable. Though many musicians and studio people take this to mean that they shouldn’t run unbalanced cables longer than this, longer runs can be okay in some applications. This depends on other factors, such as noise already introduced by other pieces of equipment. The common misconception among musicians that unbalanced cables add noise and that balanced cables don’t is a myth. Every piece of equipment, including cables, adds noise. Balanced cables just accumulate less noise over the same length than unbalanced cables. So, the rule of thumb is that you add about 1 dB of noise for every 18.5 feet because of the higher resistance of unbalanced cables. What determines an acceptable length is the individual and his/her needs when using longer cable runs.”

Okay, lets be careful here. I think we are jumping to some conclusions. Yes, all cables and all equipment add noise. And yes, balanced lines (“lines”, as opposed to “cables”, implies that the sending and receiving devices are also balanced, not just the cable), as we have discussed, are better at rejecting it. However, unbalanced lines do not inherently have more (electrical) resistance than balanced ones. That unbalanced lines are typically used in high impedance connections and balanced lines are typically used in low impedance situations is wholly incidental, yet does ultimately have an affect on noise. And yes, sometimes the cables themselves have very different impedances, but not always. There are also many varied impedances within the range of “high” and “low” impedance. As for the 1 dB of noise per 18.5 feet, while sometimes true that is a generalization (the .5 feet implies that this figure is not generalized); there are MANY factors that determine how much noise is induced along a length of cable and the variance can sometimes be quite wide. It is valid, however, that the individual’s needs, as well as the circumstances and environment will determine whether or not to run balanced cables.

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