Mga kapatiran na gustong makatipid (okey yan) sa paggawa ng XLR (balanced) terminated cable for your equipment and loudspeakers-- beware on proper PIN CONFIGURATION! Or else, you may already have this PHASE SHIFT for your loudspeakers and reverse ABSOLUTE POLARITY for your equipment set-up.
What exactly is a balanced line, and how is it different from a standard RCA cable (unbalanced line) and jack?
In an unbalanced line, the audio signal appears across the center pin of the RCA jack and the shield, or ground wire. Some unbalanced interconnects have two signal conductors and a shield, with the shield not used as a signal conductor. If this unbalanced interconnect happens to be close to fluctuating magnetic field- an AC power cord, for example- the magnetic field will induce a noise signal in the interconnect which is heard as hum and noise reproduced through the system's loudspeakers.
In a professional application, this hum, buzz and noise is unacceptable, leading to the development of an interconnection method that is immune to noise interference: the balanced line. A balanced line has three conductors: two carrying signals, and one ground. The two signals in a balanced line are identical, but 180 degrees out of phase with each other. When the signal in one of the conductors is peak positive, the signal in other conductors is at peak negative. The third conductor is signal ground. Some balanced interconnects use three conductors plus a shield.
Equipment (e.g. power amplifiers) having balanced input/output connection has this DIFFERENTIAL AMPLIFIER circuit internally. When the two identical but opposite polarity signals carried on the balanced line are input to differential amplifier, noise picked up by the interconnect is rejected. Here's why: a differential amplifier amplifies only the DIFFERENCE between the two signals. If noise is introduced into the line, the noise will be common to both conductors and the differential amplifier will reject the noise. This phenomenon of rejecting noise signals common to both conductors in a balanced line is called COMMON-MODE REJECTION.
Note that a balanced line won't make a noisy signal clean; it just prevents additional noise from being introduced in the interconnect. If the noise is common to both halves of the balanced line, common-mode rejection will eliminate the noise.
And now, the caution section; in balanced line terminated with XLR connectors, pin 1 is always signal ground. There is however, no convention for which of the two signal conductors carries the non-inverted signal and which carries the inverted signal. The non-inverted conductor is often called the "HOT" conductor. With the inverted conductor designated "COLD". Having decades of no clear standard, it was later adopted by the industry of having pin 2 carry the non-inverted (" hot") signal, pin 3 the inverted ("cold") signal.
How the balanced line is wired (pin 2 or pin 3 "hot") can determine if your system is inverting or non-inverting of absolute polarity. When DIY'ng your balanced cable from RAON, 5th Avenue, Audio Studio-Megamall, Lyn-Ann Electronics, even if its free connection, for your processors, preamps, power amplifiers or loudspeakers-- you should know if your equipment's XLR jacks are wired the same-- either pin 2 or pin 3 "hot"-- as the existing equipments or loudspeakers. The freak interconnect may wind up in another system or application where you don't want the polarity switched and worst, expect phase reversal for your loudspeakers.
Direk
