APPENDIX C: Arcane Mysteries Illuminated

BALANCED LINES

Balanced lines offer increased immunity

to external noise (specifically, hum and buzz). Because a balanced system is able to minimize noise, it is the preferred interconnect method, especially in cases where very long lengths of cable are being used. A long unbalanced cable carries with it more opportunity for noise to get into a system — having balanced inputs means very little noise will enter the system via snakes and other cables that typically must run a long length.

A balanced line is a three- wire system where two wires carry the signal and the third is a ground wire that shields the inner conductors from

EMI. The two inner conduc-

tors carry the same signal, but with opposite polarity. The balanced input amplifies only the difference between the two conductors. But a signal that is common to both conductors, and in phase, is rejected (canceled out) at the balanced input. This includes hum from AC lines or other EMI induced noise.

An unbalanced line does not have this noise-rejecting capability because it has only two conductors. One conductor carries the signal and the other is a ground wire that shields the inner conductor. Any hum or EMI noise that gets through the shield is added to the signal and amplified at the unbalanced input. Often the hum can be louder than the signal itself!

FIXED INSTALLATIONS

If you install sound systems into fixed instal- lations, there are a number of things that you can do to make your life easier and increase the likelihood of the sound system operating in a predictable manner. Even if you don’t do fixed installations, these are good practices for any sound system.

1.Do use foil-shielded snake cable for long cable runs. Carefully terminate each end, minimizing the amount of shielding removed. Protect the exposed foil shield with shrink sleeving or PVC sleeving. Prevent adjacent shields from contacting each other (electrically). Use insulating sleeving on the drain wire (the one that

connects to pin 1) to prevent it from contacting the connector shell.

2.Don’t connect the XLR connector shell to pin 1 of the XLR connector unless necessary for RFI shielding. Doing so is an invitation for a ground loop to come a-courting.

3.Do ensure that your speaker lines and AC power lines are physically separated from your microphone lines.

4.If you use floor pockets, use separate pockets for inputs and speakers, or put the connectors on opposite sides of the box so that they may be shielded separately.

5.If your speaker lines run in the open, they should be twisted pairs, at least 6 twists per foot. Otherwise, run the speaker lines in their own conduit. (Of course, conduit is not too practical for portable systems, heh-heh.)

6.Minimize the distance between the power amplifiers and the speakers.

7.Use heavy gauge, stranded wire for speaker lines. Ideally, the wire resistance should be less than 6% (0.5 dB power loss) of the load impedance. Remember that the actual run is twice as long as the physical length of the run. See below.

Maximum wire run for 0.5dB power loss in feet
wire
res. per

2

4

8

gauge

1000 ft.

Ω

Ω

Ω

10

1.00

60

120

240

12

1.59

40

75

150

14

2.5

24

48

95

16

4.02

15

30

60

 

 

 

 

 

8.Ensure that the electrician uses the star- ground system for the safety grounds in your electrical system. All of the audio system grounds should terminate at the same physical point. No other grounds may come in contact with this ground system.

9.Ensure that the AC power feeds are connected to the same transformer, and ideally, the same circuit breaker.

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