Mackie SR244 / SR324 Owner's Manual - Page 57

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 visiting. 3. Doensurethatyourspeakerlinesarephysically 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% (0.5dB 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 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 6. Ensure that the electrician uses the starground 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. 7. Ensure that the AC power feeds are connected to the same transformer, and ideally, the same circuit breaker. 8. Walk outside - look in the horizon, see any radio towers? Locate potential sources of RF interference and plan for them before you begin construction. Know what frequency, transmitter power, etc. You can get this information by calling the station. Remember that many broadcast stations change antenna coverage pattern and transmitter power at night. 9. Don't use hardware-store light dimmers. 10. Don't allow for anything other than microphone inputs at stage/altar locations. Supplying line inputs at these locations is an invitation for misuse. Make all sources look like microphones to the console. 11. Balance (or at least impedance balance) all connections that are remote from the console's immediate location. 12. If you bridge an amplifier, don't use 1/4-inch phone plugs for speaker connectors. Grounding Grounding exists in your audio system for two reasons: product safety and noise reduction. The third wire on the power cord exists for product safety. It provides a low-resistance path back to the electrical service to protect the users of the product from electrical shock. Hopefully, the resistance to ground through the safety ground (third wire) is lower than that through the user/ operator to ground. If you remove this connection (by breaking or cutting the pin off, or by using a 'ground cheater'), this alternate ground path ceases to exist, which is a safety hazard. The metal chassis of the product, the ground connections provided by the various connectors, and the shields within your connecting cables provide a low potential point for noise signals. The goal is to provide a lower impedance path to ground for noise signals than through the signal wiring. Doing so helps minimize hum, buzz, and other extraneous non-audio signals. Many "authorities" tell you that shields should only be connected at one end. Sometimes this can be true, but for most (99%) audio systems, it is unnecessary. If you do everything else correctly, you should be able to connect every component of your audio system using standard, off-the-shelf connecting cables that are available at any music store. Here are some guidelines: 1. Allreturnlinestothestageshouldbebalanced. At a minimum, they should be impedance balanced. Remember that you can balance a line by inserting a piece of equipment inline that has a balanced output. 2. Run your own AC power wiring from the stage for the mixer and related equipment. Don't use the "conveniently located" receptacle thoughtfully provided by the management for your use. You have no idea how it's wired or grounded. 55 Appendices