Mackie 408M Owner's Manual - Page 30

TS phone plug. The balanced-to-unbalanced connection has been anticipated in the wiring of Mackie jacks. A 1/4" TS plug inserted into a 1/4" TRS balanced input, for example, automatically unbalances the input and makes all the right connections. Conversely, a 1/4" TRS plug inserted into a 1/4" unbalanced input automatically ties the ring (low or cold) to ground (earth). 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 be connected only 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. Use balanced lines if at all possible. They provide better immunity to induced noise and ground loops. Remember that you can balance a line by inserting in-line a piece of equipment that has a balanced output. 2. Avoid using three-phase power lines since they are usually used for air conditioning and other heavy power equipment. If using 240VAC single phase with center-ground power, connect all audio equipment to one side of the AC power, and all lighting and other equipment to the other side. 3. Be sure all AC outlet safety grounds are connected to one common point in a star ground arrangement. This common ground point should then tie back to earth ground at the service entrance by one heavy stranded wire, #2 gauge or larger. 4. Don't cut the third pin off the power cord. Carry some ground-lifter adapters and use them only if you have to plug into an ancient two-wire outlet. 5. Cables that are too long are less likely to pick up hum if you uncoil them in their entirety, and then find a place to stow the excess. Leaving the excess coiled only helps the cable pick up hum more efficiently. 6. If you bundle your cables together, don't bundle AC wiring and audio wiring together. Bundle them separately. 7. If your sound system insists on humming, you may want to teach it the words. 30