Mackie 408M Owner's Manual - Page 26
GENERAL PRECAUTIONS AND CONSIDERATIONS, Thermal Considerations, AC Power Considerations
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GENERAL PRECAUTIONS AND CONSIDERATIONS Thermal Considerations The PPM Series Powered Mixers contain two powerful built-in amplifiers. All amplifiers produce heat, and the more power it produces, the more heat is produced. It is important to dissipate the heat away from an amplifier as quickly as possible. This results in increased reliability and longevity for the amplifier. That's what the massive die-cast aluminum heatsink on the rear panel is for. Heat from the output transistors is coupled to the heatsink, which draws the heat away from the transistors and out to the heatsink fins. Cool air is drawn through the fins due to a process called convection, warming the air and taking the heat away. In order for this convection cooling to work efficiently, it is important to provide adequate airspace behind the mixer. We recommend leaving at least six inches of air space behind the mixer when you set it up. In the unlikely event of the amplifier overheating, a thermal switch activates and shuts down the mixer until it cools off. Double-check the speaker load on each amplifier and make sure that the total impedance of each load is 2 ohms or greater. If the load impedance drops below 2 ohms, it can cause the amplifier to overheat. If the load impedance is okay, overheating may be caused by excessive ambient temperature (the room's too hot!). Try using a fan aimed at the heatsink to move air through the fins faster. AC Power Considerations Be sure the PPM Series Powered Mixer is plugged into an outlet that is able to supply the correct voltage specified for your model. If the voltage should drop below 97% of the specified line voltage, the built-in amplifiers will no longer be able to supply rated power. (They will continue to operate down to 75% of the rated voltage, but won't reach full power, resulting in lower headroom.) Under typical conditions, reproducing music where musical peaks are just below clipping, the powered mixers draw the following average currents: Average Current Amplifier Loading 406M, 408M, 408S 808M, 808S 8Ω per side 2.0 A 2.2 A 4Ω per side 3.1 A 3.3 A 2Ω per side 4.8 A 5.0 A (Above current based on using 120V AC line) NEVER bypass the AC plug's ground pin. This is dangerous! It is recommended that a stiff (robust) supply of AC power be used because the amplifiers place high momentary current demands on the AC line. The more power that is available on the line, the louder the amplifiers will play and the more peak output power will be available for cleaner, punchier bass. A suspected problem of "poor bass performance" is often caused by a weak AC supply to the amplifiers. AC Power Distribution The majority of AC outlets encountered in homes and clubs (in the U.S.) are served by a 240VAC center-tapped service entrance transformer. This provides two phases of AC power on either side of the center tap, at 120V each. If lighting is used in a show, it is preferable to power the lights from one leg of the service, and power the audio equipment from the other leg. This will help minimize noise from the lights coupling into the audio (particularly if SCRs, or light-dimmer switches, are used). In order to minimize ground loops, the safety grounds for all the outlets should be connected to a common ("star") grounding point, and the distance between the outlets and the common grounding point should be as short as possible. When setting up for a show, oftentimes you are plugging into an AC power distribution system you know nothing about. You may even be faced with 2-wire outlets that are missing the third safety ground pin. It's a good idea to have a three-wire AC outlet tester in your toolbox so you can check the outlets yourself to make sure they are wired correctly. These testers will tell you if the polarity of the hot and neutral wires is reversed and if the safety ground is disconnected. Don't use an outlet if it is wired improperly! This is to protect yourself as well as your equipment. If you find that you must plug into a two-wire outlet, you will need to use a two-wire to threewire adapter (cheater plug). These come with a metal tab that you put underneath the center screw that holds the AC outlet faceplate in place. This center screw must be grounded. You can check it by connecting the adapter to the outlet and then plugging in your handy-dandy AC outlet tester. 26