Mackie M1200 Owner's Manual - Page 25

The thickness of wire is rated in gauges. Use the chart below to determine the correct gauge of wire to use according to the distance between the speakers and the amplifier, and the impedance of the load the amplifier is driving. This ensures that the power lost across the speaker wire is less than 0.5 dB. Wire Length Up to 25 ft. Up to 40 ft. Up to 60 ft. Up to 100 ft. Up to 150 ft. Up to 250 ft. Load Impedance 2Ω 4Ω 8Ω 2Ω 4Ω 8Ω 2Ω 4Ω 8Ω 2Ω 4Ω 8Ω 2Ω 4Ω 8Ω 2Ω 4Ω 8Ω Gauge of Wire 14 gauge 16 gauge 18 gauge 12 gauge 14 gauge 18 gauge 10 gauge 12 gauge 16 gauge 8 gauge 10 gauge 14 gauge 6 gauge 8 gauge 12 gauge 4 gauge 6 gauge 10 gauge 70V DISTRIBUTION SYSTEMS A distributed sound system uses a constantvoltage, high-impedance network that feeds a number of tapped transformers which, in turn, deliver power to individual speakers. Each tap is rated in watts, so you can select the amount of power delivered to the speaker. Developed for distributed paging and public address systems, one benefit of such a system is that it eliminates complicated impedance calcula- FR SERIES POWER AMPLIFIER IN BRIDGE MODE + CH 1 - - CH 2 + RC NETWORK +C1 C2+ R1 tions when setting up a multi-speaker system. You just add up the total wattages of all the speakers in the system and make sure that it doesn't exceed the total power rating of the amplifier (allowing at least 10% for insertion losses). Another benefit is that by using high voltage and low current, losses in the speaker distribution wiring are kept to a minimum. Standard voltage levels include 25V, 70V, and 100V, but 70V systems are most commonly used in commercial sound distribution systems in the U.S. Because of the high power capability of the M•1200/M•1400 amplifiers, they can be used to directly drive 70V constant-voltage distribution systems without the use of a step-up transformer. When the M•1200 amplifier is operated in BRIDGE mode, it can produce 800 watts into 8 ohms, or 80V. This is slightly higher than the standard 70.7V for which the system was designed. You can recalculate the actual power delivered to each tap by multiplying the tap's rated wattage by a correction factor (K). The correction factor is P1/P2, where P1 is the power delivered by the amplifier into 8 ohms (BRIDGE mode), and P2 is the power delivered by 70.7V into 8 ohms (625W). M•1200: K = 800W/625W = 1.28. Thus, a 2.5W tap becomes 3.2W, a 5W tap becomes 6.4W and a 10W tap becomes 12.8W. M•1400: K = 850W/625W = 1.32. Thus, a 2.5W tap becomes 3.3W, a 5W tap becomes 6.6W and a 10 W tap becomes 13.2W. CAUTION: A characteristic of tapped transformers is that they saturate at very low frequencies, which causes their impedance to decrease, approaching the DC resistance of the copper wire. This can result in overloading the amplifier if the signal contains lots of low frequencies. When using an FR Series amplifier in a 70V distribution system, set the LOW CUT FILTER to 100Hz or higher. In addition, install an RC network at the SPEAKER OUTPUT of the amplifier, as shown in the figure below. 70V LINE RC NETWORK VALUES C1 = C2 = 660µF @ 250VDC R1 = 4Ω @ 100W CONSTANT VOLTAGE TRANSFORMER ALTERNATE RC NETWORK Note: You can substitute 2.5W 5W 10W 2.5W 5W 10W 2.5W 5W 10W a single capacitor for C1/C2. C3 C3 = 330µF @ 250VDC, POWER TAP NON-POLARIZED. SWITCH R1 + + + 70V Constant Voltage Distribution System - - - 25