Mackie FR800/FR1400/FR2500 FR Series Owner's Manual - Page 15

Owner's Manual conductors and smaller Metric WG numbers = smaller conductors. The Metric WG is equal to ten times the nominal conductor diameter in millimeters. Longer Lengths For cable lengths over 200 feet / 60 m at 8 ohms, and over 100 feet / 30 m at 4 ohms, the conductor sizes needed for less than 0.5 dB power losses are rarely practical for physical and cost reasons. As a practical compromise for these situations the recommended conductor gauge is 10 AWG or 25 metric. Speaker Impedances A speaker's impedance varies with frequency. For example, it may be 4 ohms at 500Hz, and 6 ohms at 120Hz. What you need to know is the average (or nominal) impedance across the speaker's frequency range. This will be printed somewhere on the cabinet, or in the specification section of the missing manual. If you're just dealing with one speaker per channel, then make sure that the average impedance is greater than or equal to 2 ohms. In BRIDGE MONO mode, make sure that your speaker impedance is greater than or equal to 4 ohms. If you're driving an assortment of speakers, you have to make sure that the total impedance does not go below these same levels. There are two basic ways of linking multiple speakers: series and parallel. The following sections show how to make the connections and how to work out the total impedance. SERIES "Series" means that the positive amp output connects to the first speaker's positive terminal, the first speaker's negative terminal connects to the second speaker's positive terminal, the second speaker's negative terminal goes to the amp's negative output. Series connections are not normally used in PA applications because it ruins the amplifier's ability to damp (control) the speakers. The other snag: if one speaker goes out, they all do. Doing load calculations with series configurations is easy - just add the loads. For instance, two 4-ohm speakers in series, equals 8 ohms (4 + 4 = 8). PARALLEL "Parallel" means that the positive amp output connects to the positive terminals of all the speakers, and the negative amp output connects to the negative terminals of all the speakers. If one speaker opens in a parallel configuration, the others will still work, but the load will change. That lets you breathe a little easier (the show will go on), except that you may have a dead speaker and not even know it. Calculating parallel loads is also easy, as long as each speaker has the same value - just divide the value by the number of speakers. For example, four 8-ohm speakers, connected in parallel, will equal 2 ohms (8 / 4 = 2). If the parallel loads aren't all the same, things gets a little more complicated, but nothing that you can't do with a simple calculator. The total impedance (ZT) is given by the following formula, where Z1, Z2 and Z3 are the impedances of your speakers. ZT = 1+ Z1 1 1+ Z2 1 Z3 + . . . There are other, more complicated configurations, like series-parallel (using a combination of series and parallel links to arrive at a desired load) and parallel configurations of unmatched loads (usually not recommended). But rather than get too deep into this, let's just summarize the basics, as they apply to you and your amp: • The lower the speaker impedance, the more power can be put out by the amplifier. • Driving lower impedance speakers makes the amplifier work harder and heat up quicker. • Do not connect a total impedance of under 2 ohms per channel in STEREO and MONO modes. • Do not connect a total impedance of under 4 ohms in BRIDGE mode. • Connecting speakers in series or parallel can drastically alter their frequency response. • Consider using multiple amplifiers rather than overloading one. • Reduce the low-frequency output by setting the LOW CUT FILTER to match the speaker's specifications. • Never plug the amplifier outputs into anything except speakers (unless you have an outboard box designed to accept speaker level levels). • Be careful in BRIDGE mode as both speaker wires are live. In this mode, do not connect the speaker wires to any external device which is grounded. Owner's Manual 15