Mackie M1200 Owner's Manual - Page 39

APPENDIX E: Technical Info DO THE MATH: OHMS, LOADS AND SUCH Remember: As the load gets "heavier," its value in ohms goes down. For instance, a 2-ohm speaker load is twice as "heavy" as a 4-ohm load. An ohm is a unit of resistance - the more ohms, the more resistance (impedance). The more the resistance, the less the power. It can all seem backwards at first. Just remember that a dead short means no resistance at all, or zero ohms. Since you're in the biz (or you are now, since you just bought an amp), you probably own a volt/ohm meter (or DVM, for Digital Volt Meter). It's an indispensable tool for anyone working with speakers and such. If you don't own a meter, go out and get one right now - we'll wait. If you're just dealing with one speaker (or cabinet) per output, the load in ohms will be printed on it somewhere. That's your load. You can confirm this with the volt/ohm meter you just bought - set it for ohms, set it for the lowest range (unless it's an autoranging meter) and measure across the speaker terminals. It may not agree exactly; a speaker rated at eight ohms may read between 5 and 7 ohms. (If it's a multiple-driver speaker with a built-in passive crossover, this method won't work.) If you're driving an assortment of speakers (or cabinets), things can get complicated. There are two basic ways of linking multiple loads (speakers in this case): series and parallel. "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 third, and so on, until the chain ends at 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. Doing load calculations with series configurations is easy - just add the loads. For instance, four 8-ohm speakers, connected in series, will equal 32 ohms (8 + 8 + 8 + 8 = 32). "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 fails 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 instance, four 8-ohm speakers, connected in parallel, will equal 2 ohms (8 / 4 = 2). If the loads aren't all the same, the formula gets a little more complicated, but nothing that you can't do with a simple calculator. ZT = 1+ Z1 1 1+ Z2 1 Z3 + . . . There are other, more complicated configu- rations, 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 M•1200/M•1400 Power Amplifier: • As a load gets "heavier," its impedance in ohms decreases. • The lower the impedance (ohms), the higher the power: The M•1200 has 600 watts (per side) with a 2-ohm load, 400 watts with 4 ohms, and 225 watts with 8 ohms. • Do not connect a load of under 2 ohms (in STEREO and MONO mode) or 4 ohms (in BRIDGE mode). • Never plug amplifier outputs into anything except speakers (unless you have an outboard box designed to accept speaker levels). • Never play good music through bad speak- ers. You may, however, play bad music through good speakers (but only on odd-numbered Fridays). 39