Mackie M1200 Owner's Manual - Page 37

the component and the nominal signal level. It is important to maintain a reasonable amount of headroom so that the dynamic transient peaks of the musical program can be reproduced without clipping. 10 dB of headroom is usually adequate, but some program material may require up to 20 dB. 40dB 30dB HEADROOM 20dB 10dB 0dB -10dB -20dB SIGNAL TO NOISE RATIO -30dB -40dB -50dB -60dB -70dB -80dB Dynamic Range DYNAMIC RANGE MAXIMUM OUTPUT LEVEL (M•1200 = 400W/4Ω) NOMINAL LEVEL (25W/4Ω) NOISE FLOOR The best way to accomplish this goal is to optimize the input and output levels for each component in the system. It is best to start at the beginning of the chain (the microphone) and work your way to the end (the speakers). The following procedure details how to optimize a sound system with 10 dB of headroom. A microphone is connected to the mic input on the mixing console. The gain of the mic preamp circuit, sometimes called Mic Trim, should be adjusted so that the loudest microphone signal is just below the overload point of the preamp. Most mixing consoles provide a mic preamp clipping indicator or level metering of some kind to optimize the mic preamp. Next comes the output level of the channel, controlled by a fader or rotary gain control. Faders usually have an indication in the graphics that shows the normal or nominal setting for the fader. This setting is usually 10 or 12 dB below the maximum output level of the channel. This provides the headroom needed to reproduce the transient peaks associated with music. If the channel has a meter, you can use that to visually confirm that the nominal output level of the channel is around "0" on the meter. Next set the master output level (fader) control on the mixer to the nominal level indication next to the control. Again, this should be at least 10 dB below the maximum output level of the mixer. You can double check the actual output signal level if there is an output meter on the mixer. On most professional equipment, this nominal output level will be +4 dBm (1.23V rms into 600 ohms). The output of the mixer may drive a power amplifier directly, or it may go through a signal processor first (i.e., equalizer, compressor limiter, crossover). Determine the maximum output capability of the signal processor. Hopefully, it can produce at least +14 dB to maintain the required 10 dB of headroom. Chances are it can produce considerably more than that (like +20 dB to +24 dB). If 10 dB of headroom is not available, you'll have to introduce a resistive pad between the mixer and the signal processor to reduce the signal level from the mixer so it is at least 10 dB below the maximum output of the processor. If the processor has enough headroom, set the level controls to unity gain, so with a +4 dB input it produces a +4 dB output. Keep in mind that if this is an equalizer, and you've boosted several frequency bands, the nominal output level may be more than +4 dB because of the extra energy the processor is adding to those frequencies. It may be necessary, in that case, to reduce the level controls a few dB below unity. The M•1200/M•1400 amps are designed to accept a nominal +4 dB input signal. Set the GAIN controls fully clockwise. This will provide the best signal-to-noise ratio and available headroom for the amplifier. 37