Mackie M1400/M1400i Owner's Manual - Page 11

FEATURE DESCRIPTIONS GAIN These giant knobs control the levels to the output section of the M•1400/M•1400i amplifiers. You'll notice that their travel is detented, meaning there are 40 built-in "resting points" so you can easily set both controls to the same level. Usually, these controls are set all the way up. The gain structure of the amplifier is designed so that a +4 dBu (1.23V rms) input signal drives the amplifier to 425 watts into 4 ohms (41.23V rms @ 4 ohms = 425 watts). This is how the sensitivity of an amplifier is defined. In this case, it equates to a voltage gain of about 30 dB (okay, 30.51 dB for those of you sittin' there with your calculator!). The graphics around the knob depict two different methods for setting the gain. The inner circle is marked in dB, calibrated from off (∞) to 30. This represents the amount of voltage gain from input to output. When using professional equipment with +4 dBu output levels, set the GAIN control all the way up to 30. The outer circle is labeled in volts, with indications of 1V, 2V, and 3V. These correspond to the input sensitivity of the amplifier. With the GAIN control all the way up (fully clockwise), the input sensitivity is 1.23V, which works well with professional equipment operating at a nominal +4 dBu level. On the other hand, you may want your listening level to be quieter than the M•1400/ M•1400i's maximum level. For instance, if you're using the M•1400/M•1400i as a control room amp, and your control room is the size of a telephone booth, you'll probably never want to hear the amp at its maximum level. You can set the GAIN controls as low as you like. However, reducing the GAIN controls requires an increased input level to reach full power at the amplifier's output. See the sidebar "Constant Gain vs. Constant Sensitivity" for a better understanding of how this works. Like all amplifier controls, you'll typically determine the optimal settings during installation or sound check, then leave them alone, using your signal source (usually a mixer) to control listening levels as you work. Or play. Constant Gain vs. Constant Sensitivity There are two viewpoints, or philosophies, regarding the gain structure of power amplifiers - constant gain and constant sensitivity. Constant gain means that regardless of the output power of the amplifier, the gain from input to output remains the same. (By the way, this refers to the full gain of the amplifier, with the gain or level controls all the way up.) Within a product line of constant gain power amplifiers, as the output power rating of an amplifier increases, the level of the input voltage must also increase. For example, if an amplifier is rated at 100W into an 8-ohm load, and it has 26 dB of gain, it requires an input signal of 1.4V rms to drive it to full power. This is about +5 dBu, a reasonable operating point for professional gear. Now take an amplifier rated at 200W into an 8-ohm load. If it also has a gain of 26 dB, it requires an input signal of 2.0V rms to drive it to full power, or +8 dBu. This can become problematic as the power of the amplifier increases. What if you have a power amp rated at 800W into 8 ohms? This will require an input signal of 4.0V rms to drive it to full power. This equates to a whopping +14.3 dBu!! You've just robbed your mixer of 10 dB of headroom. You'll either have to have a good limiter to keep the transient peaks down, or turn down the level from the mixer and not use all the power available from the amplifier. Constant sensitivity means that regardless of the output power of the amplifier, the input sensitivity of the amplifier (the input voltage required to attain full output power) remains the same. As the output power of an amplifier increases, the gain of the amplifier must also increase. Referring back to the previous example, an amplifier rated at 100W into 8 ohms with a gain of 26 dB requires an input signal of 1.4V rms to drive it to full power. It has an input sensitivity of 1.4V rms. In order for the 200W amplifier to reach full power into 8 ohms with a 1.4V rms input signal, it must have a gain of 29 dB. And the 800W amplifier will require a gain of 35 dB to reach full power with a 1.4V input signal. Continued on page 12 11