Feature Descriptions; Gain - Mackie M-2600 Owner's Manual

FEATURE DESCRIPTIONS

GAIN

These front panel knobs allow you to control
the levels going into the output section of the
M•2600 amplifier. Their travel is detented,
meaning there are 20 built-in "resting points" so
you can easily set both controls to the same level.
Usually, these controls are set to maximum.
The gain structure of the amplifier is designed
so that a +4 dBu (1.23V rms) input signal drives
the amplifier to 700 watts into 4 ohms. This is
how the sensitivity of an amplifier is defined.
In this case, it equates to a voltage gain of 32.7
dB. The graphics around the knob depict two
different methods for setting the gain. The in-
ner circle is marked in dB, calibrated from off
(∞) to 33. This represents the amount of voltage
gain from input to output. When using profes-
sional equipment with +4 dBu output levels,
set the GAIN control all the way up to 33.
The outer circle is labeled in volts, with in-
dications 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 listen-
ing level to be quieter than the M•2600's
maximum level. For instance, if you're using
the M•2600 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 Sensi-
tivity" for a better understanding of how this
works.
Like all amplifier controls, you'll typically
determine the optimal settings during installa-
tion 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 re-
quires 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.
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