MACKIE M1400/M1400i - Manual (Page 12)

12
You may wonder why we
didn’t use just one stereo
control to control both sides.
That’s in case your applica-
tion requires a left/right
imbalance (due to an irregu-
larly shaped room) or if you’re using the two
sides for completely different purposes (moni-
tor in channel 1 and side-fill in channel 2, for
instance). Besides, they look cool.
METERS
The M•1400/M•1400i’s meters indicate the
relative output level of the amplifier refer-
enced to full power. The numbers next to the
meter’s LEDs are in dB below full power.
Ideally, the M•1400/M•1400i’s –20, –9,
–6, and –3 LEDs will flicker at normal signal
levels, while the
OL LED may flicker occasion-
ally during peak moments.
OL is short for Overload. Overloading, or
clipping, occurs when the output voltage no
longer linearly follows the input voltage and
simply stops. This causes a sine wave to
“square off,” or get “clipped off.” Thus, the term
clipping. Fear not — this scenario is quite un-
likely. Even with the
GAIN controls fully up,
the M•1400/M•1400i amplifier easily accepts
professional “+4 dBu” operating levels.
If the OL (Overload) LED
is blinking frequently or
continuously, turn down
the source signal (i.e. the
mixer’s master faders).
So what are the pros and cons of these two
approaches? The reason some amplifier
manufacturers use the constant gain ap-
proach is because the noise specification
looks better. It’s a fact of physics that as the
gain of the amplifier increases, the circuit
noise is amplified and increases too. By main-
taining a constant gain, the noise spec for an
800W amplifier can look as good as the noise
spec for a 100W amplifier. The downside to
this is that you have to crank up your mixer
level feeding the input of the amplifier, losing
headroom and possibly increasing the noise
level from the mixer (unless you have a
Mackie mixer with low-noise VLZ circuitry!).
Conversely, constant sensitivity demands
that as the power increases, so must the gain.
Yes, the output noise of the amplifier will
increase, but you maintain the critical head-
room available from your mixer. The addition-
al noise is generally not a problem in live
sound reinforcement situations. If it is, you
can turn down the
GAIN control a few clicks
to find a happy compromise between noise
floor and headroom available. As an added
benefit, you can drive multiple amplifiers with
the same signal and get the maximum power
available from all of them.
Mackie subscribes to the philosophy of
constant sensitivity. Our amplifiers can be
driven to full power with an input level of
+4 dBu (1.23V rms).
OL
–3
–6
–9
–20
–3
–6
–9
–20
SIG
OL
SIG
CH
1
30
28
26
20
22 18
24 16
14
8
0 0
1.23v (+4dBu) SENSITIVITY
GAIN/dB CH
2
PROTECT
COLD HOT
SHORT
TEMP STATUS
INTERNAL STATUS
CH
1
CH
2
CH
1& 2
3v
2v
1v
30
28
26
20
22 18
24 16
14
8
0 0
1.23v (+4dBu) SENSITIVITY
GAIN/dB
3v
2v
1v
FULL SYMMETRY DUAL DIFFERENTIAL HIGH CURRENT DESIGN