JBL MPA275 - Manual (Page 13)

12
Controls
The front-mounted Gain controls have 11 detents for easy matching of levels. The
Gain scale shows dB of attenuation from full gain, with positions for 0 (full), -2, -4,
-6, -8, -10, -12, -14, 18, -24, and
∞
(full off). Attenuation is accurate within 1 dB
(down to -14 dB).
Gain Control Lockout
Provided with each amplifier are two lockout knob caps. Carefully remove the Gain
control knob (A), as shown in Figure 9. Cover the opening by snapping in the lock-
out knob cap (B). The Gain control can no longer be adjusted.
Displays
The LED displays for each channel operate as follows:
LED Label Color Behavior
Power Green Main power supply active on this channel.
Signal Present Green Triggers at -36 dB below full output.
-20 dB Green Triggers at -23 dB.
-10 dB Yellow Triggers at -11 dB.
Clip Red Shows activity of Limiting circuit, which responds to
both clipping and to thermal overload.
Protect Red Flashes as amplifier approaches maximum tempera-
ture. Illuminated steadily plus flashing during thermal
muting. Illuminated steadily during non-thermal
muting.
Typical Power Up, Operation, Power Down Behavior
When the amplifier is first turned on, the green POWER displays will light up for
both channels, and there will be a 3 second turn-on delay, during which all red LEDs
will be bright (PROTECT and CLIP). If a channel will not come out of muting, it may
be responding to an excessive high frequency level at the input (see page 12,
“Protection Circuits”). Turning down the Gain controls (before you power up the amp
is best) allows the muting to release, and is always a good idea to prevent an unex-
pected input signal from overdriving the load.
Signal Level will be shown as the green Signal Present, -20dB and the yellow
-10dB display illuminate with increasing signal level (see Figure 10 on next page).
When the amplifier is turned off, muting should be virtually instantaneous, with
all red LEDs illuminated until the power supply discharges.
Table 3. LED Display Operation
Operation
Figure 13. Gain
Control Lockout
Installation