JBL MPA275 - Manual (Page 16)

15
Operation
extended somewhat, and the turn-off muting has been quickened, to ensure muting
even of poorly designed preceding components. On/off muting is equally effective
whether the amplifier is turned off with its own switch or with a remote switch.
Inrush current is limited by an NTC resistor (Negative Temperature Coefficient)
which starts at a high resistance and then diminishes after turn-on to avoid loss of
power. The amplifier’s inrush current is no more severe than amplifiers of one half to
one third its power rating.
DC Fault protection uses a load grounding relay.
Careful attention has been paid to thermal tracking and response. Sensors are
located on each power transformer, as well as each channel’s heat sink, so that
overheating of either component will trigger the thermal defenses. Instead of an
abrupt amplifier shutdown at the temperature limit, we now have 4 levels of
response. At temperatures of 50° to 65° C, the fan speed is elevated from slow to full,
to improve cooling as needed with the minimum possible noise. At temperatures of
about 75° C, the red PROTECT LED begins to flash at a gradually increasing rate. At
85° C, the limiter circuit begins to reduce gain, which is shown by progressively
brighter illumination of the red CLIP LED. At 90° C, gain is reduced by the maximum
amount of 15 dB. If this does not arrest the temperature rise (in case of an output
short or blocked ventilation), the amplifier will finally enter full muting, which causes
full shutdown of the output bias circuit. Thermal tracking can be monitored at the
input slot (see below).
Clipping activates a “Clip-Limit” circuit. This permits the amp to reach the point
of clipping without premature limiting, for exceptional headroom capability.
However, once clipping is reached (for any reason), input volume is then limited to
prevent gross clipping. This circuit limits distortion caused by clipping to about 1%
to10% (depending on degree of overdrive), which is audible but not severe.
An ultrasonic detector mutes the amplifier in case of gross overloads above
20 kHz, to prevent load or amplifier burnout. The amplifier will reproduce high
frequencies into normal loads without muting, but if the amp is turned on with high-
level, high frequency input above 10 kHz, the amplifier will not come out of turn-on
muting (this applies to full-level pink noise as well). This feature is intended to pro-
tect high frequency drivers from accidental burnout due to high frequency system
oscillations, etc.
While not a protection feature in its own right, the remote monitoring capability
built into the input slot provides the foundation for computer control and monitoring
as such systems become available. The input slot has lines to read the status (for
each channel) of power-on, input level, speaker level, thermal level, clip, protect,
and mute status.