14
Maximum Long-Term Output Power
In most cases, the desired sound level can be obtained without using the full power
output of the amplifier. The level displays are then used to confirm that both chan-
nels are working as desired.
If the amplifier is operated at extreme power levels, it may overheat or the
speakers may be damaged. The following guidelines will help you determine how
much power can be delivered to the speakers without thermal limiting (which is
indicated by flashing of the red PROTECT display).
8 Ohm Loads
The amplifier can be played at practically any volume without overheating.
However, if the amplifier is pushed into continuous triggering of the CLIP display, the
average output power can reach higher levels than more most speakers will take.
4 Ohm Loads
The amplifier’s maximum long-term power capacity will be reached when the
LEVEL -10 is on almost all the time, with occasional flashing of the CLIP display. If
the CLIP display is on half the time, the amplifier is liable to start indicating thermal
limits are being approached in a few minutes (full brightness flashing of the PRO-
TECT LED).
2 Ohm Loads
To avoid overheating on those models rated at 2 ohms (see Specifications), the
LEVEL -10 display should not be on more than about half the time, with only occa-
sional clipping.
Protection Circuits
The essence of high-efficiency, lighter-weight designs is to control more power with
fewer or smaller load-bearing components. The MPA275, MPA400, MPA600, MPA750
and MPA1100 represent a considerable step forward, with high efficiency due to the
multi-step power supplies. Because of the higher power flow, effective protection
circuits become even more important.
The inputs are resistively buffered for overload and RF protection. Chassis
bypass capacitors are installed at inputs and outputs to further improve RF rejection.
All models have short circuit protection that is based on averaging the output
power. This protection circuit permits full output current into resistive or reactive
2 ohm loads, but if the output is shorted, the current is cut back to a safe level of about
25%.
Turn-on/off muting prevents transients from the amplifier or preceding devices
from reaching the speakers. The MPA275, MPA400, MPA600, MPA750 and MPA1100
use simultaneous relay and electronic muting to guarantee minimum possible on/off
noise. The limiter (see subsequent paragraphs) is fully activated during muting, so
that its release time cushions the onset of full volume. The turn-on delay has been
Operation