L-ACOUSTICS LA 24 MANUAL Version 2.2
17/11/08 7
Once the AC connector is connected to a suitable AC supply, the amplifier can be started by
pressing the AC power switch. When you power up the amplifier it takes a couple of seconds
to check its circuits (this is known as the "soft start" or "slow start" sequence), the fans then
blow at high speed before going into "idle" and the 2 bottom green LED’s come on to show
the output circuits are receiving the correct rail voltage.
Grounding
There is no ground lift switch or terminal on this amplifier. The signal ground is always floating
via a resistor to chassis and the grounding system is automatic. If a potential above 0.6 V
presents itself between signal ground and chassis ground, a short circuit is introduced between
the two, thereby enabling electrical protection. If a unit in the system is faulty, its mains fuse
will blow due to this automatic ground system.
If you want to tie the signal ground to chassis, connect the XLR-connector’s shell lug to pin 1.
In the interest of safety never disconnect the earth pin on the AC cord.
For all units that are EMC approved for radio interference there is an AC mains filter. This
filter needs the chassis ground for reference, otherwise a current loop is formed via the signal
ground.
Use the balanced input to avoid hum and interference.
Power consumption
There are three ways to determine the power/current consumption of the amplifier:
First, the peak current draw at full output power. Under this condition the power will
trip the wall breaker within 30 seconds and the amplifier will operate for less than 2 minutes
before thermally limiting. During this time, the temperature of the power supply will be
stabilized at a temperature that will have no effect on the insulation rating of the AC line cord.
Secondly, the maximum expected average current under worst case program material is
1/3 of full power according to the FTC standard. At this level the music will be in a state of
constant clip and this is therefore the highest power level one can obtain without completely
obliterating the program.
Finally, the "normal operating power", as measured according to safety standard IEC 65
and used by a majority of safety agencies. The normal operating power is measured using pink
noise, with an average output power equal to 1/8 of full power. One eighth of the total
power is as loud as you can play music while making some attempt to avoid obvious clipping.
It also corresponds to a headroom of 9 dB, which is very low for audio program.
For 2 ohm operation, the AFS-protection of the amplifier circuit will not permit long term
current draw and the component temperature rise will stabilize well below the rating.
