Power Limiter - QSC PL218A User Manual

FEATURES & SETUP-
POWER LIMITER: Each channel has its own limiter with independently adjustable threshold and time con-
stants. A ten-section switch for each channel controls the limiter settings.
LIMITER ACTIVITY LED's: Just below the CH 1 and CH 2 labels you will note a clear circle which has a yellow
LED behind it. Each LED illuminates only when its limiter is reducing gain. Therefore, the LED activity tells you
how hard the limiter is working and shows the limiter time constant (see SPEED).
POWER LIMITER ENABLE: Set this switch to ON to activate the limiter for each channel. NOTE: The Limiter
Activity LED does not illuminate until the output power reaches the limiter threshold.
POWER LIMITER MODE:
AVG mode tracks the program level with moderate attack and release rates (SPEED is set
below). This setting does not respond strongly to isolated peaks and therefore gives the
smoothest results.
PEAK mode causes limiting to increase abruptly, even on short peaks. It provides faster
protection, but causes more audible "ducking" of the program level.
POWER LIMITER THRESHOLD: These four switches control the threshold where limiting begins in 1 dB steps
relative to the amplifier's rated power. The limiting characteristic has a soft knee which begins several dB below
the threshold. The threshold is reached at 6 dB of limiting. Slope is 8:1 on AVG mode, and 15:1 on PEAK mode.
To enter the desired power, find the amplifier's rated power at your load impedance (see Specification
section), determine the desired percentage of power and consult the table at right to convert this to the nearest
dB setting. Then combine switch settings to obtain the desired setting.
EXAMPLE: You want limiting to occur at 100 watts, using a 4 ohm load. You are using a PL230A
which is rated for 900 watts into 4 ohms. Therefore, you want to limit at 11% of this power. This falls between -
9 dB and -10 dB on the table (below). We enter -10 dB by setting the -8 dB and -2 dB switches.
POWER LIMITER SPEED, ATTACK-RELEASE: Three overall response times or "speeds" are available.
Although it may seem desirable to have rapid response in all cases, a limiter should not change gain appreciably
during an individual cycle of audio, as this causes intermodulation (IM) distortion. Therefore the speed must be
slow enough to pass the lowest frequency intended, without unwanted modulation. Otherwise the low fre-
quency tones cause "garbling" of the highs. The speed settings adjust the release time which governs how much
the limiter can change from one cycle to the next. The AVG mode (described above) has a 25% attack time
relative to this release time, so the gain changes gradually in both directions. The PEAK mode has a 1 to 2%
attack time and therefore gain reduction can occur quickly for more protection. However, gain increase is still
controlled by the overall SPEED setting, and therefore, the same frequency range applies.
NOTE: The time constants for each setting are determined by internal capacitor values which can be
changed by a qualified service technician. Consult QSC Technical Services for more information.
SIDE PANEL SWITCHES:

POWER LIMITER

Limiter adjustment DIP switches.
Settings shown are for example only.
Power limiter switch settings for all possible combinations
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