QSC Audio MD-LP118 User Manual

The green SIG (signal) indicator alerts the user to the presence of an input signal to the MD 
loudspeaker.

The green SIG indicator illuminates when the input signal exceeds -25 dB.

Check Gain settings and increase gain if necessary. Check input connections and audio source 
for signal. If the red LIM LED illuminates, refer to the LIM indicator section, below. 

If the green SIG LED illuminates with no signal input, there may be system oscillations or some 
other malfunction. Disconnect the input or fully reduce the gain. If the green SIG LED remains 
on, the amp may need servicing. 

The red LIM indicator alerts the user to several abnormal conditions within the MD loud-
speaker:

• Indicates protective mute mode. 
• The speaker normally mutes for several seconds after applying power, after which the light 

should go out, and sound should be heard. 

• If the speaker enters Mute during operation, it has either overheated or developed a fault. 
• Overheating should correct itself within 1-2 minutes, after which sound should resume. See 

below for a full explanation of thermal protection. 

• Short periods of muting could indicate excessive high-frequency program with subwoofer set at 

Full-Range. If this is not the case, it could indicate a component fault; remove AC power and 
have the speaker serviced. 

• During operation, bright flashing indicates clipping (overdrive distortion). 
• This is normally due to excessive volume, and will likely be accompanied by audible distortion. 
• If the speaker mutes repeatedly during peaks, there may be a component fault. AC power should 

be removed and the speaker serviced. 

• Indicates that the internal limiter is reducing gain, due to prolonged clipping and/or excessive 

temperature. 

• After several seconds of severe clipping, the limiter will reduce power to protect the speaker 

and improve the sound. This results in a steady, half-bright red indication. Any further clipping 
will still result in bright flashes on top of the steady half-bright indication. When the program 
level is reduced, the limiter will clear after several seconds, and the red indicator will go out. 

• If the power module overheats despite fan operation, the first response is to trigger limiting, to 

reduce volume and limit further temperature rise. This results in a steady half-bright illumination 
that does not clear even after reducing program level. It may take several minutes for tempera-
ture to drop and clear the limiter. During this time, the exposed heat sink will feel uncomfortably 
hot to the touch and the fan should be running. If overheating continues, the amplifier will ulti-
mately mute, resulting in a full-bright red indication. When muting clears, the amplifier will 
resume operation, with thermal limiting still active until it further cools off. 

• Overheating is usually caused by excessive ambient temperature, since the internal temperature 

rise of the Class-D power module is relatively low. Protect the speaker from excessive tempera-
tures, such as being placed over a heater vent, or allowing direct sunlight to impinge upon the 
heat sink surface.