Troubleshooting
If you don’t already have a block diagram for your system, I recommend that you create one.
It can help you troubleshoot your sound system if the sound dies or distorts.
The general principle is this: Starting from the loudspeaker, work back toward the left in the diagram.
Find the group of components that are not indicating an input signal.
The component just to the left of that group is causing the loss of signal.
For example, suppose you hear no sound from the channel 1 loudspeaker.
Assuming that the loudspeaker is not blown, work toward the left in the block diagram (refer to Figure 4).
• Does the power amp have an input signal? No.
• How about the graphic equalizer? No.
• How about the limiter? Yes, its input LED is flashing.
The component just to the left of the components without an input signal is the limiter. That’s the problem. Either it has failed, its controls are misadjusted, or its output cable is broken or unplugged.
Now suppose that an actor is speaking on stage, but you hear nothing through the house loudspeakers. Work backwards from the speakers:
• Does the power amp have an input signal? No.
• Graphic EQ? No.
• Limiter? No.
• Mixer? No. The mic signal LED on the mixer is not flashing when the actor speaks.
At this point, the cause of the problem is somewhere to the left of the mic signal LED in the mixer. It could be the mic preamp gain in the mixer, the mic/line switch, the mic phantom power switch, the mic cable, or the mic itself.
So you check the mic preamp’s gain knob. It’s turned up pretty far, so you rule that out. The mic/line switch is set to “mic.”
You look at the phantom-power switch and see that it’s turned off, so the phantom-powered condenser mic on stage is not receiving power. You mute the mic’s channel, turn on its phantom power, and unmute the mic’s channel. Ah ha – there’s the actor!
Some sound techs prefer to work from input to output if the sound dies. That works, too.
Either way, using a block diagram to trace the signal is a vital tool.
Bruce Bartlett is a microphone engineer (www.bartlettmics.com), recording engineer, live sound engineer, and audio journalist.