In Part 1 of this two-part article, we talked about ways to troubleshoot some of the common problems that plague PA systems. This time, we’ll discuss how to identify and fix distortion, hiss, input issues, and the gremlins that sometimes lurk beneath the surface of digital consoles.
This problem will likely come from one of two places:
Many powered speakers have a switch to select between mic or line level input. If the speaker is being used with a mixer, set the switch to line level — otherwise, the mixer will overload the input, resulting in distortion.
When connecting a microphone directly to the input of a powered speaker, set the switch to mic level or you’ll hear a lot of hiss when you turn up the gain control.
While many audio interfaces and mixing consoles designed for recording offer inputs specifically designed for connecting instruments, few live sound mixers do. In that case, you’ll need to use an external DI box (short for Direct Inject) to connect instruments such as keyboards, bass or electric guitar to mixer mic inputs at the proper level. If you’re not getting signal from the DI, here are the steps to take.
Buzz or hum from a DI being used in conjunction with an instrument amplifier (as is often the case with bass) usually indicates that there’s a ground loop between the amp and the mixing console. This is due to the fact that the bass is connected to ground via the bass amp and again via the direct box connection to the mixing console. Fortunately, all DIs feature a ground lift switch that safely allows you to break this loop by disconnecting the audio ground.
Set the switch to lift the ground and see if the noise stops. If the noise persists, then change the cable.
Buzz or hum combined with low level and/or a loss of low frequencies usually indicates that one of the three connections on an XLR cable is faulty — a condition that some live sound engineers refer to as a “leg up.” If changing the cable doesn’t work, then you may need to use a hum eliminator as discussed in Part 1.
This is usually caused by the combination of a low-level source and excessive input gain. Here’s how to troubleshoot it:
This type of problem is usually caused by one of two things:
In a similar fashion, excessive compression on an input channel can cause that channel to be very low in level, even with the channel fader raised up all the way.
A defective cable is the most likely suspect when you hear intermittent audio, but another possibility could be the settings on a gate that’s inserted on a channel or output. If the threshold is set too high, the gate will remain closed and mute the channel. Audio that “chatters” on and off is also an indication that gate threshold is set too high.
Unlike analog mixers, most digital mixers provide the option to route any bus to any physical output, so it’s up to the user to decide which jacks output the main L/R and monitor mixes. For example, Yamaha TF Series models feature 16 “Omni” outputs that can be assigned to any bus.
Typically, the two highest-numbered jacks are used for the L/R mix (Omni 15 and 16 in this case), and the lowest-numbered jacks are used for the aux sends/monitor mixes. This enables matching the monitor mixes to the Omni outputs so that monitor mix 1 feeds Omni out 1, monitor mix 2 feeds Omni out 2, etc. — which helps prevent confusion. If you’re not getting signal from an aux send or L/R output, confirm that the output bus is assigned to the correct jack.
Many digital mixers also have the ability to connect to a variety of input and output devices. Yamaha QL Series, CL Series and RIVAGE PM mixers can route audio via the “local” (built-in) XLR connectors, Dante® network, or expansion card slots — or a combination of all three simultaneously.
Lack of audio to multiple input channels of a digital mixer usually indicates that the channels are not set to receive the correct input device. For example, the input channels may be set to receive signal via Dante when in fact the microphones are connected to the local inputs. This parameter can be changed using the input patch window.
Similar problems with mixer outputs can usually be traced to the output patch, which can be set using the output patch window.
It’s also worth mentioning that a mis-patched digital insert can cause a channel to stop sending signal. If a channel or output uses an insert, confirm that both the insert output and input have been properly assigned.
Troubleshooting problems in a PA system takes a bit of skill and patience, but as you gain experience, the process will become easier.
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