That’s correct, either mute your PA channel or use the stand by feature to avoid that sound.
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I have the same issue. Any problem to put it in standby and after that remove the power cables?
how do you go about connecting to PA? im scared about this as well…
Pretty much recommended:
Turn on QC, then amp/pa, etc.
Turn OFF amp/pa, etc- then QC.
(can get tricky when FOH is in the mix)
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@xush I was just about to say the same. Treat the QC just like any mixer or other pro audio gear. Never leave your speakers on with it turned off!
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I’m always suprised that this is not just standard practice for everyone.
You ALWAYS plug your guitar in before turning things on, you always power your pedals before turning the amp on, why is it suddently different when using a modeller? 
Pro Audio engineers should definitely know this too. All channels on the desk should always be muted if things are being plugged in/unplugged.
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When playing on a stage there is always the possibility of your pedal board becoming unplugged from power during change overs between bands for example. Sure, a good sound man will mute everything while set up/change over is happening but I’ve worked with plenty that don’t, so riddle me this Batman: how come the noise issue doesn’t happen when using outputs 3 & 4? Isn’t TRS just XLR in a different form factor? Shouldn’t XLR be able to function in the same noiseless way?
I said this in this thread in June 2021! They just don’t get it.
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Any soundman (or woman) who doesn’t mute everything between bands (or when bands have finished) is an absolute amateur and should not be doing it for a living. End of.
The rest is inconsequential, I don’t know how the internals of the QC are wired, and I don’t need to, because that is not the issue here.
Muting the desk when connecting cables is audio 101. Not doing it risks hundreds, thousands or even tens of thousands of pounds/dollars/whatever worth of damage to speakers.
Edit: Our keyboard player has a Roland FA06 and MicroKorg that buzz like hell if they are turned off but plugged in - we always have to turn down his channels on our practice room desk as it doesn’t have mute buttons. It’s likely a similar behaviour with a lot of devices as you essentially have a complex antenna connected to your audio inputs.
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i have posted the fix for this in another thread
this has to do with the way the power is with the QC.
To make sure you dont have this problem (for those who have it) its a simple solution
NEVER have any output devices plugged in to the QC until you power it up first. Once its booted up then plug your monitors / PA etc into the outputs and you wont have this issue.|
As soon as you turn off the QC it will strart doing it again, until you remove either the power cord or the output cables from the back of the QC
simpllified answer: always boot up QC before you hook up monitors or a PA or whatever.
this is poor advice because the sound will persist once you unmute. this has to do with the QC doing whatever it does about power and grounding when it boots up.
Boot it fully without outputs connected so it can get its ground “equilibrium” and then once its done doing that, THEN you plug in your outputs.
As others here have pointed out. Always turn off or mute your PA/monitors first before turning off any connected equipment. Although it is a nice convenience when equipment that is connected to a PA/monitor can be turned on or off or disconnected without any pops or noise, you should never count on it.
Startup: First turn on connected devices (e.g. QC), volume all the way down or low if you want to be thorough and avoid issues like a guitar feeding back, then turn on or unmute PA/monitor. Bring connected devices up to performance levels.
Shut down: Turn off or mute PA/monitor first, then turn off and/or disconnect connected devices.
Just adding another voice to the many who have already stated this.
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you have to boot up QC without any outputs connected or the people that have the power hum will still have the power hum.
you have to power the unit on without the outputs hooked up and let it boot and do its little power thing. if it detects the output cables when it boots up it gets a different reading on the outputs and it sets up its power and it has that hum, so the people affected by this must ensure there is nothing hooked up to the outputs before its powered up. after its powered up plug in outputs.
when you turn OFF the QC when you are done all that hum will come back, until you remove the power cord or the outputs after you turn it off as well.
@Marco2073 described the problem and the solution:
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I’m still unhappy with the situation and wonder what is “wrong” with the design of the QC.
I’ve bought the QC 2 weeks ago.
Our whole rehearsal room is permanently wired, why should I have now to unplug my XLR cable before turning off or use uncoated cables?
My kemper, my lrbaggs venue, my bassist’s ampeg, my lead guitarist’s fender, my keyboarder’s Yamaha, everything is permanently wired and produces no noise when turned off. Only my QC makes a loud buzz and needs now a special treatment, that’s annoying.
It uses optical ground lifts (to avoid loud pops on power on/off) which default to open when unpowered.
If you’re leaving amps on, put the QC into standby instead of off.
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Thank you for the explanation!
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