Loud buzzing from amp after powering off QC

I currently have the QC connected via output 1 to my amp using an xlr cable.
When I power off the QC I get loud hum/buzz from the amp the moment it powers off. It seems to be emanating from the power supply since if I pull out the power plugin of the QC the hum stops.
Anyone else seeing this?

The buzz stops when the device is powered on so some circuit is blocking this noise. However I expect it to be silent when powered off as well.

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I had the same issue once. It disappeared after unplugging from the wall socket and then never appeared again yet.

I’m getting a massive pop sound that can pretty much blow away my speakers easily even with the volume knob all the way down.

The power amp and speakers should always be the last to be turned on, and the first to be switched off. Connecting other devices, or switching them on or off, whilst the speakers are on will always cause a lot of noise.


I beg to differ. This piece of equipment has a ground loop by design … I talked to reputable studio and sound engineers that I happen to know and showed it to them. They couldn’t believe it …

Agree. You don’t get this with the Kemper. It’s more than just a pop. Safest thing is to put it on standby, pull out the connecting lead, then turn off the power supply. I love this thing, but this is a bad feature and makes me nervous of using it live into a PA.

I get that same pop through my monitors, has happened since day one. Yesterday i started getting static type interference through the monitors after QC powers down.

Nothing has change location cables etc.

Hi there,

I think this is normal.

You can hear a click when you go stanby. I thinks this is a relay.
This will disconnect the signal line from ground. This will cause the noise I think. :thinking:

by the way, I got less noise when upgrading to a better SMPS. I bet that a linear power supply will
cause minimal noise. On the otherhand , high-end audio isolation transformers would probably
be good also. and protect QC from any power surges from PA …

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Has anyone tried using a third party power supply yet to see if the issue is the power supply ?

I think the issue is the quality of the power supply.
I plugged it into my true sine wave UPS and plugged my amp into the same outlet as the UPS and that had a major improvement for me. It stopped the buzzing while the unit was off.
Obviously I won’t have this setup when using it elsewhere but it kind of proves that the issue is related to the power supply.

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Yes had the same this evening at rehearsal - a really loud- really loud! - buzzing when switching the unit off. It lasted 2-3 seconds then stopped. My guess is the power supply as well.
In future i’ll be disconnecting form the monitors and PA before powering off.

Realistically, any processor should be powered on prior to the monitoring and vice versa, monitors powered off or lines muted prior to powering down the QC… the real issue here is in the event that the QC has to be powered down when you are live through a PA… this is NOT a good thing when you’re playing in front of an audience and this happens… I’ve had occasions over the years with other modellers where i have had to reboot during a performance and it’s never been a real problem… unfortunately i’m having nightmares about when that day happens with the QC :flushed::grimacing:

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I am not a technician but today I discovered this thing :

if you connect a XLR with external unpainted metal chassy the noise problem disappear; on the contrary, if it is painted or made of plastic, the noise is present.

This happens because inside the XLR female socket of the Quad Cortex there is a metal tab that ground the XLR cable. If the tab comes into contact with the outline of the XLR connector it interrupts the ground loop.

So the solution to this problem is to use XLR cables with an unpainted metal outer chassy.

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Wow this is an interesting find. If so I wonder if putting masking tape on the xlr jack would help this as well.

No, it’s the exact opposite … if you don’t allow the jack to make contact with the tab, ground loop occurs. I can verify that with painted jacks the ground loop is present while with unpainted metal jacks the ground loop goes away. And again, with painted jack, if I scrape off the paint at the point of contact with the tab the ground loop goes away (even if inside the jack there is a contact between the ground and the jack itself) .


Thanks for the tip @Marco2073 ! I saw a reference to this post on TGP. Good thing to know! :sunglasses:

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