XR18 - Noisy MIDAS Pre's

[Harpman]

I've been working in the studio getting ready for my gigs this weekend and noticed a hissing coming from my QSC K-12's.  I have nothing connected to any channels. I muted all the channels and problem disappeared.  I unmuted channel 1 and the issue was back.  I pulled the gain down on channel 1 and the problem disappeared.  The gain was at +30dB which is about equivalent to 1 o'clock on a gain knob.  Never experienced this before and never on the Mackie DL1608.  All I can attribute this to is a noisy preamp.  Any thoughts?

[WK154]

Gio it's been crazy around here but here is my take. Channel 1 & 2 are Hi-Z and will be more sensitive to open inputs. Try Channel 3 and see if you get the same results. The other possibility is a ground loop between the XR18 and the K-12. When all else fails use a RMS meter for actual output. The DL has no Hi-Z inputs. I trust you have the K-12's at unity gain or less. IIRC you have Audiotools so check the FFT for content it may identify the source ie. 120Hz peak.

[Harpman]

Gio it's been crazy around here but here is my take. Channel 1 & 2 are Hi-Z and will be more sensitive to open inputs. Try Channel 3 and see if you get the same results. The other possibility is a ground loop between the XR18 and the K-12. When all else fails use a RMS meter for actual output. The DL has no Hi-Z inputs. I trust you have the K-12's at unity gain or less.

Unity gain on the K12's certainly doesn't mean all the way up. Don't believe in that old school thought. Gain structure for me starts at the source and works down to the final output. I keep the K12's at between 11 and 2 and adjust those for what I want the FOH SPL levels to be.

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[WK154]

IIRC the K series processes the input signal and also has a preamp. The 0 dB (12 o'clock) is what is unity gain, max is another 10 dB of gain. That would amplify noise as well as signal equally and with no signal  you'd hear a hiss. Just keeping you honest. I don't have any hiss with a XR12 at 60 dB gain open input non-Hi-Z channels. QSC never published a max signal input spec making gain staging more difficult.

[Harpman]

I usually keep the K12's at 11 o'clock which I would say would be -5dB and really have never taken them past unity 12'clock, with the exception on last Friday with the Stones tribute band where I took it to 1 o'clock. Why would Behringer make just channels 1 and 2 Hi-Z? Doesn't make sense. Any unused channels that don't have anything connected are muted anyway. In my studio, the XR18 is connected into my Furman 1U power strip and then connected to my CyberPower 1500 UPS.  I do have AudioTools and will check out the FFT.


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[Greg C.]

They made the first 2 inputs Hi-Z in case folks want to plug passive electric guitars directly into the mixer. My choice would have been to have them as the last 2 inputs or not at all. But they're fine as long as they're terminated as far as the noise.

[Harpman]

Good to know, thanks Greg

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[WK154]

Have you had a chance to narrow things down?

[Harpman]

Well at least I can say I will always have something connected to CH-1 and 2.  Bill in regards to how things are going, still trying to isolate why compression on vocal channels is introducing feedback.  Not saying it's causing it, but something is not correct.  Found a couple of issues with TAP points that I can't explain and can't reproduce now.  I joined a Behringer X-Air User Forum on FB who is run by a guy named Sean Thomas.  Has a lot of great videos and insight.  Busy with an IT cut-over this weekend, but would love to show you the weird TAP issue I ran into.  I will try to load a snapshot from a prior gig and see if I can post it out here. I have an early day tomorrow, but will see what I can do in the next few days. Are you back in town?


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[WK154]

Leaving today from PA.

[RoadRanger]

They made the first 2 inputs Hi-Z in case folks want to plug passive electric guitars directly into the mixer. My choice would have been to have them as the last 2 inputs or not at all. But they're fine as long as they're terminated as far as the noise.

Yah, on my XR12 they are the last two inputs.

[Harpman]

They made the first 2 inputs Hi-Z in case folks want to plug passive electric guitars directly into the mixer. My choice would have been to have them as the last 2 inputs or not at all. But they're fine as long as they're terminated as far as the noise.

Yah, on my XR12 they are the last two inputs.

Now that makes more sense (last two inputs).  Call me "Creature of Habit", but I ALWAYS put vocals starting with channel 1, followed by instruments (drums typically being last).  Personally, have them "not at all" is even a better option.  The good news is that they will always be terminated since I always have vocals in 1 and 2.

[Harpman]

Well, I can tell you, don't run vocals on channels 1 or 2.  This has been the source of my frustration. You would think that Hi-Z would kick in using 1/4" but it's apparent that XLR is Hi-Z as well. Moved vocals, including Kepi's vocals and all feedback issues disappeared. Amazing

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[Harpman]

Like Greg said in a previous post, I wish they didn't put any Hi-Z channels. Would have made things a lot better.

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