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Hummmmmmmmmmmm

A

-Asdfgh-

Apr 13, 2010
15,333
27,544
UK
#1
I had an issue at the weekend. For compactness I was intending to use my Zoom R16 at the weekend, to save space in the lighting/sound area to act as a way to send additional backing tracks out direct from a laptop and, potentially, run feedback suppression by routing to/from the laptop. The Zoom has XLR/jack combo inputs, balanced jack outputs, and can be USB, battery, or wall wart powered. It's not convenient in terms of quick tweaks of anything other than channel level, but that was all that was going to be required.

However, I had terrible issues with hum, that increased as more items were plugged in. Only one item plugged in was powered by anything that touched mains power, and that was over a wall wart. Mostly passive mics, and one active DI. The Zoom actually rebooted itself several times, which I've never seen happen before, and the Zoom has been fine, and quiet, when used as a compact mixer before.
 
#4
s0c9 said:
Could be a ground loop, could be how you powered it.. USB power from wal-warts is notoriously noisy..
Click to expand...

Yes, very likely a ground loop. My post was more a grumble about issues with doing live sound. That a mixer with full ground eliminated things confirmed the ground loop theory.

The desk was around 100 feet from the stage, so quite possibly on a different ring from it. I realise now there was one option for powering I could have tried but didn't, which was laptop on battery power, mixer via USB. I didn't have enough AA batteries to go to battery power alone in the mixer.

The issue I have with USB power and audio is just some random digital noise rather than hum. Normally the mixer is quiet, and with no stage gear connected it was also quiet.

If I'd needed to I'd have been able to run inserts on the larger mixer tracks into the second as an audio interface to kill feedback, but luckily feedback wasn't an issue so wasn't required. A feedback eliminator might be a future purchase.
 
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#5
jimfist said:
any idea what the actual voltage delivered was from the outlets in the room? Use a voltage regulator, power distribution from 220v circuit, etc?
Click to expand...

I'm in the UK, so it was all 220V. I do, in theory, have a small power conditioner for s standard plug but seemed not to have it with me, grrr.

It was all solvable with the larger mixer, but I had to bounce between the lighting desk, the audio mixer on a separate table, and my laptop for backing tracks and additional lighting effects control, and a secondary lighting desk for the spot operators. So it was a busy night! It went well apart from having muted all mics for the unplanned chat after the show finished.

The smaller desk would have allowed everything on one desk. I do have a smaller mixer of a conventional type, but it only has 4 XLRs. I was anticipating up to 5 mono over XLR, 1 stereo, and one stereo tape, so one more connection than the smaller mixer could handle.
 
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#7
seamonkey said:
Maybe it was lack of a real ground?
Seems like one input would at least have a real ground, but maybe you're unlucky.
Click to expand...

With wall warts no real ground at the mixer end until I swapped mixer to one with an explicit ground. The power amp should be grounded. It's not been an issue with this mixer before using my own snake up to about 50 feet, rather than the 100 feet thus time round, so I was surprised to have an issue. The hum increased as I added items, but luckily I was defensive enough to have taken the larger mixer just in case.
 
#8
-Asdfgh- said:
With wall warts no real ground at the mixer end until I swapped mixer to one with an explicit ground. The power amp should be grounded. It's not been an issue with this mixer before using my own snake up to about 50 feet, rather than the 100 feet thus time round, so I was surprised to have an issue. The hum increased as I added items, but luckily I was defensive enough to have taken the larger mixer just in case.
Click to expand...
Everything that has power has a ground.. the question becomes how is that ground routed.. explicitly or implicitly..
Wall warts in a rack means that the device with the ground (3-pin) becomes the defacto ground via rack rails or some other method.
You can get tons of hum like that.. too..
 
#9
s0c9 said:
Everything that has power has a ground.. the question becomes how is that ground routed.. explicitly or implicitly..
Wall warts in a rack means that the device with the ground (3-pin) becomes the defacto ground via rack rails or some other method.
You can get tons of hum like that.. too..
Click to expand...

In the UK it's the third earth prong that provides earth in an explicit way but on most wall warts now (SMPS) that's just a plastic one to switch the socket on. So I should have been prepared for hum but it's been fine before. I was concerned in case it was venue wiring but since using a mixer with an explicit ground (connected to piping) immediately cured it I presume it is likely that is common. In some places that can be properly tired to earth on one ring but not another.

I think the advice to myself is not use something not explicitly grounded as a mini mixer.

I'm still left with the DMX issue: I used the audio XLR wiring to send a DMX signal back to the stage to control lights there, but it was rare that I got more than thirty seconds of connection before it was dropped. I'm presuming that the electrical issues were overwhelming the poor little interface, so I'll need to check on its abilities and a way to resolve it in the future. Again, never an issue before over distances closer to 50 feet in modern (20th century) buildings.
 
#11
s0c9 said:
Same here in the US, but it's a round pin, not square(ish) like the UK.
Click to expand...

Square pin, round hole etc.

Our electricity is at least more refined and classy than that 110v rubbish :)

Anyway, lesson learned on using the small mixer, but I found out useful things reading the manual for my cheap hum eliminator after the fact. I've not used the eliminator much, not normally having had much hum to eliminate. Although once I had very low signal and hum but that was a mix up in my bundles of balanced and unbalanced jack cables..
 

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