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Old May 23rd, 2020, 10:54 AM   #46
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Re: in need of some expertise and guidance

Quote:
Originally Posted by Paul Mailath View Post
Is your house wiring earthed?
I assume so, all the wall sockets are 3 pin
https://www.sparkydirect.com.au/p/20...RoCajwQAvD_BwE
By the way, if your mains power sockets are switched, like the link you sent me, it's not sufficient just to turn off those switches. They will switch the line side of the power, but should not switch the neutral or safety earth connections. Even when the switch is off, those remaining connected wires may still provide a source of noise or a ground loop. Unplug means literally that.

- - - - -

Here in the states, at the local hardware store, we can buy a mains wiring tester.
https://www.amazon.com/Power-Gear-Re...c-20/din02c-20
It's roughly a 1.5" plastic cube, with three colored lights. Plug it into any mains socket, and the lights will tell you whether polarity is correct or reversed, and whether safety earth is present. They cost only a few dollars. Presumably a similar device is available where you live.

Your noise issue may end up relating to equipment grounding. (too soon yet to tell for sure) It wouldn't hurt to know for certain whether all your mains sockets are correctly wired and grounded. A tester like this would answer that question definitively.
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Old May 23rd, 2020, 01:09 PM   #47
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Re: in need of some expertise and guidance

I have been carrying one of those little cube AC line tested on live sound gigs for many moons. When noise rears it's ugly head I first check the outlets with that. If nothing can be done by using alternate outlets, out come the Sescom IL-19 ISO transformers which I never leave home without either.
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Old May 23rd, 2020, 11:36 PM   #48
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Re: in need of some expertise and guidance

Hi all,

I purchased this unit and tested all the powerpoints in the room
https://www.google.com.au/shopping/p...E,gclsrc:aw.ds

All powerpoints in the room tested OK


I then turned off and unpluged the powerpoints in the room and the surrounding 4 room. The Air cond was switched off at the breaker and the pool filter was unplugged. Lights & fridge were turned off

I then ran test 4
recorded a few sentences. paused at :43 unpluged the cable at the mike, waited 20 seconds, paused and unplugged the cable at the DBX

https://drive.google.com/open?id=1O3...3JcMVcBk6YO897

Since I have no other XLR cables to test yet I replaced the USB cable and ran test 5, I could still hear the hum in the headphones but the recording seems slowed down and I'm not sure why.

https://drive.google.com/open?id=1PD...25g6Yf57xO5kZB

I need a cuppa tea & a lie down now
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Old May 24th, 2020, 04:25 AM   #49
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Re: in need of some expertise and guidance

We've done OK on this one. The recording with the hum suddenly went quiet when you disconnected the microphone - so we have proven the DBX unit itself is silent, and the hum is entering the system before the XLR input on the DMX. Therefore the 'fault' is either the cable (likely) or the microphones (less likely), but you now need substitution. The cable as mentioned before must have all three conductors intact because the phantom power gets to the destination and we hear the audio. With all your kit turned off there should be no magnetic fields in the area from transformers, or other coil containing devices, unless the transformer in the DMX unit is the culprit.

We know you can't swap the mic cable or the mic - which would determine exactly if these are the issue, but if it's simply a poor screened cable - they exists - then moving the cable while recording - as in bringing it closer to the DMX will change the amount and character of the hum. So move the mic and cable around while recording silence and see if it changes - if it does, follow the hum volume, the closer you get to the offending item/area you will know from the sound.

Re: the pitch change, I suspect unplugging re-plugging fired the interface up in the wrong mode. You probably recorded in 48K and are replaying in 44.1K. Switching off and then on again should restore the usual defaults. It happens and is nothing to do with the cable.

The thing with cables is that screening is extremely variable in effectiveness. For years broadcasters shifted audio from centre to centre on twisted pair telecoms cable with no screening whatsoever, so it's a bit of a myth that it's critical, and balanced cables reduce hum - we think this means screening as a separate thing, but the balanced cables do most of the noise rejection, the screen just assists. Does your test meter do resistance? If there is leakage from hot or cold conductor to the screen, then the impact is on the phantom power voltage, but many mics are happy with wide phantom power voltage swings - 10-52Volts being enough to power the preamp in the mic, so a cable fault with leakage could allow the mic to energise and function, but the leakage could be enough to destroy the hum rejection. A new cable will sort this. Most faults are at the connectors, so if you can solder, cut 6" off each end and remake the connections. Swapping the mic often makes the hum go away, but does not automatically mean the humming mic is faulty - it may just react differently to cable faults.

I'm lucky enough to have trunks full of cable, and my oldest is the one that came with my first SM57 in the 70s, and that cable is still perfectly good, but over the years dozens have failed at the connectors. Perfectly normal. A simple joint gone dry, or a stray strand of screen that's touching pin 2 or 3 could be enough.

You're making progress, but now need to take action physically.
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Old May 24th, 2020, 08:31 AM   #50
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Re: in need of some expertise and guidance

Paul, I completely agree with that assessment. And this is a very strange puzzle to solve! Different USB cable did not change a thing.

The attachment shows how much lower the noise is when the mic is unplugged.

What to try next? Turn on exactly the same kit, record silence, move the mic and cable around. Exactly as Paul Johnson suggested. Does the hum change in loudness or in character?

One other test would be to leave the cable connected to the mic. Unplug the cable from the dBx, plug it back into the M-Audio, and record another test (voice + silence) using the M-Audio. While recording the silence part of that test, turn the dBx power on and off, see whether the hum is related to the dBx being turned on (even 'though it's out of the circuit).

One almost gets the impression that there is a tiny source of audible hum in the booth which is actually being picked up by the mic ... a chorus of tiny mosquitoes with perfect pitch ... but of course the OP would have said "I hear the same hum in the booth, with my ears."

There's also the possibility of a corroded connector, putting more resistance in one side of the circuit than the other. This would probably be either the mic end of the cable, or the mic connector itself. Some good spray contact cleaner (Cramolin or Deoxit), Q-Tips, and pipe cleaners would identify that.

And there's a possibility that the power supply in the dBx is bad. Yet the recorded hum goes away when the mic is unplugged, so it's a very strange kind of fault.

Unfortunately, Paul (M), it looks as if you're in line for a few more tests. ;-) We have to be getting very close now. Good luck!
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in need of some expertise and guidance-test1-vs-test4-noisespectrum.gif  

Last edited by Greg Miller; May 24th, 2020 at 12:11 PM. Reason: Added image
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Old May 24th, 2020, 03:16 PM   #51
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Re: in need of some expertise and guidance

thanks so much guys, I really appreciate it. Unfortunately I'l have to wait a ay or so before the next test - everything is back up and running again for a wedding edit I have to complete and a couple of 360 virtual tours I'm trialing for clients.

Would it help if I order a new TRS to XLR cable to try?
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Old May 24th, 2020, 03:29 PM   #52
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Re: in need of some expertise and guidance

It certainly wouldn't hurt to have a spare $20 cable on hand. If a cable fails and you don't have a spare it will bring you to a standstill.

However I think it's much more likely that the problem is the mic cable.

Also, if you can come up with your other mic, that would give us one more comparison to try.
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Old May 24th, 2020, 03:39 PM   #53
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Re: in need of some expertise and guidance

I'm always amazed people don't collect cables. From day one of my working life, I started to collect the damn things, and many are still with me. I treat them like batteries - use them till they die, then open the next one. If I need more longer ones, or really short ones, I make them, if I want something unusual, like the " to XLR, to be honest if I see one on an internet site I trust - as in companies who know what they are doing, I'll order one for pure laziness. I've probably got all the bits in the workshop.

Most of my cable failures have been physical. Somebody running heavy flight case over one crushes it, or wrenching the cable out of the connector, but usually my stock going down is simply natural wastage. I've got bins full of Neutric 3,4 and 5 pin XLRs, and speakons and both versions of powercons. ", 3.5mm jacks and RCA phonos. " to phono (RCA), to , to male XLR, to female XLR.

We do video and audio, so we've also got every kind of video cable you can imagine, and so far, somewhere will be exactly what I need. I just find it strange people don't have these things, even in much more limited numbers. We buy equipment, yet we don't buy the cables?
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Old May 24th, 2020, 03:44 PM   #54
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Re: in need of some expertise and guidance

Same here, Paul. I have old Cannon connectors with the rubber insert and the coiled spring strain relief. Yet I did not have a proper connector for an EV 666. I had to beg my brother for one from his collection. Luckily, on that particular day we were friends. ;-)
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