in need of some expertise and guidance

[Greg Miller]

Originally Posted by Paul Mailath

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.

[Rick Reineke]

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.

[Paul Mailath]

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

[Paul R Johnson]

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.

[Greg Miller]

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!

[Paul Mailath]

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?

[Greg Miller]

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.

[Paul R Johnson]

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 ¼" 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. ¼", 3.5mm jacks and RCA phonos. ¼" to phono (RCA), ¼ to ¼, ¼ to male XLR, ¼ 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?

[Greg Miller]

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. ;-)

[Paul Mailath]

sorry for the delay gentlemen, I've been trying to finish off a wedding edit and doing some tests on 360 virtual tours (trying to diversify while everything recovers)

Test 6

https://drive.google.com/file/d/1W7C...ew?usp=sharing

everything off as before, pick up the xlr cable and move it about near the dbx
mike out of the cradle and move it about
new cable
another mike (borrowed a Rode NT3)

it seems to my inexperienced ears that the mike is the problem?

[Greg Miller]

Certainly some interesting results. Changing mics certainly makes a difference. Obviously that was not my first guess, with all that AC powered equipment in close proximity.

I assume the NT3 was phantom powered for this test? i.e. you did not turn off the phantom power on the dBx preamp ... just to be extra certain about what's going on here?

My only other slim thought would be to check for corroded pins on the NT1A XLR connector. It wouldn't hurt to swipe over those a few times using a Q-tip and some contact cleaner. I guess there's a slim chance you have some resistance on one pin, causing the mic circuit to become unbalanced.

[Paul Mailath]

yep - phantom on the NT3

I checked the pins and they look pristine, I'll give them a clean but it's certainly looking like the mike - I was with you, once I started switching off everything I realised just how much powered crap I have in the room (all useful for cameras & video editing) but scary for audio recording once I realised how complex the field is. I had nightmares expecting you to ask me to crawl under the house tracing wiring next!

I'm looking at a Neumann TLM 102.

Thanks for putting up with me and all your help.

BTW I just got hold an TVC I voiced for Tasmania Wines (don't think it's been released yet)
https://drive.google.com/file/d/1TP7...ew?usp=sharing

[Greg Miller]

I had nightmares expecting you to ask me to crawl under the house tracing wiring next!

Hey, that's a great idea!

Seriously, if you're really lucky, your NT1A is registered with the factory and is still under warranty!

Once you get this straightened out, it will be interesting to see what happens as you turn things back on one by one. Given that most of the noise picked up was caused by the mic's proximity to the preamp, you might have very little trouble with most of the other stuff.

[John Nantz]

I've been following this thread because I'm going to get into Voice Over for my home video archiving project. Probably not until next winter (northern hemisphere). Been scanning lots of 35mm negatives and slides for a family history project and have some 8mm Standard and Super 8 movies without sound. So this will be a project with lots of V-O.

Anyway, ...
Checking the wiring under the floor, I like that idea too!

Speaking of the NT1-A mic warranty, Røde has a free extended warranty program that may be helpful if it's not a counterfeit. There are a lot of Chinese counterfeit mics of all types floating around and Røde has been targeted along with Shure, Sennheiser, Neuman, and I'm sure others. This is the Røde web page for signing up for the extended warranty:
https://www.rode.com/warranty/step1

The NT1-A is one of the mics eligible for the extended warranty.

[Paul R Johnson]

One of the iises with mains power issues is that electricians following their country's rules and codes are following a process only concerned with 110/240V power, and safety - nothing about noise and interference. We get concerned about hums and buzzes, but radio enthusiasts get problems with RF noise. Those folk who still use the old short wave bands for world wide communications as a hobby often get wiped out by radio noise from the same gizmos we have issues with. We worry about 50/60Hz hums and harmonics in the audio spectrum, but the radio people have the same problems up to 30MHz or so, and our local Coastguard use 156.000MHz and that is one of the nasty harmonics of the cheap and nasty PSUs. Expensive radios have good filtering, but cheaper ones just show interference all the time. Luckily, it's better out to sea!

Mains wiring offers simple paths for this crud to spread, and just think how better it could be if wiring was enclosed in the old metal conduit, or perhaps in screened cable? It's just not designed that way, and accidental combinations of kit cause the issues we're having here. It works and is safe - that is where electricians stop. Screening often is against the rules!