The hum of death!?

[Jumpmonkey]

So, I've got some hum in the system, pretty sure it wasn't there yesterday. Where could it have come from? Well... I may have been making some cables today. The thing is as soon as I noticed the hum I started looking back over my work. It was nothing pretty, but everything was in its proper place no wires touching, no streams crossed. I decided to back the chain up as far as I could and start working my way down. Mic, my old Monster XLR that's been just fine, and the Mixer. Good place to start except... I got hum even with just that. The only other thing I can think of is that when I started I did screw up my first cable. I reversed the hot and ground cables on the male end of it. Made an awful racket the moment I turned things on. Could I have burned some kind of ground loop deep into my system?

More testing tomorrow, but any advice from the sages of audio engineering would be greatly appreciated. I guess I'm just really perplexed because what I thought would make a good start for a noise free system was not.


God bless,
Adam

[djmukilteo]

Well the first thing is to get a known good cable to make sure all your interconnections are noise free.
Then test each signal path one at a time with only those known good cables and test each piece of equipment with those known good cables. Then you'll find the culprit.
A continuity ohm test of each conductor and pin of each cable is of course essential and as you've found out should always be done before ever connecting it to see if it works.
Yes you could have damaged the input or output circuitry of your equipment with mis-wired cables....!!!
Lesson learned...
Don't forget to check all pins in the connector and make sure you only get continuity on the correct pin.
Sometimes a strand of wire or solder blob can short across multiple pins and you may not see it but it will show up when you test them.
Check the Rane Notes for the proper terminations. This is a valuable sheet to have in front of you when doing your own cables or trying to figure out how to balance or unbalance a particular transmission line.

Sound System Interconnection

[Kapt.Krunch]

I decided to back the chain up as far as I could and start working my way down. Mic, my old Monster XLR that's been just fine, and the Mixer.

Actually, that's not "backed up" as far as you need to go.

We don't know if you have a mixer and amp/speakers, or if it's a mixer/amp, but either way, you need to disconnect everything plugged into them. If it's a mixer/amp, disconnect all input cables, and leave only the speakers connected. Turn it on, and see if it hums. Turn up channels, one t a time, to see if any of them hum.

If it's a mixer, and a separate amp/speakers, disconnect the mixer leaving only amp/speakers. Turn the amp on, see if it hums, and turn up the output on it, while listening. If all is OK, turn it off, and connect the mixer. Turn both on (mixer first, of course) and fiddle with levels at the main, and channels.

If it still doesn't hum, you've eliminated a lot. If it does hum at any point, you've eliminated the mic and cable as the current hum source. It has to be in the mixer/amp section.

It's unlikely that wiring a mic cable wrong would fry an input...but it's electronics. Anything can happen.

Now, plug in a mic cable, with no mic. Does it hum? Yes? That may be the problem. Try it in another channel. Does it hum? Yes? Could be the cable. Try a known-good cable. Does it hum? Yes? Are you sure all cables are good?

If no to any with only a cable, plug in a mic. Does it hum? Yes? Could be the mic. You didn't specify whether it's a dynamic or condenser mic, and if you had phantom power applied. A dynamic mic is less likely to get zapped by improper cable wiring, and no phantom power. I suppose it's possible for a condenser mic to get zapped by improper wiring, and phantom power applied.

It may even be possible for a channel input to get zapped from improper wiring, and phantom power applied to to either kind of mic.

Anyway, eliminate EVERYTHING except the amp, or mixer/amp, to start. Then, start plugging things in. If it's only an amp, and it's humming, it's something in the amp. If it's a mixer/amp, and it's humming, it's something in that.

Kapt.Krunch

[Jumpmonkey]

Kapt,

Thanks! i'll be busy with that.


Adam

[Jumpmonkey]

So, the news...

Cables: My monster cable is humming (really!?) Another purchased XLR hums if I touch the connectors with my hands or when a microphone is plugged in. All the cables I built (except the cable of death which I haven't repaired yet) work beautifully.

Mics: The mic seems to hum depending on where it is in relationship to my gear. Is this normal? I kid you not proximity and angle can take me from no hum to humsville.

Gear: I'm getting the slightest hum from my ART Pro VLA II compressor. The WA-12 is a hum machine. I'd had some issue with it when I first got it, it didnt seem to like the front plug on my Furman. The odd thing is it hums whether it's on or off, connected to a power supply or not. How can I fix this? Can I fix this? Will Lassie reach Timmy before it's too late?


Thanks,
Adam

EDIT: I have to take that back concerning the Pre being a hum-bucket. Yes it hums like nobodies business (well my business) when it's chained up to the mixer, but if I just monitor it by itself its dead silent... Now I'm more lost. Or maybe less, but I don't know it yet.

[TheJackAttack]

You have eliminated the amp and speakers themselves? Everything is plugged into the same power strip or receptacle? It is unlikely the Mackie is necessarily the cause since generally one can drive a railroad spike through a VLZ and have it still work but it can't be ruled out. Unless you have broken the SM7 opening up the basket or driving over it with a van it should not cause any hum at all.

I am guessing you have a chassis ground on some piece of equipment that is broken loose internally and every other device that connects into the system/hub via any kind of wire is creating induction hum.

[Jumpmonkey]

Jack,

I don't use speakers/amp, I monitor through a headset. It took all afternoon but I've got things managed better now (thanks for the advice DJ and Kapt.). It seems part of the issue was the furman I was running in the top rack. That took care of the nasty pre-amp hum. Still got a very light hum, it's coming from/through the compressor. How would I identify the chassis ground on a piece of equipment?


Thanks for your time!
Adam

[TheJackAttack]

What Furman unit? A conditioner or voltage regulator or.....

A Furman power conditioner should not induce any hum in the system at all. That is what they are designed to eliminate.

A chassis ground in most equipment is a point at which the mains is grounded internally against the case/chassis of the piece of equipment. Some manufacturers don't use them at all but when an input/output jack breaks loose a little a wire or a metal part of the jack field itself can then touch the case which will then start causing hum problems. This last bit is the main cause of hum in stage snakes.

[Jumpmonkey]

Jack,

The Furman is an M8x2 (conditioner.) Once I took it out that made all the difference. Faulty?

[TheJackAttack]

Possibly. It may also be amplifying a loose ground in other equipment but the Furman is certainly something to check out as hum is reduced by removal.