hum / RFI question

G

Guardian

New member
I know this is the mic forum, but this is mic related mostly..

I am getting a clear radio signal for some reason that I never noticed before, with my Neumann TLM103's..I can actually hear the radio program very well..and I am a perfectionist and it's really pissing me off greatly..it's not audible unless I have the gain/trim on my board (Mackie 32*8) higher than halfway up..

It is coming from somewhere between the mic pre and the mic..so it's gotta be in the cabling running to the studio..which is a Horizon Stage Series, 100ft snake, with 24 mic inputs on the box and 1/4inch..

http://www.musiciansfriend.com/srs7...9011405488/search/g=home/detail/base_id/35568

I had 2 speaker cables running in parallel with the snake cord, down to the studio..but I yanked those, and it helped, because they were acting as an antenna I believe..that was my first guess..but it's still there...so I know it's coming from snake somehow..any ideas on what to do?

Also, to make sure it was the snake, I plugged a 25ft XLR into the mixer directly, and turned up the preamp all the way, and the channel all the way, and..etc etc..and all I got was the typical hissing..no radio..but then I did the same for a channel connected to the snake, and the radio was right there...with the TLM 103 connected, it's like loud and clear after I go over 1/2 way on the gain/trim....it's there with other mics too, just not as much..

Anyone have suggestions? My guess is maybe a ground loop somewhere? If I have all the faders down, and pre's gain down on the mixer, but I crank the left/rix mix fader, and the control room knob, and set my amp up high..there's hiss, but also a slight hum, that increases as I pull up channel faders..so maybe there's a slight ground loop somewhere causing a problem also?

All the power for the mixer, etc is coming in through a single outlet, connected to a Furman PL-PLUS..

Thanks for any advice,

--Sal
 
OK, now what?

I found the problem..it's the electrical wiring in the studio, which is the basement of my house..it runs along the ceiling..is there any way I can build a shield around that wiring cheaply, and effectively block or reduce?

--Sal
 
More

..and the problem is only when I connect any <I tried several> mic cable to the actual snake..there is no radio audible without a cable attached to the snake box..as soon as I click one in, boom, in comes the radio reception..I'm getting 2 distinct stations, one more prominent...just for kicks, I attached a speaker wire to a metal pipe in the basement, which I'm assuming is grounded, and touched the other end to the snake box..nothing..touched it to the 1/4 output, and the reception went down..but was still there..the 2nd station that was less prominent went away! Gone!

Anyone figuring this out yet? I'm not gonna mess around any more, because I don't wanna shock/kill myself somehow....my only guess is maybe the Horizon snake is wired wrong, and the ground isn't working?

---Sal
 
EMI issues when the snake is in the loop that go away when you route around it are almost certainly grounding issues, as you suspect. A good snake should behave exactly the same as a good single mic cable, _IF_ it's built correctly.

Here's a thought. 1/4" connectors are almost always mounted metal-to-metal on the snake box, which means that their sleeve is shorted to the box's chassis. That means that the box is probably shorted to chassis ground on whatever equipment is *driving* the returns- so there's a great path for ground noise to couple into right the stage box, which is bad enough that it might be causing your whole problem.

However, even more likely is that the bozos who built the box shorted pin 1 at one or more (or maybe all of the!) XLRs to chassis ground- which dumps the EMI that rides up the snake on the 1/4" returns right into the ground lines on your mics, and that is _muy bad juju_ for getting noise into your signal.

First test: disconnect all the returns at the fan end. Does that help?

Second test: does every snake channel behave exactly the same? If not, you have a ground problem on just the line you're using.

If so, you may need to open the snake box and make sure that the mic signal grounds are not (even accidentally) getting coupled to the box chassis. Mic signals need good shielding from end to end, and that shielding needs to be connected to chassis ground at one and only one point- the input to the preamp. Having multiple places to hit ground is a recipe for EMI, and I think that's what you've got going here: there's a loop up the snake returns, and back down the mic runs. But read on, first.

Another thing I have done is replace the metal-to-metal 1/4' return jacks with Neutrik's plastic insulating jacks, and then connect the box chassis to one (and only 1!) return ground so that the box isn't floating. Shorting all the return sleeves together is bad juju *anyway*, but for rock&roll stage use, a lot of manufacturers didn't give a shit, seems like...

Pin one has to have perfect continuity from the mic pre to your mic, and it has to _float_ with respect to everything but the ground reference at the preamp. Which brings me to one other final point: mic cables need to have their connector shells *floated*with respect to the mic ground line: if the connector shell is connected to Pin 1's signal ground, and accidentally rests against the shell on a connector on a different signal, you get transient loops.

The only time you ever want the shell connected to signal ground is right at the mic, and even then you *only* want it for mics that do not properly connect signal ground to the mic case. Your TLM-103 is *not* such a mic: it shorts the ground to the case internally, because you need the case properly grounded to shield the capsule and electronics...

My point is that if your mic cables connect the connector shell to the signal ground line (Pin 1) at the male end, then _they_ are quite likely shorting Pin 1 to the stage box chassis when you plug them in. Once again, bad juju: there's your hum source. From your description of the problem occurring only when you plug a cable into the stage box, that may be the most likely of all: check your cables to see if shell and Pin 1 are shorted at the male end!

XLRs have these big lugs that beckon to us to short their shells to signal ground, and regrettably some cable manufacturers routinely do exactly that. But for mic use, the shells really need to *float* unless you have a very specific reason to do otherwise- and then, you ought to paint the shell of the offending connector red, so you know it's a wierd one.

Try these quick checks and get back to us.
 
Last edited:
Also, contact Neumann. I seem to remember some people on rec.audio.pro complaining about radio station pickup on the TLM-103. Neumann may have a quick fix for your problem.
 
Does the Neuman have its own power supply? If so make sure you are plugging it into the same outlet as the rest of your gear.

Most ground loops occur when you connect gear by audio cables that are plugged into different outlets.
 
ok

Thanks guys..I am heading out now but will play with this later on, or early AM tomorrow actually.

I got the interferrence with an NT1 as well as C300b, when I got the gain high enough..and the TLM103 doesn't have a power supply...so I doubt its the microphone..I just think the Neumanns have a REALLY hot output, which makes the problem more noticeable on them.

---Sal
 
Checked everything..

I checked the mic cable I was using, and 2 others..they are all seemingly connected properly..3 leads connected, nothing touching the shell connector (the flat thingie with the hole in it). I opened the snake box..3 leads, nothing touching any part of the metal box..the 1/4 jacks all have a plastic washer between the case and the metal..so that's not it....any other thoughts?

--Sal
 
Well, it's good to hear that somebody is actually building snakes well! That's good news- there's no need to rework the snake to get it right. That happens quite often when people try to use stage snakes in a studio setting, where "quick and dirty" just doesn't get the job done.

However, that suggests one other thing to possibly try. If the snake box really is completely floating with respect to all the signal grounds, you might be getting some coupling of RFI in on the unshielded bits: the box's electrical dimensions might actually be accidentally tuned just right for it to resonate with the one of the local stations (don't laugh, this is a real problem when you're chasing EMI/RFI issues!). Try grounding the snake box to your mixer's chassis ground with a separate piece of wire, just as a temporary trial deal, and see if that helps.

If all the other grounds are right, and there are no other loops or other things happening, that's the last remaining item I can easily think of. At that point, I'm officially stumped.
 
ok will try

I'll give it a shot..is there an actual place to ground on a Mackie 32*8, or just tape it to a metallic spot somewhere on the chassis?

Thanks for all the help btw :)

--Sal
 
As a last resort have you tried any ground lifts on the mackie? I had a mackie 1202vlz a few years back that started to buzz no matter what I did. It sounded and acted like a ground loop but would not go away at different studios, diff cables and mics, nothing. I finally sold it to GC.
 
hmm

Could it be the board? Maybe something is not grounded properly there? My father says the outlets in the house are all grounded properly, or we'd be getting problems..the ground outside is ok..I have the mackie plugged into a Furman PL-PLUS..any issues there?

It's not a huge deal, just it's annoying as hell when I'm trying to run a 'pro' home studio..
 
weird

Testing this further..it's only happening on SOME of the mic channels..not all of them...it's not the preamp or the board at all, as far as I can tell, because mic cable 3 has AM radio no matter which preamp its plugged into, but mic cable 1 doesnt have anything on either channel 1 or 3...

I checked the end of the cable, the male end, coming from the snake..and it looks fine...I even plugged the cable in without the shell touching the end, and it was happening..maybe the shield is somehow broken inside the snake somewhere? All channels have mic input tho, I double checked that with an SM57 just now...


This is really weird....

???

--Sal
 
Ahh! Not wierd at all: the foil shields on snake cables are really more appropriate for permanent install than portable use- they are just used there to keep the size and weight of the cable down. When they fail, you will indeed start to get individually noisy channels. And if they snake has been used at a couple hundred live shows before being moved into your studio, it probably *will* have a few shielding and open problems.

Sounds like you have a very experienced and well used snake there. It comes down to the old doctor joke: "Doc, it hurts when I do this...". Answer: "then don't _do_ that!". If you can't find obvious problems at the fan end or inside the stage box, then the noisy/open channels are just dead. Retire them, and move on, using other channels in the snake.

How many of your 24 snake channels are dead- and in what way (noisy or open)?

I managed to rejuvenate a little (but much abused) snake that had 4 of 8 mic channels dead, once. The usual place for a snake to die is at the fan end, where all the individual runs break out of the overall jacket. I moved the strain relief down 3 feet, stripped the cable down and cut off all the old fan ends to below where the old fan strain relief had been, and soldered on fresh XLRs to create a whole new fan. That worked fine: the flexing that happens right at the fan is a *bitch* on both the foil shielding and on the hot conductors.

If you have enough channels to reassign around the problem, then do that, of course. But you could possibly rejuvenate it if the breakages are right at the fan...
 
doh

I never used the snake for anything other than permanent install..I got it 'new' off Ebay, and then installed it..it looked/seemed brand new too..it was neatly coiled and the snake was tied with those plastic thingies you need to cut to remove..so it probably 90% was brand new...so you think it should be fixable by rewiring at the fan end, by just cutting away and re-connecting? Do you think the problem could be later on down the line? How hard is it to cut and then reconnect a male XLR..I've never done that before..?

--Sal
 
Yes, it should be very fixable. But first of all, how many channels are dead? Don't even screw with it if 3 out of 24 are dead, unless you absolutely need 24 at all times. And remember that the breakage could be well down the cable in the middle, and the fan rebuild may not fix it- it could be at the stage box strain relief, too. Which part of the snake looks the most used? Whereever it's had the most flexing (or where they rolled the piano over it!) _that's_ where the problem is.

It's trivial to desolder an XLR and move it to a fresh-stripped end. Depending on how the cable ls laid up, you might also need to get a goodly bit of heatshring tubing as well: if each pair has only the foif shield, but has no outer einsulation, you'll need to heatshrink each pair between the fan strain relief and the XLR. Otherwise, the shield won't last long.

Another Ebay "as-new" deal, eh? That's sad, but it is a very common story...
 
hm

The snake cable itself makes noise whenever I move it around much..kinda like a cracking sound..and the cable is not easy to unwind..it's curled in a few places, so it's not 100% straight end to end..I'd say about 8-10 channels have RFI on them, without looking at the list I made downstairs..

What's heatshring tubing? From what I noticed, it just looked like everything was soldered on at the male XLR end.

thead79 (theadrick@farmerstel.com)

That's who I got it from..the snake looked 100% brand new, the wrapping, the shipping, etc..his last ebay transaction was the same thing I have...should I bother trying to contact him about it? I bought it summer of 2001, approx...
 
Heatshrink tubing- sorry about the typo. If you look at the fan end, the insulation on the individual wires as they travel out to the XLRs is probably not present inside the cable's outer jacket. It was probably *added* after the cable was stripped, by slipping it over the individual pair and then heating it with a mondo hairdryer to shrink it into place. Some snake cables do have individually-jacketed pairs, but most don't- they just have a clear plastic spiral wrap over the foil sheild, and that is far too fragile to make a fan with unless you support it somehow. So you use heatshink to create a jacket.

Right at the bottom of the fan, where the overall cable jacket ends and the individual pair jackets begin, is the point of highest stress for the wires- that's where they fatigue, start to crunch, and eventually break, in most cases.

If the snake has a stiff section or a section that won't come uncurled, that's also *very* worrisome. The snake cable should be equally stiff and obnoxious over its whole length- it shouldn't be stiffer in some parts, and more flexible in others. If there's a part that is stiffer, or refuses to come uncurled (like it has been overheated and melted a little, like maybe it was laying against a stage light, or had the piano rolled over it), that's a problem.

You could contact the seller, but I doubt that it'd be much use at this point. If you aren't comfortable with trying to find the exact location of the problems, you might want to take it by a good audio tech in your area. But the next step for you will be to find out if the dead pairs are having theit problems at the fan, at the stage box, or out in the middle somewhere.

Meanwhile, you still have 16 working channels. Tie a knot in the fan end of the dead ones, put a square of duct tape over the females on the stage box, and move on for the time being...
 
Thanks

It's pretty stiff all over the length of it..From what I've gathered, it IS brand new, so maybe it was just defective...I contacted the Ebay guy, and he emailed me a copy of the Horizon Warranty, with their phone #, etc, etc..so maybe I should just try going through them before modifying, etc?
---
If service should be necessary, your Horizon product should be well packed
in a sturdy carton and returned prepaid to:

North America

Horizon Music, Inc.
230 N. Spring St.
Cape Girardeau, MO 63701
------------------------------------------------------------

The warranty is through Horizon. Try calling Horizon at 1-800-255-9822 and
ask for Carl in customer service. I'm sorry for the trouble and please let
me know if Horizon will take care of you.
 
By all means, if it is still under warranty, run that by the manufacturer. It never hurts to ask, and they may well make it right for you- that'd be by far the easiest way to solve the problem.

A good snake should behave the same on every channel, and should not have much in the way of crunchies when handled. Make sure you explain to them exactly what problems you are having, and good luck!
 
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