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60 Cycle Hum coming from 24 Channel Snake Mixers (Analog)
Old 3rd September 2011
  #1
Gear Addict
 

60 Cycle Hum coming from 24 Channel Snake

Hey guys,

I'm running into a weird issue that I just can't seem to fix. I've dealt with 60hz issues many times before and this one has just left me stumped.

I've got a 24 Channel x 4 return "Musicians Gear" snake from Musicians Friend that I bought about 3-4 weeks ago. I'm noticing that every single channel on it seems to produce a nasty buzz. I've gone through and metered each channel with my fluke to check for ground faults, shorts, etc.. and I'm coming up with nothing -- nothing is shorting to the stage box either.

I've noticed that I get this with any console I hook it up to as well. If I hook up a normal mic cable (Whirlwind, Proco, Blue Microphones, etc...) I have no hum -- it's only when this snake is hooked up.

I unscrewed the box and visually inspected it -- Even though it's cheap, it's pretty damn well built and there's not a single bad solder point. The cabling is done well too.

I attached a grounding cable to the box and hooked it straight into the mains ground connector -- no dice. I re-racked everything from my custom rack with wooden rails (I built it into a desk) to one with metal rails. It's made the buzz ever so slightly better.

Does anyone have any ideas? I'm dead on stumped at this point.
Old 3rd September 2011
  #2
Lives for gear
Quote:
Originally Posted by Matt R View Post
I re-racked everything from my custom rack with wooden rails (I built it into a desk) to one with metal rails. It's made the buzz ever so slightly better.
Bingo!

Earthing (grounding) problem. One or two cables don't show up the problem, but all 24 plus returns do, because they are not properly grounded and therefore act as one huge radio antenna. The problem is not the cable, but the earthing of the equipment - attaching 24 lengths of wire has just shown it up!

Basically, every piece of equipment in your studio has to be completely connected to Mother Earth and to the electrical earth at one and the same time. When you put in the metal racks, you improved the earthing slightly.

As you have a multimeter, you can test this for yourself. The resistance between the electrical earth and Mother Earth has to be absolute zero.

If you are in any doubt about this, get a fully qualified electrician to do this test and rewire the supply.
Old 3rd September 2011
  #3
Gear Addict
 

Quote:
Originally Posted by The Byre View Post
Bingo!

Earthing (grounding) problem. One or two cables don't show up the problem, but all 24 plus returns do, because they are not properly grounded and therefore act as one huge radio antenna. The problem is not the cable, but the earthing of the equipment - attaching 24 lengths of wire has just shown it up!

Basically, every piece of equipment in your studio has to be completely connected to Mother Earth and to the electrical earth at one and the same time. When you put in the metal racks, you improved the earthing slightly.

As you have a multimeter, you can test this for yourself. The resistance between the electrical earth and Mother Earth has to be absolute zero.

If you are in any doubt about this, get a fully qualified electrician to do this test and rewire the supply.
What's the best way to go about testing that? We recently replaced the entire circuit breaker panel here, so that may very well be causing the issues (Although I'm not finding any stray voltages from ground->neutral -- that reads 0.00V at the panel and at the outlet and I get 120V from either leg to ground)
Old 3rd September 2011
  #4
Gear Addict
 

I think it should also be noted that I'm NOT getting this same sort of thing from my 8 Channel, 50 foot, Proco subsnakes.
Old 3rd September 2011
  #5
Lives for gear
Test R between (for example) shield on the cable and a metal support in the building, or similar object.

Water pipes are usually plastic somewhere in their travels, so they are not suitable, but need to be earthed anyway.

We have (on top of the electrical earth in the wires) an open earth wire connected to all metal objects in the studio and connected to the steel support beams and to the foundations of the building. Our new mastering room will have an earth spike driven eight feet into the ground.
Old 3rd September 2011
  #6
Lives for gear
Quote:
Originally Posted by Matt R View Post
I think it should also be noted that I'm NOT getting this same sort of thing from my 8 Channel, 50 foot, Proco subsnakes.
hmmm . . .

Often, this kind of problem is a combination of things. For example, insufficient earthing and the shielding not connected, or poorly connected.

I would definitely test R between the multicore shield (XLR pin 1) and electrical ground first. Anything other than absolute zero is a no-no.
Old 3rd September 2011
  #7
Lives for gear
Quote:
Originally Posted by Matt R View Post
I attached a grounding cable to the box and hooked it straight into the mains ground connector -- no dice.
Stupid question - but are you sure that this is a pure 60Hz mains hum and not RF interference buzzing?
Old 3rd September 2011
  #8
Gear Addict
 

Quote:
Originally Posted by The Byre View Post
Stupid question - but are you sure that this is a pure 60Hz mains hum and not RF interference buzzing?
Pretty sure. I got pissed off at the whole thing so I unhooked the snake and isolated it and laid it flat out on a tile floor. From there, I hooked it into a cheap $300 Yamaha mixer, one of the MG series I think? Then, I went to the circuit breaker box and cut off EVERY SINGLE CIRCUIT minus the one the mixer was plugged into, which conveniently happened to be the only thing on that circuit.

If something is causing enough interference to turn this snake into a big ass antenna and cause what appears to be 60hz hum, Musicians Friend needs to go back to the drawing board with their design and actually shield the cables properly.
Old 4th September 2011
  #9
Lives for gear
So we are back to earthing!

What is the R between cable shield and the metal supports in the building (or Mother Earth, or anything else that is a 'real' earth)?
Old 4th September 2011
  #10
Gear Addict
 

Quote:
Originally Posted by The Byre View Post
So we are back to earthing!

What is the R between cable shield and the metal supports in the building (or Mother Earth, or anything else that is a 'real' earth)?
In the end, I've said "screw it" to this cable snake. I've never worked with one before that's had so much noise. I borrowed a friends Whirlwind and it worked beautifully. I'm sending this thing back and trading it in for a Proco since Musicians Friend doesn't carry Whirlwind.

Lesson Learned: Never assume that a company CANT screw something up, such as a cable snake. Spending $20k on mics and audio I/O only to cheap out on the snake is a bad idea tutt
Old 4th September 2011
  #11
Lives for gear
Well, sorry we never got to the bottom of this mystery - but I had a look on their website and the cables going into the XLRs looked suspiciously thin, so it may have been severely sub-standard cable or just unbalanced. Both of these eventualities is rather unusual today and of all the cheap snakes and XLR cables I have come across in the past ten to fifteen years, I have yet to come across one that was incorrectly wired.

Having just read the reviews on the Musicians Friend website, it seems that some of them really are incorrectly wired up and several people have had buzzing problems with them.
Old 4th September 2011
  #12
Gear Addict
 

Quote:
Originally Posted by The Byre View Post
Well, sorry we never got to the bottom of this mystery - but I had a look on their website and the cables going into the XLRs looked suspiciously thin, so it may have been severely sub-standard cable or just unbalanced. Both of these eventualities is rather unusual today and of all the cheap snakes and XLR cables I have come across in the past ten to fifteen years, I have yet to come across one that was incorrectly wired.

Having just read the reviews on the Musicians Friend website, it seems that some of them really are incorrectly wired up and several people have had buzzing problems with them.
I bought it anyway knowing that if it was wired improperly, I could just fix it. It seemed worth it for the $190 I spent since it was on a 50% off sale. The terminations themselves are solid -- it's very, very well cable tied together and the solder joints are amazingly well done, obviously by hand. It's impressive... minus the hum
Old 4th September 2011
  #13
Gear Addict
 

I've attached a few photos of the box itself and it's insides.

You'll notice that I attached a grounding wire to the box, to no avail.

My cat loves helping, too.
Attached Thumbnails
60 Cycle Hum coming from 24 Channel Snake-grounding-wire.jpg   60 Cycle Hum coming from 24 Channel Snake-inside-box.jpg   60 Cycle Hum coming from 24 Channel Snake-solder-joints.jpg   60 Cycle Hum coming from 24 Channel Snake-helper-cat-helping.jpg  
Old 6th September 2011
  #14
Gear Nut
 

From what I can see, it looks like the shield was wired to pin 3. It should be on pin 1, with the negative phase conductor on pin 3, not pin 1.
Doesn't surprise me you got buzz.....
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