When is a ground loop not a ground loop?

[Shez]

Had an odd one yesterday... We'd hired a four way G3 rack for a conference; it was powered via the antennae splitter from the spectacularly wide wall wart (NT1-1 I think). Plugging one of the jacks from the (plugged in but not switched on) rack in to the mixer (Folio notepad) with the channel off of course resulted in a low level mains hum coming through the PA. Sounded just like a ground loop. But the receiver rack's PSU is class II - no earth so it couldn't be a ground loop. I'd followed the usual good practice of mains wiring anyway with everything going back to the same outlet so I would have been surprised if it was an actual loop. I tried the usual swapping out of cables etc but with no luck.

 

The PSU for the Notepad is also class II but that outputs to a BSS LMS and onwards to amps that are all class I which I assumed would have the audio ground tied to earth at some point. All connections throughout the system were balanced. I surmised that as the audio ground was, at least within the radio rack & mixer, not tied to a fixed reference (earth), it was floating a little and hence humming. Does that sound feasible?

 

Based on that theory, I tried plugging a laptop's (with class I PSU) headphone output in to the mixer which completely killed the hum. Presumably that gave the audio ground a fixed reference?

 

Up until now, (hopefully through knowledge rather than luck) I've never had to deal with hums from ground loops or other sources so this one came as a bit of a surprise. Was it just that specific combination of equipment that was doomed not to work? Something else? Are there any other fixes besides the one I used?

[chelgrian]

Had an odd one yesterday... We'd hired a four way G3 rack for a conference; it was powered via the antennae splitter from the spectacularly wide wall wart (NT1-1 I think). Plugging one of the jacks from the (plugged in but not switched on) rack in to the mixer (Folio notepad) with the channel off of course resulted in a low level mains hum coming through the PA. Sounded just like a ground loop. But the receiver rack's PSU is class II - no earth so it couldn't be a ground loop. I'd followed the usual good practice of mains wiring anyway with everything going back to the same outlet so I would have been surprised if it was an actual loop. I tried the usual swapping out of cables etc but with no luck.

 

Stubby antenna or external ones on the end of coax? It's possible that the loop is coming in via the antenna.

[Stuart91]

We suffered similar sounding interference at an outdoor gig last summer. We had only recently bought our G3 sets, and were powering them from the original wall warts that came with them.

 

The hum would appear during loud passages of music - they would be fine for just speech and then a Zumba instructor got up and they were humming like crazy.

 

The best theory we could come up with was that the hum was an artefact of bad power - probably voltage drop on the feed from the genny. Running from some older, non-switch mode supplies, and the problem hasn't reoccurred.

[jonathanhill]

Switch mode power supplies tend to be more tolerant of variations in supply voltage, whereas older transformer based power supplies outputs are proportional to the supply voltage.

[nb705]

Also, if they were racked, the signal ground (I believe) is linked to the chassis

[BigKev]

What might be happening here is that one or more piece of equipment is tieing the screen to the chassis/ground/earth.

If this is the case then the screen on the connecting balanced cable is completing the ground loop between the devices.

 

Without knowing enough to draw out the system, including earthed and class II equipment to see where the loops could be I can't say what is best to do.

 

 

However, standard practice with earthing signal cables between earthed equipment is to only tie the screen at one end.

The best way to achieve this is to use an adaptor to lift the screen between the mains equipment.

 

Just simply snipping off the screen from the end of the XLR can cause major problems (both functional and safety) depending what the cable is used for in the future. I don't recommend this to anybody.

[ceecrb1]

do you own any isolation transformers?

I got myself one from thomann

http://www.thomann.de/gb/art_dti.htm

 

It has saved a lot of hassle on more than a few occasions.. especially those jobs where you are just one department on a large site with many other crews working together... for me its used a lot when I'm out with my ob truck, I just cant trust pa feeds or even sat trucks who run off their cheap generators or power inverters....

 

I learned the hard way that gound noises can come from very strange places... a few years ago it took me 2 days to find a ground loop in a projector image, in the end it was coming from the metal vga-joiner touching the truss (the original post is on here somewhere, I posted after pulling my hair out searching over and over).

 

Another common one I get is when working in the ob truck, if we plug into someone else's talkback, if you touch a metal part of the talkback kit and something inside the truck, your body closes the circuit and creates the loop. lecky tape fixed that really easily.. I have also now dedicated some plastic-cased belt packs to the truck for just that reason, I also bypass the patch bay and run a cable direct inside to make sure nothing touches anything.

 

My point being, dont just think of insulating cabling, check the chassis's of all the parts connected... to all parts of the system that is connected in any way to the mixer.. ie mic receivers, effects or eq's, amps etc etc.

 

Finally a thing to be aware off.. I own a cheap ebay oscilloscope that charges from usb, if you use it while charging from a laptop, as is in your case, the laptop give this a ground..... and if you go poking the probe into certain areas its too much for the laptop to handle and it gets fried... an extreme case but just watch out how much there is on earth in these situations.

 

edit:wrong item in the link

[Shez]

Stubby antenna or external ones on the end of coax? It's possible that the loop is coming in via the antenna.

Stubby ones mounted on the front panel. Sounds like you're referring to a hearing induction loop? Not applicable in this case.

 

The best theory we could come up with was that the hum was an artefact of bad power - probably voltage drop on the feed from the genny. Running from some older, non-switch mode supplies, and the problem hasn't reoccurred.

Very much indoors here with good power.

 

What might be happening here is that one or more piece of equipment is tieing the screen to the chassis/ground/earth.

It's the opposite problem here - neither bit of equipment in question was earthed - they were both class II. It seemed to be the absence of the earth than was a problem here rather than the presence.

[J Pearce]

The absence of any proper tie to ground may have caused some safe but noisy voltage to be apparent on the chassis/screen via capacitive coupling. This can manifest as a hum.

[OwenS]

However, standard practice with earthing signal cables between earthed equipment is to only tie the screen at one end.

The best way to achieve this is to use an adaptor to lift the screen between the mains equipment.

 

Earthing signal cable shields to chassis ground is standard, recommended, and when followed comprehensively does not cause noise issues.

 

Ground loop and other hum issues can arise from other sources, however:

• Badly designed devices which connect the cable shield to signal ground. This entirely defeats the shielding purpose of the cable and chassis by allowing the (noisy) cable shield to connect directly to the (clean) signal ground. Any earth loops/other noise current flow through the shield will show up as a signal there.
• Devices powered from Class II transformers, particularly switch mode. Switch mode transformers in particular are notorious, as for RFI reasons their output ground is coupled to neutral via an X (Engineered so, if it fails, it fails open) capacitor. What this means is that generally their 'ground' output floats at ~80V with respect to Earth. The available current is small, but when two or more of these get together, and there is no other earth tie, you can start getting various noises travelling down the shields and into the grounds of the connected appliances.
   
  This is then exacerbated because most class II appliances don't have quite the same clean/unclean earth distinction that class I audio devices do, and also because they don't have thick copper wires carrying only small currents dedicated to equipotential bonding connecting them.

Whenever racking class II devices, I generally would find a way (where possible) to earth the rack rails. This could be as simple as using a rack mount PDU. This gives them a high quality ground connection, which can eliminate this sort of problem.

[paulears]

Some useful info here

[Brian]

My CAT 2 TV has a CAT 2 2.1 speaker system connected via unbalanced audio connections. With the TV in standby it hums although not with the TV on. The hum can be cured by rotating the TV's mains connector 180 degrees. Go figure (or see OwenS's post for a clue).