Cable Construction pt2... correct ground connection?

H

HansonD

Active member
Hello,

First I'd like to thank the guys who helped provide some answers on my last cable construction question. This one seems to be more a matter of oppinion.

My question is: when soldering the ground to a connector (balanced mic cable) do you also connect the ground to the shell of the connector? - or - do you just connect the ground to pin 1 and call it a day? I've asked a number of parties about this question and have gotten mixed responses.

Personally my gut feeling is to connect the ground to the shell, because this is what the xlr was originally intended to accomplish,, that is making sure the ground line (via the shell) gets connected before any live voltage is connected. (the story I heard was that a Lockheed test pilot got electrocuted when connecting his headset radio to his instrument panel and voltage arced through his headphones before a proper ground was connected... thus they designed the xlr).

- however I've asked the repair tech where I work (who has 20 years experience repairing electronics, cameras and audio equipment) this question and he told me he rarely connects the ground to the shell because in some odd cases he claimed this could cause ground loops and problems.

- I also emailed both PSC and SoundDevices with the same question and they both responded quickly with a yes you should connect ground to shell becuase if you don't it can create a point of entry for interference to be picked up by the cable.

- Another factor is that if you take apart most pre-made mic cables you'll find that rarely does the manufacturer connect pin1 to shell.

Sorry for the long length of this post which is probably putting most of you to sleep but as I just bought a 600' roll of cable and a bag of connectors I want to make sure I'm constructing them the best way possible. Matt I'm definitely hoping for your advice in this question as I know you've made a ton of cable. Any and all feedback is welcome, thanks guys.

-d
 
Hardly anyone connects ground to the shell because that
opens up the possibility of an accidental connection to ground.
What if the connector touches another connector or a chassis?
The best approach to avoiding ground loops is to carefully
work out the various ground paths and make sure accidental
connections don't happen.

Les
 
Thanks for the input. Again it seems like the responses are pretty varied. Les, most reading I've done on the subject usually recommends connecting the chassis (or ground) from one device to the next as this is the ideal for reducing signal interference. Problems occur when devices are grounded at different earth points (ie: different outlets which may have slightly different voltage potentials). But... for my purposes where all my audio equipment is either battery powered (floated ground in this case) or powered off a single outlet using some sort of surge strip in which case all the grounds would end up the same wouldn't it be better to have cables with pin1 to chassis?

...and maybe in case I need to interface with other equipment at different ground potentials I could make a special ground lift cable adapter?

...this is also me sort of thinking out loud but if anyone can add to this or check/correct my facts that would be great.

Thanks again,

-d
 
Part of the difference in opinion is because signal ground and chassis ground are not always the same and connecting the barrel to signal ground may connect you to the chassis ground. THe chassis ground is to keep you from curling your hair if something shorts out and the chassis goes "live". The signal ground is to suck away all that bad stuff floating in the air. This is much less an issue these days since a lot of stuff is low voltage it was a much bigger issue when people were using tubes and you had some serious voltages floating around the inside of the box.
I connect the shell, though you have to remember that if you get a ground loop and start TSing by trying floating the shell on some connections to see if that is the cause.

SPX90 is a box where the signal ground is tied to the chassis (with often not so great results- it often needs to be floated in a rack or you get all kinds of ground loops).
 
Great responses guys. Thought I'd bump this just for S&Gs and see if anyone else might chime in. I still have about 500' of cable to put together!

I also emailed both Sound Devices and PSC with this same question and both responded back quickly and advised me to connect the ground to shell for RF immunity's sake.

cut & paste of the email is below...

Hi Derek,
We make all of our cables with Pin 1 tied to the shell of the XLR connector. This provides the best RF protection, especially on mixer inputs. Most of the time when you are operating portable gear, you will not have any ground loops because the gear is not being powered from AC. I would say that only 1 case in 100 will benefit from not having Pin 1 tied to the shell. If disconnecting the shell ground solves the problem, usually this just points out the fact that there is another underlying problem somewhere. By having the shell grounded, you are eliminating a point in which RF can get into the mixer via the wire terminations inside the XLR conncetor itself. I hope this helps.
Best regards
Ron Meyer
President, PSC


Derek,

We recommend grounding pin-1 to the shell. That short portion of cabling
is where interference can be injected.

Jon Tatooles
 
Ok, if pin 1 is connected to the XLR connector shell, then the shielding and pin 1 are the same potential; ground. Does that mean that the cable is now unbalanced?
 
nope, it will still be balanced.

Balancing works specifically on the fact that it is not referanced to ground.

- Mikko
 
gco said:
Are pin 1 (signal ground) and shielding are distinct from chassis ground ?
Click to expand...


Yes, pin 1 (signal ground) is used to return signal currents to their source,
as a voltage reference for signals, and for shielding signals from radio interference.
Chassis ground (AC third prong) is primarily there for safety.

Since they serve different purposes, you will often see external connectors
for making or breaking the connection. Ideally, from an audio point of view,
they shouldn't be connected, but safety is important, too. For example,
lifting the AC third prong will sometimes fix a ground hum, but then you
have no safety protection--i.e., you might touch a chassis with 110 or 220
volts on it. Surprise!

I have done a lot of medium-to-large touring sound systems, and have found
a pretty good technique that works 99.9% of the time:

1. Power the entire sound system from a single source (branch of house power),
separate from the lights.

2. Always use the safety ground (AC third prong) on every piece of audio
equipment.

3. Use balanced lines for all long cable runs.

4. Run the power along approximately the same path as the signal.
In other words, if you have a snake going to a house mixer, run the
mixer's power along the same path as the snake. NOTE: some of you
will disagree with this, but I can explain if you'd like.

5. If you do encounter a ground loop--for example plugging in a laptop
onstage produces a hum--then use a direct box to lift the SIGNAL ground
at that point. The laptop is getting two different ground connections:
disconnecting one of them usually solves the problem, and it's better
not to disconnect the safety ground.

6. In certain cases where a direct box won't work, an isolating transformer
with a ground lift will serve the same purpose.


So ideally your grounding will start at the power panel and fan out
to all of your sound system, following approximately the same path
as your signal lines. By doing this, even when there are multiple
paths to ground, they are similar paths, so they pick up similar
interference, etc. and are at equal voltage relative to your AC panel ground.

There are two situations where this approach has not worked for me:

1. If there is not enough power available, like a big production
in a very old house, then the lighting return current might be
overloading the sound feed. The solution: rent a generator for
the sound.

2. If you are in a very critical situation (huge contrast between
loud and soft) and the power amp's power supplies are not super clean.
The solution: get better power amps.

SOOOOO, if you get everything all hooked up and it's sounding clean,
it's because you've really thought about the ground and worked
hard to make sure it's good everywhere in the system. :)
So what if someone wired the XLR case to ground and one
of those cases accidentally touches a chassis or something?
What if this happens during the show/shoot? What if this
connection isn't solid (intermittent problems are the worst),
but makes and breaks as someone moves? BUMMER. You've got
a mystery hum! Where is it????? :(

This is why I don't connect the XLR case to anything.
This is why I don't connect patch bay grounds to the rack.
I want to control the ground path.

OK, that's my approach. I hope it's useful.

Peace and donuts,
Les
 
Last edited:
mikkowilson said:
nope, it will still be balanced.

Balancing works specifically on the fact that it is not referanced to ground.

- Mikko
Click to expand...


Terve,

I disagree. FLOATING means the balanced signal has no DC connection to ground.

BALANCING is different. It means that for each signal path there are two wires,
connected out of phase in order to cancel out radio interference that is picked
up along the path. Both wires are hot, so there must be some kind of ground
reference for the path to be complete.

Balancing is good practice. Floating is usually not necessary, and typically
requires the use of transformers.

Hey hey,
Les
 
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