i'm installing a new system and have found that some of the gear needs
the signal ground lifted. should i do this at the patchbay or the
gear side?

what should be done about the mixer (balanced) to pathcbay connection
(all mixer i/o go to patchbay) ?

-i've read that grounds should be connected at mixer and lifted at gear
but i'm not sure how this applies when a patchbay enters the situation.

amble wrote:
i'm installing a new system and have found that some of the gear needs
the signal ground lifted. should i do this at the patchbay or the
gear side?

Do it at the gear end of the cable. That way the shield will be carried
through the patchbay if it doesn't need to be lifted.

what should be done about the mixer (balanced) to pathcbay connection
(all mixer i/o go to patchbay) ?

Connect the shields everywhere you can.

-i've read that grounds should be connected at mixer and lifted at gear
but i'm not sure how this applies when a patchbay enters the situation.

That's old school. Leave the shield (which you mistakenly keep calling
"ground") connected all the way. If you absolutely have to lift the
shield, lift it where it will only affect that one piece of equipment.

Don't lift so many shields that you break the signal path. And don't
elsectrocute yourself.

Mike Rivers wrote:
amble wrote:

-i've read that grounds should be connected at mixer and lifted at gear
but i'm not sure how this applies when a patchbay enters the situation.

That's old school. Leave the shield (which you mistakenly keep calling
"ground") connected all the way.

It may be "old school" but it's still the best way to avoid ground
loops. Sure, you can just wire everything up with the shields connected
at both ends and it will work most of the time. Until it doesn't and
then you can spend hours chasing stubborn ground loops.

The basic idea is a "star" ground system where each piece of equipment
has exactly one path to ground*. If you connect two pieces of equipment
that are both connected to ground via their AC plug and you attach the
shield at both ends you will have a ground loop. If it's a small one,
it'll have no audible effect, although external things can change to
make it become a problem, often at the worst possible moment. So the
"professional" thing to do is to lift the shield at one end, usually at
the output. Lift the shield at the output, connect it at the input.

Note that this is different than always connecting the shield at the
mixer. Connecting the shields at the mixer only will indeed give you a
star ground system, but imagine trying to apply this rule to a
multi-studio complex with 4 mixers, each of which can be routed to any
of the others - where does it tell you to lift and where does it tell
you to connect the ground? Better to think in terms of inputs and
outputs - that's always clear. Plus there's a slight advantage of
tieing the shield near the higher impedence input than at the lower
impedance output.

Now, as far as the patchbay goes, you have two choices: carry the
shields through the bay, or continue with the approach of lifting the
shields on output and tieing them on the input (i.e. think of the bay as
a device). I've seen both approaches work, the main thing is to be
consistent.

//Walt


* If you've studied topology, this is equivalent to saying that the
ground system should have a trivial fundamental group. If you haven't,
nevermind.

Walt wrote:

It may be "old school" but it's still the best way to avoid ground
loops.

No, the best way to avoid ground loops is to not have gear that causes
ground loops when you connect the shields. Today EMI is a bigger
problem than it used to be back when one-end-only shielding was in
vogue. While you may be able to fix a ground problem by lifting a
shield, you might cause an EMI problem that's worse than the ground
loop. Or maybe not. So you trade one problem for another. But if you do
things right, you can avoid both problems.

Sure, you can just wire everything up with the shields connected
at both ends and it will work most of the time. Until it doesn't and
then you can spend hours chasing stubborn ground loops.

You have to do it systematically. If your system is quiet with all the
shields connected and then gets noisy, either you've added something
new (so you know where the problem is) or something broke and you have
to find (and fix) it. If you hook up things one at a time starting with
the monitors, if something hums when you connect it, you fix it before
you connect the next thing. Do it right and you only have to do it
once.

Or you can leave holes in your shielding. Just make sure everyone turns
off cell hones and cordless phones, and keep your computer monitors
away from your audio cables. And don't build your studio near a
broadcast transmitter.

* If you've studied topology, this is equivalent to saying that the
ground system should have a trivial fundamental group. If you haven't,
nevermind.

And if you haven't read the June 1995 AES Journal, you haven't studied
grounding. It's practical, it's not rocket science, and you can order
the issue for $15 he
http://www.aes.org/publications/journal_issues.cfm