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

amble wrote:
Thanks for your replies, the info you have provided is helpful.

One followup question--if I'm connecting some unbalanced gear to the
patchbay in most cases is it better to use balanced or unbalananced
cables?

Cables are neither balanced nor unbalanced. Cables are just cables.

If you are connecting unbalanced gear to a balanced patchbay, the easiest
thing is usually to use two-conductor cables and telescope the shield at
the unbalanced end. This uses the power line safety ground as ground
reference for the box.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

amble wrote:

One followup question--if I'm connecting some unbalanced gear to the
patchbay in most cases is it better to use balanced or unbalananced
cables?

Slight misnomer there, but it's getting so common that it's almost
acceptable. I always use two-conductor shielded cable (what you call
"balanced") and wire the connector at the unbalanced end so that the
equipment works. But it means that you need to wire the connector and
not use a pre-made cable.

Let's say you have a cable with a black and a white wire, plus the
shield. At the patchbay, connect the white to the tip and the black to
the ring. At the other end of the cable, where it's plugged into
something that's unbalanced, connect the white wire to the tip and both
the black wire and shield. That will put the signal beween the tip and
shiled at the unbalanced equipment, but between the tip and ring on the
ptachbay. If you use 2-conductor (TRS) patch cables, you'll be able to
patch between balanced and unbalanced equipment without worrying about
what's hot and what's not.

There are only a couple of exceptions. First, some older equipment was
built with XLR connectors but unbalanced outputs, and the signal was on
Pin 3 (conventially equivalent to the Tip) and nothing is on Pin 2. So
you have to wire to that accordingly. Also, there are some
electronically balanced outputs that are build so that they don't like
to have the low side grounded (which is what happens with this wiring
scheme when you patch it to an unbalanced input). If you have one of
those, you have to make an exception to the all-the-same-patch-cable
feature and use a special patch cable that has the ring open so it
doesn't get grounded at the unbalanced input.

And before you ask, you just have to figure those exceptions out. They
don't usually tell you about them (though sometimes the
balanced-unbalanced with Pin 3 floating warning is noted in a manual).
And to make your life even more interesting, there are some
electronically balanced outputs that need to have the low side
connected to ground if you're connecting them to an unbalanced input.
(All of the Mackie Onyxen except the 1220 is like this)

Scott, Mike, Walt, thanks for your help!

-Mike could you just clarify one thing for me--

At the other end of the cable, where it's plugged into
something that's unbalanced, connect the white wire to the tip and both
the black wire and shield.

let me make sure i understand--at the unbalanced side, if you are using
a TS connector, you connect the black wire and shield to the sleeve?

On 9 Mar 2006 18:42:17 -0800, "amble" wrote:

Scott, Mike, Walt, thanks for your help!

-Mike could you just clarify one thing for me--

At the other end of the cable, where it's plugged into
something that's unbalanced, connect the white wire to the tip and both
the black wire and shield.

let me make sure i understand--at the unbalanced side, if you are using
a TS connector, you connect the black wire and shield to the sleeve?

For balanced outs feeding unbalanced ins, I'd connect the white to tip
shield to shield and float the black wire altogether. If you tie the
black and shield together you are shorting out the minus side of any
balanced outputs and that is not always a good thing.

Also just to disagree with everyone else who's weighed in, I am not
opposed to breaking grounds at the patch bay. I've seen it work
really well in some studios.

Julian

amble wrote:

At the other end of the cable, where it's plugged into
something that's unbalanced, connect the white wire to the tip and both
the black wire and shield.

let me make sure i understand--at the unbalanced side, if you are using
a TS connector, you connect the black wire and shield to the sleeve?

Yes. The sleeve is connected to the chassis, which is the shield for
the unit. By connecting the cable shield to the chassis, you maintain
the shielding. all the way to the device you're connecting.

Julian wrote:

For balanced outs feeding unbalanced ins, I'd connect the white to tip
shield to shield and float the black wire altogether. If you tie the
black and shield together you are shorting out the minus side of any
balanced outputs and that is not always a good thing.

I did mention that this may be required, but it wasn't the first
choice. There are some outputs for which it's imperitive to connect the
low side to ground if you're not connecting to a balanced input. A
transformer output is a good example, though you don't see them a lot
any more. And a cross-coupled electronic balance doutput is another.
This is why you have to either know what you have, or be able to
understand when (and why) what you've done isn't working right.

Also just to disagree with everyone else who's weighed in, I am not
opposed to breaking grounds at the patch bay. I've seen it work
really well in some studios.

That was a good way to work when we had tape hiss and fairly high
levels of internal hum to cover up EMI noise. Now that everything is
quiet enough for digital, your ground system has to be, as well.
Leaving a hole in the shield for EMI to creep in is not helpful.

I do have a couple transformer outputs on my gear, perhaps I should
connect low to shield.

Rather than making alll new cables with 2 conductor to TS plugs,
perhaps i will use my existing 1 conductor instrument cable to TS
plugs, connecting T to T and S to S at the patchbay, then putting a
jumper between R and S at the patchbay. Could this be a problematic
shortcut? It would make things alot easier for me.

amble wrote:

Rather than making alll new cables with 2 conductor to TS plugs,
perhaps i will use my existing 1 conductor instrument cable to TS
plugs, connecting T to T and S to S at the patchbay, then putting a
jumper between R and S at the patchbay.

It will work, but you'll want to rewire if you change that unit to
something with a balanced input or output. And heed the warnings about
the balanced outputs that might be a problem.