Grounding on patchbay

Hi.
I know there already a thread on this, but it wasnt about patchbays, so I didnt want to hijack it.

Ive done as much soldering as I can without help now, and I still have 4 hours left to solder in, so I would appriciate some help to I can make use of that time.

This is what Ive got so far
Live room -> Patchbay top row, all soldered to all three pins in both ends.
The patchbay row under live rooms inputs has preamp inputs soldered to them, but no XLRs in the other end of those cables yet, The second section in the top row has the preamp outputs soldered to them, but no XLRs in the other end)

Which (if any) of the XLR cables do I need to leave the ground off, and Will I be able to send phantom power to my mics with this scheme ?

I was told to do this the whole way around on the bay http://home.flash.net/~motodata/patchbays/busswire2.jpg so I am going to do that when I put the patches back into the bay....unless there are objections

Last but not least. I have some presoldered D-sub to stubs. The Dsub end is going to my AD converters, the stubs should sit half normalled under the preamp outputs on the bay. Do I need to cut the ground on any ends of that ?

Its been mentioned in the other thread that its a world of "voodo", but I really need some kinda suggestion to get some work done now.

Gustav

Hi Gustav, the idea with your patchbay grounding is to prevent earth loops (multiple paths) in your studio. Ideally the only ground you want in there is via the mains power ground line. If you wire all your balanced cables with the earth connected at both ends, you are installing dozens of earth loop paths in your studio.

There are 3 common earthing schemes that I have come across.

Source earthing, where you connect the earth shield at the OUTPUTS of each piece of equipment, but not at the input end.

Destination earthing, where you earth the shields at the INPUTS of each device, and lift the shield at the output end.

Patchbay earthing, where you connect all the shields only at the patchbay, and then connect the patchbay to a good earth point so that all the shields are grounded in a star from the patchbay, and lifted at the other ends.

So, if you for instance chose Destination Earthing, you would connect all the earths at the patchbay, all the earths at the male XLR connectors that plug into the INPUTS of your various pieces of rack gear, and the console, and lift all the earths at the female XLR connectors that come from the outputs of each piece of gear (including the console). Hence each interconnection between 2 pieces of gear is shielded at it's DESTINATION. Source earthing is exactly the opposite.

You have to use common sense here - these earthing schemes are used when connecting pieces of BALANCED equipment together in the studio, ie:
Console line inputs and outputs and inserts and FX sends and returns (IF the console's sends and inserts are balanced, many desks are not) to: outboard gear, effects units, samplers and tape machines and audio interfaces, GSSL's and the like, in other words, all the stuff in your rack. Otherwise, you are just asking for hum.

Obviously, the trick is to decide where you need to connect earths - that is, anywhere you will need to send phantom power, usually mics and if you have phantom powered DI boxes. So all MIC line grounds are connected to the patchbay and to the mic inputs of your console, mic preamps etc.

So really you can lift one end of all your grounds except those from your console mic inputs and your outboard mic pre inputs. These usually go to a wall plate somewhere, and only get used with mics or phantom powered DI's, so there is no danger of loops there. As for console line inputs, if you lift the ground at one end and then connect an unbalanced piece of equipment to the input as long as you are careful making up your adaptors, the mono jack will automatically put the cold signal into the sleeve of the balanced line, so it will work OK too.

When you make up your plates to go in your studio, put specific mic input sockets there alongside the line input sockets, and connect the grounds all the way on the MIC inputs only.

VERY IMPORTANT: When you decide which scheme you want to use, as you dress each cable, strip and tin each earth lead as though you are going to connect it, and then pop a piece of heatshrink over each unconnected earth wire, and shrink it. That way if you need to connect a ground in the future, you just pull the shrink off and solder it onto the XLR pin.

To make this system work best, buy a bunch of DI boxes for your synths, basses etc.

[quote author="Steve Jones"]
VERY IMPORTANT: When you decide which scheme you want to use, as you dress each cable, strip and tin each earth lead as though you are going to connect it, and then pop a piece of heatshrink over each unconnected earth wire, and shrink it. That way if you need to connect a ground in the future, you just pull the shrink off and solder it onto the XLR pin.

[/quote]
Thanks a lot

I dont have a console, and for now Im just looking to get a mic->preamp->converters thing going. I also doubt Ill have any line level signals going thru the bay. I already got DIs to try to avoid this. (Smart DIs and a DW Fearn DI)

I wanna try leaving the ground in the XLRs going into the preamps open. Would I also need to leave the grounds on the Dsub going into the converters open, and...What about the phantom power thing. Im not quite getting that?

Gustav

Actually. I think that "put shrink wrap on the open ground" will do it. It wasnt to much for experimenting, but that little trick makes experimenting a whole lot easier....

Gustav

I wouldn't suggest lifting grounds in a D sub going to convertors, leave them on. The issue with phantom power is that it will not work if the earth is broken, so you cannot lift grounds on mic inputs, only line inputs.

Having said all this, it sounds like your studio is fairly small, so you may just be able to connect everything anyway, and if you have hum, just begin lifting grounds. The earthing schemes I mentioned are less important in a smaller installation, because there aren't so many earths to lift if there is a problem, so don't sweat it too much.

PS, one place that can cause unexpected loops are MIDI cables of all things! the centre pin is an earth, so if you have a mystery hum, try disconnecting MIDI leads, and if one is causing a problem, snap off the centre pin on one end of your MIDI cable.

[quote author="Steve Jones"]The issue with phantom power is that it will not work if the earth is broken, so you cannot lift grounds on mic inputs, only line inputs.[/quote]

Presumably there's also an inherent risk in having phantom-powered connections going through a patchbay with jack connectors - someone may pull out the wrong patch cord while the power is switched on, which would momentarily connect the wire the 'wrong way' and potentially damage the microphone. Is this correct?

PS, one place that can cause unexpected loops are MIDI cables of all things! the centre pin is an earth, so if you have a mystery hum, try disconnecting MIDI leads, and if one is causing a problem, snap off the centre pin on one end of your MIDI cable.

Click to expand...

Aren't MIDI inputs and outputs supposed to be opto-isolated to avoid such problems? I've no idea how rigorously this standard is upheld in practice, though.

Thanks for your advice earlier in the thread. I'm going to be wiring a new patchbay soon myself, so I found it very interesting.

cheers,
James.

I wont have any midi going thru the bay.
This is the layout Ive made.

Ill put some compressor and eq in it as well, (unconnected top to bottom row on 21-24 in second section), or maybe they will all go in a second bay along with some DA returns, but this layout is what I have to get working.

As you can see you a right. I dont have a lot of stuff, but I will get this bay filled, and it probably wont be long till I get another one filled up as well. The other bay will be stocked full of compressors, eqs and DA´s.



Gustav

Good luck, studio wiring is always a bit of an adventure...

Another thing that may help keep your studio noise floor down, if it is not already too late, is when you get your electrician to install all your power outlets, is to have him (or her) star earth the points. By this I mean running a seperate earth lead to each power outlet from the main earth point, rather than daisy chaining it as they normally do. The active and neutral are still daisy chained as usual, but each earth lead is seperate, so you don't have all your earths along an increasing resistive chain as you go along.

You probably have all your power in place already, but I am mentionong this in passing in case you haven't had the Sparkies in yet.

As for MIDI earthing, MIDI only requires 2 lines, it is simply a current loop flashing an LED in an opto isolator. Most gear however put an earth connection to the centre pin, which is shielding, but it creates a loop nonetheless, and I have more than once had sessions where a MIDI lead was the culprit. Busting off the centre pin fixes this, prefeably leaving the earted end connected to a grounded piece of gear, as opposed to an ungrounded piece of kit such as a merge box or splitter etc. MIDI is pretty robust, it won't suffer too much from lack of shielding because it takes a lot of interference to flash an LED!

[quote author="jlarcombe"][quote author="Steve Jones"]The issue with phantom power is that it will not work if the earth is broken, so you cannot lift grounds on mic inputs, only line inputs.[/quote]

Presumably there's also an inherent risk in having phantom-powered connections going through a patchbay with jack connectors - someone may pull out the wrong patch cord while the power is switched on, which would momentarily connect the wire the 'wrong way' and potentially damage the microphone. Is this correct?

cheers,
James.[/quote]

Yes, phantom can be a bit touchy on patchbays, particularly TRS 1/4 inch where people can plug unbalanced plugs in. There is usually enough current limiting to protect the phantom line, I haven't reallt given much thought to what it may do to various kinds of mic though. I haven't heard of any disasters though, so it may not be such a big deal.

[quote author="Steve Jones"]I haven't reallt given much thought to what it may do to various kinds of mic though. I haven't heard of any disasters though, so it may not be such a big deal.[/quote]

Apparently this is very detrimental for ribbon microphones! Happened to come across this over lunch, from the Royer website:

http://www.royerlabs.com/phantompower.html

When the TRS plug is shorted the 'wrong way' during connection or disconnection, the phantom power is applied directly across the ribbon element, which is very bad for the lifetime of that element.

I suppose the best thing to do is to make sure that the (potentially) powered section of the patchbay is clearly distinguished from the rest. If you were building your own, you could even use those lockable jack sockets that Neutrik make...

cheers,
James

FWIW: I always thought that running microphones through a patchbay was a Bad Idea. There are times when you must, but many more times when you can avoid it if you just stand on principle. Weak mike signals shouldn't get any more run-around or bad-contact than possible. In most small studio work (at least before "flavored" preamps), you can just run your board mic ins 1-8 to jacks 1-8 in the studio wall, and plug-up while setting mikes.

I confess I have run a patched-mike installation (not my design). Sometimes it was convenient, but very often it was buzz-city from poor TRS contacts. And once we did have a Phantom-like power cross (but just 10mA into a custom 60mA mike, no harm done). Over time, I changed to "patching" with XLRs direct from a wall-plate into the board, bypassing the TRS patchbay. ("Patching in the studio" was not an option: the stage was about 60 steps below the booth, I had no helper, and I wasn't running back and forth any more than possible.)

In my facility, mics (and everything else) are run through patchbays. This is just usual broadcast practice and I'm used to it. You must lay out your patchbays very carefully if you do this, though.

Also, if you're using a patchbay without "bussed grounds", and all the jack terminals are simply brought out to punchdowns, Christmas tree blocks or whatever, you have to be sure to connect the grounds between the source and destination jacks. One of my predecessors at my current job didn't do this when he wired the patchbays; I had to re-do almost all of his wiring to get the mics to work properly and noiselessly.

But that's in a big plant with a "maximum instant accessibility in case of on-air emergency" design philosophy. In a small recording studio, there's really no big advantage to running the mics through patchbays.

[quote author="PRR"]FWIW: I always thought that running microphones through a patchbay was a Bad Idea.[/quote]

One case in which this is useful is for a piece of gear that has can take microphone or line-level signals at its XLR inputs, and has some other useful processing on board (eg. compression). You'd commonly want this connected to the studio panel during tracking, but re-patched to some inserts (or whatever) during mixing. Is there a convenient alternative to wiring the mic lines through the patchbay here?

J