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Shared ground reference phantom power?
Old 1 week ago
  #1
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Shared ground reference phantom power?

Something I didn't really see addressed about mics thru patchbays:

if a patchbay mic line is normalled to a console input and the shells/shields/grounds are vertically strapped, when you patch the mic line into a outboard pre and engage
Phantom power the patch cable is connecting the 0v reference pin #1 to mic pin #1 and because of the vertical strapping also to the consoles inputs #1 pin.

Is it not a problem if the electrical ground pins are on the same outlet/circuit

How do other people with mic lines on a patchbay deal with this?

Is it possible to strap the grounds with a diode to prevent this?
Old 1 week ago
  #2
Lives for gear
 

It is not a problem and entirely a non-issue...however, shells, IMO should not be connected to pin 1
Old 1 week ago
  #3
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Quote:
Originally Posted by ripple_fx1 View Post
It is not a problem and entirely a non-issue...however, shells, IMO should not be connected to pin 1
so for a patch cable and patch point you have tip, ring and sleeve
I was assuming that the sleeve gets connected to pin #1 on a xlr, is that not right?
Old 1 week ago
  #4
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Yes, the sleeve would be connected...which becomes the metal shell on a 1/4" plug by virtue of the plug design, is insulated on a tt, and should not be strapped to the metal shell on an XLR
Old 1 week ago
  #5
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Quote:
Originally Posted by ripple_fx1 View Post
Yes, the sleeve would be connected...which becomes the metal shell on a 1/4" plug by virtue of the plug design, is insulated on a tt, and should not be strapped to the metal shell on an XLR

tip to pin 2, ring to pin 3 and sleeve to pin 1
Old 1 week ago
  #6
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Typically, yes, but don't do any polarity/balanced/unbalanced determination at the back of the patchbay or the tt cables - keep them consistent...'hot' & 'cold'/bal/unbal determination should be done in the termination at each individual piece of equipment.
Old 1 week ago
  #7
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Quote:
Originally Posted by ripple_fx1 View Post
Typically, yes, but don't do any polarity/balanced/unbalanced determination at the back of the patchbay or the tt cables - keep them consistent...'hot' & 'cold'/bal/unbal determination should be done in the termination at each individual piece of equipment.
So those #1 pins being shared isn't a issue when you patch the mic into a pre then turn on the phantom and it's connecting to the consoles pin 1 as well?
Old 1 week ago
  #8
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Ah.., sorry, now I understand what you're up to. Pursuit of aesthetics. I still doubt that it will be an issue, but if for some reason it does become one, you simply clip your vertically strapped jumpers and gaze sadly at the hated rows of patch cables. Not that it matters, and perhaps you weren't saying so, but engaging phantom power will not employ a switch pole connected to the shield, only the twisted pair.
Old 1 week ago
  #9
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All it will do is connect the audio ground of the pre and the console. This won't make a difference to 48V since it's only looking for a path back to where it originated, but it could however create a ground loop. One of the reasons why normaling mic inputs isn't a good idea.

Quote:
Originally Posted by SunkenCity View Post
so for a patch cable and patch point you have tip, ring and sleeve
I was assuming that the sleeve gets connected to pin #1 on a xlr, is that not right?
There's a 4th connection on XLRs to the metal shell - it's the tab in the middle on cable connectors and the tab above pin 1 on panel connectors. This is permanently connected to chassis which can be useful in special circumstances but shouldn't be connected normally.
Old 1 week ago
  #10
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Quote:
Originally Posted by ripple_fx1 View Post
Ah.., sorry, now I understand what you're up to. Pursuit of aesthetics. I still doubt that it will be an issue, but if for some reason it does become one, you simply clip your vertically strapped jumpers and gaze sadly at the hated rows of patch cables. Not that it matters, and perhaps you weren't saying so, but engaging phantom power will not employ a switch pole connected to the shield, only the twisted pair.
Aesthetics? Switch power pole? Some of your posts have been confusing.
Old 1 week ago
  #11
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Quote:
Originally Posted by samwinston123 View Post
All it will do is connect the audio ground of the pre and the console. This won't make a difference to 48V since it's only looking for a path back to where it originated, but it could however create a ground loop. One of the reasons why normaling mic inputs isn't a good idea.



There's a 4th connection on XLRs to the metal shell - it's the tab in the middle on cable connectors and the tab above pin 1 on panel connectors. This is permanently connected to chassis which can be useful in special circumstances but shouldn't be connected normally.
Will it only make a ground loop if the console and the pre are on different outlets/circuits, isn't that when ground loops are actually ground loops?

I figured that was what he was trying to get at with the xlr case and got mixed up in terminology.
Old 1 week ago
  #12
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By "aesthetics", I was commenting that it appears your focus is to have a patchbay with no semi-permanent patch cables messing up the clean look of the rack. By "...switch pole" I was trying to make clear that with your statement "when you patch the mic line into a outboard pre and engage Phantom power the patch cable is connecting the 0v reference pin #1 to...", that engaging phantom power does not physically connect "the 0v reference pin" due to the act of throwing the switch..."the 0v reference pin" is already connected as a matter of course. I assumed you knew that, but wanted to clarify, JIC. Sorry for any confusion.... Definitely, the "starrier" the ground the better...difficult to implement in a complex installation, but as long as you haven't scheduled a session for the next day, you'll have time to sort it out.

If you're right handed, solder left to right, bottom (inputs) row first.
Old 1 week ago
  #13
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Quote:
Originally Posted by ripple_fx1 View Post
By "aesthetics", I was commenting that it appears your focus is to have a patchbay with no semi-permanent patch cables messing up the clean look of the rack. By "...switch pole" I was trying to make clear that with your statement "when you patch the mic line into a outboard pre and engage Phantom power the patch cable is connecting the 0v reference pin #1 to...", that engaging phantom power does not physically connect "the 0v reference pin" due to the act of throwing the switch..."the 0v reference pin" is already connected as a matter of course. I assumed you knew that, but wanted to clarify, JIC. Sorry for any confusion.... Definitely, the "starrier" the ground the better...difficult to implement in a complex installation, but as long as you haven't scheduled a session for the next day, you'll have time to sort it out.

If you're right handed, solder left to right, bottom (inputs) row first.
ah got ya.

Do you have first hand experience with this not being an issue?

Is this not an issue if the consoles electrical ground and the pres electrical ground are on the same outlet/circuit?
Old 1 week ago
  #14
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I do, although in that particular installation, the top and bottom row ground lugs were bused and tied together...and not by me. Re: loops, there's less chance of it being an issue...and best case if you can get everything control room audio related onto one circuit...and all system gear onto one phase of electrical power, and anything with motors or ballasts on the other....
Old 1 week ago
  #15
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Quote:
Originally Posted by ripple_fx1 View Post
I do, although in that particular installation, the top and bottom row ground lugs were bused and tied together...and not by me. Re: loops, there's less chance of it being an issue...and best case if you can get everything control room audio related onto one circuit...and all system gear onto one phase of electrical power, and anything with motors or ballasts on the other....
I guess now it's a wait a see thing

Thanks
Old 1 week ago
  #16
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It is important to remember that in a balanced system, pin 1 not a signal connection, it is just a screen. The only signal carrying pins are 2 and 3.

Cheers

Ian
Old 1 week ago
  #17
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Quote:
Originally Posted by SunkenCity View Post
Will it only make a ground loop if the console and the pre are on different outlets/circuits, isn't that when ground loops are actually ground loops?

I figured that was what he was trying to get at with the xlr case and got mixed up in terminology.
A ground loop can happen (or not) in almost any circumstances, if the audio grounds of gear are connected together. It's just what happens when there are multiple paths to ground shared by multiple audio circuits. There is definitely a greater risk if they're on different electrical circuits, but it's not necessary. Factors within each piece of gear (ground trace layout, solder joint quality, wire gauge, power consumption, changes in the weather...) can cause it to happen with two pieces on the same circuit in the same rack. The only cure all is to not connect the audio grounds.
Old 6 days ago
  #18
Gear Nut
 

A more serious problem is placing a mike on a TRS patchbay in the first place. What happens when P48 is on and you insert the patchcord? In the process of inserting a patchcord, if the patchcord ring is touching the patchbay sleeve, and patchcord tip is touching the patchbay ring, you have 48V across pins 2 and 3 of the mike.

IMHO, mike patching should only be done using XLR connectors. Otherwise, you can blow up a lot of stuff in a lot of different ways.
Old 5 days ago
  #19
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Quote:
Originally Posted by Monte McGuire View Post
A more serious problem is placing a mike on a TRS patchbay in the first place. What happens when P48 is on and you insert the patchcord? In the process of inserting a patchcord, if the patchcord ring is touching the patchbay sleeve, and patchcord tip is touching the patchbay ring, you have 48V across pins 2 and 3 of the mike.

IMHO, mike patching should only be done using XLR connectors. Otherwise, you can blow up a lot of stuff in a lot of different ways.
Hot swaping has been covered many times and isn't a good idea even with xlr

Do you have any insight on the ground loop created by patching in this scenario?
Old 5 days ago
  #20
I used to turn off the lights when patching mics in a bay. The sparks were great. The repair pay even better.
Old 5 days ago
  #21
Gear Nut
 

Quote:
Originally Posted by SunkenCity View Post
Hot swaping has been covered many times and isn't a good idea even with xlr
And it will always happen, sooner or later - just a friendly warning. Mike amps are supposed to handle hot patching and P48 transients, also possibly caused by a flaky cable, but there are examples of gear that lack or have really poorly done over-voltage clamping circuits. Apply a big transient to a large geometry input stage transistor (like the first stage of a transformer-less mike amp) and it can fail, but often not completely. Or, the clamp diode could partially fail, and induce distortion from odd DC leakage. The net problem is that the mike or the mike input could become noisier or have more distortion, which is actually worse than if it blew up outright.

Again, a friendly warning. I've seen these failures.

XLR will at least not connect the wrong pins together, which TRS will always violate, but XLR will not guarantee the order of pin connection, causing a smaller set of problems that are still serious.

Quote:
Originally Posted by SunkenCity View Post
Do you have any insight on the ground loop created by patching in this scenario?
A patchbay is usually 'declared' to be the master ground in a system, and between jacks, the pin 1 connection is usually solid and very low impedance, created by thick machined metal that mounts the jacks or at least stout bus wire connecting the sleeve connections.

Given that, the idea of a ground loop is more about how the preamp or console is connected to the patchbay. For a console with a built-in patchbay, you don't have to worry about it, since the console designer already did the work.

If you have a standalone patchbay system, system ground should emanate from the patchbay, so the normal handling of how devices are connected to the patchbay is what we're talking about. In general, neatly bundled groups of cables have very small loop area, so there's usually no problem treating each cable's pin 1 as the 'same' ground, and terminating all of them at both ends. Yes, you have a ground loop, but the loop area is very low, so it's not a problem. You also win since you're using all of the shield metal in the cable to connect ground at the two ends of the cable together, reducing the ground impedance between the patchbay and the remote device.

To summarize, use quality cable, bundle multiple runs neatly using cable ties, sleeving, or lacing cord, and you should not have any problems. This is not a special situation from any other studio wiring situation, so the standard rules will serve you well.

As a practical fact, I had a studio long ago using a Neotek Series II, which is a completely unbalanced console - all IO, except for the mike inputs, is unbalanced. I used star quad (Mogami 2799) cable wired with signal on pin 2, ground on pin 3 and used what's known as hybrid grounding for the shield - a direct connection from cable shield to pin 1 at the remote end, and a 0.1µF leaded MLCC capacitor connecting the shield to the console ground at the console end. The system was extremely quiet, despite having timecode and video signals moving between various gear, as well as a 50kW AM transmitter (1.03MHz) only a mile or two away. IMHO, ground loops don't have to mean noise problems - the entire Series II console is a huge ground loop and it's dead quiet.
Old 4 days ago
  #22
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Monte, I think we are all wasting our time. In spite of very good advice here, its pretty clear that sunken city is going to go ahead and do a TT patch bay. You can take the horse to water, but you cant make him drink!
Old 4 days ago
  #23
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Quote:
Originally Posted by Radardoug View Post
Monte, I think we are all wasting our time. In spite of very good advice here, its pretty clear that sunken city is going to go ahead and do a TT patch bay. You can take the horse to water, but you cant make him drink!
Why would it be a waste of time to elaborate on concerns of these ground loops if not just adding to reasons not to run mic lines thru a patchbay?

the main reason being hotswapping(whether accidental or not) the general consensus is to eliminate the possibility entirely, correct?
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