Multi room studio grounding. Where to lift?

Hi there. I need some expert advise on how to plan this right.
Here's the scenario:
We are in the process of designing a 3 studio complex for post production purposes. We are gonna have a machine room where all the video recorders etc are gonna live, as well as a patchbay that will be accessible from each studio via tie-lines. Each room will have a rack with outboard gear and a DAW system all accessible via patchbay.
We are getting each room's AC ground direct to the main power box which is 20meters away from the machine room.(and I think it connects to a water pipe) And the machine room is kind of surrounded by the studios.
SSLTech mentioned somewhere that he likes to collect the signal grounds at the patchbay. In other words he lifts the shields at the rack or outboard end of connections.
Now this is where I get a little confused. Our TT patchbays don't allow for shield soldering, there's no lugs for it. OK let's say we stripped the shileds of each cable and connected them to the bay somehow. And then what? Do I carry the ground from the bay to the mains ground?

Also what about the connection between each control room patchbay and the machine room patchbay? Which side do I lift? Seems like I need to lift the control room side. And then lift the tape machines as well.

Or do I forget about all this and follow a more confusing lift the outputs connect the ins kind of a scheme?

I am really confused. :?

I recently found this reading on Jensen very usefull for a better general understanding were to ground.

http://www.jensentransformers.com/an/generic%20seminar.pdf

Maybe it can help you
:guinness:

Himm, interesting read. I'll take a look. Cheers.

I recommend lifting the shields of all balanced, line level inputs and connecting the shields everywhere else. We've been doing this on installs including multi room facilities for 20 years and it works very well.

Hi David,
You mean lifting them at the gear side?
I was checking our Patchbay rows and some of them have the shield lugs and some of them don't. In that case would it be more feasable to lift grounds at the patchbay side?

Or better yet how about lifting all the shields coming to the bays except for the machine room side and have the bays attach to the mains ground?

Yes, I should have been more specific about that. Lift shields on the gear side -- at the male XLR connectors, the 1/4" TRS input connectors, the DB25 input connectors, etc. Do the same on the AES inputs. This will minimize ground loop currents, especially between rooms.

Don't lift or buss shields at the bays, and definitely don't connect them to a mains ground. Just connect high, low, and ground to TRS and where you have normals, connect the shields between those two jacks.

Sorry, but I disagree with the above comment about isolating rack gear from the racks.

There are many problems that tend to come up if you try to do this. Tops and bottoms of rack gear can contact each other (intermittently), causing crackles and intermittent hum. The insulators tend to break and get lost and it's hard to get things in and out of the rack quickly. Since the random voltage differences between chassis don't have a big thick rack rail to pass through, they can pass through SPDIF, word clock, and unbalanced audio cables instead, which is worse.

If everything is wired right you can just bolt everything into the rack, and not sweat it, and the system will usually be very quiet (individual equipment problems notwithstanding).

I do agree with the caution about computer gear, though. It's good to be extra careful there. Consider using balancing amps with the audio I-O's on a PC or laptop.

"Without this isolation you would then have an electrical ground connection between all racked units running through the case flanges and electrical grounds connected at the AC mains outlets. An invitation for an unwanted ground loop. "

Yes, but as I said, if the rack rails are not there as a low impedance "buss", then current will tend to take other paths like unbalanced audio shields, word clock cables, and SPDIF, which is worse.

Regarding the equipment touching each other, imagine two 1176LN's in a rack. They have steel chassis and the tops and bottoms are fastened with hex head screws, whose heads stick up on the top and down on the bottom. If these units are adjacent, one of three things will happen.

1. The chassis won't touch each other, they are insulated from each other by air.

2. The chassis will touch each other and their grounds will be connected together.

3. The chassis will lightly brush one another, causing an intermittent connection, and a grounding situation that changes at random.

I used 1176LN's as an example, but the same thing can happen with almost any two pieces of gear. Heavy pieces of gear will tend to droop in back and rest on the piece underneath.

The problem is that there's no control over this. Links between adjacent chassis are random, and may change from day to day. This makes the grounding system an unknown quantity that is not entirely designed by the installer, but partly dictated by which bits of hardware happen to contact each other in the rack.

99% of the time, telescoping shields (cutting them at balanced line level input connectors) will yield super quiet results. The other 1% is usually caused by gear that's unbalanced, needs to be serviced, or has a poorly designed power supply. These issues can usually be dealt with on a case by case basis without compromising the overall grounding scheme. In rare cases, the only practical solution is to insulate a piece of gear from a rack, but I would put this near the last resort category. You won't see major studios or broadcast facilities using this method.

We've installed around 250 studios over the last 26 years so I can assure you that the above comments are based on much field experience, not to mention many instances of learning things the hard way.

Hey Roger,

Those are good questions to raise here. (You sound technical but I will put this in a basic way for others who may read this.)

Re #1, imagine two pieces of rack gear sitting side by side on the floor and powered up. Due to small leakage currents on the power transformer, capacitance in the wiring harness, and the vagaries of how they are grounded internally, there may be slight differences between their ground potentials. If you measure between the two chassis with a DVM, there will probably by a small AC voltage. Maybe it's just 25 millivolts, but that's enough to cause ground noise. If you connect a wire between the two chassis, some small amount of current will flow through the wire.

(The currents will tend to be greater with big hefty pieces of gear like power amps, high current effects units, etc.)

If those two units are mounted in a rack the usual way, and also have their chassis connected together by (for example) an unbalanced audio cable the noise current will tend to flow through the steel rack rail, since it's the path of least resistance. But if the units are insulated from the rack rail, that current can only flow through the cables, so it may be imposed on the audio signals.

Re #2, we don't go out of our way to check the resistance of the connection to the rack rail. But at the conclusion of an install, we listen to each output of each piece of gear at very high monitor levels, to evaluate the background noise. If we hear background noise on a certain unit we'll test to determine the cause. If the cause is poor connection to the rack rail then yes, we'll use a star washer, or maybe add a thick ground wire, or something.

People have different grounding methods. It sounds like you deal with some very interesting applications.

Yeah David thank you very much.
On that note though..do you recommend attaching a bad ass grounding connection to the rail itself?

[quote author="David Kulka"]...
The problem is that there's no control over this. Links between adjacent chassis are random, and may change from day to day. This makes the grounding system an unknown quantity that is not entirely designed by the installer, but partly dictated by which bits of hardware happen to contact each other in the rack.
[/quote]

At Mount Wilson Observatory above Los Angeles, there was rack-mounted instrumentation and a healthy amount of lore surrounding its use in the very intense RF environment, the mountain also serving as home to a great many major TV and other transmitters.

One I recall was how tightening a particular screw on the rack adjusted the offset voltage in a particular system. The rotation of the dome was also a crucial variable.

I used to joke that they issued dosimeters to the astronomers in the form of Ramona brand machine-dispensed cheese burritos. You had one in your pocket, and when they cheese melted you knew it was time to get off the mountain.

When someone contracted with a firm in Arizona, which was started by some guys that had worked for Kitt Peak National Observatory and developed instrumentation, to make a detector system for Mt. Wilson, I counseled that the contract stipulate that the system would work within a few dB of its lab performance when installed on the mountain. It didn't, not even close in fact, and I was allowed in to see what I could do. By the contrivance of a bunch of Al foil and tape, I showed where a more robust shielding system might be deployed. I did get to within about 3 dB of what had been alleged to be the low-RFI performance.

I wasn't terribly astute politically in those days. The disgruntled tech took my example literally, and I was reviled as someone who recommended slipshod arrangements of aluminum foil etc. These were days before email, so I should have documented my advice in writing.

[quote author="Mundox"]Yeah David thank you very much.
On that note though..do you recommend attaching a bad ass grounding connection to the rail itself?[/quote]

No, I don't recommend that. In general we avoid adding ground wires of any kind and my experience has been that often they add noise, rather than take it away.

We have seen installs where exotic and very expensive ground systems were installed, with elaborate buried elements and thick wires going to racks. Ironically, I think this does more harm than good and is usually a waste of money. In some cases, studios have installed elaborate systems like this but then left them disconnected, because they backfired.

The people at Jensen transformers call this the Sump Theory. There's an urban myth that if you use thick ground wires, and enough of them, noises will magically be diverted into the ground.

The best thing is not more ground connections, but fewer of them. In a balanced system with telescoped shields, ground currents are stopped before they start. An octopus of ground wires going every which way can cause ground currents to flow every which way.

This may seem to contradict my earlier advice to maintain bonded rack rail connections to all the gear, but it doesn't really, because the situation is different.

Notice that my recommendations for a quiet system also save time. Telescoping the balanced inputs makes your job easier because there's one less wire to solder. Simply bolting gear in the rack is easier than using special hardware. Avoiding extra ground wires saves time and reduces clutter. Form follows function?

Of course, studio grounding is not black and white, and there are exceptions to all these things. The best thing is to begin with proven methods, test very carefully, and then rectify any problems on a case by case basis.

Lot's of good advice here, but I would slightly reframe the perspective of this discussion. IMO there aren't ground problems, as much as equipment or equipment and wiring problems (ignoring the rare actual ground wiring problem).

There are plenty of good guidelines based on safety concerns for mains grounding, and proper equipment design is tasked with operating well while safety grounds are in place. This isn't a new concept but historical performance of studio gear is a little shaky in this regard. Getting a mild shock between sundry units was all to common, but hopefully most of that gear is in some museum (or dump).

When dealing with poorly shielded and/or unbalanced wiring there is perhaps merit to brute force approaches but with proper balanced wiring and well designed gear "ground loops" or modest ground potential differences are harmlessly ignored as common mode to valid signals and removed by differential receivers. Lifting shields on one end, if indeed the shield is a redundant ground, may open a LF sensitive loop while at the same time reducing effectiveness of HF shielding, so YMMV.

There may be some older legacy gear around studios that doesn't handle grounds well, but this is not the ground wiring's fault and can be mitigated with external audio transformers or perhaps wiring tricks, but I would advise against ever lifting safety grounds as people can get exposed to dangerous voltages by equipment power faults if there is no robust path to ground to quickly trip breakers.

I don't mean to trivialize this subject and there are good books on the general subject, perhaps lacking chapters on studio specifics. I would be inclined to test studio gear for sensitivity to noise currents injected at pin 1 of balanced I/O. This should give you an early warning regarding what to expect.

There are some typical pin 1 test circuits published on the web but you can roll your own from a low voltage AC wall wart with some series resistance to current limit and generate an artificial ground loop connected between pin 1 and chassis or safety ground. Perhaps a diode and LED in one polarity of the AC current will dirty up the loop waveform and make it more audible. A proper product design should provide a low impedance path from pin 1 to chassis/safety ground, and not corrupt signal between pin 2 and pin 3. I wouldn't go crazy and dump amps into pin 1 but tens of milliamps seems a fair test. A LED would give visual confirmation of quality of pin 1 ground connection.

If pin 1 is floated inside the gear it will open any ground loop so no loop current will flow, but shield currents will have no where to go either so effectiveness of shields is compromised.

I expect there may be plenty of legacy equipment that is not up to this current design thinking that AFAIK only became widely publicized and discussed over the last several years. You will need to do what it takes on a case by case (chassis by chassis?) basis. As other's have noted extraordinary attention to mains wiring may be attacking the wrong end of the problem.

Have fun and good luck.

JR

Agree with John---if equipment were properly designed lifting shields would almost never be necessary. The AES workshop and subsequent dedicated journal issue on Ground Noise, years ago, brought this home emphatically.

One interim fix BTW---if you are plagued with RF while still needing to do a shield lft at one end, try a small capacitor on the otherwise-lifted end, to act as a low Z ground for high frequencies.