The Problem: How to accommodate as many forms of internal equipment grounding designs as possible in your sound system while addressing the open ended shield problem in the system cabling and the signal grounded shield problem in the equipment.
According to Stephen Macatee (of AES fame) in his digestible "Grounding and Shielding Audio Devices" (Note 151) for Rane's web site, the "absolute best right way to do it" would be to modify every piece of audio equipment not having its internal shields routed properly to the chassis bond point and have the shields connected at both ends. Absolutely, I agree. However, until the manufacturers get it together, I'd rather "bring Mohammed to the mountain" for large sound systems. For recording studios or a small one system oufit sure, modify, but for a sound company or very large PA system, that can be a mammoth technical and time consuming undertaking. It also makes the system a bit unforgiving with respect to the gear one is able to deploy as most of the gear out there isn't designed properly. Another way to circumvent this problem would be to have custom multi-pin line level Input/Output rack panels made that redirect the incoming shields to a specified high quality AC ground contact point within the rack housing the equipment instead of the shield terminating at the equipment signal ground ( Giddings "G&S for S&V" 6.2.3) . This gets the shield connected at both ends and bypasses the equipment issue altogether. These panels could accommodate balanced pro audio gear of virtually any internal grounding design and when deployed, the gear would suffer very little downtime in the process.
The FOH Drive Rack to Amp Rack Interface.
The most urgent need for these types of panels are in long signal cable runs such as the interface between the FOH drive rack and the stage left and right amplifier racks, which in stadium shows can easily reach lengths of 300' feet from the FOH to the stage wings. Many mobile arena systems have at least 250 feet or 100 meter input and crossover return snakes as standard. Some local sound companies whose primary income source is "industrial" applications routinely put their crossovers in the amp racks to simplify their grounding and cabling from the FOH to the amp racks. Some sound companies, rather than try to employ esoteric ground schemes to beat the buzz in the drive chain, simply put transformers at the inputs to every amp rack. Ultimately as laptops & software drive programmable digital crossovers such as the XTA 226 through data lines, crossovers can be located at the amp racks and still give the FOH engineer the ability to "tweak" them - which is the ONLY reason why they arrived at the FOH to begin with. Many regional sound companies put their crossovers in their amp racks much to the chagrin of experienced engineers.
Existing Methods: (Other than having the crossover in the amp racks)
- In the FOH crossover drive rack to stage amp rack chain, the most common method is to drop the shield at the input to the amplifier as shown in the diagram of a Simple Shield Telescoped System. The shields are simply dropped at the amplifier input, typically in the male XLR connector or a terminal block.
- However, there are sound companies who elect to lift the shield just before the crossover output multicore exits the FOH drive rack, often after an output transformer. The shields then run continuous to the amplifier inputs. The main justifications for dropping the crossover return shields at the FOH drive rack end of the shield are this:
- It's easier and cheaper to modify the few drive racks than all the amp racks.
- If the crossover snake from the drive rack is inadvertently disconnected or needs to be disconnected quickly in an emergency, that disconnection is more likely to be harmless and generally silent (especially when using crossovers with output transformers). It would also maintain the shielding along the length of the crossover multicore to the stage amplifiers. If the shields were, by design, lifted at the amplifier end of the shield and the crossover return multi became disconnected at the Front of House end, the amplifiers would be getting unterminated input from more than 100' (generally more than 250') of unshielded signal cable and the disconnection would most likely be a loud one. I don't particularly recommend dropping the shield at the FOH drive rack end because the noise susceptibility at the amplifier input end far outweighs the benefits of being able to disconnect the system from the drive rack while powered up - something you shouldn't want to do anyway. Also, as I said, this would render the system amplifiers susceptible to noise if the amplifiers were of poor design with respect to internal shield handling. If the design of the sound system is to drain the crossover return shields to the stage amp end (not uncommon but I've used several systems like that) then care should be taken that the amplifier design is such that its internal shield connections are routed directly to the amplifiers' chassis/earth ground and not connected directly to the signal/audio ground of the amplifier's circuit. Unfortunately, many pro-audio amplifiers route the input shields to the audio ground by design - a byproduct of their built-in factory Solutions #3 or #4 (Design types #6 or #7) of isolating audio ground and chassis ground. Happily, many sound companies choose not to use that factory supplied solution and adhere to input shield dropping conventions for the amplifiers. See Section Three for test procedures to determine internal shield routing of equipment.
Also, if the shields are intact at the amplifier end, you can create a ground loop between stage left and right amp racks if one of the racks are running in stereo. For example, the right mid crossover feed can end up being grounded on both sides of the stage creating multiple shield terminations on the same signal line which forms a ground loop between the mid power amps among the interconnecting signal cable and the common AC grounds of the power amps.
- A more expensive alternative is to install line transformers in each amplifier rack to provide isolation from both the FOH drive rack and all other amplifier racks connected to the same crossover outputs. Simple shield telescoping or the redirect panels explained below will give you very good performance but if you want "over the top" hum prevention this is it. However, it can be very expensive to outfit every house amplifier rack in an inventory with as many as 8 high quality line transformers if the rack is to power a stereo 4 way speaker system. Of course the number would drop in half if they are to be a dedicated mono 4 way rack. One very beneficial thing that can be done with this arrangement is "tuning" the entire interface between the FOH crossover outputs and the amplifier inputs with some simple resistors and capacitors along with the transformer. This can be an ideal way to provide superior isolation as well as and negating the frequency response problems caused by the capacitance of long signal cable runs (Cable Anatomy I) from the FOH drive rack to the amp racks. For more information on how to set this up, see the following link on the Jensen Transformers web site. JT-11P-1 Receiver for Very Long Line Applications (17KB PDF) Naturally you would need a separate transformer setup for each crossover line entering the rack. Personally, I think one can achieve acceptable performance without having to do this. In the days before balanced inputs on amplifiers became the standard, I imagine this practice was more common.
Diagram of Shield Redirection for FOH to Amp Rack Interface. An incoming shield is normally lifted at the input end in a shield telescoping scheme but in order to allow shield termination at both ends, the issue of equipment with signal grounded shields must be addressed. This can be accomplished by redirecting the shield via a quality ground path directly to the incoming AC/Earth ground of the rack containing the equipment - bypassing the equipment altogether. The rack's internal signal distribution cable then proceeds to the equipment's input(s) and is pin1 lifted at that point via a bridged resistor capacitor network. The ISO switch is there to add complete grounding flexibility allowing the rack to instantly emulate a shield lifted scheme if need be for any reason while the inline iso networks minimize RF contamination if this function is utilized. When the switch is open, the shield is no longer directly AC grounded and simply resembles shield telescoping with an iso network bridged across what would normally be a open shield. If the amp racks need to be pressed into service with systems of different grounding, they can resemble a virtual shield telescoping scheme at the flip of a switch. The drive rack also has this function for 100% grounding flexibility although the switch would normally remain closed in most situations. I came up with this design although I haven't had the opportunity to build them yet. However, based on the new AC grounded shield conventions, these should work very well as it also provides for conventional isolation of the shield.
The Console to Insert and Effects Racks Interface.
These interfaces are local and typically 15 to 30 feet and you may not realize that much of and increase in signal to noise performance over simple shield telescoping. I would consider this if you are still experiencing problems as regular shield lifting should be more than acceptable for signal runs under 30'. Additionally, in effects and insert racks, there will be a necessarily higher level of complexity because you have output shields as well as input shields to redirect. But, if you want to go all the way and standardize all interfaces to reflect a system-wide scheme, you could implement the same system for these racks.
Diagram of an insert or effects rack interface. Because these interfaces are with mixing consoles they need the added flexibility to isolate the send and return shields at the rack end as opposed to the console. This is because modifying the console is not an attractive option. The iso switches are a good option here as redirecting at the console end is problematic.
The level of complexity of the above panels is completely dependent on design parameters required. There are some non-redirecting amp rack panel designs having provisions to select (via mechanical switching) which set of crossover outputs the amplifiers see as input. Some even allow the selection of different banks of crossovers in multi-array systems. This would add considerably to panel design complexity (as well as the probability of failure!). There are also some engineers who are not huge fans of bussing shields together. To avoid bussing the shields you could have discrete switching on every shield line and then bus the switch outputs to the same single AC ground point but the end result is that they all end up grounded to the same point anyway. With discrete shield switching, the only time there would be discrete shield paths is in the ISO or open position of the switch. For comparison see diagram. It is in my humble opinion that the reliability of a rack I/O panel is inversely proportional to the level of design complexity. Keep it as simple and straightforward as possible.
This method greatly reduces open ended shield lengths. This allows and accommodates most grounded balanced I/O equipment even with poor internal grounding designs to be used without modification to the equipment itself. It does require that all the equipment be AC ground referenced. There may be some equipment exceptions to be addressed in any inventory such as two pronged AC cord gear or unbalanced gear. The equipment is still individually Pin1 Lifted or resistively bridged at its actual input connector but that end of the shield wire would be typically less than about 3' from the earth grounded redirect point within the rack instead of as much 250' or more of open ended shields. A custom panel design might be more complex than this diagram to suit the individual needs of a given system with respect to the number of lines and if it wants to accommodate both inputs and outputs but this is the basic routing.
It also establishes, in addition to the distributed AC earth ground, a sizable low impedance ground contact between racks that collectively can be of higher gauge than the actual distributed AC earth ground if multicore connecting cable is used depending on the number of shielded pairs of signal lines in the cable. As an example of this, I had an occasion where we were fixing a PA system than was using the questionable practice of AC ground lifting just about EVERYTHING! Shields were connected at both ends everywhere but only one thing in the system saw an earth ground - the system ground was made via a passive hardwired microphone splitter (non-transformer) that bussed all I/O shields together and AC grounded them at the splitter rack. (see Other Ground Schemes:). Sadly, that was the only ground path for the entire system. Because the shields were connected at both ends on all cable runs that system necessitated lifting AC ground on virtually everything else in the system, consoles, amp racks - the lot, just to keep it from buzzing. We disconnected the splitter's AC ground and shield lifted the split to the monitor console allowing us to ground both the Front of House and Monitor systems. Although this method was certainly much much safer (!), it was simply not as quiet as the dangerous earth lifting method even after careful maintenance of the AC system ground paths and connections (although a resistor-capacitor iso network bridging the shield lifted monitor side finally cured the remaining noise). I think one reason for the initial performance disparity was the huge cumulative ground plane between the consoles formed by up to forty-eight 20 (or 22) gauge shield wires directly linking the two consoles' shield inputs together via the splitter - acting like a huge ground bus bar forcing them to the same potential! You don't need to worry about that sort of thing at all if you use a good transformer isolated microphone splitter but this just serves as an example of a what a huge ground bus that many shields can look like.
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