Conventional wisdom has dictated that, since our audio equipment uses a common distributed AC ground that gets bonded to the internal audio ground in almost all our gear, and that all gear should be AC grounded, we should use the AC ground as the forced common audio reference. As previously shown, this presents the problem of dual references between the AC ground and the shield. However, since we must use the AC ground, then it's goodbye to the shield connection. Other alternatives such as modifying all of the equipment are time consuming, expensive, and relatively inflexible. In order to accommodate and encompass the most of amount of gear, the quickest, cheapest, safest, most practical way to avoid hums & buzzes in professional live sound systems is with shield telescoping.
Shield telescoping or "Pin 1 lifting" (as in XLR pin1), is where the shield of the balanced signal line is disconnected at one consistent end (Possible Solution #5). Current thinking dictates lifting the shield at the input end of the line. In a shield telescoped system, the equipment and the major areas of the system are shield isolated from each other so as to not form the common ground loops as described in Section 2 . If the internal grounding of all your equipment resembled Design example #10 in Section 3, shield telescoping might not be necessary because the properly designed equipment would be resistant to ground loops. Unfortunately, I have never seen a full system comprised of only this type of equipment design. As we in the live audio industry are all painfully aware, this is a pie in the sky expectation as there is so much gear around with poorly designed shield handling (Design example #1), the lowest common denominator of these designs must be considered when considering a system wide approach to grounding. That is to say unless you are prepared to spend a LOT of time and money to modify every piece of gear you own or will ever own. There are already enough modifications to be done to create a good ground scheme without adding extensive internal equipment modifications. Taking the gear modification approach becomes a strain on your electronics technicians and ties them up doing that instead of equipment repair and maintenance. It should go without saying however that all new equipment should be inspected for the integrity of its grounding.
Here are some guidelines to follow when employing or troubleshooting a system of this type. I hesitate to call them "rules" because it is unavoidable that they will be sometimes bent or broken but they will serve as an "in an ideal system" concept:
Since you are relying on AC earth ground as your distributed audio reference all audio equipment connected to the system distributed AC power should also be connected to AC earth ground so that the chassis of the equipment is securely grounded. This is for both safety reasons and good grounding practices. b
Applications:
- Since a quality audio circuit reference depends on quality AC ground connections, all ground connections in the audio and AC distribution systems should be of the highest possible quality.
- All AC plugs & outlets in equipment racks and AC distribution boxes should be of specification grade or better. For example; using "hospital grade" Edison duplex outlets is worth the extra money. They not only provide superior contact strength and integrity but that contact strength helps to keep the cables from falling out of the sockets from road vibration and the occasional errant foot.
- All ground terminals in AC outlets should be hard-wired to the incoming AC ground wire and NOT rely on mechanical contact of the receptacle housings.
- Routine inspection and maintenance of all AC systems should be scheduled and mandatory - especially before a tour.
- Any major signal path critical equipment owned in quantity and not manufactured with a three prong AC cable should be refitted with one or the existing cable modified so that it has a grounded AC plug that carries an additional ground conductor connected at least to the equipment chassis and possibly to the audio 0 volt reference if the unit remains quiet and stable.
- Example: A piece of gear that has a two prong AC plug and has a chassis not common to the audio circuit ground should, if possible, be modified so that it does. This type of unit (unmodified) would not have an audio reference if it is totally shield isolated since the unit is not directly AC grounded. The equipment design rationale there is that if there is no AC ground connection, there will be no ground loop.
Since the distributed audio reference is the AC ground and wants to be at the same electrical potential in all your equipment, the audio reference relies on the quality of the equipment's incoming AC earth ground connection to the internal zero volt reference of the equipment. All equipment's internal zero volt audio reference from the DC supply and the equipment chassis must be securely bonded to the incoming AC ground of the unit. It is in most three pronged AC cord pro gear but it should be inspected.
Applications:
- All equipment, upon commissioning, should be inspected for secure internal zero volt (audio ground) to AC ground reference at the chassis bonded connection point. Modifications should be made as needed. Some 0V to chassis/AC ground connections are attempted through paint or anodizing resulting in poor conductivity hence poor reference (it might buzz).
- There will be special situations with certain types of equipment to be addressed. They should be tested and or modified on a case by case basis with respect to how the equipment will properly function in your ground scheme. In small installations or temporary situations, exceptions can be made for special cases where it is easier to modify the connecting signal wiring for a few units instead of the equipment itself. However, a consistent, well implemented ground scheme will be less prone to problems.
- Equipment powered by remote DC power packs will be handled on a case by case basis on their ability to conform to all the guidelines. Generally treat them as a two prong AC cord unit. Because they are difficult to modify and since there probably isn't too many of those types in your pro audio inventory, it is easier to modify the signal cable connections to those units. See Adapter Page.
In order to avoid multi-reference ground loops and audio circuit ground contamination from equipment with poorly designed internal shield routing (Design example #1): Where an audio circuit is completed between two pieces of audio equipment, there should be only one common audio reference established between the two units' audio grounds. That is actually pretty close to a truism no matter what kind of system you are working with but in a shield telescoped system it can be restated more specifically as: Where an audio circuit is completed between two pieces of AC ground referenced audio equipment, the shield (XLR pin 1) of the connecting circuit should be lifted at one end - generally at the input end. There used to be quite a debate on the whether or not to drop the shield at the input or the output of the device. As the more informed view about how seriously bad incoming shield contamination of the audio ground became more widely known, conventional wisdom dictates dropping the shield at the input to avoid the problem.
Applications:
- The wiring standard should be to drop the shield at the input to the device. The most practical and convenient location to do this is in the input XLR cable connector. Until such time as audio equipment manufacturers come more universally into compliance with correct internal grounding designs, this convention will prove to be the most reliable for transportable sound systems used in a variety of professional situations even though it is acknowledged to be not as good having the shield "properly" routed to the chassis ground at both ends. The problem is that too many audio equipment manufacturers do not handle internal shield routing correctly. Transportable sound systems, especially rental systems, see such a wide variety of equipment and interfaces that the lowest common denominators of equipment design must be acknowledged and accommodated to insure the highest probability of success on a daily basis. This method will also minimize the mountain of equipment modifications needed to adhere to a more theoretically correct standard. I am by no means suggesting that this is the correct method for studio or permanent installations as the time and expense are often well worth the effort in those comparatively stable environments for more esoteric solutions. Mobile live audio systems encounter a wide range of situations and variables with respect to different types of temporary equipment interfaces, grounding conventions of other systems, and the sometimes transient nature of the configuration of the system itself.
- Signal connections between equipment should be via appropriate length cable with pin 1 lifted in the male XLR with some sort of visual indication that the line is shield lifted at that particular end. Some sound companies use color coded wire or XLR shells, heat shrunken labels, or special wire markers to designate the cable end as being shield lifted.
- Many units have ground lift switches on them or barrier strip audio ground isolation links. Some switches separate AC ground from audio ground, some lift the XLR pin 1 input, and some lift the pin 1 output. Know the equipment you are dealing with. Each unit should be tested for compatibility with your ground scheme. See Section Three for equipment ground status test procedures.
- Equipment manuals should be consulted to determine whether a ¼" input or output is balanced or unbalanced. See the Section 5 about unbalanced equipment interfaces.
- All standard microphone cables to be wired pin to pin at both ends so as to insure proper microphone connections. XLR pin 1 must not short to the shell! If a mic cable must be used to connect two pieces of grounded equipment, an XLR pin 1 lift adapter should be used with the mic cable - don't wreck a perfectly good microphone cable by opening it up and hacking off the pin 1 connection.
- The majority of systems in the world use XLR pin 2 hot as the standard. All wiring and adapter connectors should reflect only one standard.
- Smaller portable or rental auxiliary racks should be fitted with input panels with shield lift switches on the inputs as they often interface with other companies' equipment and need to be somewhat shield flexible. Cables from the input panel to the equipment should carry the shields intact after the switches; contrary to the guidelines as the shield lifting is to be situational. You can't second guess what some clients might have in the way of a ground scheme when they rent small racks from you. The external lift switch should make an obvious and convenient way for other sound people to integrate the equipment without having to tear it apart to see how it's grounded.
All audio equipment required to receive input from a source greater than roughly 6' away should have balanced inputs.
Applications:
- Equipment requiring balanced inputs can be modified with small internal electronic add on balancing circuits or through the use of an external balancing transformer with leads about a foot long between it and input connector. See Section 5 on transformers and unbalanced input and output interface options. Transformers wherever needed should be of the highest quality. Any and all equipment modifications should be fully documented and submitted for permanent record to the company technical manager.
- If you are sending or receiving an audio signals to or from outside of your system (as defined by anything not powered by your system distributed AC power) use a 1:1 isolation transformer. Use an XLR pin 1 lift (shield lift) adapter cable just ahead of the transformer unless you are absolutely certain that the shield has been lifted at the other end. When in doubt, use an XLR pin 1 lift. A flick of a lift switch (buzz) in some remote place can have you pulling your hair out in the middle of the show. Transformers are available from Jensen Transformers , or Bauer Communications 1124 Barstow Ave. Eugene Oregon 97404 800-627-7277 or 541-689-0893. Jensen also provide excellent schematics and application notes via their web site..
Implementation exceptions you will encounter;
- Microphone Splitters;
Microphone splitters pose a unique problem to implementing a coherent ground scheme due to their more complex signal routing duties. Their grounding methods are absolutely a key to understanding effective system-wide grounding practices. See the special section on Microphone Splitters for more details. Also check out the section on Troubleshooting Loops Between the FOH and Monitor consoles.- Crossover to amp rack signal chains;
In the FOH crossover drive rack to stage amp rack chain, the most desirable method is to drop the shield at the input to the amplifier. However, there are sound companies who elect to drop the shield just before the crossover output multicore exits the FOH drive rack, often after an output transformer. Later in this section I'll discuss crossover to amp rack interfacing and a method of signal input/output panels which would avoid problem in this area and allow amplifiers of various shield input grounding designs to be used successfully without modification while addressing the issue of open shield RF contamination at the same time. Also check out the section on Troubleshooting "ground loops" between the FOH PA system drive rack and the system amp racks.
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