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Sound Advice

Star Grounds, Loop Areas, & Electrical Safety In Project Studios, Edit Suites, & Other Compact Audio Installations Part II by Neil A. Muncy

A Star grounding scheme, in which all equipment in an installation is bonded to a central ground hub, can be useful for minimizing low frequency common mode voltages between various pieces of equipment if it’s properly implemented. If not properly implemented, star grounding can result in performance, which in some cases is actually worse than that resulting from a completely haphazard approach.

Any secondary grounding system installed in parallel with already existing equipment U-Ground conductors in an installation has the instant effect of causing far more potential ground loops between equipment than would otherwise exist. Sometimes it makes a difference, sometimes it doesn’t. The $64 question is whether it reliably, and without exception, makes noise go away permanently and completely. Not likely.

A popular Star Grounding practice involves using separate ground wires to bond all equipment in the ensemble to a central hub, and then connecting this hub to a dedicated earth-grounding terminal, which is not bonded to the main building ground system. This practice is very dangerous and is completely illegal in the context of North American Electrical Codes.

One connection between an ensemble of equipment and building ground is all that is needed to make the system safe in terms of both the letter and intent of applicable electrical codes. Most installations usually involve more than one AC power circuit, whether actually required due to the size of the total load or not.

What is not considered in such a scenario is how long and by what path(s) the power circuits and their respective equipment ground conductors take before they get back together at the breaker panel. Just because two outlets are within a few feet of each other does not necessarily mean that they are on the same circuit.

In smallish installations in which all equipment is in one area/room and the longest audio cables are perhaps less than 100′ in length, and assuming that the breaker panel is somewhere else in the building, a very effective approach is to arrange to have all of the power circuits end up at a point in one box in the middle of the equipment ensemble. Very often, this middle point would be in the floor trench under the tabletop of the producer’s table equipment cabinet behind where the engineer/producer sits.

Install as many circuits as you think you need. What this scheme buys you is that by bringing all circuits into one multi-gang outlet box, all of the associated equipment ground conductors (one per circuit) also end up in the same box, all bonded together as prescribed by code. This star point becomes your one connection back to building ground, with the added advantage that now you have a demonstrably lower impedance path back to building ground by virtue of having X paralleled equipment ground conductors.

From this central box, 3-wire branch circuits are then run out to each grouping of equipment. If at all possible, all of these circuits should be in one continuous raceway/conduit, so that the associated equipment ground conductors are daisy-chained throughout the facility. This ensures that the total length of the equipment ground conductors between different equipment locations within the room is as short as possible. For only a few circuits, series-connected power bars are acceptable for this application, but use good ones and try to stay away from conventional “Surge Protected” ones, which employ Metal Oxide Varistors (MOVs) – they have been known to start fires when they ultimately outlive their service life. This ensures that the total length of the equipment ground conductors between different equipment locations within the room is as short as possible. As simple as this seems, this approach may eliminate enough residual noise so as to end the effort to go any further.

“OK wise guy, so what happens when I then run shielded audio cables all over the place?” you ask. “Don’t I end up with a big bunch of ground loops anyway?” Yes you do. Minimize the areas of the resulting ground loops by selectively cutting cable shields at one end or the other, the One-End-Only (OEO) approach. This is a simple way of smothering the symptoms of Pin-1 problems, and while in larger systems it may be required for other reasons, it’s usually not necessary in a small installation – besides which, it’s a pain in the ass and you can’t do it anyway in unbalanced single-conductor shielded cable installations for reasons which should be obvious. What you can do to minimize these loop areas is to simply run all of the low-level audio cables parallel and adjacent to your new branch power cables.

Be sure to pick up the October issue of PS where Muncy delves into the Pin-1 problem and other RFI solutions.

Neil Muncy has been around since the days when recorded sound was analog mono and vacuum tubes ruled the audio landscape. He has been a consultant in the audio field for many years, and can be contacted by e-mail at: nmuncy@allstream.net.

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