On Solid Ground, part 1 (continued)

Isolated grounding can help eliminate ground-based noise by isolating the studio gear from other sources of noisy ground currents, such its large appliances. However, this noise reduction is predictable only in equipment with balanced audio connections. If you are building a professional-level studio from the ground up (so to speak), establishing an isolated ground might be the first thing to do. However, it usually is a last resort in an existing facility.

Chassis Ground
In a well-designed audio system, the chassis ground (negative terminals) of the power supplies are connected together and all audio-cable shields are connected together (signal ground). Somewhere along the way, these two grounding systems should be connected together as well. (If all your gear is properly designed, these grounds will already be connected for you, but don't bet on it.) The resulting system ground point is the zero signal reference potential. The whole system is connected to the building's ground, which ideally is at the same potential as the earth (earth ground).

On most electrical equipment, the metal chassis should be connected to the safety ground line through the third prong of the power cable, but sometimes the safety ground comes loose. One quick way to verify this connection is to use an ohmmeter to check the continuity between the third prong of the AC plug and the metal case of the equipment. (Make sure to test a part of the case that is not painted.)

It's important to make sure that safety grounds are connected, as they protect you in the event of a short circuit between the chassis and the hot line. Should a short circuit occur inside the equipment, the current will pass through the safety ground to earth, and not through whoever touches the equipment. (Remember, electrons always take the easiest path to ground.) This need for a safety ground also is a strong argument against the use of 3-prong-to-2-prong AC ground lifters (a.k.a. AC plug adapters), which defeat the third-prong safety ground.

If you have older gear with 2-conductor AC connectors, you could have a shock hazard between the chassis of the device and the chassis of, say, a mic. (Newer UL-approved, 2-conductor gear should be okay, but take precautions anyway.)  A microphone case is grounded through the cable shield to the mixer or preamp, so if the latter is properly grounded, the mic should be safe. However, if an older device such as a tube-based guitar amp or a powered mixer uses a poorly grounded, 2-conductor power cord, you might become the easiest path to ground, which could give you a hot time in the old town. Microphones with properly wired, balanced XLR connectors don't pose much of a problem, as the two main conductors carry signals that are 180 degrees out of phase, and the shield does not carry a signal.

To test the ground connection of a mic, use a neon circuit-tester (see Fig. 3) or high-impedance AC voltmeter, and without touching both chassis at the same time with your body, place one lead on the amp or mixer chassis in question and the other lead on the mic chassis. (For added safety, use a clip to attach a lead to the amp chassis, instead of hand-holding the lead. Put one hand in your pocket so your arm can't become a path to ground.) If you find even a few volts difference, you could have a serious problem. First, try reversing the amp's AC plug in the outlet, or if the amp has a ground-reverse switch, try flipping that. If these remedies don't work, try plugging the amp into a different outlet on the same circuit. If nothing helps, don't use the amp; any alternative beats electrocution.

A Great Leap Forward
Well, that about covers the fundamentals of your electrical system. If you have tested your outlets and, with help from your friendly local electrician, corrected any faulty conditions, your studio has come a giant leap closer to electrical sanity. Next month, we'll turn our attention to discovering and correcting ground loops and other wiring conditions that make your studio gear hum and your ears burn.

(The authors would like to thank Alan Gary Campbell, Chris Meyer, Kevin Kaiser, Larry the O, and Charlie Bolois.)

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