Grounding Techniques

Hum and buzz are the biggest enemies you face when interconnecting a large number of different pieces of equipment to a central audio mixer. This is because each piece of equipment may operate at a marginally different voltage (this difference is called potential) and, when two devices at slightly different potential are physically connected with audio cabling, the end result can be nasty, extraneous noise (mind you, connecting two devices at very different potential can result in a major electrical shock!).

However, there are several steps you can take to avoid grounding problems. First, assuming you have an isolated electrical circuit that can handle the electri- cal demands of your mixer and all connected audio equipment (these needs will usually be modest), you should always plug your mixer and all connected equip- ment into the same circuit. If possible, nothing else but this equipment should be connected to that circuit. If you can’t do this, at least avoid plugging your mixer and audio equipment into the same circuit that is already powering things like heavy machinery, air conditioners, heaters, refrigerators, washing machines, neon signs or fluorescent light fixtures. One particular culprit that will almost cer- tainly create problems is the standard light dimmer (the kind that uses silicon controlled rectifiers). Where low-level lighting is desired, use incandescent fix- tures with autotransformer-type dimmers (sometimes called Variacs) instead— these cost considerably more than the standard dimmer you’ll find at your local hardware store, but are well worth the extra expense.

Three-prong plugs (such as the one used by the MPL 1640) should always be used as is; don’t use adapters to lift the ground (unless you’re using a “star ground network”—see below). If you hear hum or buzz from a device that uses a two-prong plug (or an external two-prong AC/DC adapter), you can try revers- ing the plug in the socket. If that doesn’t work, you may need to physically ground that device’s chassis by connecting a wire (called a strap) from it to a grounded piece of metal such as rack ears. Some pieces of equipment have a screw-type ground post to which the strap can be connected; if not, you can attach some kind of metallic binding post to the case itself. If you are using rack- mounted audio devices and are experiencing hum or buzz, there's a simple test to determine the source of the problem: while keeping all devices powered on and connected with audio cabling, physically remove each device, one by one, from the rack. If the hum disappears when a particular device is removed, you'll know that that device is the culprit.

We also recommend that you use balanced audio cabling and connectors wher- ever possible. The MPL 1640 provides electronically balanced inputs for all channel mic and line inputs as well as for its Main Mix outputs. The wiring dia- gram in the “Connecting The MPL 1640” section of this manual (page 10) shows how 1/4” TRS (Tip/Ring/Sleeve) connectors should be wired for use with these inputs and outputs.

In addition, you can minimize possible interference by planning your audio, elec- trical, and computer cable runs so that they are as far apart from one another as possible and so they don’t run parallel to one another. If they have to cross, try to ensure that they do so at a 90° angle (that is, perpendicular to one another). In particular, try to keep audio cabling away from external AC/DC adapters.